6239 3 IV Bondar DA Leopold BJ Richmond JD Victor NK Logothetis 2009-12-00 12 4 1 10 PLoS One Many neurons in primate inferotemporal (IT) cortex respond selectively to complex, often meaningful, stimuli such as faces and objects. An important unanswered question is whether such response selectivity, which is thought to arise from experience-dependent plasticity, is maintained from day to day, or whether the roles of individual cells are continually reassigned based on the diet of natural vision. We addressed this question using microwire electrodes that were chronically implanted in the temporal lobe of two monkeys, often allowing us to monitor activity of individual neurons across days. We found that neurons maintained their selectivity in both response magnitude and patterns of spike timing across a large set of visual images throughout periods of stable signal isolation from the same cell that sometimes exceeded two weeks. These results indicate that stimulus-selectivity of responses in IT is stable across days and weeks of visual experience. no notspecified http://www.kyb.tuebingen.mpg.de/ published 9 15017 15421 6096 3 K-M Müller F Schillinger DH Do DA Leopold 2009-07-00 7 4 1 8 PLoS ONE Neurons in the visual cortex are responsive to the presentation of oriented and curved line segments, which are thought to act as primitives for the visual processing of shapes and objects. Prolonged adaptation to such stimuli gives rise to two related perceptual effects: a slow change in the appearance of the adapting stimulus (perceptual drift), and the distortion of subsequently presented test stimuli (adaptational aftereffects). Here we used a psychophysical nulling technique to dissociate and quantify these two classical observations in order to examine their underlying mechanisms and their relationship to one another. In agreement with previous work, we found that during adaptation horizontal and vertical straight lines serve as attractors for perceived orientation and curvature. However, the rate of perceptual drift for different stimuli was not predictive of the corresponding aftereffect magnitudes, indicating that the two perceptual effects are governed by distinct neural processes. Finally, the rate of perceptual drift for curved line segments did not depend on the spatial scale of the stimulus, suggesting that its mechanisms lie outside strictly retinotopic processing stages. These findings provide new evidence that the visual system relies on statistically salient intrinsic reference stimuli for the processing of visual patterns, and point to perceptual drift as an experimental window for studying the mechanisms of visual perception. no notspecified http://www.kyb.tuebingen.mpg.de/ published 7 15017 15422 15017 15421 5840 3 J Cui M Wilke NK Logothetis DA Leopold H Liang 2009-01-00 2 49 228 236 Vision Research We investigated how the perceptual visibility of a target influences the pattern of microsaccadic eye movements expressed during generalized flash suppression. We found that the microsaccade rate was highly dependent on the reported visibility of the target. In the visible trials, the microsaccade rate promptly rebounded to the pre-onset level, whereas on the invisible trials the rate remained low, reaching pre-onset levels hundreds of milliseconds later. In addition, the directional distributions of microsaccades were biased to the target positions in the visible condition. The present findings indicate that the microsaccade behavior is highly correlated with the perceptual state of target visibility, and suggest that the measured microsaccade rate and direction are reliable indicators of the perception. no notspecified http://www.kyb.tuebingen.mpg.de/ published 8 15017 15421 4672 3 A Maier NK Logothetis DA Leopold 2007-03-00 13 104 5620 5625 Proceedings of the National Academy of Sciences of the United States of America no notspecified http://www.kyb.tuebingen.mpg.de/ published 5 15017 15421 4396 3 M Wilke NK Logothetis DA Leopold 2006-11-00 46 103 17507 17512 Proceedings of the National Academy of the United States of America Neurophysiological and functional imaging experiments remain in apparent disagreement on the role played by the earliest stages of the visual cortex in supporting a visual percept. Here, we report electrophysiological findings that shed light on this issue. We monitored neural activity in the visual cortex of monkeys as they reported their perception of a high-contrast visual stimulus that was induced to vanish completely from perception on a subset of trials. We found that the spiking of neurons in cortical areas V1 and V2 was uncorrelated with the perceptual visibility of the target, whereas that in area V4 showed significant perception-related changes. In contrast, power changes in the lower frequency bands (particularly 9–30 Hz) of the local field potential (LFP), collected on the same trials, showed consistent and sustained perceptual modulation in all three areas. In addition, for the gamma frequency range (30–50 Hz), the responses during perceptual suppression of the target were correlated significan tly with the responses to its physical removal in all areas, although the modulation magnitude was considerably higher in area V4 than in V1 and V2. These results, taken together, suggest that low-frequency LFP power in early cortical processing is more closely related to the representation of stimulus visibility than is spiking or higher frequency LFP activity. no notspecified http://www.kyb.tuebingen.mpg.de/ published 5 15017 15421 4398 3 DA Leopold IV Bondar MA Giese 2006-07-00 7102 442 572 575 Nature The rich and immediate perception of a familiar face, including its identity, expression and even intent, is one of the most impressive shared faculties of human and non-human primate brains. Many visually responsive neurons in the inferotemporal cortex of macaque monkeys respond selectively to faces sometimes to only one or a few individuals while showing little sensitivity to scale and other details of the retinal image. Here we show that face-responsive neurons in the macaque monkey anterior inferotemporal cortex are tuned to a fundamental dimension of face perception. Using a norm-based caricaturization framework previously developed for human psychophysics we varied the identity information present in photo-realistic human faces, and found that neurons of the anterior inferotemporal cortex were most often tuned around the average, identity-ambiguous face. These observations are consistent with face-selective responses in this area being shaped by a figural comparison, reflecting structural differences be tween an incoming face and an internal reference or norm. As such, these findings link the tuning of neurons in the inferotemporal cortex to psychological models of face identity perception. no notspecified http://www.kyb.tuebingen.mpg.de/ published 3 15017 15421 4674 3 A Maier NK Logothetis DA Leopold 2005-10-00 9 5 668 677 Journal of Vision Certain pairs of visual patterns, when superimposed as if transparent, elicit a wavering percept in which one or the other pattern can enjoy temporary periods of exclusive visibility. This multistable perceptual phenomenon is called monocular rivalry or pattern rivalry (PR) and is most pronounced when the component patterns are faint and devoid of detail. The principal mechanisms that give rise to PR continue to be a topic of speculation. In the present study, we examine the determinants of exclusive dominance during PR using a novel stimulus in which a central portion is free of conflict. By observing the properties of suppression in this so-called rivalry-free region, we demonstrate that perception is driven largely by the global and holistic interpretation of the patterns, rather than by the need to resolve local spatial conflict. The suppression of this central region was often complete and varied as a function of the parameters of the global stimulus, including the size of the surround region, its ocular configuration, and stereoscopic depth ordering. Suppression also varied as a function of pattern continuity across the central region as well as with the temporal offset of the overlapping components. These findings demonstrate that the visibility or invisibility of a pattern is not fundamentally a product of local processing, but is instead shaped by the brain&lsquo;s global interpretive assumptions regarding the composition of the stimulus. no notspecified http://www.kyb.tuebingen.mpg.de/ published 9 15017 15421 4676 3 DA Leopold G Rhodes K-M Müller L Jeffery 2005-05-00 1566 272 897 904 Proceedings of the Royal Society of London B Several recent demonstrations using visual adaptation have revealed high-level aftereffects for complex patterns including faces. While traditional aftereffects involve perceptual distortion of simple attributes such as orientation or colour that are processed early in the visual cortical hierarchy, face adaptation affects perceived identity and expression, which are thought to be products of higher-order processing. And, unlike most simple aftereffects, those involving faces are robust to changes in scale, position and orientation between the adapting and test stimuli. These differences raise the question of how closely related face aftereffects are to traditional ones. Little is known about the build-up and decay of the face aftereffect, and the similarity of these dynamic processes to traditional aftereffects might provide insight into this relationship. We examined the effect of varying the duration of both the adapting and test stimuli on the magnitude of perceived distortions in face identity. We found that, just as with traditional aftereffects, the identity aftereffect grew logarithmically stronger as a function of adaptation time and exponentially weaker as a function of test duration. Even the subtle aspects of these dynamics, such as the power-law relationship between the adapting and test durations, closely resembled that of other aftereffects. These results were obtained with two different sets of face stimuli that differed greatly in their low-level properties. We postulate that the mechanisms governing these shared dynamics may be dissociable from the responses of feature-selective neurons in the early visual cortex. no notspecified http://www.kyb.tuebingen.mpg.de/ published 7 15017 15421 2276 3 DA Leopold A Maier NK Logothetis 2003-10-00 9-10 10 115 130 Journal of Consciousness Studies Understanding how activity in the brain leads to a subjective percept is of great interest to philosophers and neuroscientists alike. In the last years, neurophysiological experiments have approached this problem directly by measuring neural signals in animals as they experience well-defined visual percepts. Stimuli in these studies are often inherently ambiguous, and thus rely upon the subjective report, generally from trained monkeys, to provide a measure of perception. By correlating the responses of a neuron or group of neurons to this report, one can speculate on the role of