% % This file was created by the Typo3 extension % sevenpack version 0.7.14 % % --- Timezone: CEST % Creation date: 2017-05-23 % Creation time: 10-52-56 % --- Number of references % 96 % @Article { TakemuraPWKLSYBLFLW2016, title = {Occipital White Matter Tracts in Human and Macaque}, journal = {Cerebral Cortex}, year = {2017}, month = {3}, volume = {Epub ahead}, abstract = {We compare several major white-matter tracts in human and macaque occipital lobe using diffusion magnetic resonance imaging. The comparison suggests similarities but also significant differences in the tracts. There are several apparently homologous tracts in the 2 species, including the vertical occipital fasciculus (VOF), optic radiation, forceps major, and inferior longitudinal fasciculus (ILF). There is one large human tract, the inferior fronto-occipital fasciculus, with no corresponding fasciculus in macaque. We could identify the macaque VOF (mVOF), which has been little studied. Its position is consistent with classical invasive anatomical studies by Wernicke. VOF homology is supported by similarity of the endpoints in V3A and ventral V4 across species. The mVOF fibers intertwine with the dorsal segment of the ILF, but the human VOF appears to be lateral to the ILF. These similarities and differences between the occipital lobe tracts will be useful in establishing which circuitry in the macaque can serve as an accurate model for human visual cortex.}, department = {Department Logothetis}, web_url = {https://academic.oup.com/cercor/article-lookup/doi/10.1093/cercor/bhx070}, DOI = {10.1093/cercor/bhx070}, author = {Takemura, H and Pestilli, F and Weiner, KS and Keliris, GA and Landi, SM and Sliwa, J and Ye, FQ and Barnett, MA and Leopold, DA and Freiwald, WA and Logothetis, NK and Wandell, BA} } @Article { OrtizRiosAKBMKLR2017, title = {Widespread and Opponent fMRI Signals Represent Sound Location in Macaque Auditory Cortex}, journal = {Neuron}, year = {2017}, month = {2}, volume = {93}, number = {4}, pages = {971–983}, abstract = {In primates, posterior auditory cortical areas are thought to be part of a dorsal auditory pathway that processes spatial information. But how posterior (and other) auditory areas represent acoustic space remains a matter of debate. Here we provide new evidence based on functional magnetic resonance imaging (fMRI) of the macaque indicating that space is predominantly represented by a distributed hemifield code rather than by a local spatial topography. Hemifield tuning in cortical and subcortical regions emerges from an opponent hemispheric pattern of activation and deactivation that depends on the availability of interaural delay cues. Importantly, these opponent signals allow responses in posterior regions to segregate space similarly to a hemifield code representation. Taken together, our results reconcile seemingly contradictory views by showing that the representation of space follows closely a hemifield code and suggest that enhanced posterior-dorsal spatial specificity in primates might emerge from this form of coding.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S0896627317300375}, DOI = {10.1016/j.neuron.2017.01.013}, author = {Ortiz-Rios, M and Azevedo, FAC and Kuśmierek, P and Balla, DZ and Munk, MH and Keliris, GA and Logothetis, NK and Rauschecker, P} } @Article { ShahBKKJVV2015, title = {Cholinergic and serotonergic modulations differentially affect large-scale functional networks in the mouse brain}, journal = {Brain Structure and Function}, year = {2016}, month = {7}, volume = {221}, number = {6}, pages = {3067–3079}, abstract = {Resting-state functional MRI (rsfMRI) is a widely implemented technique used to investigate large-scale topology in the human brain during health and disease. Studies in mice provide additional advantages, including the possibility to flexibly modulate the brain by pharmacological or genetic manipulations in combination with high-throughput functional connectivity (FC) investigations. Pharmacological modulations that target specific neurotransmitter systems, partly mimicking the effect of pathological events, could allow discriminating the effect of specific systems on functional network disruptions. The current study investigated the effect of cholinergic and serotonergic antagonists on large-scale brain networks in mice. The cholinergic system is involved in cognitive functions and is impaired in, e.g., Alzheimer’s disease, while the serotonergic system is involved in emotional and introspective functions and is impaired in, e.g., Alzheimer’s disease, depression and autism. Specific interest goes to the default-mode-network (DMN), which is studied extensively in humans and is affected in many neurological disorders. The results show that both cholinergic and serotonergic antagonists impaired the mouse DMN-like network similarly, except that cholinergic modulation additionally affected the retrosplenial cortex. This suggests that both neurotransmitter systems are involved in maintaining integrity of FC within the DMN-like network in mice. Cholinergic and serotonergic modulations also affected other functional networks, however, serotonergic modulation impaired the frontal and thalamus networks more extensively. In conclusion, this study demonstrates the utility of pharmacological rsfMRI in animal models to provide insights into the role of specific neurotransmitter systems on functional networks in neurological disorders.}, department = {Department Logothetis}, web_url = {http://link.springer.com/content/pdf/10.1007\%2Fs00429-015-1087-7.pdf}, DOI = {10.1007/s00429-015-1087-7}, author = {Shah, D and Blockx, I and Keliris, GA and Kara, F and Jonckers, E and Verhoye, M and Van der Linden, A} } @Article { PapanikolaouKLLS2015, title = {Nonlinear population receptive field changes in human area V5/MT + of healthy subjects with simulated visual field scotomas}, journal = {NeuroImage}, year = {2015}, month = {10}, volume = {120}, pages = {176–190}, abstract = {There is extensive controversy over whether the adult visual cortex is able to reorganize following visual field loss (scotoma) as a result of retinal or cortical lesions. Functional magnetic resonance imaging (fMRI) methods provide a useful tool to study the aggregate receptive field properties and assess the capacity of the human visual cortex to reorganize following injury. However, these methods are prone to biases near the boundaries of the scotoma. Retinotopic changes resembling reorganization have been observed in the early visual cortex of normal subjects when the visual stimulus is masked to simulate retinal or cortical scotomas. It is not known how the receptive fields of higher visual areas, like hV5/MT +, are affected by partial stimulus deprivation. We measured population receptive field (pRF) responses in human area V5/MT + of 5 healthy participants under full stimulation and compared them with responses obtained from the same area while masking the left superior quadrant of the visual field (“artificial scotoma” or AS). We found that pRF estimations in area hV5/MT + are nonlinearly affected by the AS. Specifically, pRF centers shift towards the AS, while the pRF amplitude increases and the pRF size decreases near the AS border. The observed pRF changes do not reflect reorganization but reveal important properties of normal visual processing under different test-stimulus conditions.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S105381191500590X}, DOI = {10.1016/j.neuroimage.2015.06.085}, author = {Papanikolaou, A and Keliris, GA and Lee, S and Logothetis, NK and Smirnakis, SM} } @Article { AzevedoOLLK2015, title = {A Potential Role of Auditory Induced Modulations in Primary Visual Cortex}, journal = {Multisensory Research}, year = {2015}, month = {7}, volume = {28}, number = {3-4}, pages = {331-349}, abstract = {A biologically relevant event is normally the source of multiple, typically correlated, sensory inputs. To optimize perception of the outer world, our brain combines the independent sensory measurements into a coherent estimate. However, if sensory information is not readily available for every pertinent sense, the brain tries to acquire additional information via covert/overt orienting behaviors or uses internal knowledge to modulate sensory sensitivity based on prior expectations. Cross-modal functional modulation of low-level auditory areas due to visual input has been often described; however, less is known about auditory modulations of primary visual cortex. Here, based on some recent evidence, we propose that an unexpected auditory signal could trigger a reflexive overt orienting response towards its source and concomitantly increase the primary visual cortex sensitivity at the locations where the object is expected to enter the visual field. To this end, we propose that three major functionally specific pathways are employed in parallel. A stream orchestrated by the superior colliculus is responsible for the overt orienting behavior, while direct and indirect (via higher-level areas) projections from A1 to V1 respectively enhance spatiotemporal sensitivity and facilitate object detectability.}, department = {Department Logothetis}, web_url = {http://booksandjournals.brillonline.com/content/journals/10.1163/22134808-00002494}, DOI = {10.1163/22134808-00002494}, author = {Azevedo, FAC and Ortiz-Rios, M and Li, Q and Logothetis, NK and Keliris, GA} } @Article { OrtizRiosKDAASJKR2015, title = {Functional MRI of the vocalization-processing network in the macaque brain}, journal = {Frontiers in Neuroscience}, year = {2015}, month = {4}, volume = {9}, number = {113}, pages = {1-10}, abstract = {Using functional magnetic resonance imaging in awake behaving monkeys we investigated how species-specific vocalizations are represented in auditory and auditory-related regions of the macaque brain. We found clusters of active voxels along the ascending auditory pathway that responded to various types of complex sounds: inferior colliculus (IC), medial geniculate nucleus (MGN), auditory core, belt, and parabelt cortex, and other parts of the superior temporal gyrus (STG) and sulcus (STS). Regions sensitive to monkey calls were most prevalent in the anterior STG, but some clusters were also found in frontal and parietal cortex on the basis of comparisons between responses to calls and environmental sounds. Surprisingly, we found that spectrotemporal control sounds derived from the monkey calls (“scrambled calls”) also activated the parietal and frontal regions. Taken together, our results demonstrate that species-specific vocalizations in rhesus monkeys activate preferentially the auditory ventral stream, and in particular areas of the antero-lateral belt and parabelt.}, department = {Department Logothetis}, web_url = {http://journal.frontiersin.org/article/10.3389/fnins.2015.00113/pdf}, DOI = {10.3389/fnins.2015.00113}, author = {Ortiz-Rios, M and Kuśmierek, P and DeWitt, I and Archakov, IA and Azevedo, FA and Sams, M and J{\"a}{\"a}skel{\"a}inen, I and Keliris, GA and Rauschecker, JP} } @Article { LeePKS2014, title = {Topographical estimation of visual population receptive fields by fMRI}, journal = {Journal of Visualized Experiments}, year = {2015}, month = {2}, number = {96}, pages = {1-8}, abstract = {Visual cortex is retinotopically organized so that neighboring populations of cells map to neighboring parts of the visual field. Functional magnetic resonance imaging allows us to estimate voxel-based population receptive fields (pRF), i.e., the part of the visual field that activates the cells within each voxel. Prior, direct, pRF estimation methods1 suffer from certain limitations: 1) the pRF model is chosen a-priori and may not fully capture the actual pRF shape, and 2) pRF centers are prone to mislocalization near the border of the stimulus space. Here a new topographical pRF estimation method2 is proposed that largely circumvents these limitations. A linear model is used to predict the Blood Oxygen Level-Dependent (BOLD) signal by convolving the linear response of the pRF to the visual stimulus with the canonical hemodynamic response function. PRF topography is represented as a weight vector whose components represent the strength of the aggregate response of voxel neurons to stimuli presented at different visual field locations. The resulting linear equations can be solved for the pRF weight vector using ridge regression3, yielding the pRF topography. A pRF model that is matched to the estimated topography can then be chosen post-hoc, thereby improving the estimates of pRF parameters such as pRF-center location, pRF orientation, size, etc. Having the pRF topography available also allows the visual verification of pRF parameter estimates allowing the extraction of various pRF properties without having to make a-priori assumptions about the pRF structure. This approach promises to be particularly useful for investigating the pRF organization of patients with disorders of the visual system.}, department = {Department Logothetis}, web_url = {https://www.jove.com/video/51811/topographical-estimation-of-visual-population-receptive-fields-by-fmri}, DOI = {10.3791/51811}, EPUB = {e51811}, author = {Lee, S and Papanikolaou, A and Keliris, GA and Smirnakis, SM} } @Article { BahmaniMLK2014, title = {Binocular Flash Suppression in the Primary Visual Cortex of Anesthetized and Awake Macaques}, journal = {PLoS ONE}, year = {2014}, month = {9}, volume = {9}, number = {9}, pages = {1-8}, abstract = {Primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies. However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1. In particular, only a moderate percentage of neurons in V1 were found to modulate in parallel with perception with magnitude substantially smaller than the physical preference of these neurons. It is yet unclear whether these small modulations are rooted from local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. We found that a small but significant modulation was present in both the anesthetized and awake states during the flash suppression presentation. Furthermore, the relative amplitudes of the perceptual modulations were not significantly different in the two states. We suggest that these early effects of perceptual suppression might occur locally in V1, in prior processing stages or within early visual cortical areas in the absence of top-down feedback from higher cognitive stages that are suppressed under anesthesia.}, department = {Department Logothetis}, web_url = {http://www.plosone.org/article/fetchObject.action?uri=info\%3Adoi\%2F10.1371\%2Fjournal.pone.0107628\&representation=PDF}, DOI = {10.1371/journal.pone.0107628}, EPUB = {e107628}, author = {Bahmani, H and Murayama, Y and Logothetis, NK and Keliris, GA} } @Article { SchmidK2014, title = {Filling-in versus filling-out: patterns of cortical short-term plasticity}, journal = {Trends in Cognitive Sciences}, year = {2014}, month = {7}, volume = {18}, number = {7}, pages = {342–344}, abstract = {Investigations of topographic cortical plasticity following peripheral nervous injury predominantly report receptive field (RF) shifts toward the intact periphery. A recent study on visual cortex plasticity following retinal lesions by Botelho et al. finds RF coverage of the lesion affected space when global retinotopic mapping strategies are used.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S1364661314000308}, DOI = {10.1016/j.tics.2014.01.013}, author = {Schmid, MC and Keliris, GA} } @Article { PapanikolaouKPSKPSBSLS2014, title = {Population receptive field analysis of the primary visual cortex complements perimetry in patients with homonymous visual field defects}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, year = {2014}, month = {4}, volume = {111}, number = {16}, pages = {E1656–E1665}, abstract = {Injury to the primary visual cortex (V1) typically leads to loss of conscious vision in the corresponding, homonymous region of the contralateral visual hemifield (scotoma). Several studies suggest that V1 is highly plastic after injury to the visual pathways, whereas others have called this conclusion into question. We used functional magnetic resonance imaging (fMRI) to measure area V1 population receptive field (pRF) properties in five patients with partial or complete quadrantic visual field loss as a result of partial V1+ or optic radiation lesions. Comparisons were made with healthy controls deprived of visual stimulation in one quadrant [“artificial scotoma” (AS)]. We observed no large-scale changes in spared-V1 topography as the V1/V2 border remained stable, and pRF eccentricity versus cortical-distance plots were similar to those of controls. Interestingly, three observations suggest limited reorganization: (i) the distribution of pRF centers in spared-V1 was shifted slightly toward the scotoma border in 2 of 5 patients compared with AS controls; (ii) pRF size in spared-V1 was slightly increased in patients near the scotoma border; and (iii) pRF size in the contralesional hemisphere was slightly increased compared with AS controls. Importantly, pRF measurements yield information about the functional properties of spared-V1 cortex not provided by standard perimetry mapping. In three patients, spared-V1 pRF maps overlapped significantly with dense regions of the perimetric scotoma, suggesting that pRF analysis may help identify visual field locations amenable to rehabilitation. Conversely, in the remaining two patients, spared-V1 pRF maps failed to cover sighted locations in the perimetric map, indicating the existence of V1-bypassing pathways able to mediate useful vision.}, department = {Department Logothetis}, web_url = {http://www.pnas.org/content/111/16/E1656.full.pdf+html}, DOI = {10.1073/pnas.1317074111}, author = {Papanikolaou, A and Keliris, GA and Papageorgiou, TD and Shao, Y and Krapp, E and Papageorgiou, E and Stingl, K and Bruckmann, A and Schiefer, U and Logothetis, NK and Smirnakis, SM} } @Article { LeePLSK2013, title = {A new method for estimating population receptive field topography in visual cortex}, journal = {NeuroImage}, year = {2013}, month = {11}, volume = {81}, pages = {144–157}, abstract = {We introduce a new method for measuring visual population receptive fields (pRF) with functional magnetic resonance imaging (fMRI). The pRF structure is modeled as a set of weights that can be estimated by solving a linear model that predicts the Blood Oxygen Level-Dependent (BOLD) signal using the stimulus protocol and the canonical hemodynamic response function. This method does not make a priori assumptions about the specific pRF shape and is therefore a useful tool for uncovering the underlying pRF structure at different spatial locations in an unbiased way. We show that our method is more accurate than a previously described method (Dumoulin and Wandell, 2008) which directly fits a 2-dimensional isotropic Gaussian pRF model to predict the fMRI time-series. We demonstrate that direct-fit models do not fully capture the actual pRF shape, and can be prone to pRF center mislocalization when the pRF is located near the border of the stimulus space. A quantitative comparison demonstrates that our method outperforms the direct-fit methods in the pRF center modeling by achieving higher explained variance of the BOLD signal. This was true for direct-fit isotropic Gaussian, anisotropic Gaussian, and difference of isotropic Gaussians model. Importantly, our model is also capable of exploring a variety of pRF properties such as surround suppression, receptive field center elongation, orientation, location and size. Additionally, the proposed method is particularly attractive for monitoring pRF properties in the visual areas of subjects with lesions of the visual pathways, where it is difficult to anticipate what shape the reorganized pRF might take. Finally, the method proposed here is more efficient in computation time than direct-fit methods, which need to search for a set of parameters in an extremely large searching space. Instead, this method uses the pRF topography to constrain the space that needs to be searched for the subsequent modeling.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S105381191300520X20X}, DOI = {10.1016/j.neuroimage.2013.05.026}, author = {Lee, S and Papanikolaou, A and Logothetis, NK and Smirnakis, SM and Keliris, GA} } @Article { ShaoKPFZ2013, title = {Visual cortex organisation in a macaque monkey with macular degeneration}, journal = {European Journal of Neuroscience}, year = {2013}, month = {11}, volume = {38}, number = {10}, pages = {3456–3464}, abstract = {The visual field is retinotopically represented in early visual areas. It has been suggested that when adult primary visual cortex (V1) is deprived of normal retinal input it is capable of large-scale reorganisation, with neurons inside the lesion projection zone (LPZ) being visually driven by inputs from intact retinal regions. Early functional magnetic resonance imaging (fMRI) studies in humans with macular degeneration (MD) report > 1 cm spread of activity inside the LPZ border, whereas recent results report no shift of the LPZ border. Here, we used fMRI population receptive field measurements to study, for the first time, the visual cortex organisation of one macaque monkey with MD and to compare it with normal controls. Our results showed that the border of the V1 LPZ remained stable, suggesting that the deafferented area V1 zone of the MD animal has limited capacity for reorganisation. Interestingly, the pRF size of non-deafferented V1 voxels increased slightly (\verb=~=20\% on average), although this effect appears weaker than that in previous single-unit recording reports. Area V2 also showed limited reorganisation. Remarkably, area V5/MT of the MD animal showed extensive activation compared to controls stimulated over the part of the visual field that was spared in the MD animal. Furthermore, population receptive field size distributions differed markedly in area V5/MT of the MD animal. Taken together, these results suggest that V5/MT has a higher potential for reorganisation after MD than earlier visual cortex.}, department = {Department Logothetis}, web_url = {http://onlinelibrary.wiley.com/doi/10.1111/ejn.12349/pdf}, DOI = {10.1111/ejn.12349}, author = {Shao, Y and Keliris, GA and Papanikolaou, A and Fischer, MD and Zobor, D and J{\"a}gle, H and Logothetis, NK and Smirnakis, SM} } @Article { MaierPTK2012, title = {Introduction to research topic – binocular rivalry: a gateway to studying consciousness}, journal = {Frontiers in Human Neuroscience}, year = {2012}, month = {9}, volume = {6}, number = {263}, pages = {1-3}, abstract = {In 1593, Neapolitan polymath Giambattista della Porta publicly lamented that he was unable to improve his impressive productivity (he had published in areas as diverse as cryptography, hydraulics, pharmacology, optics, and classic fiction). Della Porta was trying to read two books simultaneously by placing both volumes side-by-side, and using each eye independently. To his great surprise, his setup allowed him to only read one book at a time. This discovery arguably marks the first written account of binocular rivalry (Wade, 2000) – a perceptual phenomenon that more than 400 years later still both serves to intrigue as well as to illuminate the limits of scientific knowledge. At first glance, binocular rivalry is an oddball. In every day vision, our eyes receive largely matching views of the world. The brain combines the two images into a cohesive scene, and concurrently, perception is stable. However, when showing two very different images (such as two different books) to each eye, the brain resolves the conflict by adopting a “diplomatic” strategy. Rather than mixing the views of the two eyes into an insensible visual percept, observers perceive a dynamically changing series of perceptual snapshots, with one eye’s view dominating for a few seconds before being replaced by its rival from the other eye. With prolonged viewing of a rivalrous stimulus, one inevitably experiences a sequence of subjective perceptual reversals, separated by random time intervals, and this process continues for as long as the sensory conflict is present.