individual neurons and groups of neurons in the formation and maintenance of a particular percept. However, in order to draw valid conclusions from such experiments, it is critical that the responses accurately and reliably reflect what is perceived. For this reason, a number of behavioral paradigms have been developed to control and evaluate the truthfulness of responses from behaving animals. Here we describe several approaches to optimizing the reliability of a monkey's perceptual report, and argue that their combination provides an invaluable approach in the study of subjective visual perception. no notspecified http://www.kyb.tuebingen.mpg.de/ published 15 15017 15421 2331 3 M Wilke NK Logothetis DA Leopold 2003-09-00 6 39 1043 1052 Neuron A pattern of light striking the retina of an alert observer is normally readily perceived. While a handful of conditions exist in which even salientvisual stimuli can be rendered invisible, the mechanisms underlying such suppression remain poorly understood. Here, we describe experiments using a novel stimulation sequence that gives rise to the sudden and reliable subjective disappearance of a wide range of visual patterns. We found that a parafoveal target immediately vanished from perception following the abrupt onset of a surrounding texture. The probability of disappearance was influenced by the ocular configuration of the target and surround, as well as their spatial separation. In addition, suppression was critically dependent upon several hundred milliseconds of stimulus-specific adaptation. These findings demonstrate that the all-or-none disappearance of a salientvisualtarget, which is reminiscent of a high-level selection process, is inextricably linked to topographic stimulus representations, presumably in the early visual cortex. no notspecified http://www.kyb.tuebingen.mpg.de/ published 9 15017 15421 2056 3 A Maier M Wilke NK Logothetis DA Leopold 2003-07-00 13 13 1076 1085 Current Biology Background: Continuous viewing of ambiguous patterns is characterized by wavering perception that alternates between two or more equally valid visual solutions. However, when such patterns are viewed intermittently, either by repetitive presentation or by periodic closing of the eyes, perception can become locked or "frozen" in one configuration for several minutes at a time. One aspect of this stabilization is the possible existence of a perceptual memory that persists during periods in which the ambiguous stimulus is absent. Here, we use a novel paradgim of temporally interleaved ambiguous stimuli to explore the nature of this memory, with particular regard to its potential impact on perceptual organization. Results: We found that the persistence of a perceptual configuration was robust to interposed visual patterns and, further, that at least three ambiguous patterns, when interleaved in time, could undergo parallel, stable time courses. Then, using an interleaved presentation paradigm, we established that the occasional reversal in one pattern could be coupled with that of its interleaved counterpart, and that this coupling was a function of the structural similarity between the patterns. Conclusions: We postulate that the stabilization observed with repetitive presentation of ambiguous patterns can be at least partially accounted for by processes that retain a recent perceptual interpretation, and we speculate that such memory may be important in natural vision. We further propose tha the interleaved paradigm introduced here may be of great value to gauge aspects of stimulus similarity that appeal to particular mechanisms of perceptual organization. no notspecified http://www.kyb.tuebingen.mpg.de/ published 9 15017 15421 2275 3 DA Leopold 2003-06-00 6 6 548 549 Nature Neuroscience Neural activity in the enigmatic lateral intraparietal area (LIP) of the monkey brain is now reported to reflect the perceived direction of ambiguous motion more faithfully than does activity in ohter areas apparently specialized for motion analysis. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf2275.pdf published 1 15017 15421 2054 3 DA Leopold Y Murayama NK Logothetis 2003-04-00 4 13 422 433 Cerebral Cortex We examined fluctuations in band-limited power (BLP) of local field potential (LFP) signals recorded from multiple electrodes in visual cortex of the monkey during different behavioral states. We asked whether such signals demonstrated coherent fluctuations over time-scales of seconds and minutes, and would thus serve as good candidates for direct comparison with data obtained from functional magnetic resonance imaging (fMRI). We obtained the following results. (i) The BLP of the local field displayed fluctuations at many time-scales, with particularly large amplitude at very low frequencies (<0.1 Hz). (ii) These fluctuations exhibited high coherence between electrode pairs, particularly for BLP signals derived from the gamma (g) frequency range. (iii) Coherence in the BLP, unlike that in the raw LFP, did not fall off sharply as a function of cortical distance. (iv) The structure and coherence of BLP changes were highly similar under distinctly different behavioral states. These results demonstrate the existence of widespread coherent activity fluctuations in the brain of the awake monkey over very long time-scales. We propose that such signals may make a significant contribution to the high variability observed in the time course of physiological signals, including those measured with functional imaging techniques. The results are discussed in the context of combined fMRI/electrophysiological recordings. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf2054.pdf published 11 15017 15421 2274 3 DA Leopold NK Logothetis 2003-02-00 1-2 14 195 205 Reviews in the Neurosciences Spontaneous activity among visually responsive neurons is often considered to consist of random neural events, or perhaps to reflect an irrelevant by-product of brain homeostasis. However, recent studies have emphasized that such ongoing activity is strongly synchronized over large cortical distances, and can have a marked impact on the responsiveness of neurons to visual stimuli, suggesting that such activity may indeed be highly relevant to the brain's interpretation of its sensory input. In the current study, we examined the spatiotemporal nature of local field potential (LFP) fluctuations in the visual cortex of two macaque monkeys that were awake, but in a state of relaxation with minimal visual stimulation. Using an array of 16 electrodes spaced by several millimeters, we simultaneously monitored the LFP at many sites over a large region of the visual cortex. In agreement with the literature, we found that the coherence in the raw LFP signal fell off quickly with both frequency and distance. However, when we examined slower fluctuations in the LFP power, we found that power signals, including those derived from the high γ-range frequencies, had high coherence that fell off only very slowly with cortical distance. Finally, we performed an additional experiment, with several electrodes placed on either side of a sulcus, to demonstrate that the decline in local field synchrony with cortical distance was so reliable that the interruption in the cortical sheet corresponding to the opening of the sulcus could be easily identified by monitoring just a few minutes of spontaneous LFP activity. These experiments reveal that a significant portion of spontaneous LFP fluctuations in the visual cortex is contributed by global mechanisms, imposing synchrony that is, first and foremost, a function of cortical separation between any two points. no notspecified http://www.kyb.tuebingen.mpg.de/ published 10 15017 15421 2050 3 DA Leopold 2003-01-00 1 13 10 12 Current Biology A new psychophysical study has examined the free flow of perception as observers viewed stimuli with several possible visual interpretations. The results suggest that our subjective impression of such ambiguous patterns may be more closely linked to the brain's encoding of complex shape than previously appreciated. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf2050.pdf published 2 15017 15421 2049 3 DA Leopold 2002-09-00 17 12 R582 R584 Current Biology Neurophysiological studies in human patients, with experiments of the kind traditionally reserved for monkeys, are beginning to provide valuable insight into the workings of the brain. Taking center stage is the question of which neurons lie at the heart of perception itself. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf2049.pdf published 0 15017 15421 847 3 DA Leopold M Wilke A Maier NK Logothetis 2002-06-00 6 5 605 609 Nature Neuroscience During the viewing of certain patterns, widely known as ambiguous or puzzle figures, perception lapses into a sequence of spontaneous alternations, switching every few seconds between two or more visual interpretations of the stimulus. Although their nature and origin remain topics of debate, these stochastic switches are generally thought to be the automatic and inevitable consequence of viewing a pattern without a unique solution. We report here that in humans such perceptual alternations can be slowed, and even brought to a standstill, if the visual stimulus is periodically removed from view. We also show, with a visual illusion, that this stabilizing effect hinges on perceptual disappearance rather than on actual removal of the stimulus. These findings indicate that uninterrupted subjective perception of an ambiguous pattern is required for the initiation of the brain-state changes underlying multistable vision. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf847.pdf published 4 15017 15421 922 3 DA Leopold HK Plettenberg NK Logothetis 2002-04-00 3 143 359 372 Experimental Brain Research We used optokinetic responses and functional magnetic resonance imaging (fMRI) to examine visual processing in monkeys whose conscious state was modulated by low doses (1–2 mg/kg) of the dissociative anesthetic ketamine. We found that, despite the animal’s dissociated state and despite specific influences of ketamine on the oculomotor system, optokinetic nystagmus (OKN) could be reliably elicited with large, moving visual patterns. Responses were horizontally bidirectional for monocular stimulation, indicating that ketamine did not eliminate cortical processing of the motion stimulus. Also, results from fMRI directly demonstrated that the cortical blood oxygenation level-dependent (BOLD) response to visual patterns was preserved at the same ketamine doses used to elicit OKN. Finally, in the ketamine-anesthetized state, perceptually bistable motion stimuli produced patterns of spontaneously alternating OKN that normally would be tightly coupled to perceptual changes. These results, taken together, demonstrate that after ketamine administration cortical circuits continue to processes visual patterns in a dose-dependent manner despite the animal’s behavioral dissociation. While perceptual experience is difficult to evaluate under these conditions, oculomotor patterns revealed that the brain not only registers but also acts upon its sensory input, employing it to drive a sensorimotor loop and even responding to a sensory conflict by engaging in spontaneous perception-related state changes. The ketamine-anesthetized monkey preparation thereby offers a safe and viable paradigm for the behavioral and electrophysiological investigation of issues related to conscious perception and anesthesia, as well as neural mechanisms of basic sensory processing. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf922.pdf published 13 15017 15421 46 3 DA Leopold AJ O'Toole T Vetter V Blanz 2001-01-00 1 4 89 94 Nature Neuroscience We used high-level configural aftereffects induced by adaptation to realistic faces to investigate visual representations underlying complex pattern perception. We found that exposure to an individual face for a few seconds generated a significant and precise bias in the subsequent perception of face identity. In the context of a computationally derived 'face space,' adaptation specifically shifted perception along a trajectory passing through the adapting and average faces, selectively facilitating recognition of a test face lying on this trajectory and impairing recognition of other faces. The results suggest that the encoding of faces and other complex patterns draws upon contrastive neural mechanisms that reference the central tendency of the stimulus category. no notspecified http://www.kyb.tuebingen.mpg.de/ published 5 15017 15422 15017 15421 201 3 DA Leopold NK Logothetis 1999-07-00 7 3 254 264 Trends In Cognitive Sciences Traditional explanations of multistable visual phenomena (e.g. ambiguous figures, perceptual rivalry) suggest that the basis for spontaneous reversals in perception lies in antagonistic connectivity within the visual system. In this review, we suggest an alternative, albeit speculative, explanation for visual multistability – that spontaneous alternations reflect responses to active, programmed events initiated by brain areas that integrate sensory and non-sensory information to coordinate a diversity of behaviors. Much evidence suggests that perceptual reversals are themselves more closely related to the expression of a behavior than to passive sensory responses: (1) they are initiated spontaneously, often voluntarily, and are influenced by subjective variables such as attention and mood; (2) the alternation process is greatly facilitated with practice and compromised by lesions in non-visual cortical areas; (3) the alternation process has temporal dynamics similar to those of spontaneously initiated behaviors; (4) functional imaging reveals that brain areas associated with a variety of cognitive behaviors are specifically activated when vision becomes unstable. In this scheme, reorganizations of activity throughout the visual cortex, concurrent with perceptual reversals, are initiated by higher, largely non-sensory brain centers. Such direct intervention in the processing of the sensory input by brain structures associated with planning and motor programming might serve an important role in perceptual organization, particularly in aspects related to selective attention. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf201.pdf published 10 15017 15421 242 3 DA Leopold NK Logothetis 1998-11-00 3 123 341 345 Experimental Brain Research Saccadic eye movements in primates continually shift the location at which a given stimulus strikes the retina. Even during periods of steady fixation, microsaccades frequently jerk the center of gaze by small but resolvable distances, yet perception remains stable and continuous, uninterrupted by sudden jumps or shifts. The effect of such fixational eye movements on the activity of single neurons was examined in several regions of the visual cortex in macaque monkeys. We found that the firing of many neurons in striate and extrastriate cortex is profoundly influenced by saccades much smaller than the neurons’ receptive fields. In striate cortex (V1) many cells showed a transient decrease in their firing shortly following a saccade. In sharp contrast, cells in the extrastriate areas V2 and V4 showed strong excitatory responses that closely coincided in time with the striate depression. No appreciable activity change was observed in the inferotemporal cortex (IT) following saccades. This activity pattern is consistent with the notion that topographic extrastriate areas receive extraretinal input associated with saccadic events. Such signals may be necessary for the stable perception of objects and scenes during eye movements, mediating the mapping between central object representations and the constantly changing retinotopic input. no notspecified http://www.kyb.tuebingen.mpg.de//fileadmin/user_upload/files/publications/pdf242.pdf published 4 15017 15421 368 3 NK Logothetis DA Leopold DL Sheinberg 1997-07-00 3 4 99 110 Cognitive Studies Pictures that spontaneously change in appearance, such as depth or figure-ground reversals, have always been thought of as powerful tools for understaning the nature of the perceptual system. The cause of the perceptual multistability that is experienced when viewing such figures most likely lies in the brain's physical organization; an organization that imposes several constraints on the processing of visual information. Why is it that our visual system fails to lock onto one aspect of an ambiguous figure? What accounts for the spontaneous changes of interpretation? What are the neural events that underlie such changes? Are there neurons in the visual pathways the activity of which reflects the visual awareness of the stimulus? In my paper I describe some combined psychophysical and physiological experiments that were motivated by these questions. In specific, we report on experiments in which neural activity in early visual cortex and in the inferior temporal cortex of monkeys was studied, while the animals experienced binocular rivalry. Our results provide us with new evidence not only on the neural mechanisms of binocular rivalry (one example of multistable perception), but also on the neural processes underlying image segmentation and perceptual grouping. no notspecified http://www.kyb.tuebingen.mpg.de/ published 11 918 3 NK Logothetis DA Leopold DL Sheinberg 1996-04-00 6575 380 621 624 Nature When different images are presented to the two eyes, they compete for perceptual dominance, such that one image is visible while the other is suppressed. This binocular rivalry is thought to reflect competition between monocular neurons within the primary visual cortex1. However, neurons whose activity correlates with perception during rivalry are found mainly in higher cortical areas, and respond to input from both eyes2,3. Thus rivalry may involve competition between alternative perceptual interpretations at a higher level of analysis. To investigate this, we tested the effect of rapidly alternating the rival stimuli between the two eyes. Under these conditions, the perceptual alternations exhibit the same temporal dynamics as with static patterns, and a single phase of perceptual dominance can span multiple alternations of the stimuli. Thus neural representations of the two stimuli compete for visual awareness independently of the eye through which they reach the higher visual areas. This finding places binocular rivalry in the general category of multi stable phenomena, such as ambiguous figures, and provides a new way to study the neural cause and resolution of perceptual ambiguities. no notspecified http://www.kyb.tuebingen.mpg.de/ published 3 916 3 DA Leopold NK Logothetis 1996-00-00 379 549 553 Nature no notspecified http://www.kyb.tuebingen.mpg.de/ published 4 911 3 NK Logothetis DA Leopold 1995-00-00 125 1 20 C.B.C.L. Paper, AI Memo, No: 1553 no notspecified http://www.kyb.tuebingen.mpg.de/ published 19 912 3 DA Leopold JC Fitzgibbons NK Logothetis 1995-00-00 126 1 17 C.B.C.L. Paper, AI Memo, No: 1554 no notspecified http://www.kyb.tuebingen.mpg.de/ published 16 919 7 NK Logothetis DA Leopold Tucson, AZ, USA1997-00-00 309 319 The Second Tucson Discussions and Debates 1996 no notspecified http://www.kyb.tuebingen.mpg.de/ published 10 LeopoldB2005 2 DA Leopold I Bondar Oxford University Press Oxford, UK 2005-00-00 189 212 Fitting the mind to the world: adaptation and after-effects in high-level vision no notspecified http://www.kyb.tuebingen.mpg.de/ published 23 15017 15421 2277 2 D Leopold M Wilke A Maier NK Logothetis MIT Press Cambridge, MA, USA 2004-11-00 231 259 Binocular Rivalry no notspecified http://www.kyb.tuebingen.mpg.de/ published 28 15017 15421 2101 2 NK Logothetis D Leopold DL Sheinberg Benjamins Amsterdam, Netherlands 2003-01-00 87 103 Neural Basis of Consciousness no notspecified http://www.kyb.tuebingen.mpg.de/ published 16 15017 15421 5002 7 Z Wang A Maier NK Logothetis DA Leopold H Liang San Diego, CA, USA2007-11-00 37th Annual Meeting of the Society for Neuroscience (Neuroscience 2007) Extensive studies have shown that the stimulus-evoked neuronal activity in area MT is causally related to motion perception. Yet little is known about how ongoing (pre- and inter-stimulus) activity in area MT influences the subjects’ ability to perceive and respond to stimuli. Here we ask whether the activity level of area MT preceding stimulus onset has an influence on the subject’s psychophysical performance during physically identical, but perceptually ambiguous visual stimulation. Two rhesus monkeys were well trained to indicate the perceived direction of rotation of bistable structure-from-motion (SFM) stimuli by pulling one of two levers. During this task, multi-channel multi-unit activity (MUA) and local field potentials (LFP) were recorded from area MT while monitoring the animal&lsquo;s psychophysical performance and response times. We investigated the effect of trial-to-trial differences in prestimulus activity (MUA and LFP) on the variability of psychophysical performance. We found that the level of prestimulus alpha and gamma power as well as MUA are significantly correlated with response times. This correlation between response times and the level of ongoing neuronal activity was found to be positive. In other words, the lower the power of the MUA and alpha and gamma LFP, the shorter it took the animal to report the perceived rotation of the ambiguous stimulus. These findings suggest that the state of area MT before stimulus onset can predict to some extent the subjects’ psychophysical performance in a perceptual task. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 LiangWWLLM2006 7 H Liang X Wang Z Wang NK Logothetis DA Leopold AV Maier Atlanta, GA, USA2006-10-00 36th Annual Meeting of the Society for Neuroscience (Neuroscience 2006) Certain continuously moving objects appear to spontaneously reverse their motion direction (e.g., the illusion of structure-from-motion, SFM). The neuronal mechanisms underlying these perceptual fluctuations still remain elusive. Spiking activity from individual neurons in cortical middle temporal visual area (MT) has been shown to correlate to a certain degree with an animal’s perceptual report during stimulation with SFM and related paradigms of bistable perception. Similarly, certain frequency bands of the local field potentials (LFPs) can be used to predict these perceptual judgments (Maier et al., SFN 2005). Here we ask whether spiking activity or local field potentials are better correlated with the perceptual outcome of bistable stimulation. Two macaque monkeys were trained to report the perceived direction of motion in an SFM task while single-unit activity (SUA), multiunit activity (MUA) and LFPs were recorded from area MT. Using receiver operating characteristic (ROC) analysis, we compared the ability of each neural signal to predict the monkeys’ perceptual report on individual trials (choice probability, CP). Percept-related LFP changes were assessed by evaluating the power spectrum of the LFP as well as the spike-field coherence before and around the time of perceptual report. We found that both LFP power and spike-field coherence in the gamma band showed choice probabilities significantly greater than expected by chance. Lower frequency bands revealed smaller or insignificant values, thus revealing reduced coupling with the outcome of perceptual analysis. Choice probability of all sub-band of the LFP was lower than that of the spiking activity measured in SUA and MUA on the same electrodes. These results suggest that the LFP in area MT reflects the perceptual state of an animal, however, to a lesser degree than its spiking activity. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 WangMLLL2006 7 Z Wang AV Maier NK Logothetis DA Leopold H Liang Atlanta, GA, USA2006-10-00 36th Annual Meeting of the Society for Neuroscience (Neuroscience 2006) The relationship between single neuronal activity in area MT and motion perception is a well studied phenomenon. Less is known about how larger fractions of neurons interact to produce a certain perceptual outcome. Here we asked whether pooling the responses of a large population of MT neurons with widely varying properties could improve the predictability of perceptual decisions during ambiguous visual stimulation. Two well trained rhesus monkeys indicated the perceived direction of rotation of bistable structure-from-motion (SFM) stimuli by pushing one of two levers. During this task, multi-channel intracortical recordings including single-unit activity (SUA), multi-unit activity (MUA), and local field potentials (LFP) were collected from area MT. We sorted the neural data according to the monkeys’ behavioral choices and utilized the measure of choice probability (Britten et al., 1996) to quantify the relationship between the different signals and perceptual report. We found that SUA, MUA and LFP all had a rather modest capability of predicting the monkeys’ perceptual report when considered in isolation. We developed optimal predictors for each type of neural signal by selecting the weight for each channel and combining the signals from multiple channels. We found that the combination of simultaneously collected data greatly improved the prediction accuracy of each of the signals. Furthermore, we found that by combining all these three types of neural signals from multiple channels, choice probability increased even further in a systematic way. The accuracy and statistical power of determining the monkeys’ perception increased with the number of channels as well as with the types of neural signals used for analysis. Our results demonstrate that simultaneous collection of multiple neural responses in area MT can be used to reliably determine perceptual states. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 WilkeLL2005 7 M Wilke NK Logothetis DA Leopold Washington, DC, USA2005-11-00 35th Annual Meeting of the Society for Neuroscience (Neuroscience 2005) Salient visual stimuli often subjectively disappear following the abrupt onset of a spatially separated pattern (Generalized Flash Suppression, GFS). Unlike other forms of induced perceptual suppression such as monocular or binocular rivalry, the vanishing stimulus is not replaced by a competing pattern, but simply disappears and remains continuously invisible for up to several seconds. In agreement with studies measuring single cell activity during binocular rivalry (Leopold & Logothetis, 1996; Gail et al., 2004), we reported previously that only a small minority of neurons in V1 showed activity changes corresponding to the subjective disappearance of a stimulus during GFS, while this fraction was slightly higher in area V4 (Wilke, 2003). In all cases, perceptual modulation was considerably smaller than in a control condition where the stimulus was physically removed. In the present study, we investigate the expression of perceptual suppression in the local field potential (LFP), comparing it to the changes in spiking activity observed at the same recording sites. Using transdural multielectrode recording techniques, we simultaneously measured spiking activity (single- and multi-unit) and local field potentials in areas V1, V2 and V4 in three monkeys reporting subjective target visibility during GFS. We found that perceptual suppression was more reliably expressed in the multiunit signal compared with single cells. In all visual areas, the magnitude of the evoked LFP response to the suppressing pattern was often correlated with the visibility. We examined band-limited power (BLP) of the local field potential, and found perception-related modulation to be variable, both in its magnitude and in the band in which it was expressed. Modulation in the gamma band (30 to 80 Hz) generally covaried with spiking activity on a trial by trial basis. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 MaierLL2005 7 AV Maier NK Logothetis DA Leopold Washington, DC, USA2005-11-00 35th Annual Meeting of the Society for Neuroscience (Neuroscience 2005) Single-cell recordings in the awake monkey during bistable visual perception have revealed a diversity of roles for individual neurons with regard to an experienced percept. We have recently shown that this perceptual modulation is not only neuron-specific but also stimulus-specific: MT neurons showing perceptual modulation to one stimulus often do not show significant modulation to closely related ambiguous patterns that differ only in the value of a single parameter (Maier et al., SFN 2003). In the present study, we explore how this stimulus-specificity for perceptual modulation might be reflected in the local field potential (LFP) during the perception of ambiguous 3-D rotation, as well as during binocular rivalry flash suppression. Following the presentation of several combinations of rivaling patterns, as well as 3-D rotation, to the same neurons, we analyzed percept-related changes in band-limited power (BLP) during these paradigms. LFP recorded in area MT of two monkeys was band-pass filtered into multiple frequency bands and subsequently rectified to compute power changes with high temporal resolution. While all stimuli evoked power changes throughout the entire spectrum (ranging from delta to gamma bands), perceptual modulation of BLP was often more pronounced at higher frequencies. This enables us to study whether percept-related changes in LFP power transfers across paradigms, or whether it is similarly specific as single unit activity. Findings will be discussed in context with other findings on percept-related activity modulation in early visual cortex. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 3245 7 K-M Müller MO Ernst DA Leopold Sarasota, FL, USA2005-09-00 250 Fifth Annual Meeting of the Vision Sciences Society (VSS 2005) High-level visual aftereffects (AEs) arise when adaptation to stimuli such as shapes, faces, or spatial configurations affect the subsequent perception of comparable figures. Like classical AEs, high-level AEs are characterized by a percept that is distorted in feature space in the opposite direction of the vector between adaptation and test stimulus. Shape-contrast AEs have been reported for aspect ratio, convexity and taper, but the combined effect of such parameters remained unexplored. In the present experiment the adapting stimulus consisted of two arcs. While keeping the flexion of the arcs constant we varied the distance between them in a range of ±8.2°, allowing us to examine the effects of (1) aspect ratio and (2) convexity/concavity. The test stimulus was a closed ellipsoid of about 7.8° in diameter. Perceptual distortions were assessed with repetitive adaptation and testing in the context of a staircase procedure converging to the point of subjective circularity. We found significant main effects for both factors and the interaction, i.e. smaller aspect ratios lead to stronger effects and convex stimuli result in larger effects than concave ones. The direct spatial correspondence between the location of the adapting and test stimulus was not critical for inducing an AE. We repeated the experiment with a set of smaller stimuli of roughly 1° and the distance between the curves scaled down proportionately. Results in both experiments were comparable, although the interaction was not significant in experiment 2. The results suggest that simple stimulus metrics, such as the absolute size of the curves and distance between them, are less important in creating this AE than the overall shape created by the pair of arcs together. no notspecified http://www.kyb.tuebingen.mpg.de/ published -250 15017 1542215017 18824 5543 7 MA Giese R Sigala C Wallraven D Leopold Sarasota, FL, USA2004-08-00 213 Fourth Annual Meeting of the Vision Sciences Society (VSS 2004) Some psychological models for face recognition assume that faces are encoded as vectors in face spaces relative to an average face, or face prototype [T Valentine, Q J Exp Psychol A, 43, 161 (1991)]. So far it has been largely unclear how such a prototype-referenced encoding can be realized at a neural level. Recent electrophysiological data supports the relevance of such encoding in monkey visual cortex. Neurons in area IT, after training with human faces, show monotonic tuning with respect to the caricature level of face stimuli [D Leopold et al., Soc. of Neurosci., Poster 590.7 (2003)]. A neural model is presented that accounts for these electrophysiological results. The model consists of a hierarchy of layers with physiologically plausible neural feature detectors. The complexity of the extracted features increases along the hierarchy. Neurons on the highest level encode example views of faces. The tuning of these neurons is determined by the difference between the feature vector representing the test face, and an average feature vector that is computed from the previous history of stimulation. The neurons are tuned monotonically with respect to the length of the difference vector, and show angular tuning with respect to its direction in feature space. The model was tested with gray-level images generated with a