}, department = {Department Logothetis}, web_url = {http://www.frontiersin.org/Human_Neuroscience/10.3389/fnhum.2012.00263/full}, DOI = {10.3389/fnhum.2012.00263}, author = {Maier, A and Panagiotaropoulos, TI and Tsuchiya, N and Keliris, GA} } @Article { FischerZKSSJLS2012, title = {Detailed functional and structural characterization of a macular lesion in a rhesus macaque}, journal = {Documenta Ophthalmologica}, year = {2012}, month = {8}, volume = {125}, number = {3}, pages = {179-194}, abstract = {Animal models are powerful tools to broaden our understanding in disease mechanisms and to develop future treatment strategies. Here we present detailed structural and functional findings of a rhesus macaque suffering from a naturally occurring bilateral macular dystrophy (BMD), partial optic atrophy and corresponding reduction of central V1 signals in visual fMRI experiments when compared to data in a healthy macaque (CTRL) of similar age. Fluorescence and indocyanine green angiography showed reduced macular vascularization with significantly larger foveal avascular zones (FAZ) in the affected animal (FAZBMD = 8.85 mm2 vs. FAZCTRL = 0.32 mm2). Optical coherence tomography showed bilateral thinning of the macula within the FAZ (total retinal thickness, TRTBMD = 174 ± 9 \(\mu\)m) and partial optic nerve atrophy when compared to control (TRTCTRL = 303 ± 45 \(\mu\)m). Segmentation analysis revealed that inner retinal layers were primarily affected (inner retinal thickness, IRTBMD = 33 ± 9 \(\mu\)m vs. IRTCTRL = 143 ± 45 \(\mu\)m), while the outer retina essentially maintained its thickness (ORTBMD = 141 ± 7 \(\mu\)m vs. ORTCTRL = 160 ± 11 \(\mu\)m). Accordingly, a strong central reduction in the multifocal electroretinography and a specific attenuation of cone-derived signals in Ganzfeld electroretinography was found, whereas rod function remained normal. We provided detailed characterization of a primate macular disorder. This study aims to stimulate awareness and further investigation in primates with macular disorders eventually leading to the identification of a primate animal model and facilitating the preclinical development of therapeutic strategies.}, department = {Department Logothetis}, web_url = {http://link.springer.com/content/pdf/10.1007\%2Fs10633-012-9340-3}, DOI = {10.1007/s10633-012-9340-3}, author = {Fischer, MD and Zobor, D and Keliris, GA and Shao, Y and Seeliger, MW and Haverkamp, S and J{\"a}gle, H and Logothetis, NK and Smirnakis, SM} } @Article { StoewerGKBLDS2012, title = {An Analysis Approach for High-Field fMRI Data from Awake Non-Human Primates}, journal = {PLoS One}, year = {2012}, month = {1}, volume = {7}, number = {1}, pages = {1-13}, abstract = {fMRI experiments with awake non-human primates (NHP) have seen a surge of applications in recent years. However, the standard fMRI analysis tools designed for human experiments are not optimal for analysis of NHP fMRI data collected at high fields. There are several reasons for this, including the trial-based nature of NHP experiments, with inter-trial periods being of no interest, and segmentation artefacts and distortions that may result from field changes due to movement. We demonstrate an approach that allows us to address some of these issues consisting of the following steps: 1) Trial-based experimental design. 2) Careful control of subject movement. 3) Computer-assisted selection of trials devoid of artefacts and animal motion. 4) Nonrigid between-trial and rigid within-trial realignment of concatenated data from temporally separated trials and sessions. 5) Linear interpolation of inter-trial intervals and high-pass filtering of temporally continuous data 6) Removal of interpolated data and reconcatenation of datasets before statistical analysis with SPM. We have implemented a software toolbox, fMRI Sandbox (http://code.google.com/p/fmri-sandbox/), for semi-automated application of these processing steps that interfaces with SPM software. Here, we demonstrate that our methodology provides significant improvements for the analysis of awake monkey fMRI data acquired at high-field. The method may also be useful for clinical applications with subjects that are unwilling or unable to remain motionless for the whole duration of a functional scan.}, department = {Department Logothetis}, web_url = {http://www.plosone.org/article/info\%3Adoi\%2F10.1371\%2Fjournal.pone.0029697}, DOI = {10.1371/journal.pone.0029697}, EPUB = {e29697}, author = {Stoewer, S and Goense, J and Keliris, GA and Bartels, A and Logothetis, NK and Duncan, J and Sigala, N} } @Article { StoewerGKBLDS2011_2, title = {Realignment strategies for awake-monkey fMRI data}, journal = {Magnetic Resonance Imaging}, year = {2011}, month = {12}, volume = {29}, number = {10}, pages = {1390-1400}, abstract = {Functional magnetic resonance imaging (fMRI) experiments with awake nonhuman primates (NHPs) have recently seen a surge of applications. However, the standard fMRI analysis tools designed for human experiments are not optimal for NHP data collected at high fields. One major difference is the experimental setup. Although real head movement is impossible for NHPs, MRI image series often contain visible motion artifacts. Animal body movement results in image position changes and geometric distortions. Since conventional realignment methods are not appropriate to address such differences, algorithms tailored specifically for animal scanning become essential. We have implemented a series of high-field NHP specific methods in a software toolbox, fMRI Sandbox (http://kyb.tuebingen.mpg.de/\verb=~=stoewer/), which allows us to use different realignment strategies. Here we demonstrate the effect of different realignment strategies on the analysis of awake-monkey fMRI data acquired at high field (7 T). We show that the advantage of using a nonstandard realignment algorithm depends on the amount of distortion in the dataset. While the benefits for less distorted datasets are minor, the improvement of statistical maps for heavily distorted datasets is significant.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science?_ob=MiamiImageURL\&_cid=271222\&_user=29041\&_pii=S0730725X11001809\&_check=y\&_origin=\&_coverDate=31-Dec-2011\&view=c\&wchp=dGLzVBA-zSkWb\&md5=faaec51a67a063db4ac8f1979129b81b/1-s2.0-S0730725X11001809-main.pdf}, DOI = {10.1016/j.mri.2011.05.003}, author = {Stoewer, S and Goense, J and Keliris, GA and Bartels, A and Logothetis, NK and Duncan, J and Sigala, N} } @Article { 6683, title = {The Role of the Primary Visual Cortex in Perceptual Suppression of Salient Visual Stimuli}, journal = {Journal of Neuroscience}, year = {2010}, month = {9}, volume = {30}, number = {37}, pages = {12353-12365}, abstract = {The role of primary visual cortex (area V1) in subjective perception has intrigued students of vision for decades. Specifically, the extent to which the activity of different types of cells (monocular versus binocular) and electrophysiological signals (i.e. local field potentials versus spiking activity) reflect perception is still debated. To address these questions we recorded from area V1 of the macaque using tetrodes during the paradigm of binocular flash suppression, where incongruent images presented dichoptically compete for perceptual dominance. We found that the activity of a minority (20\%) of neurons reflect the perceived visual stimulus and these cells exhibited perceptual modulations substantially weaker in comparison to their sensory modulation induced by congruent stimuli. Importantly, perceptual modulations were found equally often for monocular and binocular cells, demonstrating that perceptual competition in V1 involves mechanisms across both types of neurons. The power of the local field pot ential (LFP) also showed moderate perceptual modulations with similar percentages of sites showing significant effects across frequency bands (18-22\%). The possibility remains that perception may be strongly reflected in more elaborate aspects of activity in V1 circuits (e.g. specific neuronal subtypes) or perceptual states might have a modulatory role on more intricate aspects of V1 firing patterns (e.g. synchronization), not necessarily altering the firing rates of single cells or the LFP power dramatically.}, department = {Department Logothetis}, web_url = {http://www.jneurosci.org/cgi/reprint/30/37/12353}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1523/JNEUROSCI.0677-10.2010}, author = {Keliris, GA and Logothetis, NK and Tolias, AS} } @Article { 6257, title = {Decorrelated Neuronal Firing in Cortical Microcircuits}, journal = {Science}, year = {2010}, month = {1}, volume = {327}, number = {5965}, pages = {584-587}, abstract = {Correlated trial-to-trial variability in the activity of cortical neurons is thought to reflect the functional connectivity of the circuit. Many cortical areas are organized into functional columns, in which neurons are believed to be densely connected and to share common input. Numerous studies report a high degree of correlated variability between nearby cells. We developed chronically implanted multitetrode arrays offering unprecedented recording quality to reexamine this question in the primary visual cortex of awake macaques. We found that even nearby neurons with similar orientation tuning show virtually no correlated variability. Our findings suggest a refinement of current models of cortical microcircuit architecture and function: Either adjacent neurons share only a few percent of their inputs or, alternatively, their activity is actively decorrelated.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, web_url = {http://www.sciencemag.org/cgi/reprint/327/5965/584.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1126/science.1179867}, author = {Ecker, AS and Berens, P and Keliris, GA and Bethge, M and Logothetis, NK and Tolias, AS} } @Article { 5614, title = {Feature selectivity of the gamma-band of the local field potential in primate primary visual cortex}, journal = {Frontiers in Neuroscience}, year = {2008}, month = {12}, volume = {2}, number = {2}, pages = {199-207}, abstract = {Extra-cellular voltage fluctuations (local field potentials; LFPs) reflecting neural mass action are ubiquitous across species and brain regions. Numerous studies have characterized the properties of LFP signals in the cortex to study sensory and motor computations as well as cognitive processes like attention, perception and memory. In addition, its extracranial counterpart – the electroencelphalogram (EEG) – is widely used in clinical applications. However, the link between LFP signals and the underlying activity of local populations of neurons remains largely elusive. Here, we review recent work elucidating the relationship between spiking activity of local neural populations and LFP signals. We focus on oscillations in the gamma-band (30-90Hz) of the local field potential in the primary visual cortex (V1) of the macaque that dominate during visual stimulation. Given that in area V1 much is known about the properties of single neurons and the cortical architecture, it provides an excellent opportunity to study the mechanisms underlying the generation of the local field potential.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, web_url = {http://frontiersin.org/neuroscience/paper/10.3389/neuro.01/037.2008/pdf/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.3389/neuro.01.037.2008}, author = {Berens, P and Keliris, GA and Ecker, AS and Logothetis, NK and Tolias, AS} } @Article { 5205, title = {Comparing the feature selectivity of the gamma-band of the local field potential and the underlying spiking activity in primate visual cortex}, journal = {Frontiers in Systems Neuroscience}, year = {2008}, month = {6}, volume = {2}, number = {2}, pages = {1-11}, abstract = {The local field potential (LFP), comprised of low-frequency extra-cellular voltage fluctuations, has been used extensively to study the mechanisms of brain function. In particular, oscillations in the gamma-band (30–90 Hz) are ubiquitous in the cortex of many species during various cognitive processes. Surprisingly little is known about the underlying biophysical processes generating this signal. Here, we examine the relationship of the local field potential to the activity of localized populations of neurons by simultaneously recording spiking activity and LFP from the primary visual cortex (V1) of awake, behaving macaques. The spatial organization of orientation tuning and ocular dominance in this area provides an excellent opportunity to study this question, because orientation tuning is organized at a scale around one order of magnitude finer than the size of ocular dominance columns. While we find a surprisingly weak correlation between the preferred orientation of multi-unit activity and gamma-band LFP recorded on the same tetrode, there is a strong correlation between the ocular preferences of both signals. Given the spatial arrangement of orientation tuning and ocular dominance, this leads us to conclude that the gamma-band of the LFP seems to sample an area considerably larger than orientation columns. Rather, its spatial resolution lies at the scale of ocular dominance columns.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, web_url = {http://www.frontiersin.org/systemsneuroscience/paper/10.3389/neuro.06/002.2008/pdf/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.3389/neuro.06.002.2008}, author = {Berens, P and Keliris, GA and Ecker, AS and Logothetis, NK and Tolias, AS} } @Article { 4788, title = {Recording Chronically from the same Neurons in Awake, Behaving Primates}, journal = {Journal of Neurophysiology}, year = {2007}, month = {12}, volume = {98}, number = {6}, pages = {3780-3790}, abstract = {Understanding the mechanisms of learning requires characterizing how the response properties of individual neurons and interactions across populations of neurons change over time. In order to study learning in-vivo, we need the ability to track an electrophysiological signature that uniquely identifies each recorded neuron for extended periods of time. We have identified such an extracellular signature using a statistical framework which allows quantification of the accuracy by which stable neurons can be identified across successive recording sessions. Our statistical framework uses spike waveform information recorded on a tetrode’s four channels in order to define a measure of similarity between neurons recorded across time. We use this framework to quantitatively demonstrate for the first time the ability to record from the same neurons across multiple consecutive days and weeks. The chronic recording techniques and methods of analyses we report can be used to characterize the changes in brain circuits du e to learning.}, department = {Department Logothetis}, web_url = {http://jn.physiology.org/cgi/reprint/00260.2007v1}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1152/jn.00260.2007}, author = {Tolias, AS and Ecker, AS and Siapas, AG and Hoenselaar, A and Keliris, GA and Logothetis, NK} } @Article { 4294, title = {Functional MR imaging in the awake monkey: effects of motion on dynamic off-resonance and processing strategies}, journal = {Magnetic Resonance Imaging}, year = {2007}, month = {7}, volume = {25}, number = {6}, pages = {869-882}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science?_ob=MImg\&_imagekey=B6T9D-4NJP3PF-8-1\&_cdi=5112\&_user=29041\&_orig=browse\&_coverDate=07\%2F31\%2F2007\&_sk=999749993\&view=c\&wchp=dGLbVzW-zSkzS\&md5=65c3bcf3da054d8435af25a5cff90ab8\&ie=/sdarticle.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1016/j.mri.2007.03.002}, author = {Pfeuffer, J and Shmuel, A and Keliris, GA and Steudel, T and Merkle, H and Logothetis, NK} } @Article { 4433, title = {Robust Controlled Functional MRI in Alert Monkeys at High Magnetic Field: Effects of Jaw and Body Movements}, journal = {NeuroImage}, year = {2007}, month = {7}, volume = {36}, number = {3}, pages = {550-570}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science?_ob=MImg\&_imagekey=B6WNP-4NFXDGH-1-1G\&_cdi=6968\&_user=29041\&_orig=browse\&_coverDate=07\%2F01\%2F2007\&_sk=999639996\&view=c\&wchp=dGLzVlz-zSkWb\&md5=54c45acb296ee126362acb0f149797d8\&ie}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1016/j.neuroimage.2007.02.057}, author = {Keliris, GA and Shmuel, A and Ku, S-P and Pfeuffer, J and Oeltermann, A and Steudel, T and Logothetis, NK} } @Article { 4480, title = {Reply to ''Motion processing in macaque V4''}, journal = {Nature Neuroscience}, year = {2005}, month = {9}, volume = {8}, number = {9}, pages = {1125-1125}, department = {Department Logothetis}, web_url = {http://www.nature.com/neuro/journal/v8/n9/pdf/nn0905-1125b.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, language = {en}, DOI = {10.1038/nn0905-1125b}, author = {Tolias, AS and Keliris, GA and Smirnakis, SM and Logothetis, NK} } @Article { 3326, title = {A binocular rivalry study of motion perception in the human brain}, journal = {Vision Research}, year = {2005}, month = {8}, volume = {45}, number = {17}, pages = {2231–2243}, abstract = {The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocularrivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocularrivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Our results demonstrate that motionperception is able to modulate the activity of several of the visual areas which are known to be involved in motion processing. More specifically, in addition to area V5 which showed the strongest modulation, a higher activity during the perception of motion than during the perception of noise was also clearly observed in areas V3A and LOC, and less so in area V3. In previous studies, these areas had been selectively activated by motion stimuli but whether their activity reflects motionperception or not remained unclear; here we show that they are involved in motionperception as well. The present findings therefore suggest a lack of a clear distinction between ‘processing’ versus ‘perceptual’ areas in the brain, but rather that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S0042698905001069}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, institution = {Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany}, DOI = {10.1016/j.visres.2005.02.007}, author = {Moutoussis, K and Keliris, GA and Kourtzi, Z and Logothetis, NK} } @Article { 3329, title = {Neurons in macaque area V4 acquire directional tuning after adaptation to motion stimuli}, journal = {Nature Neuroscience}, year = {2005}, month = {5}, volume = {8}, number = {5}, pages = {591-593}, abstract = {Macaque area V4 neurons are generally not selective for direction of motion, as judged from their response to directional stimuli presented after a baseline condition devoid of movement (classical paradigm). We used a motion-adaptation paradigm to investigate whether stimulation history influences direction-of-motion selectivity. We found that classically non-directional V4 neurons develop direction-of-motion selectivity after adaptation. This underscores the dynamic nature of functional cortical architecture.}, department = {Department Logothetis}, web_url = {http://www.nature.com/neuro/journal/v8/n5/pdf/nn1446.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, institution = {Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany}, language = {en}, DOI = {10.1038/nn1446}, author = {Tolias, AS and Keliris, GA and Smirnakis, SM and Logothetis, NK} } @Inbook { PanagiotaropoulosLK2014, title = {Neural approaches to perceptual organization}, year = {2016}, month = {12}, pages = {-}, department = {Department Logothetis}, web_url = {http://www.oxfordhandbooks.com/view/10.1093/oxfordhb/9780199829347.001.0001/oxfordhb-9780199829347}, editor = {Gepshtein, S. , L. Maloney, M. Singh}, publisher = {Oxford University Press}, address = {New York, NY, USA}, booktitle = {Oxford Handbook of Computational Perceptual Organization}, state = {}, ISBN = {978-0-19-998343-8}, DOI = {10.1093/oxfordhb/9780199829347.001.0001}, author = {Panagiotaropoulos, TI and Logothetis, NK and Keliris, GA} } @Poster { deAzevedoOABLLK2016, title = {Simultaneous resting-state and visually-driven functional networks in the macaque brain}, year = {2016}, month = {11}, day = {16}, number = {834.10}, abstract = {The primate brain is a complex dynamical system displaying long-range temporally-correlated functional networks. In the absence of external stimulation, several so called resting state functional networks of spontaneous activity have been identified. Their origin and function are not well understood, but such intrinsic architecture could reflect neural fluctuations within anatomically connected areas or active mechanisms related to perception and awareness. On the other hand, when the brain is being stimulated, a different pattern of stimulus-evoked activity emerges. How the brain orchestrates those interwoven patterns of activity is still unclear. We sought to assess the relationship between resting-state and stimulus-driven functional networks by investigating their topographical correspondence by using functional magnetic resonance imaging (fMRI) under specific stimulus paradigms. To this end, we scanned two monkeys (Macaca mulatta), while anesthetized or awake, stimulated with three main paradigms: a) no visual stimulation, b) visual stimulation using a one-minute block-design displaying natural movie clips alternated with gray background, and c) continuous visual stimulation using uninterrupted natural movies. Using independent component analysis (ICA), we were able to recover topographically similar patterns of resting state networks contained in stimulus-driven datasets. This suggests that, under certain circumstances, the primate brain is able to cope with both types of functional networks independently. Moreover, our results provide implications for bidirectional causal influences between stimulus-driven and spontaneous activity. This work provides insights of how the brain organizes its functional architecture and we expect it can stimulate further analysis and reinterpretation of a wide range of existing neuroimaging and physiological data.}, department = {Department Logothetis}, web_url = {http://www.abstractsonline.com/pp8/index.html\#!/4071/presentation/20730}, event_place = {San Diego, CA, USA}, event_name = {46th Annual Meeting of the Society for Neuroscience (Neuroscience 2016)}, author = {de Azevedo, FA and Ortiz-Rios, M and Azevedo, LC and Balla, DZ and Lohman, G and Logothetis, NK and Keliris, GK} } @Poster { TakemuraPWKLSYBLFLW2015, title = {Occipital vertical fiber system in human and macaque}, year = {2015}, month = {10}, day = {21}, volume = {45}, number = {700.01}, abstract = {The large size of the human brain imposes computational constraints that are reflected in its structural organization. For example, specialized visual processing of spatial and categorical information is partially segregated in dorsal and ventral regions in the occipital and temporal lobes; these regions are widely separated in cortex. For effective vision and action, the processing performed in these regions must be coordinated. Classical as well as recent studies identify the human Vertical Occipital Fasciculus (VOF), as a likely white matter bundle that includes axons that communicate between the dorsal and ventral regions. In this study, we compare the human vertical occipital pathways with similar tracts in the much smaller macaque brain in order to better understand similarities and differences across species. Methods. We obtained diffusion MRI (dMRI) at several spatial resolutions, and we used fiber tractography to estimate the trajectories of several different occipital pathways. DMRI data were acquired from 4 macaque monkeys and many humans (Takemura et al., 2015). We analyzed the data using constrained spherical deconvolution (CSD) and an ensemble of probabilistic tractography methods. We optimized the tractography results and tested statistical hypotheses using Linear Fascicle Evaluation methods (Pestilli et al., 2014). Results. A substantial bundle of vertical occipital fibers could be found in all the macaque and human datasets. The location of the macaque VOF (mVOF) is consistent with a schematic description in a post-morterm monkey brain described by Wernicke (1881). The mVOF terminates near cortical areas V3d, V3A, V4d and MT dorsally and V4v/TEO ventrally. The pattern of mVOF terminations is similar to those of human VOF, which are principally V3A/B dorsally and hV4 ventrally. In both species, the VOF is lateral to the optic radiation (sagittal stratum). In human, the VOF is also lateral to the Inferior Longitudinal Fasciculus (ILF) and clearly distinguishable from the ILF; but the mVOF intermingles with the vertical component of macaque ILF and does not form a very distinct bundle. The estimates of the position, size and cortical terminations of the mVOF depend on instrumental parameters, such as dMRI resolution; but in all cases the core of the tract can be identified and the estimates are consistent. These findings suggest that the human and macaque vertical occipital fiber systems diverged from a common ancestor. The human system significantly enlarged and became separated from the ILF. The change in white matter may be part of the general evolution of the size and position of extrastriate maps with the increase in brain size.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2015/}, event_place = {Chicago, IL, USA}, event_name = {45th Annual Meeting of the Society for Neuroscience (Neuroscience 2015)}, author = {Takemura, H and Pestilli, F and Weiner, KS and Keliris, GA and Landi, S and Sliwa, J and Ye, FQ and Barnett, M and Leopold, DA and Freiwald, WA and Logothetis, NK and Wandell, BA} } @Poster { PapanikolaouKLLS2015_2, title = {Population receptive field changes in hV5/MT+ of healthy subjects with simulated visual field scotomas}, year = {2015}, month = {10}, day = {21}, volume = {45}, number = {700.02}, abstract = {An important question is whether the adult visual cortex is able to reorganize in subjects with visual field defects (scotomas) as a result of retinal or cortical lesions. Functional magnetic resonance imaging (fMRI) methods provide a useful tool to study the population receptive field (pRF) properties and assess the capacity of the human visual cortex to reorganize following injury. However, these methods are prone to biases near the boundaries of the scotoma. Retinotopic changes resembling reorganization have been observed in the early visual cortex of normal subjects when the visual stimulus is masked to simulate retinal or cortical scotomas. It is not known how the receptive fields of higher visual areas, like hV5/MT+, are affected by partial stimulus deprivation. Here, we measured responses in human area V5/MT+ in five healthy subjects under two stimulation conditions. FMRI measurements were obtained under the presentation of a moving bar stimulus spanning the entire visual field while the subjects were fixating. In a second session the stimulus was masked in the left upper quadrant of the visual field to simulate a quadrantanopic scotoma (“artificial scotoma” or AS) occurring often as a result of partial V1 or optic radiation lesions. PRF estimates were obtained using a recent method of pRF topography estimation (Lee et al., A new method for estimating population receptive field topography in visual cortex, NeuroImage, 2013) which is consistent with other pRF methods. Responses obtained under the AS condition were compared with simulations obtained from a linear AS model (or LAS model). The LAS model provides an estimation of the pRF changes expected to occur as a result of the truncated stimulus assuming that the pRF linearly integrates the AS. We found that pRFs in hV5/MT+ are nonlinearly affected by the truncated stimulus presented: pRF centers shifted towards the border of the AS, pRF size decreased and pRF amplitude increased near the AS border. In addition, using the full bar stimulus to estimate the pRF topography (when in fact the stimulus presented included the AS) produced erroneous pRF estimates inside the region of the artificial scotoma. These biases are not the result of a trivial methodological artifact but appear to originate partly from asymmetric BOLD responses occurring when the stimulus moves from seeing to non-seeing locations of the visual field. Distinguishing between pRF changes that occur as the result of true reorganization versus different test-stimulus presentation conditions is an important task that needs to be undertaken when studying visual cortex organization in patients with visual field deficits.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2015/}, event_place = {Chicago, IL, USA}, event_name = {45th Annual Meeting of the Society for Neuroscience (Neuroscience 2015)}, author = {Papanikolaou, A and Keliris, GA and Lee, S and Logothetis, NK and Smirnakis, SM} } @Poster { LiLK2015, title = {Neural signals of motion integration are modulated by perception}, year = {2015}, month = {10}, day = {20}, volume = {45}, number = {600.11}, abstract = {A very important feature of primate vision is the ability to integrate motion signals into a coherent percept. To this end, a two-stage motion integration model has been proposed that selectively integrates local signals over time and space to reconstruct the global motion pattern. It has been suggested that the first stage responsible for local motion detection takes place in lower level visual area(s), while a second stage in higher area(s) integrates the local motion signals in order to extract the global motion direction. Support for this hypothesis stems mainly from recordings in anesthetized non-human primates while evidence in awake-behaving ones is very limited. Furthermore, very little is known about how motion integration is influenced by perception. In this study, we designed a novel pseudo-plaid stimulus that can parametrically modulate coherent or transparent motion perception by changing local feature information. The stimulus consists of two types of apertures over a line plaid display. The first group of apertures allows only single contours to pass through while the second only intersections. Human psychophysics demonstrated that the motion perception changes parametrically with the proportion of the two types of apertures from 100\% transparent when only single-contour apertures are present to 100\% coherent when only intersection apertures are displayed. Then, we used this stimulus and performed multi-electrode recordings in areas V1 and MT of alert macaques. Analysis of the firing rates during the whole trial (1000 ms) demonstrated that MT neurons were strongly modulated by the proportion of different aperture types reflecting the perception. Specifically, MT neural responses increased when motion perception was more coherent. In contrast V1 neurons did not show any significant changes by using this measure. These data corroborate the hierarchical organization of motion integration and demonstrate the relationship of neural signals with subjective perception.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2015/}, event_place = {Chicago, IL, USA}, event_name = {45th Annual Meeting of the Society for Neuroscience (Neuroscience 2015)}, author = {Li, Q and Logothetis, NK and Keliris, GA} } @Poster { OrtizAABLLK2015, title = {Dynamic Functional Connectivity Reflects Complex Audiovisual Scenes Changes during Cognitive Processing}, year = {2015}, month = {7}, day = {10}, volume = {9}, number = {PO1002}, department = {Department Logothetis}, department2 = {Department Scheffler}, web_url = {http://ibro2015.org/?page_id=434}, event_place = {Rio de Janeiro, Brazil}, event_name = {9th IBRO World Congress on Neuroscience (IBRO 2015)}, author = {Ortiz, M and Azevedo, FAC and Azevedo, LC and Balla, DZ and Lohmann, G and Logothetis, NK and Keliris, GA} } @Poster { AzevedoOABLLK2015, title = {Eingenvector Centrality Mapping during Natural Viewing in the Macaque Brain}, year = {2015}, month = {7}, day = {10}, volume = {9}, number = {PO1078}, department = {Department Logothetis}, department2 = {Department Scheffler}, web_url = {http://ibro2015.org/?page_id=434}, event_place = {Rio de Janeiro, Brazil}, event_name = {9th IBRO World Congress on Neuroscience (IBRO 2015)}, author = {Azevedo, F and Ortiz-Rios, M and Azevedo, LC and Balla, D and Lohmann, G and Logothetis, NK and Keliris, GA} } @Poster { BahmaniLLK2015, title = {Spike-Field Coherence Reflects Perceptual State in Monkey Primary Visual Cortex}, year = {2015}, month = {7}, day = {8}, volume = {9}, number = {PO221}, department = {Department Logothetis}, web_url = {http://ibro2015.org/?page_id=434}, event_place = {Rio de Janeiro, Brazil}, event_name = {9th IBRO World Congress on Neuroscience (IBRO 2015)}, author = {Bahmani, H and Lakshminarasimhan, KJ and Logothetis, NK and Keliris, GA} } @Poster { BahmaniMLK2014_2, title = {Binocular Flash Suppression in the Primary Visual Cortex of Anesthetized and Awake Macaques}, year = {2014}, month = {10}, volume = {15}, pages = {24}, abstract = {Primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies. However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1. In particular, only a moderate percentage of neurons in V1 were found to modulate in parallel with perception with magnitude substantially smaller than the physical preference of these neurons. It is yet unclear whether these small modulations are rooted from local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. We found that a small but significant modulation was present in both the anesthetized and awake states during the flash suppression presentation. Furthermore, the relative amplitudes of the perceptual modulations were not significantly different in the two states. We suggest that these early effects of perceptual suppression might occur locally in V1, in prior processing stages or within early visual cortical areas in the absence of top-down feedback from higher cognitive stages that are suppressed under anesthesia.}, department = {Department Logothetis}, web_url = {http://www.neuroschool-tuebingen-nena.de/fileadmin/user_upload/Dokumente/neuroscience/Abstractbook_NeNa2014_final.pdf}, event_place = {Schramberg, Germany}, event_name = {15th Conference of Junior Neuroscientists of T{\"u}bingen (NeNa 2014)}, author = {Bahmani, H and Murayama, Y and Logothetis, NK and Keliris, GA} } @Poster { AzevedoFLK2014, title = {Dynamics Changes of Bold Functional Connectivity during Natural Viewing in the Awake Macaque Brain}, year = {2014}, month = {6}, pages = {51}, abstract = {The primate brain is a dynamic system interconnected by temporally correlated functional networks. The structure of this correlated activity depends on the brain’s internal state and on stimulus input. In the absence of external stimulation, functional networks of spontaneous activity, i.e. the salience network, the executive control network and the default mode network, can be observed. Their origin and function are not well understood, but they could reflect neural noise within anatomically connected areas or active mechanisms related to perception and awareness. On the other hand, when the brain is being stimulated, a different pattern of activity emerges. Exactly how this patiotemporal transition happens is still unclear. The objective of this study is to characterize the dynamic changes of BOLD based functional connectivity between resting-state and natural-stimuli-driven networks in the awake monkey brain. Due to its high spatial resolution, BOLD-fMRI is a powerful tool to study large-scale correlated brain network activity. We used a paradigm containing sequences of movie-clips with different contexts including natural and artificial environments as well as periods devoid of any visual stimulation (resting) in order to identify the global activation patterns reflecting the interplay between different populations of neurons under these conditions. For our experiments, two macaque monkeys (Macaca mulatta) were trained in a mock scanner to remain headposted and motionless in a custom-made fMRI chair while a RI-compatible periscope presented a movie clip, a gray background (FOV 30 x 23, 60 Hz, eff. res. 530 400 fibers) or nothing. After the behavioral training was completed, the monkeys were scanned under the same conditions in a Bruker 4.7 T vertical MRI scanner with a custom-designed whole-head coil (single-shot GE-EPI, TR 1000 ms, TE 18 ms, 128 64 18 voxels, 1 1 2 mm). Each run lasted 10 min (600 volumes). We collected 30 functional runs of resting state activity (without any visual stimulation) and 30 functional runs of stimulus driven activity (1 min of a natural movie presentation alternated with 1 min of gray background) for each monkey. All the volumes containing artifacts were pre-selected and excluded from the data analysis. For the visual stimulation condition, we selected the scans with strong visual activation based on a generalized linear model (GLM). Functional connectivity data analysis (group-level ICA with 20 components) of the scans devoid of stimulation revealed resting-state networks consistent with previous reports in humans and monkeys (Mantini et al., 2013, J. Neurosci.). Furthermore, preliminary analysis of the scans with visual stimulation revealed components reflecting visually driven networks. Currently, we are employing the eigenvector centrality mapping (ECM), which is a parameter-free effective connectivity method (Lohmann et al., 2010, PLoS ONE) as well as models based dynamic causal modeling (DCM) (Friston et al., 2003, Neuroimage) to delineate differences across stimulation with different contexts and to characterize the physiological mechanisms behind the transition of brain states.}, department = {Department Logothetis}, web_url = {http://areadne.org/2014/home.html}, event_place = {Santorini, Greece}, event_name = {AREADNE 2014: Research in Encoding and Decoding of Neural Ensembles}, author = {Azevedo, FAC and Florin, E and Logothetis, NK and Keliris, GA} } @Poster { KelirisSPLS2013, title = {Estimation of average single-unit receptive field size by fMRI}, year = {2013}, month = {11}, day = {13}, volume = {43}, number = {824.16}, abstract = {Receptive fields (RF) are a fundamental property characterizing sensory neurons. In the visual domain, numerous electrophysiological and computational studies established the spatio-temporal characteristics of RFs and modeled early visual neurons in primates as Gabor filters with well-defined properties. Recently functional magnetic resonance imaging (fMRI) techniques have been introduced to estimate aggregate (voxel-based) “population” receptive field (pRF) sizes in humans (Dumoulin SO, Wandell BA, 2008). Population receptive field estimates are a function of: 1) the receptive field properties of single units belonging to a voxel, and 2) the scatter in the location of receptive field centers across units. In this study, we estimate RF sizes by exploiting the spatial-frequency selectivity of visual RFs modeled as Gabor functions. Blood oxygen level dependent (BOLD) measurements were collected from humans fixating in the center of band-limited white noise stimuli presented in a block-design (12 seconds ON, 20 seconds OFF). In different blocks, we modulated the spatial frequency content of the stimuli by changing the size of the pixels of the white noise. We found that the BOLD signal amplitude in retinotopic visual cortex changes dramatically with different stimulation conditions in a way consistent with the spatial frequency sensitivity of the Gabor RF models. We modeled the BOLD signal of each voxel as a sum of Gabors with homogeneous orientation distribution followed by a compressive non-linearity and fit this model to our data. The standard deviation of the Gaussian envelope of the Gabor function provides an estimate of RF size. Estimates obtained this way were compared to pRF estimates derived from additional experiments with moving bar stimuli. RF size estimates obtained with our method increased linearly with eccentricity as expected, but were significantly smaller in comparison to standard pRF measurements (Dumoulin SO, Wandell BA, 2008). The reason is that our method is not sensitive to the receptive field scatter that happens between units belonging to the same voxel. Similar experiments performed in anesthetized macaques provided RF size estimates comparable to electrophysiological measurements of single-unit RFs. We conjecture that our method estimates for the fist time average single-unit RF sizes in the human.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/annual-meeting/neuroscience-2013}, event_place = {San Diego, CA, USA}, event_name = {43rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)}, author = {Keliris, GA and Shao, Y and Papanikolaou, A and Logothetis, NK and Smirnakis, M} } @Poster { LakshminarasimhanLK2013, title = {Stimulus-dependent gamma-frequency shifting in the macaque v1}, year = {2013}, month = {11}, day = {10}, volume = {43}, number = {259.15}, abstract = {The phase of spikes in the gamma cycle has previously been shown to be modulated by stimulus orientation in macaque V1 (Vinck et al. 2010, Womelsdorf et al. 2012). These studies suggested that such stimulus-dependent phase shifts selectively facilitate the impact of neurons that fire earlier in the gamma cycle, on their targets. However, such phase coding schemes implicitly depend on the generation of a consistent gamma oscillation frequency across stimulus conditions. To test this, we examined whether the stimulus orientation and the eye of presentation affected the peak gamma frequency in V1. Two macaque monkeys were trained to passively fixate on a central spot, while one of two orthogonally oriented gratings was presented monocularly through a mirror stereoscope for a period of one second. In different trials either the eye of presentation or the orientation, were changed. Local field potential (LFP) signals recorded from 168 sites across multiple sessions were found to exhibit significant coherence with concurrently recorded single-unit spikes in the gamma frequency range. The power spectral density of each of those LFPs was fit as the sum of a power function and a gaussian function, and the center of the gaussian was taken as the peak gamma frequency. We found that, across sites the peak frequency varied between 30 Hz and 45 Hz in both monkeys. Moreover, within each site there was a significant shift in the peak frequency both with orientation (median shift \verb=~=1.99Hz) as well as the eye of presentation (median shift \verb=~=0.89Hz). There was no systematic relationship between the direction of shift and stimulus preference. Given that the orientation-dependent phase shifts reported earlier were of a very small magnitude (only a few degrees), it follows that such frequency changes could be detrimental for phase-shift coding schemes that rely on entrainment of multiple cortical areas to a single ‘clock-like’ signal. Alternatively, neural mechanisms implementing phase computations could be relatively invariant to frequency changes by using more intricate signal properties like instantaneous phase.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/annual-meeting/neuroscience-2013}, event_place = {San Diego, CA, USA}, event_name = {43rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)}, author = {Lakshminarasimhan, K and Logothetis, NK and Keliris, GA} } @Poster { ShaoKSPLS2013, title = {Population receptive field measurements in the visual cortex of macaque monkeys with and without retinal lesions}, year = {2013}, month = {11}, day = {9}, volume = {43}, number = {64.01}, abstract = {Visual receptive fields have dynamic properties and it has been reported that receptive field organization can change following chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two healthy adult macaque monkeys, and two monkeys with binocular central retinal lesions. FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli and expanding ring stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In general, there is good agreement between maps obtained by fMRI and previous results obtained by anatomical and physiological methods. FMRI pRF sizes and electrophysiology measurements in healthy animals show similar trends. For the monkeys with a photocoagulation induced retinal lesion, a former study has shown that the fMRI defined lesion projection zone (LPZ) border in V1 did not shift following the lesion (Smirnakis et al. Nature 2005). We reanalyzed these data using pRF methods and suggest that pRF size in the non-deafferented V1 showed little, if any, change on average. However, voxels inside the LPZ of areas V2/V3 do show visual modulation over time following the lesion, suggesting that area V2/V3 has more capacity for plasticity than area V1. Further investigation using fMRI and standard electrophysiology methods is in progress.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/annual-meeting/neuroscience-2013}, event_place = {San Diego, CA, USA}, event_name = {43rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)}, author = {Shao, Y and Keliris, GA and Schmid, MC and Papanikolaou, A and Logothetis, NK and Smirnakis, SM} } @Poster { BahmaniLK2013_2, title = {Effects of binocular flash suppression in awake and anesthetized macaque}, year = {2013}, month = {9}, pages = {127-128}, abstract = {The primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies (Lehky, 1988; Blake, 1989; Polonsky et al., 2000; Tong and Engel, 2001). However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1 (Leopold and Logothetis, 1996; Sheinberg and Logothetis, 1997). In particular, only a moderate percentage of neurons in V1 was modulated in parallel with perception and the magnitude of their modulation was substantially smaller than the physical preference of these neurons (Keliris et al., 2010). It is yet unclear whether these small modulations are rooted in local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of awake and anesthetized macaque monkeys during the paradigm of binocular flash suppression. The results showed that the pattern of perceptual modulation of neurons in V1 under the conditions of general anesthesia is almost identical to those recorded from awake monkeys. This suggests a role of local processes in V1 in perceptual suppression. Alternatively, these modulations could be caused by feedback from higher areas independent of conscious state.}, department = {Department Logothetis}, web_url = {https://portal.g-node.org/abstracts/bc13/\#/doi/nncn.bc2013.0124}, event_place = {T{\"u}bingen, Germany}, event_name = {Bernstein Conference 2013}, DOI = {10.12751/nncn.bc2013.0124}, author = {Bahmani, H and Logothetis, NK and Keliris, GA} } @Poster { LiLK2013_2, title = {Parallel processes may underly pattern motion perception}, year = {2013}, month = {9}, pages = {128-129}, abstract = {Local measurements by small receptive fields induce ambiguous and noisy one-dimensional motion estimation. This problem can be overcome by selective integration or pooling over time and space to reconstruct the global pattern (Adelson \& Movshon, 1982). However, it remains unclear if the local signals from intersections could influence the global pattern motion perception. Many studies used multiple apertures in order to investigate motion integration over space (Alais, Van der Smagt, Van den Berg, \& Van de Grind, 1998; Maruya, Amano, \& Nishida, 2010; Mingolla, Todd, \& Norman, 1992; Takahashi, 2004), but none took this issue into consideration. Here we developed a novel stimulus and try to answer this question. We used a mask with multiple transparent apertures over a moving plaid. The plaid consisted of two overlapping moving gratings with directions 135\(^{\circ}\) apart. The apertures were small (0.4\(^{\circ}\)) and were placed in locations that allowed either only single contours (AP1) or intersections (AP2) to pass through. We hypothesized that if motion integration takes place only at higher stages with larger receptive fields, the probability of coherent pattern motion perception would not be affected by the relative ratios of the aperture types. Our results indicate that motion perception is largely affected by the ratio of aperture types. We conjecture that parallel processes at different stages are involved in motion integration.