morphable 3D face model [V Blanz, T Vetter, SIGGRAPH '99, 187–194 (1999)], replicating the stimulus set from the electrophysiological study. We conclude that prototype-referenced encoding, compared with the encoding in shape spaces with absolute coordinates, increases coding efficiency by optimally exploiting the available neural hardware. no notspecified http://www.kyb.tuebingen.mpg.de/ published -213 15017 15421 5547 7 GR Sigala A. D Leopold C Wallraven MA Giese Tübingen, Germany2004-02-00 144 7th Tübingen Perception Conference (TWK 2004) Conceptual models of face recognition have assumed that faces are encoded as points of an abstract face space relative to an average face, or face prototype (e.g. [1]). So far it has been largely unclear how such a prototype-referenced encoding of faces could be implemented with real neurons. Recent electrophysiological evidence seems to support the relevance of prototype-referenced encoding. Neurons in macque inferotemporal cortex, which have been trained with human faces, tend to show a monotonic tuning with the caricature level of the stimuli [2]. We present a neural model that accounts for these new electrophysiological results. The hierarchical model consists of multiple layers of neural detectors modeling properties of neurons in the dorsal visual processing stream. The rst layer models simple cells using Gabor lters with with physiologically realistic parameters. A second layer combines responses of Gabor lters that carry signicant information about a training stimuli into more complex features. The complex features in the model are based on the Principal Components of the Gabor responses, which could be extracted using simple Hebbian-like learning rules. The highest hierarchy layer models neurons in area IT. The responses of these neural detectors increase monotonically with the distance of the input feature vector, from the previous layer, and the average feature vector over all training faces. In addition, neural detectors on the highest hierarchy level show a broad tuning with resepect to the direction of the difference vector between input feature vector and this average vector. The model was tested with gray-level images that were generated using a morphable 3D face model [3]. The model was trained with 98 randomly chosen faces from a data basis with 200 faces. It was tested with caricatures and anti-caricatures of 4 selected faces. In addition we tested lateral caricatures of the faces, which lie on curves in face space that connect the four selected example faces. Exactly the same stimuli had been used in the electrophysiological experiments [2]. After training, a signicant number of the neural units on the highest level of the model show a monotonic tuning with the caricature level of the faces, and a moderate tuning with respect to facial identity, consistent with the electriophysiological results. The model provides a physiologically plausible concrete neural implementation of face spaces. Future work will explore its computational properties and coding efciency in comparison with classical neural models for face recognition. no notspecified http://www.kyb.tuebingen.mpg.de/ published -144 15017 15421 2940 7 K-M Müller DA Leopold Tübingen, Germany2004-02-00 139 7th Tübingen Perception Conference (TWK 2004) Exposure to a face can alter the perception of another subsequently presented face. Using a computational 3-D model derived from a database of 200 scanned faces [1,2], we have previously demonstrated the existence of face identity aftereffect (FIAE) [3]. In a multidimensional “face-space”, the average prototype face occupies the central position, while individual real faces are represented as points or vectors emanating from the center. In this context, by inverting a vector corresponding to an individual face, one can create a so-called “anti-face” [4], which served as the adapting stimulus in our previous study. While the FIAE has much in common with more traditional aftereffects, such as its negative sign and its storage during brief unstimulated periods [3], its dynamic aspects have not yet been studied. The present investigation aimed to determine the effects of adaptation and test durations on this phenomenon. Subjects learned to recognize four faces at different identity levels over a period of several sessions, until they could correctly name low-identity faces. We then tested the effects of adaptation and testing duration on the FIAE. In each trial, one of the four names was shown on the screen, followed by the presentation of an “adaptation” face (the anti-face of the named individual) for a period between 1.0 and 16.0 s. Immediately following adaptation, a “test” face (the average face) appeared for between 0.1 and 1.6 s. Subjects were required to rate, on a scale of 1 to 7, the degree to which the test face resembled the individual named at the beginning of the trial. They were specically told to restrict their judgment to the instant that the test stimulus disappeared. The results indicate that the FIAE resembles other aftereffects in that it is increased in magnitude by long adaptation times, as well as by short test times. Mean ratings showed good ts to the adaptation and test times with positive and negative logarithmic functions, respectively. However, despite this similarity, the magnitude of this proportionality in the FIAE was considerably smaller than in low-level aftereffects. no notspecified http://www.kyb.tuebingen.mpg.de/ published -139 15017 15421 3289 7 MA Giese R Sigala C Wallraven DA Leopold 2004-00-00 no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 2488 7 A Maier NK Logothetis DA Leopold New Orleans, LA, USA2003-11-00 33rd Annual Meeting of the Society for Neuroscience (Neuroscience 2003) Neurophysiological studies using bistable visual patterns have revealed a diversity of roles for individual neurons with respect to the ultimate percept. Furthermore, studies have demonstrated that the incidence of percept-related neurons in different areas appears to be similar during ambiguous perception brought about by binocular rivalry (BR) and structure from motion (SFM). One interpretation of these results is that there exists a specialized subset of neurons whose responses are inextricably linked with perception, regardless of the exact stimulus. The present experiments address this hypothesis by examining perception-related responses at individual recording sites during the presentation of two or more fundamentally different ambiguous patterns. Multielectrode recordings were performed from parietal areas in two macaque monkeys, and neurons were tested with bistable binocular rivalry flash suppression (BRFS) and SFM patterns. In agreement with previous studies, we found that in area MT, activity at a subset of neural sites was modulated according to the perceptual state. However, although their relative fraction was similar in the two cases, the particular sites were only partially overlapping, even when the stimuli were adjusted to contain same directions of motion. Moreover, during BRFS, at some sites perception-related activity was idiosyncratic for specific pattern pairs. For example, perceptual dominance was marked by activity increases for one suppressed pattern but not for another. These results suggest that the expression of perceptual dominance in the visual cortex is closely linked to the resolution of ambiguity among specific visual features, and may therefore be closely related to mechanisms of perceptual organization in natural vision. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 GhanzafarLHL2003 7 AA Ghanzafar DA Leopold MD Hauser NK Logothetis New Orleans, LA, USA2003-11-00 33rd Annual Meeting of the Society for Neuroscience (Neuroscience 2003) The vocal behavior of macaque monkeys has been studied extensively in both field and laboratory settings. To date, however, no investigation has systematically examined the neural mechanisms underlying species-specific recognition of their vocalizations. To bridge this gap, we are investigating the auditory representation of conspecific vocalizations in the neocortex of behaving rhesus monkeys. Subjects performed a simple auditory detection task during which, on each trial, they listened to a conspecific vocalization followed by a non-vocal, target sound. The stimulus set consisted of call types familiar to our captive-bred subjects but from unknown individuals: three exemplars from each of 7 call categories. Our preliminary investigation using these natural stimuli (Ghazanfar et al, SFN 2002) revealed that ~45% of auditory belt cortical multi-units were selective for 1 or 2 call categories. This suggests that the macaque auditory cortex is specialized to process communication sounds. To assess the degree of this specialization and the specificity of vocalization-sensitivity, we are comparing single-unit responses to vocalizations with time-reversed versions, pure-tones and band-passed noise. For each vocalization, a pure-tone and a band-passed noise stimulus were generated to match the signal’s duration, dominant frequency and average RMS power. Recordings from both core and belt cortical areas will determine whether vocalization-sensitive neurons in these regions are driven by simple features of the signal or by more complex interactions between spectral and temporal properties. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 BondarLBL2003 7 IV Bondar DA Leopold AA Boldyrev NK Logothetis New Orleans, LA, USA2003-11-00 33rd Annual Meeting of the Society for Neuroscience (Neuroscience 2003) Single neurons in the inferotemporal visual cortex of monkeys are known to modulate their activity in response to highly specific patterns, including complex shapes, objects, and faces. While numerous single-unit studies have explored feature selectivity and temporal patterning of responses in these areas, traditional recording techniques have provided only indirect information regarding their long-term stability. Here we address this issue using chronic implanted microelectrodes, which allowed us to monitor the activity of individual visually responsive neurons over periods of days and weeks. Each monkey was implanted with a thin bundle of 64 microwires (for one monkey in the anterior inferotemporal cortex (AIT), for the other in the superior temporal sulcus (STS)), and trained to view a small fixation point for several seconds while a stimulus was presented. The stimuli consisted of a diverse set of images that the monkey had seen many times prior to the recordings, including faces, animals, objects, and geometrical patterns. Activity was recorded over a period of approximately one year in one animal and 4 months in the other. Individual neurons were found to be highly stimulus-selective in the temporal profiles of their responses, showing robust differences in latency, transience, and amplitude. We recorded for several consecutive days from 55 isolated neurons, 10 of which were monitored for a week or more. We found that the fine temporal patterning for different stimuli was remarkably consistent during this period. While previous physiological experiments have demonstrated that representations in the inferotemporal areas can be shaped by experience and learning, the present findings demonstrate that the specific role of individual neurons in the analysis of complex patterns is not under a state of continual reorganization. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 2413 7 M Wilke NK Logothetis DA Leopold New Orleans, LA, USA2003-11-00 33rd Annual Meeting of the Society for Neuroscience (Neuroscience 2003) Following the onset of a large surrounding pattern, a wide range of salient visual targets can be induced to suddenly disappear. This phenomenon, resembling binocular rivalry flash suppression in its time course, and motion-induced blindness in its spatial arrangement, can be elicited in the absence of any interocular or local stimulus conflict (Wilke et al., 2002). In the present experiments, we investigate the neurophysiological basis of this phenomenon by performing multielectrode recordings from early visual areas of two trained adult monkeys. Using transdural multielectrode recording techniques, we monitored activity simultaneously in areas V1, V2, and V4 while monkeys kept fixation. Based on the parameters derived from both human and monkey psychophysical experiments, we first adjusted the stimuli to compare neural responses under two different stimulus conditions, one in which the stimulus disappeared, and another in which it remained continuously visible. Then, exploiting the subjective reports provided by the monkey, we next used an ambiguous set of parameters, and compared trials in which the stimulus disappeared to those in which it did not. We found that in area V1 only a small minority of neurons showed activity changes corresponding to the subjective disappearance of the stimulus, while in area V4, this fraction of neurons was significantly higher. We additionally found that the expression of perceptual suppression was often intricately linked with a reduction in responses to the flashed surround pattern, which generally elicited excitatory responses even when its closest elements were several degrees from the classical receptive field. It thus appears that the neural modulation associated with target visibility entails a complex interplay, either within or between visual areas, involving local receptive field activation and responses to global surround elements. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 LeopoldBML2003 7 DA Leopold IV Bondar G Mahlknecht NK Logothetis New Orleans, LA, USA2003-11-00 33rd Annual Meeting of the Society for Neuroscience (Neuroscience 2003) Many neurophysiological studies have revealed single-unit responses to complex objects such as faces in the primate inferotemporal cortex. Nonetheless, the basic principles underlying the encoding of such stimuli remain a topic of active research. Of particular interest is the brain's ability to differentially process highly similar patterns, such as faces, that ultimately leads to the recognition of an individual. We have recently demonstrated using a high-level adaptational aftereffect that the perception of face identity is strongly influenced by an unseen central tendency or prototype face representation (Leopold et al, 2001). We thus speculated that the brain's analysis of complex patterns might entail the comparison of an incoming sensory pattern with a stored neural prototype representation. Here we investigate this hypothesis using a combined psychophysical and physiological approach. Two monkeys were trained to indicate the identity of four individual faces. Their psychophysical responses indicated that their perception of human face identity was similar to human subjects, with and without prior adaptation. Using chronically implanted electrodes in the anterior portion of area TE, we found that many neurons responded to the test faces. The response magnitude often varied monotonically as a function of the amount individual identity present in a face (i.e. the distance of a face from the average prototype). Five seconds of prior adaptation to a different face, a condition which produced consistent perceptual aftereffects, resulted in a diversity of effects on the neural responses, with some neurons showing a shift in their tuning that directly mirrored the perceptual distortions. These results, taken together, provide evidence that the neural analysis of identity for faces and perhaps other complex patterns, exploits a prototype-based strategy. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 2128 7 A Maier NK Logothetis DA Leopold Paris, France2003-09-00 115 26th European Conference on Visual Perception The paradigm of monocular rivalry (MR), also called pattern rivalry, has been used to illustrate selective suppression of one of two superimposed visual patterns or images. Much like the related phenomenon of binocular rivalry (BR), perception during MR is dynamic, and often marked by dominance of only one of the competing patterns, with complete invisibility of its rival. However, MR distinguishes itself from its counterpart in that it arises in the absence of any interocular conflict, but is highly restricted in the types of stimuli that will engage in spontaneous alternation. Here we report several experiments that attempt to identify the principles by which perception selects and/or suppresses superimposed patterns during MR. Stimuli generally consisted of superimposed orthogonal gratings, which have been shown previously to engage in vigorous rivalry. However, in the present study, some of the patterns were complete, covering the extent of the stimulus, while other patterns were 'partial', covering only a portion of the stimulus (eg containing a blank window). Superimposing complete and partial stimuli provided a heterogeneous MR stimulus, with some regions containing pattern conflict and others containing only one pattern. We report that, upon viewing these stimuli, perceptual dominance of a partial pattern often entailed the complete suppression of its rival, even in those regions (up to several degrees in size) devoid of local competition. These results illustrate that, in resolving perceptual conflict, the brain dissects a superimposed stimulus according to inferences regarding the global structure of the competing patterns. Additional experiments revealed similar results with complete and partial competing patterns during BR. no notspecified http://www.kyb.tuebingen.mpg.de/ published -115 15017 15421 2359 7 M Wilke DA Leopold NK Logothetis Paris, France2003-09-00 137 26th European Conference on Visual Perception Under certain conditions, a salient visual pattern presented at or near the centre of gaze, which would normally be easily seen, can be rendered completely invisible for perception. Combining principles from existing demonstrations of visual suppression, we present here a novel visual stimulus in which the immediate disappearance of highly salient visual patterns can be induced by the presentation of a second, non-overlapping pattern. This effect, which we term generalised flash suppression (GFS), resembles the previously reported binocular-rivalry flash suppression (BRFS) in its dynamic properties, but differs in its spatial requirements. In the main condition, the target is a foveally presented patch or image, shown alone on a black background, and remains continuously visible. After 2000 ms, randomly distributed dots are flashed in the periphery, an event that often causes the immediate perceptual disappearance of the target. While disappearance is aided by dichoptic presentation, it is neither contingent upon interocular conflict, nor any direct spatial conflict at all. These properties, along with the all-or-none nature of target disappearance, closely resemble the recently described spontaneous target disappearance present in motion-induced blindness (MIB). We characterise several aspects of GFS, exploring its generality, the spatial and temporal requirements for suppression, the role of ocular configuration, and the specificity of adaptation. no notspecified http://www.kyb.tuebingen.mpg.de/ published -137 15017 15421 1557 7 M Wilke DA Leopold NK Logothetis Orlando, FL, USA2002-11-00 32nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002) Under certain viewing conditions, visual stimuli that are normally highly salient can be rendered invisible for perception. In the paradigm of binocular rivalry (BR), for instance, the unique perception of a stimulus shown to one eye entails the subjective disappearance of that shown to the other. In a variant of BR termed “flash suppression” (FS, Wolfe, 1984), the balance of perceptual dominance can be adjusted by introducing a delay of several hundred milliseconds between the presentation of the two eyes’ patterns. Central to FS is the existence of interocular spatial conflict between the competing patterns. Here we report that asynchronous stimulus presentation can initiate perceptual disappearance of highly salient targets in the absence of either interocular or spatial conflict. Specifically, we found that a variety of visual patterns, including faces, gratings, and rotating geometric objects, disappear immediately from view when a small number of randomly distributed dots are flashed in the periphery. While this effect was present for both monocular and binocular viewing, it was most consistent when the target was presented monocularly and the dots binocularly. The duration of target disappearance, as well as its dependence on stimulus onset asynchrony, closely resembled FS, but was distinctly different from metacontrast masking. We are currently investigating the neurophysiological basis of this phenomenon in a macaque monkey trained to report the disappearance of a variety of visual targets in the context of this paradigm. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1558 7 A Maier DA Leopold NK Logothetis Orlando, FL, USA2002-11-00 32nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002) Multistable perception is generally considered to be the inevitable consequence of prolonged inspection of ambiguous displays. We have recently demonstrated that in human observers perceptual alternation can be strongly retarded, and in some cases nearly eliminated, when the inducing pattern is perceived intermittently rather than continuously (Leopold et al., Nat. Neurosci. 2002). The present study extends these results to the macaque monkey (macaca mulatta), and examines what types of neural mechanisms might underlie such stabilization. We recorded broad-band signals with multiple electrodes from the early visual areas of a rhesus monkey trained to report his percepts during perceptual rivalry. Ambiguous patterns were presented intermittently with “on” and “off” periods each lasting 1-5 seconds. Given our previous finding that under these conditions perception of such patterns can become stable, and a perceptual configuration can predominate over many subsequent presentations, we examined whether neural activity during the blank periods (i.e. delay-period activity) might reflect such a prolonged perceptual bias. We will present our initial results on this topic, with emphasis on how immediate perceptual history might affect subsequent activity in the early visual areas. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1551 7 AA Ghazanfar J Pauls DA Leopold MD Hauser NK Logothetis Orlando, FL, USA2002-11-00 32nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002) The design of the primate auditory system should reflect the specialized functions that it evolved to carry out. One such function is conspecific vocal recognition. Measuring the neural selectivity to vocalizations is a way to identify the mechanisms underlying this auditory specialization. Previous studies examining call selectivity in the primate auditory cortex suffered from two drawbacks: an impoverished stimulus set and/or the use of anesthetized or passive-listening paradigms. We adopted a simple behavioural task to study how neurons in the rhesus monkey auditory cortex respond to conspecific vocalizations. Two monkeys were trained to listen to sound sequences composed of one or two conspecific vocalizations (mean SPL=79.9 dB) and an artificial horn sound (frequency bandwidth: 788Hz-11kHz; 300ms in duration; SPL=69.7 dB) presented in free-field. They were rewarded if they pulled a lever within 1 sec following the horn sound. Both monkeys were able to do this task at >95%-correct performance levels. This task requires that the monkey attend in the auditory domain without the confounds of over-training on vocalizations and the response lability that occurs during passive-listening. Our stimuli consist of 3 exemplars from each of 7 call categories (coos, pant-threats, grunts, aggressive barks, shrill barks, noisy screams and harmonic arches). Using this paradigm, we are investigating whether the responses of auditory ‘belt’ cortical neurons reflect category-level selectivity to conspecific vocalizations. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1552 7 D Leopold M Augath NK Logothetis Orlando, FL, USA2002-11-00 32nd Annual Meeting of the Society for Neuroscience (Neuroscience 2002) We developed a novel method to visualize neural networks of endogenous activity that contribute to the variability observed in electrophysiological and functional imaging experiments. We compared activity recorded from a single electrode in the visual cortex of anesthetized monkeys with the time course of simultaneously recorded fMRI measurements. The electrophysiological signal we used was the band limited power (BLP) signal of the local field potential (LFP), which we have previously shown to exhibit slow and highly coherent fluctuations over large cortical distances. Whole-brain EPI images were collected in a 4.7 T scanner, while electrical activity was monitored with a single intracortical electrode. The maxima of cross-covariation between the BLP at that electrode and blood oxygen level-dependent (BOLD) signals for each voxel (1x1x2mm) in the imaged volume were used to generate maps of brain regions that were functionally linked with the spontaneous fluctuations on the electrode. We found that electrical activity at a single site was highly correlated with voxels throughout the brain. While covariation magnitude was greatest near the electrode tip, it remained significant even in distant brain regions, with cortical and subcortical sites showing different covariation patterns. Albeit preliminary, these results suggest that the electrophysiologically measured spontaneous activity in the visual cortex may result from large-scale fluctuations in global brain networks. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 LeopoldBOL2002 7 DA Leopold IV Bondar AJ O'Toole NK Logothetis Glasgow, UK2002-08-00 60 25th European Conference on Visual Perception As a first step toward investigating the neural encoding of faces and other complex objects, we examined the effects of visual adaptation on the perception of human faces. We found that following a few seconds of exposure to one face, the perceived identity of a second face was systematically distorted along a specific trajectory in multidimensional 'face space'. This trajectory passed through the central tendency of all faces, and its direction thus defined a particular identity. The results suggested that the visual system considers the average prototype face to be a reference point in its representation of faces, and led us to speculate that neural decoding of faces is a fundamentally comparative process. Such a scheme might constitute a fast and economical storage strategy for the brain to contend with a myriad of very similar shapes. With the aim of investigating this hypothesis more directly by neurophysiological methods, we recently trained a monkey to perform the same task, again with human faces. We found that, while the monkey's identification thresholds were slightly higher than the mean threshold for humans, his perception was affected by adaptation in exactly the same way as that of the human subjects. no notspecified http://www.kyb.tuebingen.mpg.de/ published -60 15017 1542115017 15422 955 7 I Bondar DA Leopold NK Logothetis Paris, France2002-07-00 3rd Forum of European Neuroscience (FENS 2002) It is clear from a large number of electrophysiological studies in primates that complex patterns and objects are selectively represented in the firing of neurons in the visual cortex. However, less clear are the computational strategies by which the brain encodes and differentiates between highly similar patterns, such as that used in recognizing the identity of an individual face. One possibility is that such neural encoding proceeds in a parametric manner, exploiting the naturally occurring intra-category variability. We have recently demonstrated that adaptational aftereffects can systematically and precisely bias the perception of faces (Leopold et al., 2001). Our results suggest a privileged role for the prototype or central tendency face in the decoding of face identity. We speculated that the analysis of a complex sensory pattern may involve a comparison with a prototype representation implicitly stored in the sensory apparatus. The present study is a first step to investigating this hypothesis in alert, behaving, monkeys. Two monkeys were trained to indicate the identity of up to four individual faces by pressing one of four buttons. In the first experiment, they were required to identify briefly presented, identity-depleted faces. In the second experiment a 4-second adaptation to a different face preceded each test face presentation. Baseline discrimination thresholds as a function of face identity were similar to those of humans performing the same task. When the discrimination was preceded by a 4-second adaptation with a different face, perception was biased according to the structure of the adapting stimulus, again in a manner similar to humans. These results suggest that monkeys and humans employ similar strategies in the neural processing of human faces. Current studies are underway using multielectrode bundles implanted in the inferotemporal cortex of both monkeys to elucidate the role of the prototype in the neural representation of faces. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 956 7 F Haiss DA Leopold Y Murayama NK Logothetis Tübingen, Germany2002-02-00 214 5. Tübinger Wahrnehmungskonferenz (TWK 2002) Sensory processing, as well as motor planning and execution, are well-studied aspects of primate brain function. However, it is well known that cortical and subcortical structures engage in significant activity unrelated to a sensory stimulus or motor response. Much of this activity is not random, but rather reflects the brain’s own homeostatic mechanisms, typically involving interactions between diverse cortical and subcortical structures. Such activity changes are particularly pronounced during transitions of consciousness, such as those occurring during natural sleep or anesthesia. In the present study, we examined the dynamics of changes in sensory processing accompanying the loss of consciousness in monkeys undergoing anesthetic induction. We measured the auditory evoked potential (AEP) to a series of short clicks (5 Hz, 0.1 msec.) in monkeys during anesthetic induction. In all cases, the animals were brought to the setup awake and maintained spontaneous respiration throughout the course of the experiment. On different days, the anesthetics ketamine (KET), propofol (PRO), isoflurane (ISO), and sevoflurane (SEVO) were administered while the animal was awake. KET and PRO were delivered intravenously using a remote controlled syringe pump. ISO and SEVO were provided via mask from an anesthesia machine. Prior to anesthesia, consistent peaks appeared at 30 ms (Pa) and 50 ms (Nb) in the AEP. The amplitude and latency of these peaks are known to be affected by anesthetic level. We found that during induction and emergence with ISO and PRO, the changes in both the latency and amplitude were more closely related to the animal’s visible state than to the measured anesthetic concentration. The results suggest that, at least for light anesthetic levels, conscious transitions draw upon endogenous mechanisms that are impacted by, but not unambiguously determined by, anesthetic concentration. no notspecified http://www.kyb.tuebingen.mpg.de/ published -214 15017 15421 957 7 IV Bondar DA Leopold JM Pauls NK Logothetis Tübingen, Germany2002-02-00 88 5. Tübinger Wahrnehmungskonferenz (TWK 2002) Adaptational aftereffects have often been described as the “psychophysicist’s electrode” because of their ability to isolate specific populations of neurons related to perception. We have recently demonstrated that adaptational aftereffects can systematically and precisely bias the perception of complex patterns such as faces (Leopold et al., 2001). These results suggested a privileged role for the prototype or ‘central tendency’ of an object category in the representation of faces, which may be expressed in the selective responses of neurons in the inferotemporal cortex. Specifically, the analysis of a complex sensory pattern may involve a comparison with a prototype representation implicitly stored in the sensory apparatus. The present study is a first step to investigating this hypothesis in alert, behaving, monkeys. Two monkeys were trained to indicate the identity of up to four individual faces by pressing one of four buttons. In the first experiment they were shown brief presentations of faces whose identity was modulated between the mean face and each individual, and required to identify the face. In the second experiment a 4-second adaptation to a different face preceded each test face presentation. Without adaptation, thresholds for discriminating between the memorized faces were evaluated in both monkeys, and were very similar to those of humans performing the same task. Following adaptation, perception