}, department = {Department Logothetis}, web_url = {https://portal.g-node.org/abstracts/bc13/\#/doi/nncn.bc2013.0125}, event_place = {T{\"u}bingen, Germany}, event_name = {Bernstein Conference 2013}, DOI = {10.12751/nncn.bc2013.0125}, author = {Li, Q and Logothetis, NK and Keliris, GA} } @Poster { BahmaniLK2013, title = {Effects of binocular flash suppression in the anesthetized macaque}, journal = {Perception}, year = {2013}, month = {8}, volume = {42}, number = {ECVP Abstract Supplement}, pages = {145}, abstract = {The primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression in numerous psychophysical and imaging studies (Lehky, 1988; Blake, 1989; Polonsky et al., 2000; Tong and Engel, 2001). However, neurophysiological results in awake monkeys provided evidence for competition mainly between neurons in areas beyond V1 (Leopold and Logothetis, 1996; Sheinberg and Logothetis, 1997). In particular, only a moderate percentage of neurons in V1 was modulated in parallel with perception and the magnitude of their modulation was substantially smaller than the physical preference of these neurons (Keliris et al., 2010). It is yet unclear whether these small modulations are rooted in local circuits in V1 or influenced by higher cognitive states. To address this question we recorded multi-unit spiking activity and local field potentials in area V1 of anesthetized macaque monkeys during the paradigm of binocular flash suppression. The results showed that the pattern of perceptual modulation of neurons in V1 under the conditions of general anesthesia is almost identical to those recorded from awake monkeys. This suggests a role of local processes in V1 in perceptual suppression. Alternatively, these modulations could be caused by feedback from higher areas independent of conscious state.}, department = {Department Logothetis}, web_url = {http://pec.sagepub.com/content/42/1_suppl.toc}, event_place = {Bremen, Germany}, event_name = {36th European Conference on Visual Perception (ECVP 2013)}, DOI = {10.1177/03010066130420S101}, author = {Bahmani, H and Logothetis, NK and Keliris, GA} } @Poster { LiLK2013, title = {Pattern motion signals from V1 receptive fields}, journal = {Perception}, year = {2013}, month = {8}, volume = {42}, number = {ECVP Abstract Supplement}, pages = {145}, abstract = {Local measurements by small receptive fields (RFs) in V1 are thought to induce ambiguous and noisy one-dimensional motion estimation. This necessitates integration at higher brain stages for computation of global pattern motion. Electrophysiological evidence from monkeys viewing plaid stimuli is consistent with this hypothesis finding a small percentage of cells in V1 responding to pattern motion but the percentage is increasing in higher motion responsive areas MT and MST. We conjectured that a subset of V1 RFs residing on specific stimulus features could directly respond to the pattern motion thus biasing motion integration at higher stages. We used a novel stimulus to mimic V1 RF responses to plaids. It comprised of a mask with multiple transparent apertures (0.4\(^{\circ}\)) over a moving plaid. The aperture locations were chosen in advance to be of two types: AP1 were chosen to “see” only single grating components at any given time while AP2 were chosen to “see” only grating intersections. We manipulated the percentage of these two types in different trials to test how they influence motion perception. We found that the motion perception of subjects changes sigmoidally from 100\% transparent when all apertures are AP1 to 100\% coherent when all apertures are AP2.}, department = {Department Logothetis}, web_url = {http://pec.sagepub.com/content/42/1_suppl.toc}, event_place = {Bremen, Germany}, event_name = {36th European Conference on Visual Perception (ECVP 2013)}, DOI = {10.1177/03010066130420S101}, author = {Li, Q and Logothetis, NK and Keliris, GA} } @Poster { deAzevedoALK2013, title = {Effeccts of Visual Attention on Neural Processing in Rhesus' V1 by Simultaneous Electrophysiology and Bold-FMRI}, year = {2013}, month = {3}, day = {16}, department = {Department Logothetis}, web_url = {http://nwg.glia.mdc-berlin.de/media/pdf/conference/Program2013.pdf}, event_place = {G{\"o}ttingen, Germany}, event_name = {10th G{\"o}ttingen Meeting of the German Neuroscience Society, 34th G{\"o}ttingen Neurobiology Conference}, author = {de Azevedo, FAC and Azevedo, LC and Logothetis, NK and Keliris, G} } @Poster { AzevedoALK2012_2, title = {Effects of spatial attention on neural processing in rhesus’ V1: a simultaneous electrophysiology and fMRI study}, year = {2012}, month = {11}, pages = {19}, abstract = {Attention is a cognitive function thought to enhance our ability to select, process, and perceive only a behaviorally relevant fraction of the immense sensory input impinging on our receptors (Knudsen, 2007). Early electrophysiological studies in primates demonstrate that attention can modulate substantially the firing rate of single cells in extrastriate visual areas but has no or little impact in the primary visual cortex (Moran \& Desimone, 1985). In contrast, attention has been linked to strong bloodoxygenlevel-dependent (BOLD) signal modulations in human subjects (Gandhi et al., 1999). Our goal is to understand how selective visual spatial attention modulates the neuronal activity in primary visual cortex (V1) and how these effects are reflected in the different signals (single unit activity, local field potentials, and BOLD). To this end, we have trained two rhesus macaques to perform an orientation-change detection task in high field fMRI scanners (4.7T, 7T) while we can simultaneously acquire highresolution fMRI maps and electrophysiological signals. Preliminary results suggest that attention modulates the BOLD and electrophysiological signals in distinct ways. We are currently trying to address the layer specificity of the effects by using MRI compatible multicontact probes and implanted RF coils that provide ultra-high resolution maps of the fMRI activations.}, department = {Department Logothetis}, web_url = {http://www.danielabalslev.dk/workshop/Abstract_booklet.pdf}, event_place = {T{\"u}bingen, Germany}, event_name = {ERNI-HSF Science Meeting: Orienting of Attention: Neural Implementation, Underlying Mechanisms and Clinical Implications}, author = {Azevedo, F and Azevedo, L and Logothetis, NK and Keliris, GA} } @Poster { AzevedoALK2012, title = {Effects of visual attention on neural processing in Rhesus' V1 by simultaneous electrophysiology and BOLD-fMRI}, year = {2012}, month = {11}, volume = {13}, pages = {36}, abstract = {Attention is a cognitive function thought to enhance our ability to select, process, and perceive only a behaviorally relevant fraction of the immense sensory input impinging on our receptors (Knudsen, 2007). Early electrophysiological studies in primates demonstrate that attention can modulate substantially the firing rate of single cells in extrastriate visual areas but has no or little impact in the primary visual cortex (Moran \& Desimone, 1985). In contrast, attention has been linked to strong bloodoxygen-level-dependent (BOLD) signal modulations in human subjects (Gandhi et al., 1999). Our goal is to understand how selective visual spatial attention modulates the neuronal activity in primary visual cortex (V1) and how these effects are reflected in the different signals (single unit activity, local field potentials, and BOLD). To this end, we have trained two rhesus macaques to perform an orientation-change detection task in high field fMRI scanners (4.7T, 7T) while we can simultaneously acquire high-resolution fMRI maps and electrophysiological signals. Preliminary results suggest that attention modulates the BOLD and electrophysiological signals in distinct ways.We are currently trying to address the layer specificity of the effects by using MRI compatible multicontact probes and implanted RF coils that provide ultra-high resolution maps of the fMRI activations.}, department = {Department Logothetis}, web_url = {http://www.neuroschool-tuebingen-nena.de/}, event_place = {Schramberg, Germany}, event_name = {13th Conference of the Junior Neuroscientists of T{\"u}bingen (NeNA 2012)}, author = {Azevedo, FAC and Azevedo, LAC and Logothetis, NK and Keliris, G} } @Poster { LiFLK2012, title = {Multi-Stable Visual Motion Perception}, journal = {Frontiers in Computational Neuroscience}, year = {2012}, month = {9}, day = {14}, volume = {Conference Abstract: Bernstein Conference 2012}, pages = {190}, abstract = {Perceptual multi-stability is established when the brain fails to reach a single interpretation of the input from the external world. This issue intrigued scientific minds for more than two hundred years. This phenomenon has been found in vision (Leopold \& Logothetis, 1999), audition (Repp, 2007), olfaction (Zhou \& Chen, 2009) and speech (Warren \& Gregory, 1958). Crucial features are similar within and across modalities (Schwarts et al., 2012). In the visual modality, a number of ambiguous visual patterns have been described such as the Necker cube, motion plaids, and binocular rivalry. Multi-stable stimuli can provide unique insights into visual processing, as changes in perception are decoupled from changes in the stimulus. Understanding of how multi-stable perception occurs might help one to understand visual perception in general. A key question in multi-stable perception is what the brain processes are responsible in the identification and alternation of the percepts. Some investigators suggest that both top-down and bottom-up processes are involved (Garc{\'i}a P{\'e}rez, 1989) but others argue that multi-stable perception does not need high-level processing but happens automatically as low-level competition between the stimulus features (Akman et al., 2009; Wilson et al, 2000). Furthermore, it is well known that changes in stimulus features can bias perception in one or another direction, (Klink, et al., 2012). In order to explore this question, we used multi-stable motion stimuli and specifically moving plaids consisting of three superimposed gratings moving in equidistant directions (difference of 120 deg). These stimuli induce the perception of component and pattern motion simultaneously since any two component gratings bind together and are perceived to move in the opposite direction of the third grating component. We modulated properties of the stimuli such as grating speed and size and recorded the responses of human subjects reporting the direction of the single grating using one of three buttons for each direction. Preliminary results show that perceptual dominance is greatly affected by the selection of grating speeds. Grating size did not greatly change the predominance of the different gratings. We find that gratings with speed closer to physiological values have greater probability to be perceived and that gratings with similar speeds tend to group more often than gratings with different speeds. Further manipulations of other stimulus features like contrast and spatial frequency allow parametric variations of the relative probabilities of different interpretations. Our future goal is to use this information to built models of perceptual alternations using probabilistic inference.}, department = {Department Logothetis}, department2 = {Department B{\"u}lthoff}, web_url = {http://www.frontiersin.org/10.3389/conf.fncom.2012.55.00058/event_abstract}, event_place = {M{\"u}nchen, Germany}, event_name = {Bernstein Conference 2012}, DOI = {10.3389/conf.fncom.2012.55.00058}, author = {Li, Q and Fleming, RW and Logothetis, NK and Keliris, GA} } @Poster { SmirnakisKSPL2012, title = {Population receptive field measurements in macaque visual cortex}, journal = {Journal of Vision}, year = {2012}, month = {8}, volume = {12}, number = {9}, pages = {1397}, abstract = {Visual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two normal adult macaque monkeys and one macaque with binocular central retinal lesions due to a form of juvenile macular degeneration (MD). FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. FMRI measurements of normal monkeys agree with published electrophysiological results, with pRF sizes and electrophysiology measurements showing similar trends. For the MD monkey, the size and location of the lesion projection zone (LPZ) was consistent with the retinotopic projection of the retinal lesion in early visual areas. No significant BOLD activity was seen within the V1 LPZ, and the retinotopic organization of the non-deafferented V1 periphery was regular without distortion. Interestingly, area V5/MT of the MD monkey showed more extensive activation than area V5/MT of control monkeys which had part of their visual field obscured (artificial scotoma) to match the scotoma of the MD monkey. V5/MT PRF sizes of the MD monkey were on average smaller than controls. PRF estimation methods allow us to measure and follow in vivo how the properties of visual areas change as a function of cortical reorganization. Finally, if there is time, we will discuss a different method of pRF estimation that yields additional information.}, department = {Department Logothetis}, web_url = {http://www.journalofvision.org/content/12/9/1397.abstract}, event_place = {Naples, FL, USA}, event_name = {12th Annual Meeting of the Vision Sciences Society (VSS 2012)}, DOI = {10.1167/12.9.1397}, author = {Smirnakis, SM and Keliris, GA and Shao, Y and Papanikolaou, A and Logothetis, NK} } @Poster { BahmaniLK2012, title = {The role of parietal visual cortex in perceptual transitions during bistable perception}, journal = {Journal of Vision}, year = {2012}, month = {8}, volume = {12}, number = {9}, pages = {683}, abstract = {Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during bistable perception. To investigate further the possible role of parietal visual areas in perceptual alternations, we recorded extracellular neural activity in the lateral intraparietal area (LIP) of the rhesus macaque. The subject was initially presented with congruent patterns to the two eyes. Then the stimulus was switched for either one or both eyes (binocular flash suppression versus physical alternation), both resulting in perception of the newly presented stimulus. The recorded cells typically showed an initial burst of activity at stimulus onsets as well as stimulus switches. In contrast to previous reports by a number of fMRI studies, we found strong transient activity during physical alternations at the single cell level. This signal was also present during binocular flash suppression but to a lesser extent. Importantly, the amplitude of the signal dropped substantially in control conditions where physical changes were introduced in the stimuli but did not induce concomitant changes in perception. The transient response of the recorded neurons was followed by a tonic response which exhibited independent dynamics. Interestingly, this sustained activity was significantly lower during incongruent versus congruent stimulation. We conjecture that areas at the high end of the dorsal pathway might be involved in multistable perception in a different way in comparison with feature and object selective areas of the ventral pathway. The transient signal recorded in LIP neurons during perceptual transitions could potentially trigger reorganization of activity in constellations of feature selective neurons in the ventral pathway. In addition, the suppression of the sustained activity in LIP during incongruent stimulation may reflect inhibitory processes involved in the resolution of conflict between the two stimuli or indicate a failure to bind the sensory input into a coherent percept.}, department = {Department Logothetis}, web_url = {http://www.journalofvision.org/content/12/9/683.abstract}, event_place = {Naples, FL, USA}, event_name = {12th Annual Meeting of the Vision Sciences Society (VSS 2012)}, DOI = {10.1167/12.9.683}, author = {Bahmani, H and Logothetis, NK and Keliris, GA} } @Poster { PapanikolaouKPSKSLS2012, title = {Population Receptive Field Mapping in Human Subjects after Lesions of the Visual Pathway}, year = {2012}, month = {6}, pages = {76}, abstract = {Cortical damage of the visual pathway as a result of stroke typically leads to a loss of conscious vision in the affected region of the contralateral visual hemifield (scotoma). The most common visual injury involves the primary visual cortex (V1), the major relay of visual information to the rest of the cortex. However, in spite of this, several higher visual areas have been shown to be modulated by visual stimuli presented inside the scotoma. This suggests that there are alternate pathways to transmit information from the retina to the cortex that bypass V1 and transmit information directly to extrastriate visual areas. A much debated issue is whether adult visual cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the observed reorganization. The purpose of this study is to map visual cortex organization after injury, gathering information about the role that specific networks of brain areas play in cortical reorganization and recovery. To this end, we use functional magnetic resonance imaging (fMRI) methods to study several subjects with quadrandanopia and hemianopia and compare them to a group of normal controls. FMRI measurements were obtained during the presentation of a moving bar stimulus, which traverses the visual field while the subjects are fixating. These measurements are used to estimate voxel based population receptive field centers and radii using a direct isotropic Gaussian method introduced by Dumoulin and Wandell (1). In select controls an area of the stimulus is obscured (“artificial scotoma”) to simulate as much as possible the real scotoma of each patient. Preliminary results suggest that receptive field measurements obtained both in patients and in subjects examined under the artificial scotoma condition differ from measurements obtained in controls. In general, there appear to be no significant retinotopic map alterations in the borders of early visual areas of patients suffering from cortical lesions. However, there are some differences in the organization of higher visual areas such as hV5/MT+ compared to those of normal subjects. This may in part reflect the fact that some of the input to hV5/MT+ receptive fields has been lost with the V1+ lesion, but there are also suggestions that V1 bypassing pathways contribute. In addition, population receptive field size of some of the patients’ spared visual areas show deviations from the normal range of population receptive field sizes derived from the control subjects with and without the artificial scotoma condition.}, department = {Department Logothetis}, web_url = {http://areadne.org/2012/home.html}, event_place = {Santorini, Greece}, event_name = {AREADNE 2012: Research in Encoding and Decoding of Neural Ensembles}, author = {Papanikolaou, A and Keliris, GA and Papageorgiou, DT and Shao, Y and Krapp, E and Schiefer, U and Logothetis, N and Smirnakis, SM} } @Poster { ShaoKPFNJALS2012, title = {Population receptive field measurements in the visual cortex of macaque monkeys with and without retinal lesions}, year = {2012}, month = {6}, pages = {82}, abstract = {Visual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two adult normal macaque monkeys and one with binocular central retinal lesions due to a formof juvenile macular degeneration (MD). FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis, et al., Nature Neuroscience, 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In addition we used a new spatiotemporal dynamic modulation method to measure pRF sizes as comparison. In general fMRI measurements from the normal monkeys agree with electrophysiological results in the literature, with fMRI pRF sizes and electrophysiology measurements showing similar trends. For the MD monkey, the size and location of the fMRI defined lesion projection zone (LPZ) in early visual areas is consistent with the retinotopic projection of the retinal lesion. No significant activity is found within V1 LPZ of the MD monkey, and the retinotopic organization of the non-deafferented V1 periphery is regular without distortion. Higher level visual areas (V5/MT) of the MD monkey show more extensive activation than areas of control monkeys with an artificial scotoma (to obscure part of the stimuli from the visual field as a simulation of the real scotoma) of comparable size. PRF sizes in the nondeafferented V5/MT of the MD monkey are on average slightly smaller than controls. Further investigation using fMRI and standard electrophysiology methods is in progress.}, department = {Department Logothetis}, web_url = {http://areadne.org/2012/home.html}, event_place = {Santorini, Greece}, event_name = {AREADNE 2012: Research in Encoding and Decoding of Neural Ensembles}, author = {Shao, Y and Keliris, GA and Papanikolaou, A and Fischer, DM and Nagy, D and Jaegle, H and Augath, M and Logothetis, NK and Smirnakis, SM} } @Poster { PapanikolaouKSKPSLS2011, title = {Population receptive field mapping in human subjects with lesions of the visual cortex}, year = {2011}, month = {11}, volume = {41}, number = {851.07}, abstract = {Damage to the primary visual cortex (V1) as a result of stroke typically leads to the inability to perceive visual stimuli in the affected region of the contralateral visual hemifield (scotoma). However, in spite of this, several higher visual areas have been shown to be modulated by visual stimuli presented inside the scotoma. A much debated issue is whether adult visual cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the observed reorganization. Here we use functional magnetic resonance imaging (fMRI) methods to study visual cortex reorganization after injury in adult human subjects. To this end we applied a method introduced by Dumoulin and Wandell (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of neuronal populations voxel by voxel in the visual cortex. FMRI measurements were obtained during the presentation of a moving bar stimulus which traversed the visual field while the subjects were fixating and these measurements were used to derive an estimate of the voxel based population receptive field centre and radius. We studied several subjects with quadrandanopsia and hemianopsia resulting from cortical lesions and compared them to the range of measurements obtained from a group of normal controls. In general, retinotopic maps in the patients’ spared early visual cortex appear to be consistent with retinotopic maps obtained in control subjects. The organization of higher level visual areas, such as V3a/b and MT show preliminary some differences compared to those of normal subjects. Also preliminary results on the population receptive field size of some of the patients’ spared visual areas show deviations from the normal range of population receptive field sizes derived from the control subjects. We are in the process of obtaining further measurements to confirm these findings and to assess to what degree they correspond to cortical reorganization.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2011/}, event_place = {Washington, DC, USA}, event_name = {41st Annual Meeting of the Society for Neuroscience (Neuroscience 2011)}, author = {Papanikolaou, A and Keliris, GA and Shao, Y and Krapp, E and Papageorgiou, E and Schiefer, U and Logothetis, NK and Smirnakis, SM} } @Poster { ShaoKPALS2011, title = {Population receptive field measurements in visual cortex of macaque monkeys}, year = {2011}, month = {11}, volume = {41}, number = {851.09}, abstract = {Visual receptive fields have dynamic properties that may change with the conditions of visual stimulation or with the state of chronic visual deprivation. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of two adult normal macaque monkeys and one with binocular central retinal lesions due to a form of juvenile macular degeneration (D06). FMRI experiments were performed under light remifentanyl induced anesthesia (Logothetis et al. Nat. Neurosci. 1999). Standard moving horizontal/vertical bar stimuli were presented to the subjects and the population receptive field (pRF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and pRF sizes in early visual areas. In addition we used a new spatiotemporal dynamic modulation method to measure pRF sizes as comparison. In general fMRI measurements from the normal monkeys agree with electrophysiological results in the literature, with fMRI pRF sizes and electrophysiology measurements showing similar trends. For the macular degeneration monkey (D06), the size and location of the fMRI defined lesion projection zone in early visual areas is consistent with the retinotopic projection of the retinal lesion. No significant activity was found within V1 LPZ of D06, and the retinotopic organization of the non-deafferented V1 periphery is regular without distortion. Higher level visual areas of D06 (V5/MT) show more extensive activation than areas of control monkeys with an artificial scotoma (to obscure part of the stimuli from the visual field as a simulation of the real scotoma) of comparable size. PRF sizes in the non-deafferented V5/MT of monkey D06 are on average slightly smaller than controls. Further investigation using fMRI and standard electrophysiology methods is in progress.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2011/}, event_place = {Washington, DC, USA}, event_name = {41st Annual Meeting of the Society for Neuroscience (Neuroscience 2011)}, author = {Shao, Y and Keliris, GA and Papanikolaou, A and Augath, M and Logothetis, NK and Smirnakis, SM} } @Poster { BahmaniLK2011, title = {Neural correlates of binocular rivalry in parietal cortex}, journal = {Frontiers in Computational Neuroscience}, year = {2011}, month = {10}, volume = {5}, number = {Conference Abstract: BC11}, pages = {123-125}, abstract = {When dissimilar images are presented to the two eyes, perception starts alternating spontaneously between each monocular view, a phenomenon called binocular rivalry (Leopold and Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during binocular rivalry (Lumer et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area (LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010). The paradigm ensures excellent control over the subject’s perceptual state. Preliminary results confirm the transient change of brain activity around perceptual reversals at the single cell level. The recorded cells typically showed an initial burst of activity after the onset of a stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley, 2004). The transient response of recorded neurons has a short latency, lasts a few hundred milliseconds and is always positive while the sustained response is suppressive in some cells and excitatory in others. We speculate that these responses may reflect two separate underlying processes. The short latency response may reflect a fast sensory signal conveying the information in a bottom-up manner, while the sustained activity may represent top-down influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance imaging (fMRI) studies performed previously could not dissociate these two tightly overlapping signals because of the poor temporal resolution of the technique. Analysis of the firing rates of single and multi-units indicate that the transient part of the response predicts well the change in perception while the sustained activity does not show a significant correlation with perceptual state. This might be explained by the little selectivity of the sustained response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed that LIP neurons provide a representational map of saliency, integrating bottom-up and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We argue that the transient response of LIP neurons after perceptual switches is an indication for a role of this region in providing a change signal to higher areas. It is possible, that the intraparietal activation observed in humans around perceptual transitions may simply reflect the elevation of neural activity as a result of a novel percept rather than a causal role of the region in driving the switches. We are therefore planning to extend the binocular flash suppression paradigm to normal binocular rivalry and monitor the activity around spontaneous perceptual alternations in order to delineate what happens without any concomitant physical change in the stimulus. Furthermore, local field potentials, temporal dynamics of single unit activity and synchronization between neurons might provide a better understanding of the top-down influences of prefrontal cortex especially during the sustained response. This analysis is currently in progress.}, department = {Department Logothetis}, web_url = {http://www.bccn-2011.uni-freiburg.de/proceedings.pdf}, event_place = {Freiburg, Germany}, event_name = {Computational Neuroscience \& Neurotechnology Bernstein Conference \& Neurex Annual Meeting 2011}, DOI = {10.3389/conf.fncom.2011.53.00134}, author = {Bahmani, H and Logothetis, NK and Keliris, GA} } @Poster { BahmaniLK2011_2, title = {Neural Correlates of Binocular Rivalry in Parietal Cortex}, year = {2011}, month = {10}, volume = {12}, pages = {22}, abstract = {When dissimilar images are presented to the two eyes, perception starts alternating spontaneously between each monocular view, a phenomenon called binocular rivalry (Leopold and Logothetis, 1999). Several imaging studies in humans have shown the involvement of a frontoparietal network of cortical areas in perceptual transitions during binocular rivalry (Lumer et al., 1998). Here we investigate the possible role of parietal visual areas in perceptual alternations during rivalry in the rhesus macaque. Neural activity in the lateral intraparietal area (LIP) was recorded extracellularly while the subject was presented dichoptically and asynchronously with two rivalrous patterns, resulting in flash suppression (Keliris et al., 2010). The paradigm ensures excellent control over the subject{\^a}s perceptual state. Preliminary results confirm the transient change of brain activity around perceptual reversals at the single cell level. The recorded cells typically showed an initial burst of activity after the onset of a stimulus as well as at stimulus/perceptual changes, followed by a sustained response (Bisley, 2004). The transient response of recorded neurons has a short latency, lasts a few hundred milliseconds and is always positive while the sustained response is suppressive in some cells and excitatory in others. We speculate that these responses may reflect two separate underlying processes. The short latency response may reflect a fast sensory signal conveying the information in a bottom-up manner, while the sustained activity may represent top-down influences originating from higher areas in the prefrontal cortex. The functional magnetic resonance imaging (fMRI) studies performed previously could not dissociate these two tightly overlapping signals because of the poor temporal resolution of the technique. Analysis of the firing rates of single and multi-units indicate that the transient part of the response predicts well the change in perception while the sustained activity does not show a significant correlation with perceptual state. This might be explained by the little selectivity of the sustained response of parietal neurons towards particular stimuli (Lehky and Sereno, 2007). It is believed that LIP neurons provide a representational map of saliency, integrating bottom-up and top-down information to guide the allocation of spatial attention (Bisley et al., 2011). We argue that the transient response of LIP neurons after perceptual switches is an indication for a role of this region in providing a change signal to higher areas. It is possible, that the intraparietal activation observed in humans around perceptual transitions may simply reflect the elevation of neural activity as a result of a novel percept rather than a causal role of the region in driving the switches. We are therefore planning to extend the binocular flash suppression paradigm to normal binocular rivalry and monitor the activity around spontaneous perceptual alternations in order to delineate what happens without any concomitant physical change in the stimulus. Furthermore, local field potentials, temporal dynamics of single unit activity and synchronization between neurons might provide a better understanding of the top-down influences of prefrontal cortex especially during the sustained response. This analysis is currently in progress.}, department = {Department Logothetis}, event_place = {Heiligkreuztal, Germany}, event_name = {12th Conference of Junior Neuroscientists of T{\"u}bingen (NeNA 2011)}, author = {Bahmani, H and Logothetis, NK and Keliris, GA} } @Poster { PapanikolaouKSKSLS2011, title = {Population receptive field mapping in human subjects with visual cortical lesions}, year = {2011}, month = {10}, volume = {12}, pages = {37}, abstract = {Damage to the primary visual cortex (V1) as a result of stroke typically leads to the inability to perceive visual stimuli in the affected region of the contralateral visual hemifield (scotoma). However, in spite of this, several higher visual areas have been shown to be modulated by visual stimuli presented inside the scotoma. A much debated issue is whether adult visual cortex is able to reorganize after injury, and if so, what is the extent and the mechanism of the observed reorganization. We use functional magnetic resonance imaging (fMRI) methods to study visual cortex reorganization after injury in adult human subjects. To this end we applied a method introduced by Dumoulin and Wandell (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of neuronal populations voxel by voxel in the visual cortex. FMRI measurements were obtained during the presentation of a moving bar stimulus which traversed the visual field while the subjects were fixating and these measurements were used to derive an estimate of the voxel based population receptive field center and radius. We studied several subjects with quadrandanopsia and hemianopsia resulting from cortical lesions and compared them to the range of measurements obtained from a group of normal controls. In general, retinotopic maps in the patients’ spared early visual cortex appear to be consistent with retinotopic maps obtained in control. subjects. The organization of higher level visual areas, such as V3a/b and MT show preliminary some differences compared to those of normal subjects. Also preliminary results on the population receptive field size of some of the patients’ spared visual areas show deviations from the normal range of population receptive field sizes derived from the control subjects. We are in the process of obtaining further measurements to confirm these findings and to assess to what degree they correspond to cortical reorganization.}, department = {Department Logothetis}, event_place = {Heiligkreuztal, Germany}, event_name = {12th Conference of Junior Neuroscientists of T{\"u}bingen (NeNA 2011)}, author = {Papanikolaou, A and Keliris, G and Shao, Y and Krapp, E and Schiefer, U and Logothetis, NK and Smirnakis, S} } @Poster { 7064, title = {Assessing the spatio-temporal dynamics of visual receptive fields by fMRI}, year = {2010}, month = {11}, volume = {40}, number = {371.6}, abstract = {A fundamental problem of neuroscience is being able to understand the input-output relationship of early sensory areas. Central to this understanding is the notion of the receptive field (RF). Previous electrophysiological studies in the visual system of primates have demonstrated that receptive fields do not have fixed properties but are dynamically changing as a function of the stimulation conditions or behavioral tasks. In the human, aggregate (voxel-based) receptive field sizes (pRF) of early visual cortex were estimated by fMRI using standard retinotopic stimuli (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008) but it would be desirable to obtain more detailed estimates of RF spatio-temporal properties. In this study, we have used flickering checkerboard stimuli (white noise) in order to measure the spatio-temporal response properties of population RFs by fMRI. Blood oxygen level dependent (BOLD) measurements were performed both in anesthetized macaques (4.7 Tesla vertical scanner) and awake-fixating human subjects (3 Tesla Siemens Trio). We found that the BOLD-signal amplitude in early visual cortex changes as a function of the checker-size of the stimulus and this can provide estimates of the central excitatory integration area as well as the surround suppression of the aggregate RFs. The estimates of the size of the central portion of the RFs were found to increase with eccentricity within each visual area as well as from lower to higher visual areas. The results were comparable to pRF-size estimates derived from additional experiments with classical retinotopic stimuli (expanding rings, rotating wedges, moving horizontal and vertical bars). Current work focuses on evaluating how RF parameter estimates change as a function of the temporal frequency and contrast of the stimuli. We believe that this method can provide robust estimates of RF parameters which can be used in longitudinal studies for the assessment of cortical reorganization and plasticity in patients suffering from retinal and cortical lesions.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2010/index.aspx?pagename=abstracts_main}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {40th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)}, language = {en}, author = {Keliris, GA and Shao, Y and Papanikolaou, A and Peng, X and Logothetis, NK and Smirnakis, SM} } @Poster { 7055, title = {Decorrelated neuronal firing in cortical microcircuits}, year = {2010}, month = {11}, volume = {40}, number = {73.20}, abstract = {Correlated trial-to-trial variability in the activity of cortical neurons is thought to reflect the functional connectivity of the circuit. Many cortical areas are organized into functional columns, in which neurons are believed to be densely connected and share common input. Numerous studies report a high degree of correlated variability between nearby cells. We developed chronically implanted multi-tetrode arrays offering unprecedented recording quality to re-examine this question in primary visual cortex of awake macaques. We found that even nearby neurons with similar orientation tuning show virtually no correlated variability. In a total of 46 recording sessions from two monkeys, we presented either static or drifting sine-wave gratings at eight different orientations. We recorded from 407 well isolated, visually responsive and orientation-tuned neurons, resulting in 1907 simultaneously recorded pairs of neurons. In 406 of these pairs both neurons were recorded by the same tetrode. Despite being physically close to each other and having highly overlapping receptive fields, neurons recorded from the same tetrode had exceedingly low spike count correlations (rsc = 0.005 ± 0.004; mean ± SEM). Even cells with similar preferred orientations (rsignal > 0.5) had very weak correlations (rsc = 0.028 ± 0.010). This was also true if pairs were strongly driven by gratings with orientations close to the cells’ preferred orientations. Correlations between neurons recorded by different tetrodes showed a similar pattern. They were low on average (rsc = 0.010 ± 0.002) with a weak relation between tuning similarity and spike count correlations (two-sample t test, rsignal < 0.5 versus rsignal > 0.5: P = 0.003, n = 1907). To investigate whether low correlations also occur under more naturalistic stimulus conditions, we presented natural images to one of the monkeys. The average rsc was close to zero (rsc = 0.001 ± 0.005, n = 329) with no relation between receptive field overlap and spike count correlations. We obtained a similar result during stimulation with moving bars in a third monkey (rsc = 0.014 ± 0.011, n = 56). Our findings suggest a refinement of current models of cortical microcircuit architecture and function: either adjacent neurons share only a few percent of their inputs or, alternatively, their activity is actively decorrelated.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, web_url = {http://www.sfn.org/am2010/index.aspx?pagename=abstracts_main}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {40th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)}, language = {en}, author = {Ecker, AS and Berens, P and Keliris, GA and Bethge, M and Logothetis, NK and Tolias, AS} } @Poster { 7065, title = {Population receptive field mapping in a macaque monkey with macular degeneration}, year = {2010}, month = {11}, volume = {40}, number = {371.7}, abstract = {Macular degeneration (MD) is a common cause of human visual impairment. Typically MD deprives the foveal part of the primary visual cortex from retinal input. It has been reported that visual areas undergo extensive plastic reorganization in response to such deprivation (Baker et al., J. Neurosci. 2005), but this question remains not conclusively settled. We used 4.7 Tesla functional magnetic resonance imaging (fMRI) to study the visual cortex of an adult macaque monkey with binocular central retinal lesions due to a form of juvenile MD. FMRI experiments were performed under light remifentanyl induced anesthesia. Standard moving horizontal/vertical bar stimuli were presented to the subject and the population receptive field (RF) method (Dumoulin and Wandell, Neuroimage 2008) was used to measure retinotopic maps and population receptive field sizes in early visual areas. RF size was plotted as a function of eccentricity in early visual areas. As expected, population based RFs increase in size as a function of eccentricity within each visual area, and as we move from lower to higher visual areas at a fixed eccentricity. In general, there is good agreement between maps obtained by fMRI and previous results obtained by anatomical and physiological methods. The pattern of activity elicited in the MD monkey was compared to the pattern of activity elicited in two control monkeys. The primary visual cortex of the MD animal shows an extensive area devoid of BOLD (blood oxygen level dependent) activity that includes the fovea and roughly corresponds to the expected size of the retinal lesion projection zone (LPZ). Visually driven activity starts beyond the border of the calcarine sulcus, at approximately 9 degrees eccentricity a distance of \verb=~= 36 mm from foveal V1 in this animal. RF size maps derived from non-deafferented cortex abutting the retinal LPZ were comparable to RF size maps derived from the corresponding area in control subjects. Further investigation using fMRI and standard electrophysiology methods is in progress.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2010/index.aspx?pagename=abstracts_main}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {40th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)}, language = {en}, author = {Shao, Y and Keliris, GA and Papanikolaou, A and Fischer, DM and Nagy, D and J{\"a}gle, H and Seeliger, MW and Augath, M and Logothetis, NK and Smirnakis, SM} } @Poster { 7063, title = {Population receptive field mapping in human subjects with visual cortical lesions}, year = {2010}, month = {11}, volume = {40}, number = {371.5}, abstract = {Damage to the primary visual cortex (V1) as a result of stroke or other brain diseases can lead to a loss of conscious vision in the contralateral visual hemifield. Cortical blindness affects many activities on a patient's daily life and is considered to be a heavy burden while there are few, if any, options for rehabilitation and recovery. A much debated issue is whether the visual cortex is able to reorganize after injury in adult human subjects, and if so, what may be the mechanism of reorganization. Here we apply an important new approach introduced by Dumoulin and Wandell (Doumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008), which uses functional magnetic resonance imaging (fMRI) to measure the aggregate receptive field properties of neuronal populations voxel by voxel in the visual cortex. The purpose of this study is to compare receptive field measurements in patients with cortical lesions with controls and to investigate whether these measurements change following injury. Patients were fixating in the magnet and fMRI measurements were obtained during the presentation of standard visual stimuli used in retinotopic mapping (rotating wedges, expanding rings, horizontally and vertically moving bars). The patient’s intact hemisphere, as well as normal subjects were used as controls. In some controls an area of the stimulus was obscured (“artificial scotoma”) to simulate as much as possible the real scotoma of the patients. Preliminary results suggest that receptive field measurements obtained in patients and in subjects examined under the artificial scotoma condition differ from measurements obtained in controls under the normal visual stimulation condition. We are in the process of obtaining further control tests and measurements to confirm these findings and to assess to what degree they correspond to cortical reorganization. Future research will focus on using these methods to study the capacity of various visual rehabilitation training methods to induce visual cortex reorganization.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2010/index.aspx?pagename=abstracts_main}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {40th Annual Meeting of the Society for Neuroscience (Neuroscience 2010)}, language = {en}, author = {Papanikolaou, A and Keliris, GA and Peng, X and Shao, Y and Krapp, E and Papageorgiou, E and Schiefer, U and Logothetis, NK and Smirnakis, SM} } @Poster { 6810, title = {Decorrelated Firing in Cortical Microcircuits}, year = {2010}, month = {6}, volume = {2010}, pages = {58}, abstract = {Correlated trial-to-trial variability in the activity of cortical neurons is thought to reflect the functional connectivity of the circuit. Many cortical areas are organized into functional columns, in which neurons are believed to be densely connected and share common input. Numerous studies report a high degree of correlated variability between nearby cells. We developed chronically implanted multi-tetrode arrays offering unprecedented recording quality to re-examine this question in primary visual cortex of awake macaques. We found that even nearby neurons with similar orientation tuning show virtually no correlated variability. In a total of 46 recording sessions from two monkeys, we presented either static or drifting sine-wave gratings at eight different orientations. We recorded from 407 well isolated, visually responsive and orientation-tuned neurons, resulting in 1907 simultaneously recorded pairs of neurons. In 406 of these pairs both neurons were recorded by the same tetrode. Despite being physically close to each other and having highly overlapping receptive fields, neurons recorded from the same tetrode had exceedingly low spike count correlations (rsc = 0.005 ± 0.004; mean ± SEM). Even cells with similar preferred orientations (rsignal > 0.5) had very weak correlations (rsc = 0.028 ± 0.010). This was also true if pairs were strongly driven by gratings with orientations close to the cells’ preferred orientations. Correlations between neurons recorded by different tetrodes showed a similar pattern. They were low on average (rsc = 0.010 ± 0.002) with a weak relation between tuning similarity and spike count correlations (two-sample t test, rsignal < 0.5 versus rsignal > 0.5: P = 0.003, n = 1907). To investigate whether low correlations also occur under more naturalistic stimulus conditions, we presented natural images to one of the monkeys. The average rsc was close to zero (rsc = 0.001 ± 0.005, n = 329) with no relation between receptive field overlap and spike count correlations. We obtained a similar result during stimulation with moving bars in a third monkey (rsc = 0.014 ± 0.011, n = 56). Our findings suggest a refinement of current models of cortical microcircuit architecture and function: either adjacent neurons share only a few percent of their inputs or, alternatively, their activity is actively decorrelated.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, web_url = {http://www.areadne.org/2010/home.html}, editor = {Hatsopoulos, N. G., S. Pezaris}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Santorini, Greece}, event_name = {AREADNE 2010: Research in Encoding And Decoding of Neural Ensembles}, language = {en}, author = {Ecker, AS and Berens, P and Keliris, GA and Bethge, M and Logothetis, NK and Tolias, AS} } @Poster { KapoorWPKL2009, title = {Comparing inter-ocular switch and classical binocular rivalry in the human brain using EEG}, year = {2009}, month = {11}, volume = {10}, number = {3}, pages = {22}, abstract = {When disparate visual stimuli are presented to corresponding retinal locations, perception fluctuates between the presented stimuli. This phenomenon, called binocular rivalry, is an exquisite tool to dissociate sensory stimulation from visual perception. It has therefore been extensively used for studying the neural correlates of visual awareness. Initial theories have tried to explain binocular rivalry by hypothesizing the resolution of competition in V1 through inhibitory interactions between monocular neurons. However, inter-ocular switch rivalry, a paradigm where the rivaling stimuli are rapidly exchanged between the eyes also results in stable percepts that span several swaps of the visual stimuli. This has demonstrated that competition also involves higher-level stimulus representations, and not just eye based sensory information. In this study, we compared the electrophysiological correlates underlying stable visual percepts during inter-ocular switch and binocular rivalry. Delineating the differences and similarities between the two paradigms of rivalry will provide us with valuable information on the nature of competition during incongruent visual stimulation. We recorded EEGs while human subjects experienced inter-ocular switch and classical binocular rivalry elicited with dichoptic presentation of orthogonally oriented sinusoidal gratings. The subjects reported their percepts via button presses. We extracted and analyzed trials, where subjects reported at least one second long stable percepts. During this time window, we assessed the normalized spectrogram to visualize mean event-related changes in spectral power across a broad frequency range (1 - 45 Hz). We observed a strong and sustained increase in spectral power between 12 - 30 Hz across the two conditions approximately 300 ms following the reported perceptual switch. Low Resolution Brain Electromagnetic Tomography (LORETA) was used to localize the cortical sources of the observed changes. The maps of the localized cortical sources of this increase in the spectral power during inter-ocular switch and binocular rivalry were remarkably similar and showed no significant differences. We therefore propose that both types of rivalry have similar EEG correlates in the 12 - 30 Hz frequency band during a stable visual percept. 22}, department = {Department Logothetis}, web_url = {http://www.neuroschool-tuebingen-nena.de/}, event_place = {Ellwangen, Germany}, event_name = {10th Conference of Junior Neuroscientists of T{\"u}bingen (NeNa 2009)}, author = {Kapoor, V and Whittingstall, K and Panagiotaropoulos, T and Keliris, G and Logothetis, NK} } @Poster { 6286, title = {Comparing inter-ocular switch and classical binocular rivalry in the human brain using eeg}, year = {2009}, month = {10}, volume = {39}, number = {380.11}, abstract = {When disparate visual stimuli are presented to corresponding retinal locations in the two eyes, perception fluctuates between the presented stimuli. This phenomenon, called binocular rivalry, is an exquisite tool to dissociate sensory stimulation from visual perception. It has therefore been extensively used for studying the neural correlates of visual awareness. Initial theories have tried to explain binocular rivalry by hypothesizing the resolution of competition in V1 through inhibitory interactions between monocular neurons. However, inter-ocular switch rivalry, a paradigm where the rivaling stimuli are rapidly exchanged between the eyes also results in stable percepts that span several swaps of the visual stimuli. This has demonstrated that competition also involves higher level stimulus representations, and not just eye based sensory information. In this study, we compared the electrophysiological correlates underlying stable visual percepts during inter-ocular switch and classical binocular rivalry. Delineating the differences and similarities between the two paradigms of rivalry will provide us with valuable information on the nature of competition during incongruent visual stimulation. We recorded EEGs while human subjects experienced inter-ocular switch and classical binocular rivalry elicited with dichoptic presentation of orthogonally oriented sinusoidal gratings. The subjects reported their percepts via button presses. We extracted and analyzed trials, where subjects reported at least one second long stable percepts. During this time window, we assessed the normalized spectrogram to visualize mean event-related changes in spectral power across a broad frequency range (1-45 Hz). We observed a strong and sustained increase in spectral power between 12-30 Hz across the two conditions approximately 300 ms following the reported perceptual switch. Low Resolution Brain Electromagnetic Tomography (LORETA) was used to localize the cortical sources of the observed changes. The maps of the localized cortical sources of this increase in the spectral power during inter-ocular switch and binocular rivalry were remarkably similar and showed no significant differences. We therefore propose that both types of rivalry have similar EEG correlates in the 12-30 Hz frequency band during a stable visual percept.}, department = {Department Logothetis}, web_url = {http://www.abstractsonline.com/Plan/ViewAbstract.aspx?sKey=f5dec3fb-2bb6-482d-8553-db756136f1a1\&cKey=a119d583-ca30-4a52-801f-b1aed085e303}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Chicago, IL, USA}, event_name = {39th Annual Meeting of the Society for Neuroscience (Neuroscience 2009)}, language = {en}, author = {Kapoor, V and Whittingstall, K and Panagiotaropoulos, T and Keliris, GA and Logothetis, NK} } @Poster { 6290, title = {Primary visual cortex contributions in perceptual supppression}, year = {2009}, month = {10}, volume = {39}, number = {805.4}, abstract = {Understanding the neural underpinnings of conscious perception has long intrigued the students of the brain from philosophers to modern neuroscientists. In the visual domain, the primary visual cortex (V1) is by far the most extensively studied cortical area. It entails the main gateway of visual information to higher cortical areas and we understand a lot about its function in sensory processing. Nevertheless, the role of V1 in perceptual awareness remains intensely debated. Under certain stimulus conditions perception alternates between two or multiple stimulus interpretations. Notably such perceptual alternations happen while the sensory input is kept constant, offering thus a clear dissociation of sensory stimulation and subjective awareness. A celebrated example of such a perceptual phenomenon is binocular rivalry (BR). It involves the dichoptic presentation of two different stimuli at corresponding retinal locations and results in the perceptual suppression of one of the two stimuli at different times. A slight variant of BR, binocular flash suppression (BFS), ensures excellent control over the subjects’ perceptual state by intermittent presentation of monocular and binocular stimuli. We have trained rhesus macaques to report their perception during BFS and BR to study the effects of perceptual suppression in V1. We have recorded the spiking activity of a large number of well isolated single units (SUA) and acquired simultaneous local field potentials (LFPs) during the dichoptic presentation of orthogonal orientation gratings. We found that during BFS, 20\% of the single units modulated their activities in consonance with the perceptual state. Furthermore, the magnitude of the perceptual effect was small (15\%) in comparison to the sensory preference of the neurons. Analysis of the ocularity preferences demonstrated that both monocular and binocular classes of cells show perceptual modulations with equal probability. In addition, cells modulating during perceptual suppression encode information matching their sensory preferences and therefore can be used for decoding both the orientation and/or the eye of presentation of the perceived grating. Results of the LFPs were very similar to the single units showing a similar percentage of sites modulating with perception in all analyzed frequency bands. We conclude that footprints of perception are evident in both the SUA and LFP signals in V1 but in a much smaller degree than their corresponding sensory selectivity. Perceptual states might have a modulatory role on more intricate aspects of V1 firing patterns, not necessarily altering the firing rates of single cells or the LFP power dramatically.}, department = {Department Logothetis}, web_url = {http://www.abstractsonline.com/Plan/ViewAbstract.aspx?sKey=b32a2863-3104-401e-a2e7-5263bb970bc4\&cKey=32ca0e40-6724-42ff-90dd-e00783866960}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Chicago, IL, USA}, event_name = {39th Annual Meeting of the Society for Neuroscience (Neuroscience 2009)}, language = {en}, author = {Keliris, GA and Tolias, AS and Logothetis, NK} } @Poster { PanagiotaropoulosKKTL2009, title = {High frequency local field potentials and multi unit activity reflect visual awareness in the macaque prefrontal cortex}, journal = {Frontiers in Behavioral Neuroscience}, year = {2009}, month = {9}, volume = {Conference Abstract: 41st European Brain and Behaviour Society Meeting}, abstract = {Binocular rivalry (BR) has been successfully combined with extracellular electrophysiological recordings in awake, behaving macaques to study the cortical mechanisms of subjective visual perception. Here we used binocular flash suppression (BFS), a highly controlled variant of BR, to explore the neuronal correlates of visual awareness in the inferior prefrontal convexity (icPFC) of the macaque brain while simultaneously recording multi unit activity (MUA) and local field potentials (LFP). We found that MUA was perceptually modulated in 67\% of the visually selective recording sites. During BFS in 92\% of MUA modulated sites we observed higher firing rates when the preferred stimulus was perceived. An explicit representation of the perceptually dominant stimulus was also provided by the power modulation of high frequency LFP’s only at the MUA modulated sites. Specifically, sensory selectivity of the LFP power increased as a function of frequency with the highest selectivity observed between 150 and 450Hz. The same pattern in LFP power selectivity was observed when the preferred stimulus was perceived during BFS. A correlation analysis between MUA and LFP power selectivity showed significant correlation in sensory selectivity for frequencies >60Hz that saturated at 150Hz and followed the same pattern during BFS. While spikes measure cortical output, LFP’s are thought to reflect input and intracortical processing in a given cortical area. According to this scheme our results suggest that icPFC sites providing perceptually modulated output are also the sites that receive and process a representation of the perceived stimulus during BFS. Inferior temporal cortex (IT) output is also known to reflect the perceived stimulus during ambiguous visual stimulation and could thus be the source of the modulated icPFC input reflected in the LFP’s. Our results suggest a highly organized network involving IT and icPFC that mediates visual awareness during subjective visual perception.}, department = {Department Logothetis}, web_url = {http://www.frontiersin.org/10.3389/conf.neuro.08.2009.09.251/event_abstract}, event_place = {Rhodos, Greece}, event_name = {41st European Brain and Behaviour Society Meeting}, DOI = {10.3389/conf.neuro.08.2009.09.251}, author = {Panagiotaropoulos, T and Kapoor, V and Keliris, GA and Tolias, A and Logothetis, NK} } @Poster { 6177, title = {SANDBOX, an interactive fMRI data visualization toolbox}, year = {2009}, month = {3}, volume = {2009}, department = {Department Logothetis}, web_url = {http://www2.mrc-lmb.cam.ac.uk/groups/srw/cns/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Cambridge, UK}, event_name = {21st Cambridge Neuroscience Seminar: New Approaches in Neuroscience (CNS 2009)}, language = {en}, author = {Stoewer, S and Duncan, J and Bartels, A and Keliris, GA and Logothetis, NK and Sigala, N} } @Poster { 5443, title = {Neurophysiological substrates of visual awareness in the macaque prefrontal cortex}, year = {2008}, month = {7}, volume = {6}, number = {220.12}, abstract = {Human fMRI studies during binocular rivalry have demonstrated an involvement of prefrontal cortex (PFC) in the processing of subjective visual perception. In this study we used binocular flash suppression, a version of binocular rivalry that permits the robust induction of a visual percept, to study the neuronal correlates of visual awareness in the macaque prefrontal cortex (PFC) and specifically in the inferior prefrontal convexity. We found that the firing rate of almost 70\% of the visually selective neurons closely followed the induced visual percept. This percentage is significantly higher than the respective percentage of perceptually modulated cells found in the striate and extrastriate visual cortex (V1, V2 and V4) but smaller than that found in the inferior temporal cortex (IT) (almost 90\%). Interestingly, we observed that the neuronal responses following a perceptual alternation were transient, similar to the transient BOLD response observed during perceptual transitions in the human binocular rivalry fMRI studies. Our finding provides further evidence in support of a role of higher brain areas in processing an explicit perceptual representation during ambiguous visual stimulation. In addition, it points to a potential neuronal network consisting of perceptually modulated cells in IT and PFC that process an explicit representation of a visual percept. The existence of such a network is not surprising since area TE of inferior temporal cortex is anatomically connected to the inferior convexity (areas 12/45) through feedforward and feedback pathways. Finally, in an effort to explore whether the perceptual modulation observed in primary visual cortex (V1) is influenced by a feedback signal from PFC we will also present data from simultaneous PFC and V1 neurophysiological recordings during binocular flash suppression.}, department = {Department Logothetis}, web_url = {http://fens2008.neurosciences.asso.fr/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Geneva, Switzerland}, event_name = {6th Forum of European Neuroscience (FENS 2008)}, language = {en}, author = {Panagiotaropoulos, T and Kapoor, V and Keliris, GA and Tolias, A and Logothetis, NK} } @Poster { 5511, title = {The Role of Primary Visual Cortex (V1) in Perceptual Suppression}, year = {2008}, month = {7}, volume = {6}, number = {220.8}, abstract = {When two incongruent stimuli are presented simultaneously at corresponding retinal locations in the two eyes, one typically experiences a perceptual alternation of the two stimuli; a phenomenon known as binocular rivalry. Binocular flash suppression (BFS) is a variant of binocular rivalry and refers to the sudden and persistent perceptual suppression resulting when two rivalrous patterns are presented dichoptically and asynchronously to the two eyes. Under these conditions, the latter pattern dominates perceptually over the first. The binocular flash suppression paradigm ensures excellent control over the subject’s perceptual state without the need for subjective reports which involve decision making, action preparation and action execution. The role of primary visual cortex (V1) in perceptual suppression remains controversial. In this study, we assessed quantitatively the effects of perceptual suppression on neural activity in V1 of the macaque using BFS. We have analyzed both the spiking activity of a large number of single neurons (SUA) and different frequency bands of the local field potentials (LFPs). The main result for SUA was that only a small minority (\verb=~=20\%) modulates in consonance with the perceptual suppression of static orientation gratings. Furthermore, the magnitude of the perceptual effect was small (\verb=~=15\%) in comparison to the sensory preference of the neurons. LFPs showed comparable percentages. The amplitude of LFP modulations was independent of frequency although gamma frequencies showed greater selectivity during physical alternation of the stimuli. Our results provide evidence against the hypothesis that competition is happening at the level of monocular neurons at the input layers of primary visual cortex.}, department = {Department Logothetis}, web_url = {http://fens2008.neurosciences.asso.fr/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Geneva, Switzerland}, event_name = {6th Forum of European Neuroscience (FENS 2008)}, language = {en}, author = {Keliris, GA and Tolias, AS and Logothetis, NK} } @Poster { 5442, title = {Single units reflect visual awareness in the macaque prefrontal cortex}, year = {2008}, month = {6}, pages = {80}, department = {Department Logothetis}, web_url = {http://www.areadne.org/2008/home.html}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Santorini, Greece}, event_name = {AREADNE 2008: Research in Encoding and Decoding of Neural Ensembles}, language = {en}, author = {Panagiotaropoulos, T and Kapoor, V and Keliris, GA and Tolias, A and Logothetis, NK} } @Poster { 5510, title = {The Role of Primary Visual Cortex in Perceptual Awareness}, year = {2008}, month = {6}, pages = {61}, abstract = {Under certain stimulus conditions a single interpretation of the external world cannot be unambiguously designated. When the brain is presented with such stimuli typically only one possible interpretation is perceived and after a few seconds the percept switches abruptly to another. Notably such perceptual alternations happen while the sensory input is kept constant, offering thus a clear dissociation of sensory stimulation and subjective awareness. A celebrated example of such a perceptual phenomenon is binocular rivalry (BR). It involves alternations of visual perception between two different images presented dichoptically at corresponding retinal locations. Based on many psychophysical studies over decades the primary visual cortex (V1) was implicated as an important candidate for the site of perceptual suppression. However, the first neurophysiological evidence performed in monkeys did not corroborate this but instead found only a small percentage of neurons modulating their activity with the subjective awareness reported by the animals. On the contrary, studies using human functional magnetic resonance imaging (fMRI), have found V1 to be modulating to a large extent, creating an apparent controversy. Therefore, the role of primary visual cortex (V1) in subjective perception remains controversial. In this study, we studied the effects of perceptual suppression on neural activity in V1 of the macaque. We have used the binocular flash suppression (BFS) paradigm, a variant of BR which ensures excellent control over the subject’s perceptual state. We have recorded the spiking activity of a large number of well isolated single units (SUA) and acquired simultaneous local field potentials (LFPs) during the dichoptic presentation of orthogonal orientation gratings. Our design enabled us to determine a) which neurons and LFP bands are correlated with the percept and b) how this is related to their orientation and ocularity preferences. We find that only a small minority of about 20\% of the single units modulate in consonance with the perceptual suppression. Furthermore, the magnitude of the perceptual effect was small (\verb=~=15\%) in comparison to the sensory preference of the neurons. Results of the LFPs were very similar to the single units showing a similar percentage of sites modulating with perception. Analysis of the orientation and ocularity preference of neurons did not show a particular class of cells to be having a greater probability to show perceptual modulations.}, department = {Department Logothetis}, web_url = {http://www.areadne.org/2008/home.html}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Santorini, Greece}, event_name = {AREADNE 2008: Research in Encoding and Decoding of Neural Ensembles}, language = {en}, author = {Keliris, GA and Tolias, AS and Logothetis, NK} } @Poster { 5512, title = {Binocular Flash Suppression in area V1 of the macaque}, year = {2008}, month = {2}, department = {Department Logothetis}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Pittsburgh, PA, USA}, event_name = {First Annual inter-Science of Learning Center (iSLC): Student and Postdoc Conference}, language = {en}, author = {Keliris, GA and Tolias, AS and Logothetis, NK} } @Poster { 4591, title = {On the spatial scale of the local field potential - orientation and ocularity tuning of the local field potential in the primary visual cortex of the macaque}, year = {2007}, month = {11}, volume = {37}, number = {176.7}, abstract = {The local field potential (LFP) and, in particular, the gamma-band frequency range (30-90 Hz) have recently received much attention, as numerous studies have shown correlations between LFP and sensory, motor and cognitive variables in various cortical regions. However, the extent to which it reflects the activity of local populations of neurons remains elusive. The issue of spatial scale is central for understanding the origins of the LFP and how this signal can be used to study the functional organization of the brain. We addressed this question by simultaneously recording multi-unit spiking activity (MUA) and LFP from the primary visual cortex (V1) of awake, behaving macaques using arrays of tetrodes. Oriented gratings were used for visual stimulation, applied either binocular or monocular. The columnar organization of stimulus orientation and ocularity in V1 provides an excellent opportunity to study the spatial precision of the LFP signal, because neurons with similar orientation preference are organized at the fine spatial scale of cortical microcolumns (50-100 \(\mu\)m), whereas ocular dominance columns span around 450 \(\mu\)m. As shown before, we find that the increase of LFP gamma-band power is a function of orientation and ocularity of the stimulus. However, the power of the gamma-band contains much less information about the orientation of the stimulus than the MUA recorded at the same site. The average discriminability d' between preferred and orthogonal orientation was 2.46±0.15 for MUA and 1.01±0.05 for LFP (mean ±std). Moreover, we find only a weak correlation between the preferred orientation of the MUA tuning function and that of the LFP (r=0.21, p<0.05). In contrast, we find a strong correlation between the preferred ocularity of the two signals (r=0.53, p<1e-9). We therefore conclude that the gamma-power of the LFP does not reflect well the local activity on the scale of orientation columns but does capture the ocular dominance structure of V1. We suggest that gamma-band activity is generated by ensembles of neurons larger than 50-100 \(\mu\)m. In agreement with a previous study (Liu \& Newsome, 2006) we find that it more likely resembles the activity of neurons from an area spanning a few hundred micrometers.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2007/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {37th Annual Meeting of the Society for Neuroscience (Neuroscience 2007)}, language = {en}, author = {Berens, P and Ecker, AS and Keliris, GA and Logothetis, NK and Tolias, AS} } @Poster { 4733, title = {Recording chronically from the same neurons in awake, behaving primates}, year = {2007}, month = {11}, volume = {37}, number = {176.8}, abstract = {Understanding the mechanisms of learning and memory consolidation requires characterizing how the response properties of individual neurons and interactions across populations of neurons change over time, during periods spanning multiple days. We used multiple chronically implanted tetrodes to record single unit activity from area V1 of the awake, behaving macaque and developed a method to quantitatively determine recording stability. Our method is based on a statistical framework which uses similarity of action potential waveforms to detect stable recordings given a pre-defined type I error rate. The similarity measure that was used takes into account both the shape of the action potential waveform and the amplitude ratio across channels, which depends on the location of the neuron relative to the tetrode. 271 well-isolated single units were recorded from 7 tetrodes during two periods of up to 23 days. We computed the distribution of pairwise similarities of average waveforms recorded on consecutive recording sessions during the first 34 days after implantation of the chronic drive. During this period, there was no recording stability due to regular adjustments of the tetrodes. We used this distribution as an empirical null distribution for hypothesis testing. Using this statistical procedure and a type I error rate of alpha = 0.05, we find that of all single units recorded on a given day, 51\% could be recorded for at least 2 days, 40\% for at least 3 days, and 25\% for at least 7 days. In addition, we adapted a recently proposed multivariate statistical test (Gretton et al., 2007) to test whether the waveforms obtained at consecutive days come from the same underlying probability distribution. Using this test we obtained qualitatively similar results. To validate these results, we compared orientation tuning functions of neurons that were tracked across days. Consistent with the claim that the same neurons were recorded across days and the fact that the monkey was not performing a learning task, the distribution of tuning differences of stable and orientation-tuned neurons across days was highly significantly different (Wilcoxon rank sum test, n1 = 79, n2 = 582, p < 10\verb=^=-34) from the distribution of tuning differences across different neurons. Our results show that using only waveform information it is possible to reliably track stable neurons across days with a limited type I error probability. This statistical approach is particularly important since, in a learning experiment, properties of neurons such as orientation tuning are potentially changed and therefore cannot be used to evaluate stability.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/am2007/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {San Diego, CA, USA}, event_name = {37th Annual Meeting of the Society for Neuroscience (Neuroscience 2007)}, language = {en}, author = {Ecker, AS and Siapas, AG and Hoenselaar, A and Berens, P and Keliris, GA and Logothetis, NK and Tolias, AS} } @Poster { 4272, title = {A Data Management System for Electrophysiological Data Analysis}, journal = {Neuroforum}, year = {2007}, month = {4}, volume = {13}, number = {Supplement}, pages = {1222}, abstract = {Recent advances in both electrophysiological recording techniques and hardware capabilities have enabled researchers to simultaneously record from a large number of neurons in different areas of the brain. This opens the door for a wide range of complex analyses potentially leading to a better understanding of the principles underlying neural network computations. At the same time, due to the increasing amount of data with increasing complexity, significantly more emphasis has to be put on the data analysis task. Although high-level scripting languages such as Matlab can speed up the development of analysis tools, in our experience, a too large amount of time is still spent on (re)structuring and (re)organizing data for specific analyses. Therefore, our goal was to develop a system which enables experimental neuroscientists to spend less time on organizing their data and more on data collection and creative analysis. We developed an object oriented Matlab toolbox which supplies the user with basic data types and functions to organize and structure various types of electrophysiological data. By using an object oriented, hierarchical layout, basic functionality, such as integration of metadata, or storage and retrieval of data and results, is implemented independent of specific data formats or experimental designs. This provides maximal flexibility and compatibility with future experiments and new data formats. All data and experimental results are stored in a database, so the experimenter can choose which data to keep in memory for faster access and which to save to disk to save resources. Additionally, we have created an extensive library of basic analysis and visualization tools that can be used to get an overview of the data.}, url = {http://www.kyb.tuebingen.mpg.de/fileadmin/user_upload/files/publications/EckerTolias_2007_ADataManagement_4272[0].pdf}, department = {Department Logothetis}, web_url = {http://nwg.glia.mdc-berlin.de/media/pdf/conference/Proceedings-Goettingen2007.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {G{\"o}ttingen, Germany}, event_name = {7th Meeting of the German Neuroscience Society, 31st G{\"o}ttingen Neurobiology Conference}, language = {en}, author = {Ecker, AS and Berens, P and Keliris, GA and Logothetis, NK and Tolias, AS} } @Poster { 4273, title = {Orientation tuning of the local field potential and multi-unit activity in the primary visual cortex of the macaque}, journal = {Neuroforum}, year = {2007}, month = {4}, volume = {13}, number = {Supplement}, pages = {735}, abstract = {Oscillations in the local field potential (LFP) are abundant across species and brain regions. The possible relationship of these low-frequency extracelluar voltage fluctuations with the activity of the underlying local population of neurons remains largely elusive. To study this relationship, we used an array of chronically implanted tetrodes spanning a distance of 700 \&\#956;m and simultaneously recorded action potentials from multiple well-isolated single units, multi unit activity (MUA) and LFP from area V1 of the awake, behaving macaque. Moving and static gratings of different orientations were used for visual stimulation. In agreement with previous studies we find that the increase of LFP gamma-band power is a function of the orientation of the stimulus. However, the power of the gamma-band contains much less information about the orientation of the stimulus than the MUA and SUA recorded at the same site (Figure 1A). The average discriminability d‘ between preferred and orthogonal orientation was 2.46 for MUA, 2.45 for SUA and 1.01 for the LFP. Moreover, in contrast to recent results from area MT (Liu and Newsome, 2006) we find only a weak correlation between the preferred orientation of the MUA tuning function and that of the LFP (Figure 1B, different colors indicate different animals). Interestingly, all nearby LFP recording sites in our array were tuned to a similar orientation while the preferred orientations of MUA tuning functions were widely scattered. These results suggest that the power of LFP signals does not capture local population activity at the scale of orientation columns in area V1.}, url = {http://www.kyb.tuebingen.mpg.de/fileadmin/user_upload/files/publications/T16-4C_[0].pdf}, department = {Department Logothetis}, web_url = {http://nwg.glia.mdc-berlin.de/media/pdf/conference/Proceedings-Goettingen2007.pdf}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {G{\"o}ttingen, Germany}, event_name = {7th Meeting of the German Neuroscience Society, 31st G{\"o}ttingen Neurobiology Conference}, language = {en}, author = {Berens, P and Keliris, GA and Ecker, AS and Logothetis, NK and Tolias, AS} } @Poster { 4360, title = {Perceptual Suppression in area V1 of the Macaque}, year = {2006}, month = {6}, pages = {58}, abstract = {Under certain stimulus conditions we encounter pronounced perceptual suppression of suprathreshold visual stimuli. The brain mechanisms underlying these phenomena are poorly understood. Binocular rivalry (BR) and Binocular Flash Suppression (BFS) provide us excellent behavioural tools to study this phenomenon. During these paradigms visual stimuli are completely extinguished from our awareness for a substantial amount of time despite being physically present on our retinas. Therefore, we can study the dissociation between the neural responses that underlie a mere sensory representation of the visual input and what is perceived. Primary visual cortex (V1) has been implicated as an important candidate for the site of perceptual suppression. However, interestingly electrophysiological studies in V1 have found only a very small percentage of neurons to be correlated with the percept[1]. In contrast, human fMRI studies[2,3] have shown that the BOLD signal during such perceptual alternations modulates almost as much as when the stimuli are non-ambiguously presented separately. These contradicting results led to the speculation that the local field potential (LFP) signals, which have been shown to correlate with the BOLD signal, will also show correlations with perception in agreement with the BOLD results and thus potentially solve the apparent controversy. To this end, a recent study[4] claimed that low frequency (<30Hz) LFP signals in V1 correlate well with the subjective experience of macaques during BR. We have used BFS and recorded neural activity from large populations of well-isolated single neurons (SUA) from V1 using chronically implanted and non-chronic tetrodes in awake behaving macaques. In addition to the SUA we also simultaneously recorded multi-unit (MUA) and LFP signals. In agreement with previous electrophysiology experiments we find a very small percentage of single neurons (12\%, t-test: p<0.05) as well as MUA sites (15\%) to be correlated with the animals¹ percept during the binocular presentation of two gratings of orthogonal orientations. Interestingly, an even smaller percentage (7\%) of gamma-band LFP sites show a significant modulation and no other LFP band (e.g. alfa or beta-bands) showed stronger perceptually related modulation. In addition, the amplitude of the normalized population response in all three signals shows a small fractional modulation in comparison with the monocular presentation of the gratings (see figure). We therefore conclude that the activity in V1 is not a good predictor of the perceptual alternations at least using the classical simple measures of firing rate and power modulations of the signals.}, department = {Department Logothetis}, web_url = {http://www.areadne.org/2006/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Santorini, Greece}, event_name = {AREADNE 2006: Research in Encoding and Decoding of Neural Ensembles}, language = {en}, author = {Keliris, GA and Logothetis, NK and Tolias, A} } @Poster { 3949, title = {Spikes are phase locked to the gamma-band of the local field potential oscillations in the primary visual cortex of the macaque}, year = {2006}, month = {6}, pages = {39}, abstract = {Oscillations in the local field potential (LFP) are abundant across species and brain regions. The possible role of these oscillations in information processing in the primary visual cortex (V1) of the macaque still remains largely elusive despite that V1 is one of the most extensively studied brain areas. To this end, we used chronically implanted, multiple tetrodes and recorded the spiking activity of single neurons and LFPs from area V1 of the awake, behaving macaque. Moving and static gratings of different orientations were used for visual stimulation. In agreement with previous reports we find that the increase of the LFP gamma-band power is a function of the orientation of the stimulus. Surprisingly though, there is only a weak correlation between the peak of the multi-unit spiking activity orientation tuning functions and the peak of the orientation tuning function of the gamma-band power of the LFP. There is however a different kind of relationship between spikes and LFP. Namely, the timing of the spikes is not randomly distributed in time but instead is locked to the phase of the gamma-band of the LFP. Specifically, the spikes of 60 out of 151 well-isolated single units showed significant phase locking to the LFP (P<0.05, circular Rayleigh test). On average, the spikes occurred on the downward slope of the LFP oscillation. In contrast to the presence of phase precession reported in the rat hippocampus, the phase tuning in V1 is stable over time. Specifically, the preferred phase of the spikes does not seem to change over time during the presentation of the stimulus. Moreover, the preferred phase is not significantly modulated as a function of the orientation of the stimulus (Figure A). This temporal structuring of the spiking activity of neurons in V1 could allow coding of information in the temporal regime (Panzeri \& Schultz, 2001). In addition it could also potentially synchronize populations of neurons (Fries 2005). We are currently investigating these conjectures.}, department = {Department Logothetis}, web_url = {http://www.areadne.org/2006/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Santorini, Greece}, event_name = {AREADNE 2006: Research in Encoding and Decoding of Neural Ensembles}, language = {en}, author = {Berens, P and Ecker, AS and Hoenselaar, A and Keliris, GA and Siapas, AG and Logothetis, NK and Tolias, AS} } @Poster { ToliasEKSSL2006, title = {Structure of interneuronal correlations in the primary visual cortex of the rhesus macaque}, year = {2006}, month = {3}, pages = {13}, abstract = {Despite recent progress in systems neuroscience, basic properties of the neural code still remain obscure. For instance, the responses of single neurons are both highly variable and ambiguous (similar responses can be elicited by different types of stimuli). This variability/ambiguity has to be resolved by considering the joint pattern of firing of multiple single units responding simultaneously to a stimulus. Therefore, in order to understand the underlying principles of the neural code it is important to characterize the correlations between neurons and the impact that these correlations have on the amount of information that can be encoded by populations of neurons. Here we applied the technique of chronically implanted, multiple tetrodes to record simultaneously from a number of neurons in the primary visual cortex (V1) of the awake behaving macaque, and to measure the correlations in the trial-to-trial fluctuations of their firing rates under the same stimulation conditions (noise correlations). We find that, contrary to widespread belief, noise correlations in V1 are very small (around 0.01) and do not change systematically neither as a function of cortical distance (up to 600 um) nor as a function of the similarity in stimulus preference between the neurons (uniform correlation structure). Interestingly, a uniform correlation structure is predicted by theory to increase the achievable encoding accuracy of a neuronal population and may reflect a universal principle for population coding throughout the cortex.}, department = {Department Logothetis}, web_url = {http://www.cosyne.org/c/index.php?title=Cosyne_06}, event_place = {Salt Lake City, UT, USA}, event_name = {Computational and Systems Neuroscience Meeting (COSYNE 2006)}, author = {Tolias, AS and Ecker, A and Keliris, GA and Siapas, TG and Smirnakis, SM and Logothetis, NK} } @Poster { 3722, title = {Directional selectivity of human visual areas after adaptation to motion stimuli: an fMRI study}, year = {2005}, month = {11}, volume = {35}, number = {619.12}, abstract = {Motion processing is a fundamental property of the visual system. Classical electrophysiology studies in the macaque as well as fMRI studies in the human have revealed an extensive network of visual areas that contain neurons selective for direction of motion. Recent evidence from macaque fMRI (Tolias et al., J Neurosci 2001) and electrophysiology (Tolias et al. Nat Neurosci 2005) suggests that the direction-of-motion selectivity of macaque visual areas is not a fixed property but can change dynamically as a function of the state of adaptation to a moving stimulus. Here we used a visual motion adaptation paradigm to study the direction-of-motion selectivity of human visual areas with functional magnetic resonance imaging (fMRI). The visual stimuli we used consisted of expanding/contracting dot kinematograms at 100\% coherence. These were presented passively while the subject performed an attentionally demanding task at the fovea. After moving unidirectionally (expanding or contracting) for about 160 sec the kinematogram abruptly reversed direction of motion. By measuring the blood oxygen level dependent (BOLD) signal response elicited by the direction of motion reversal and comparing it to the initial response elicited when the adapting stimulus turns on, we were able to assess the degree of direction-of-motion selectivity in the various visual areas. We found that an extensive network of visual areas shows BOLD rebound when the direction of motion reverses after adaptation, including areas that according to classical electrophysiology do not show strong direction-of-motion selectivity (for example area V4). Our results agree qualitatively with the findings in the macaque (Tolias et al., J Neurosci 2001), and together underscore the dynamic nature of functional cortical architecture.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/absarchive/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Washington, DC, USA}, event_name = {35th Annual Meeting of the Society for Neuroscience (Neuroscience 2005)}, language = {en}, author = {Keliris, GA and Smirnakis, SM and Tolias, AS and Logothetis, NK} } @Poster { 3327, title = {Motion processing in area V4 revealed with adaptation: Tetrode recordings in the awake, behaving macaque}, year = {2004}, month = {10}, volume = {34}, number = {301.4}, abstract = {The ability to detect motion in our environment is a crucial function of the visual system. Motion processing is usually studied by comparing the activity elicited by various motion stimuli relative to a baseline (“no-movement”) condition. However, during natural vision the sensory input is not broken into a series of discrete presentations that are simply switched on and off. By using a motion adaptation paradigm we studied how stimulation history influences the directional selectivity of single neurons in area V4. We found that V4 neurons which classically would be thought as non-directionally selective can in fact acquire directional selectivity after adaptation. We recorded from area V4 of two monkeys using tetrodes and characterized the directional tuning properties of single units using drifting coherent random dot patterns. In agreement with previous studies we find that the majority of area V4 neurons are weakly tuned to the direction of motion when their properties are characterized using the classical stimulation paradigm. The same neurons though, express stronger directional tuning if previously adapted to a moving stimulus for a period of one second. To quantify the amount of directional information present in the activity of V4 neurons we used a Bayesian population decoding method to predict the direction of motion of the stimulus trial by trial using the activity of a population of neurons in a four hundred millisecond window. The average test error dropped significantly when computed after adaptation. It is important to characterize the properties of neuronal circuits under adaptation to better understand the mechanisms of natural vision.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/absarchive/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, institution = {Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany}, event_place = {San Diego, CA, USA}, event_name = {34th Annual Meeting of the Society for Neuroscience (Neuroscience 2004)}, language = {en}, author = {Keliris, GA and Smirnakis, SM and Logothetis, NK and Tolias, AS} } @Poster { MoutoussisKKL2004, title = {The involvement of different areas of the human visual brain in motion perception}, year = {2004}, month = {10}, volume = {34}, number = {865.14}, abstract = {The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocular rivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocular rivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Across the different visual areas, we have found varying degrees of correlation between the neural activity and the visual percept. Areas V3A, V5 (MT) and LOC showed a much stronger activation when subjects perceived coherent motion than when they perceived motion noise. A similar but not as strong an effect was observed in area V3, whereas a much less pronounced difference between the two conditions was found in areas V1, V2 and V4. These results demonstrate that motion perception is able to modulate the activity of most visual areas known to be involved in motion processing. Instead of a clear distinction between ‘processing’ and ‘perceptual’ areas, we found a gradual increase in the correlation between neural and perceptual events as one moves towards the higher areas of the motion pathway. We thus conclude that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.}, department = {Department Logothetis}, department2 = {Department B{\"u}lthoff}, web_url = {http://www.sfn.org/absarchive/}, event_place = {San Diego, CA, USA}, event_name = {34th Annual Meeting of the Society for Neuroscience (Neuroscience 2004)}, author = {Moutoussis, K and Keliris, G and Kourtzi, Z and Logothetis, NK} } @Poster { 3328, title = {Neural correlates of motion perception in the human visual brain}, journal = {NeuroImage}, year = {2004}, month = {6}, volume = {22}, number = {Supplement 1}, pages = {e1023-e1024}, abstract = {Introduction One of the most fascinating problems in visual neuroscience is finding a direct relationship between brain activity and perception. When a visual stimulus is presented to the eyes, it elicits a series of responses in many and different parts of the visual system, from the retina to the ’higher’ cortical areas, leading to a conscious visual percept. Dissociating which part of the visual brain activity is reflecting our perception is thus hard, since at the same time this activity is directly related to the processing of the visual stimulus itself. To try and answer this question, binocular rivalry has been used in the past, where the stimulus (which is different for each eye) remains constant but the perception alternates between the two rivalring monocular inputs. In this way one can dissociate the stimulus from the percept and, by studying the alternations in brain activation under such conditions, get an insight into which brain areas correlate their activity with what the subject actually perceives. Methods Binocular rivalry was used in fMRI experiments that were performed on a Siemens Trio 3T system. A different random dot kinematogram was shown to each eye, one consisting of red and the other of green dots. In one of the kinematograms 50\% of the dots moved in the same direction producing a coherent motion signal whereas in the other all dots moved in random directions thus producing pure motion noise. As binocular rivalry developed between red dots in one eye and green dots in the other, subjects inside the scanner used two different buttons to report whether they perceived one the another color. In this way we could relate the BOLD signal we recorded in the magnet to the subjects’ percept and investigate how motion perception is reflected in the cortical activation of the various visual areas. Results Averaging the event-related time-courses across all subjects showed a range of different responses, with no significant effect in areas V1, V2 and V4, only a slight difference in area V3, and a much more clear difference in areas V3a, V5 and LOC (Fig. 1). Discussion In this study we were able to show that a number of visual areas are involved in motion perception. In general, the more involved an area is in motion processing, the more it is modulated by motion perception, supporting the idea that processing and perceptual areas are not distinct and separable, but rather the same areas are involved in both processing and perception.}, department = {Department Logothetis}, web_url = {http://www.sciencedirect.com/science/article/pii/S1053811905700161}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, institution = {Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany}, event_place = {Budapest, Hungary}, event_name = {Tenth Annual Meeting of the Organization for Human Brain Mapping (HBM 2004)}, DOI = {10.1016/S1053-8119(05)70016-1}, author = {Moutoussis, K and Keliris, GA and Kourtzi, Z and Logothetis, NK} } @Poster { 2730, title = {FMRI Correlates of Perceptual Filling-in in a Moving Random Dot Paradigm}, year = {2002}, month = {11}, volume = {32}, number = {457.7}, abstract = {Perceptual filling-in refers to the fading of stabilized retinal patterns and their replacement by non-stabilized surrounding patterns. We used functional magnetic resonance imaging (fMRI) to investigate neuronal correlates of perceptual filling-in induced by a dynamic random dot pattern. The stimulus consisted of a moving random dot pattern on dark background surrounding a region devoid of dots (artificial scotoma). The subjects fixated at an eccentrically located spot, and they reported the time of onset of filling-in by button press. We controlled for attention by dimming the fixation spot at random points in time, which the subjects reported via a separate button press. Catch trials in which the stimulus physically filled the artificial scotoma were interspersed with filling-in trials to gauge the subjects’ performance. General linear model techniques with appropriate predictors were used to define areas of interest for analysis. Filling-in trials for each subject were divided in two groups of 30 trial s each, based on whether filling in occurred earlier (\<8 s) or later (8-24 s) in a trial. The stimulus was identical for all trials. Preliminary results suggest that the fMRI signal from area V1 rises initially in both groups but then dips and remains low for the group with early filling-in. This suggests that filling-in is associated with a relative suppression of cortical activity. Other interpretations will be discussed.}, department = {Department Logothetis}, web_url = {http://www.sfn.org/annual-meeting/past-and-future-annual-meetings}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, institution = {Max-Planck Institute for Biological Cybernetics, Tuebingen, Germany}, event_place = {Orlando, FL, USA}, event_name = {32nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002)}, language = {en}, author = {Keliris, GA and Smirnakis, SM and Kourtzi, Z and Tolias, AS and Logothetis, NK} } @Thesis { Keliris2007, title = {Investigating the Neural Correlates of Visual Perception}, year = {2007}, month = {11}, department = {Department Logothetis}, institution = {Eberhard Karls Universit{\"a}t T{\"u}bingen}, type = {PhD}, author = {Keliris, GA} } @Thesis { Keliris2003, title = {Evaluation of Single Unit Isolation: Tetrode Recordings in Awake Behaving Macaques}, year = {2003}, month = {2}, department = {Department Logothetis}, institution = {Eberhard Karls Universit{\"a}t T{\"u}bingen}, type = {Master}, author = {Keliris, GA} } @Conference { BelloyNKAKVV2017, title = {Dynamic resting state fMRI in mice: detection of Quasi-Periodic Patterns}, year = {2017}, month = {4}, day = {26}, pages = {566-567}, abstract = {We report, to our knowledge, the first application of a dynamic rsfMRI approach in mice and show the reliable detection of Quasi-Periodic Patterns (QPP) at both the group-level as well as in single-subject data. These patterns are consistent with QPPs detected previously in humans and rats, displaying a high intensity wave spreading across the cortex from lateral towards upward medial, followed by a low intensity wave in the same direction. These findings are promising for the application of QPPs towards investigating mouse resting state functional connectivity and its development as a potential new pre-clinical tool.}, talk_type = {Abstract Talk}, web_url = {http://www.ismrm.org/17/program_files/O77.htm}, event_place = {Honolulu, HI, USA}, event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)}, author = {Belloy, M and Naeyaert, M and Keliris, G and Abbas, A and Keilholz, S and van der Linden, A and Verhoye, M} } @Conference { TakemuraPWKLSYBLFLW2017, title = {Using diffusion MRI and tractography to identify macaque vertical occipital fasciculus}, year = {2017}, month = {4}, day = {24}, pages = {95}, abstract = {We evaluated the ability of diffusion MRI-based tractography to identify macaque vertical occipital fasciculus (VOF), an important but little-studied white-matter tract connecting dorsal and ventral visual cortex. We analyzed four macaque diffusion MRI datasets with different resolution. The high-resolution post-mortem dataset reliably detects the macaque VOF, in a consistent manner with previous invasive anatomical studies. Lower resolution in vivo data showed qualitatively consistent results, but the estimated tract endpoints are restricted to sulcus. Taken together, our results demonstrate that the need for high-resolution diffusion MRI to identify certain critical white matter tracts.}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.ismrm.org/17/program_files/O71.htm}, event_place = {Honolulu, HI, USA}, event_name = {25th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM 2017)}, author = {Takemura, H and Pestilli, F and Weiner, K and Keliris, G and Landi, S and Sliwa, J and Ye, F and Barnett, M and Leopold, D and Freiwald, F and Logothetis, NK and Wandell, B} } @Conference { Keliris2015, title = {fMRI responses to dynamic checkerboards reveal average single neuron receptive fields in early visual cortex}, year = {2015}, month = {7}, day = {7}, department = {Department Logothetis}, talk_type = {Invited Lecture}, web_url = {http://ibro2015.org/?page_id=434}, event_place = {Rio de Janeiro, Brazil}, event_name = {9th IBRO World Congress on Neuroscience (IBRO 2015)}, author = {Keliris, G} } @Conference { PapanikolaouKLPSLS2014, title = {Organization of human area V5/MT+ and sensitivity to motion coherence after lesions of the primary visual cortex}, year = {2014}, month = {11}, day = {19}, volume = {44}, pages = {772.07}, abstract = {Partial loss of the primary visual cortex (V1) and/or its inputs leads to a scotoma of the contralateral visual hemifield, the extent of which corresponds retinotopically to the region affected. However, some patients have been found to retain a small amount of residual visual sensitivity within the blind field, a phenomenon termed blindsight, suggesting the existence of alternate pathways that transmit information from the retina to cortex effectively bypassing V1. Blindsight has been associated with activity observed in the middle temporal area complex (V5/MT+) following V1 lesions. An important issue is how the properties of area hV5/MT+, like retinotopic organization and sensitivity to motion, change following V1 lesions. We measured responses in human area V5/MT+ in 5 patients with homonymous visual field defects as a result of area V1 or optic radiation lesions using functional magnetic resonance imaging (fMRI). First, we investigated whether the organization of area hV5/MT+ changes following V1 damage. To do so, we used a recent method that estimates population receptive field (pRF) topography in the visual cortex (Lee et al., A new method for estimating population receptive field topography in visual cortex, NeuroImage, 2013). FMRI measurements were obtained during the presentation of a moving bar stimulus while the subjects were fixating. The pRF topography of area hV5/MT+ was compared with that of control subjects stimulated with matching “artificial scotomas”. In addition, we measured the sensitivity of area hV5/MT+ to coherent motion using random dot kinematograms (RDK) for both patients and controls. RDK patches were presented either inside the visual field scotoma or in the contralateral healthy part of the visual field. Subjects were instructed to report the direction of motion of the presented RDK while fixating. In both cases we found responses in hV5/MT+ arising inside the scotoma, independent of area V1 input, suggesting the existence of a functional alternate pathway bypassing area V1. The retinotopic organization of hV5/MT+ differed between patients and controls under the artificial scotoma condition, suggesting a degree of reorganization. The blood oxygen level-dependent (BOLD) response of area hV5/MT+ to RDK coherent motion stimuli also differed between patients and controls, and was dependent on which side was attended. Studying how the properties of visual areas change after injury may allow us to design better rehabilitative strategies in the future.}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.sfn.org/annual-meeting/neuroscience-2014}, event_place = {Washington, DC, USA}, event_name = {44th Annual Meeting of the Society for Neuroscience (Neuroscience 2014)}, author = {Papanikolaou, A and Keliris, GA and Lee, S and Papageorgiou, TD and Schiefer, U and Logothetis, NK and Smirnakis, SM} } @Conference { Keliris2013_2, title = {Measuring average neuronal receptive field sizes with fMRI and how it can be used to study plasticity and reorganization}, year = {2013}, month = {11}, day = {20}, department = {Department Logothetis}, talk_type = {Invited Lecture}, event_place = {T{\"u}bingen, Germany}, event_name = {ESMRMB-Lectures on Magnetic Resonance, Advanced methods for acquisition and analysis of fMRI data}, author = {Keliris, G} } @Conference { PapanikolaouKPSKPSLS2013, title = {Human area V5/MT+ organization changes following lesions of the primary visual cortex}, year = {2013}, month = {11}, day = {12}, volume = {43}, number = {602.07}, abstract = {Damage of the primary visual cortex (V1) and/or its inputs leads to a visual field loss (scotoma) in the corresponding, homonymous, region of the contralateral visual hemifield. The resulting visual field defect often involves the whole hemifield (hemianopia) or one quadrant (quadrantanopia). However, some patients have been found to retain a small amount of residual visual sensitivity within the blind field, a phenomenon termed blindsight, suggesting that there are alternate pathways that effectively bypass V1 and transmit visual information directly to extrastriate visual cortex. Blindsight has been associated with activity observed in the middle temporal area complex (V5/MT+) following V1 lesions. An important issue is how the organization of area V5/MT+, including how it covers the visual field, changes following V1 lesions. We used the population receptive field (pRF) method (Dumoulin SO, Wandell BA, Population receptive field estimates in human visual cortex, Neuroimage 39, 2008) to study the organization of human area V5/MT+ after V1 injury in adulthood. Functional magnetic resonance imaging (fMRI) measurements were obtained during the presentation of a moving bar stimulus while the subjects were fixating. We mapped the retinotopic organization of area V5/MT+ in 5 patients with approximate quadrantanopia and compared them to control subjects stimulated with matching “artificial scotomas”. We investigated i) whether area hV5/MT+ remains responsive following the V1 lesion, ii) whether it retains its retinotopic organization, and iii) whether hV5/MT+ organization changes by recruiting inputs from intact portions of area V1, or from the contralateral hV5/MT+. In three patients we found responses in hV5/MT+ arising inside the scotoma, independent of area V1 input, suggesting the existence of a functional alternate pathway bypassing area V1. hV5/MT+ of the other patients responded only to locations outside of the perceptual scotoma. The retinotopic organization of V5/MT+ for all 5 patients was different than that seen in controls stimulated with the “artificial scotoma.” Specifically, the pRF center distribution was shifted across the horizontal meridian towards locations outside the visual field scotoma for all patients. PRF size distributions differed between patients and controls under the artificial scotoma condition, with some subjects having larger pRFs on average, while others smaller. Finally, changes were observed in the retinotopic organization of the contra-lesional area hV5/MT+, likely mediated via trans-callosal connections.}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.sfn.org/annual-meeting/neuroscience-2013}, event_place = {San Diego, CA, USA}, event_name = {43rd Annual Meeting of the Society for Neuroscience (Neuroscience 2013)}, author = {Papanikolaou, A and Keliris, GA and Papageorgiou, GT and Shao, Y and Krapp, E and Papageorgiou, E and Schiefer, U and Logothetis, NK and Smirnakis, SM} } @Conference { Keliris2013, title = {Receptive field mapping using fMRI: what can it tells us about functional organization}, year = {2013}, month = {9}, day = {14}, department = {Department Logothetis}, talk_type = {Invited Lecture}, web_url = {http://www.neuroschool-tuebingen-cogni.de/index.php?id=374}, event_place = {T{\"u}bingen, Germany}, event_name = {Networks! 2013: 4th German Neurophysiology PhD Meeting}, author = {Keliris, G} } @Conference { LeeKPSL2012, title = {Visualization of the population receptive field structures in human visual cortex}, year = {2012}, month = {10}, day = {17}, volume = {42}, number = {723.08}, abstract = {Functional resonance imaging (fMRI) has been used to measure the retinotopic structures of the human visual cortex in vivo. Recently, this stream of research has been advanced by introduction of a computational model to fit a predefined population receptive field (pRF) model to fMRI signals observed. This method has advantages over the previous methods by providing receptive field (RF) size as well as more accurate retinopic maps. However, this model is limited because this method need assume the pRF as a certain model (e.g., circular Gaussian). To overcome this limitation, in the present study, we introduce a new method to visualize the pRF structure prior to modeling. This method estimates the pRF structure by fitting a set of weights representing the pRF topography in space to observed fMRI signals. For that, let vector p and s represent a pRF topography and a stimulus aperture. When visual stimuli present through the aperture, the pRF response is given as r = ps. As the pRF response is observed in the form of fMRI signal, it is required to convolve it with a canonical hemodynamic response function h. Therefore, the final pRF prediction x is given: x = h*r = h*(ps) Here, * denotes convolution. From this model, vector p is estimated by using the ridge regression. Application of our method yielded clear pRF structures which include the pRF center and surround regions. In addition, some pRF centers looked elliptic while the previous method assumed the pRF is isotropic.Therefore, this approach allows scientists to select a more appropriate pRF model based on the pRF topography observed. This application resulted in more accurate eccentricity map than the one by the previous method (directly-fitting circular Gaussian model). Furthermore, we could observe pRF properties such as elongation and orientation of the pRF center, and the surround suppression.}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.abstractsonline.com/Plan/ViewAbstract.aspx?sKey=27352f56-f434-4222-ba5e-b101143718e0\&cKey=d35aa700-8a6e-45b7-834a-fd60a4aad7ee\&mKey=70007181-01c9-4de9-a0a2-eebfa14cd9f1}, event_place = {New Orleans, LA, USA}, event_name = {42nd Annual Meeting of the Society for Neuroscience (Neuroscience 2012)}, author = {Lee, S and Keliris, GA and Papanikolaou, A and Smirnakis, SM and Logothetis, NK} } @Conference { 5290, title = {On the neural mechanisms of binocular rivalry}, journal = {Frontiers in Human Neuroscience}, year = {2008}, month = {9}, volume = {Conference Abstract: 10th International Conference on Cognitive Neuroscience}, abstract = {Binocular rivalry is scientifically attractive because it allows the study of an entirely subjective experience using objective measurements: During rivalry the visual percept changes dramatically – from one image to another – while the two stimuli presented to the eyes remain constant. There are at least two aspects whose neural origin would be worthwhile understanding: 1. The mechanisms that lead to the stochastic, spontaneous, and sometimes abrupt alternations of the percept from one stimulus to the other; 2. The mechanisms that keep one stimulus dominant, perceived, and the other suppressed. Previous psychophysical studies have elegantly demonstrated that both monocular and binocular sites contribute to perceptual alternations and to perceptual dominance. Recordings from single neurons, from monocular cells in V1 to cells in the prefrontal cortex show signals representing both the suppressed as well as the dominant stimuli. The proportion of neurons exhibiting percept-modulated responses rises from V1, through V4/V5, IT to prefrontal cortex. Additionally, some studies have reported that certain bands of local field potentials in V1 contain more information about the percept than spikes, while fMRI results in the human brain even show perceptual modulations in the LGN. Like psychophysics, physiology points toward a potentially complex interaction of several neural sites involved in rivalry. We will present the latest recordings from hundreds of neurons in V1, as well as initial recordings from prefrontal cortex. We will mainly focus however, on new psychophysical results shedding light on the eye-versus-percept debate. These results suggest a time-dependence of eye and percept contributions in binocular rivalry. During a dominance period, it appears that it is initially a given monocular channel that has major influence on dominance, regardless of the percept. Over time, this reverses, with image-related, eye-independent processes increasingly controlling any perceptual switch. Our results lead us to suggest that monocular effects – as observed here and in previous studies – may directly depend on higher-level effects and vice versa, because monocular as well as higher-level perceptual influences on dominance vary in parallel but with opposite signs over time. Therefore, the monocular and binocular effects observed in binocular rivalry may reflect different ends of a single process affecting several neural stages. A potential model could be that an initially strong stimulus representation is stabilized by a reinforcing, noise-reducing loop between binocular and monocular stages. As the stability of this process weakens, both the monocular channel loses influence, and the binocular stimulus representation weakens, increasingly favoring a perceptual switch.}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.frontiersin.org/10.3389/conf.neuro.09.2009.01.048/event_abstract}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Bodrum, Turkey}, event_name = {10th International Conference on Cognitive Neurosciences (ICON 2008)}, DOI = {10.3389/conf.neuro.09.2009.01.048}, author = {Bartels, A and Panagiotaropoulos, T and Keliris, GA and Logothetis, NK} } @Conference { ToliasEKPPL2007, title = {Population codes, correlations and coding uncertainty}, year = {2007}, month = {9}, pages = {16}, abstract = {Despite progress in systems neuroscience the neural code still remains elusive. For instance, the responses of single neurons are both highly variable and ambiguous (similar responses can be elicited by different types of stimuli). This variability/ambiguity has to be resolved by considering the joint pattern of firing of multiple single units responding simultaneously to a stimulus. Therefore, in order to understand the underlying principles of the neural code it is imperative to characterize the correlations between neurons and the impact that these correlations have on the amount of information encoded by populations of neurons. We use chronically implanted tetrode arrays to record simultaneously from many neurons in the primary visual cortex (V1) of awake, behaving macaques. We find that the correlations in the trial-to-trial fluctuations of their firing rates between neurons under the same stimulation conditions (noise correlations) in V1 were very small (around 0.01 in 500 ms bin window) during passive viewing of sinusoidal grating stimuli. We are also measuring correlations in extrastriate visual areas and investigating the impact of correlations on encoding stimulus uncertainty by neuronal populations, under different stimulus and behavioral conditions.}, department = {Department Logothetis}, department2 = {Research Group Bethge}, talk_type = {Abstract Talk}, web_url = {http://www.gatsby.ucl.ac.uk/nccd/nccd07/abstract_book.pdf}, event_place = {Hossegor, France}, event_name = {Neural Coding, Computation and Dynamics (NCCD 07)}, author = {Tolias, AS and Ecker, A and Keliris, GA and Panagiotaropolulos, F and Panzeri, S and Logothetis, NK} } @Conference { 3723, title = {Structure of interneuronal correlations in the primary visual cortex of the Rhesus macaque}, year = {2005}, month = {11}, volume = {35}, number = {591.12}, abstract = {Despite recent progress in systems neuroscience, basic properties of the neural code still remain obscure. For instance, the responses of single neurons are both highly variable and ambiguous (similar responses can be elicited by different types of stimuli). This variability/ambiguity has to be resolved by considering the joint pattern of firing of multiple single units responding simultaneously to a stimulus. Therefore, in order to understand the underlying principles of the neural code it is important to characterize the correlations between neurons and the impact that these correlations have on the amount of information that can be encoded by populations of neurons. Here we applied the technique of chronically implanted, multiple tetrodes to record simultaneously from a number of neurons in the primary visual cortex (V1) of the awake behaving macaque, and to measure the correlations in the trial-to-trial fluctuations of their firing rates under the same stimulation conditions (noise correlations). We find that, contrary to widespread belief, noise correlations in V1 are very small (around 0.01) and do not change systematically neither as a function of cortical distance (up to 600 m) nor as a function of the similarity in stimulus preference between the neurons (uniform correlation structure). Interestingly, a uniform correlation structure is predicted by theory to increase the achievable encoding accuracy of a neuronal population and may reflect a universal principle for population coding throughout the cortex. Support Contributed By: MPI, NEI(NIH)}, department = {Department Logothetis}, talk_type = {Abstract Talk}, web_url = {http://www.sfn.org/absarchive/}, institute = {Biologische Kybernetik}, organization = {Max-Planck-Gesellschaft}, event_place = {Washington, DC, USA}, event_name = {35th Annual Meeting of the Society for Neuroscience (Neuroscience 2005)}, language = {en}, author = {Tolias, AS and Keliris, GA and Ecker, AS and Siapas, AG and Smirnakis, SM and Logothetis, NK} }