was biased according to the structure of the adapting stimulus. The nature and magnitude of the adaptation effects were very similar to that observed in humans. These results suggest that mechanisms underlying face recognition in the monkey are similar to those present in humans, even when it is across species. Current studies are underway using multielectrode bundles implanted in the inferotemporal cortex of both monkeys to elucidate the role of the prototype in the neural representation of faces. no notspecified http://www.kyb.tuebingen.mpg.de/ published -88 15017 15421 MaierWLL2002 7 A Maier M Wilke DA Leopold NK Logothetis Tübingen, Germany2002-02-00 67 5. Tübinger Wahrnehmungskonferenz (TWK 2002) The visual instability that results from viewing ambiguous or conflicting patterns is thought to reflect dynamic processes that are critical for perceptual organization during normal vision. We have recently discovered a method to prolong states of perceptual dominance up to two orders of magnitude by periodically switching off and on the inducing bistable pattern. In the current study we used this paradigm to temporally interleave the presentation of pairs of ambiguous stimuli at the same location in visual space. Bistable patterns including rotating three-dimensional objects, depth reversals, and binocular rivalry, were shown alternately with appropriate blanking periods to prolong phases of perceptual dominance. We were interested under what conditions the time courses of two parallel bistable visual processes would be independent. We found that for certain pairs of ambiguous stimuli, such as 3-D balls rotating around orthogonal axes, the time course of perceptual alternation was largely parallel and independent. However, for other patterns, e.g. those differing only in color, speed, size, or position, the two stimuli were synchronized in their reversals. These results suggest that for a given visual location, perceptual organization can be biased for multiple different bistable stimuli simultaneously, but that the independence between pairs of stimuli depends upon their similarity. We will discuss the results with respect to the critical stimulus dimensions that determine whether or not interference occurs. no notspecified http://www.kyb.tuebingen.mpg.de/ published -67 15017 15421 1071 7 M Wilke A Maier DA Leopold NK Logothetis Tübingen, Germany2002-02-00 68 5. Tübinger Wahrnehmungskonferenz (TWK 2002) A fundamental property of ambiguous visual patterns is the inevitability of perceptual reversal. The rate of alternation generally depends both upon the physical structure of the stimulus as well as the inherent rate of an individual. Here we present evidence that critical to the reversal process is the prolonged physical presence of the inducing stimulus. We systematically investigated the influence of stimulus on and off durations on the stability of such patterns, finding that minimizing the duration of the stimulus “on time” was the predominant factor in perceptual stabilization. We demonstrate that when stimuli are shown only intermittently, the rate of perceptual alternation sharply declines to nearly zero in some subjects. In particular, we found that rotating three-dimensional bistable patterns with mean dominance time less than 10 seconds could be stabilized for periods lasting over ten minutes by correctly adjusting the duration of on and off periods. Since either perceptual state could be maintained in this way, this effect did not simply reflect a perceptual bias on the part of the observer. This trend was also present for other bistable patterns, including those involving geometric depth reversals and apparent motion correspondence. We suggest that upon removal of the inducing stimulus a visual memory process retains and hold the last perceptual state, which persists and governs the perceptual interpretation during the subsequent presentation. Electrophysiological studies are currently underway to better elucidate the neural mechanisms contributing to this phenomenon. no notspecified http://www.kyb.tuebingen.mpg.de/ published -68 15017 15421 O039TooleLVB2001 7 AJ O'Toole DA Leopold T Vetter V Blanz Sarasota, FL, USA2001-12-00 332 First Annual Meeting of the Vision Sciences Society (VSS 2001) Prototype referenced adaptation effects were found among face stimuli in a computationally derived multidimensional face space based on a 3D morphing algorithm. Individual faces can be described as points or vectors in this space. An “identity trajectory” connecting a face to the average of all faces, defines a gradient of face individuality. Anti-caricatures lie along the “identity” trajectory between an individual face and the average face. “Anti-faces” lie along this trajectory, but on the “other side of the mean”. While anti-caricatures look like less distinctive versions of the original face, anti-faces have the appearance of an entirely different individual. For example, faces with light complexions and light eyes yield anti-faces with dark complexions and dark eyes, and faces with roundish shapes yield anti-faces with a gaunt, skinny appearance. We found that pre-exposure to an “anti-face”, specifically facilitated the identification of briefly presented anti-caricatures along the same trajectory, while diminishing performance for other non-colinear faces. The perceptual bias following anti-face adaptation was strong enough to cause systematic mislabeling of the average face as the face complement to the pre-exposed anti-face. Additional experiments showed that the adaptation effect survived a range of translations in size and retinal location between pre-exposure and test. Combined, the results suggest that the subordinate perception and recognition of faces, and perhaps other objects, may draw upon contrastive neural mechanisms that reference the central tendency of the stimulus category. no notspecified http://www.kyb.tuebingen.mpg.de/ published -332 15017 1542215017 15421 1068 7 DA Leopold Y Murayama NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) Neural firing in the visual cortex is highly irregular. Much work suggests that the spontaneous activity changes experienced by a cortical neuron are highly correlated with those experienced by its neighbors. We have previously shown that multiunit activity between distant sites in monkey visual cortex can display significant covariation during the performance of a visual task. In the present study, we recorded from electrodes separated by up to 10 mm in striate and extrastriate visual cortex under a variety of behavioral conditions including 1) actively performing a visual task, 2) relaxing between trials, 3) in light sleep, and 4) under general anesthesia. We recorded single and multiunit activity, as well as local field potentials. We were primarily interested to understand how identifiable events contribute to covariation observed in the various states. These included external events such as changes in the sensory stimulus, internal events such as spontaneous perceptual changes during binocular rivalry, behavioral events such as blinks and eye movements, and physiological events such as sleep spindles. We found that inter-site correlation was diminished during the visual task compared other states. It was particularly diminished during the perceptual instability accompanying binocular rivalry. Preliminary results also demonstrated significant anti-correlation between areas accompanying blinks and eye movements. A substantial portion of the common variability could not be accounted for by events we could identify. We will discuss these results in terms of the magnitude, frequency coherence, and spatial distribution of the covariation patterns. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1060 7 I Bondar D Leopold J Pauls NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) The fast and accurate recognition of face identity is one of the hallmarks of primate visual performance. While numerous studies have revealed face-responsive neurons throughout the monkey brain, particularly in the superior temporal sulcus (STS) and inferotemporal (IT) lobe, the basic encoding strategies for faces remain poorly understood. Recent results using high-level adaptational aftereffects have suggested that the perception of identity involves reference to a stored prototype representation (Leopold et al., Nat. Neuro., 4:1, 2001). In the current study, we investigated the neurophysiological mechanisms underlying face perception in monkeys. We taught a monkey to discriminate between four human faces, and to indicate which face it perceived by pressing one of four buttons. We then measured the responses of IT and STS neurons to morphed faces that were modulated in their identity strength, as determined by their position in a computationally derived face-space. The monkey thus classified caricatures, anti-caricatures and anti-faces as one of the learned faces. Recordings were carried out with an implanted bundle of 64 high impedance Ni-Cr-Al microwire electrodes (Bondar & Logothetis, Soc. Neurosci. Abstr., 2000). Individual electrodes were different lengths, and the position of the bundle was adjustable in all directions, allowing for simultaneous measurement of single and multiple units, as well as local field potentials, from various areas in the temporal lobe. We will report on the modulation of temporal neurons to faces morphed along perceptually meaningful trajectories in face space. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1054 7 A Maier M Wilke DA Leopold S Treue NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) The visual instability that results from viewing ambiguous or conflicting patterns is thought to reflect dynamic processes that are critical for perceptual organization during normal vision. We have recently discovered a method to prolong states of perceptual dominance up to two orders of magnitude by periodically switching off and on the inducing bistable pattern. In the current study we used this paradigm to temporally interleave the presentation of pairs of ambiguous stimuli at the same location in visual space. Bistable patterns including rotating three-dimensional objects, depth reversals, and binocular rivalry, were shown alternately with appropriate blanking periods to prolong phases of perceptual dominance. We were interested under what conditions the time courses of two parallel bistable visual processes would be independent. We found that for certain pairs of ambiguous stimuli, such as 3-D balls rotating around orthogonal axes, the time course of perceptual alternation was largely parallel and independent. However, for other patterns, e.g. those differing only in color, speed, size, or position, the two stimuli were synchronized in their reversals. These results suggest that for a given visual location, perceptual organization can be biased for multiple different stimuli simultaneously, but that the independence between pairs of stimuli depends upon their similarity. We will discuss the results with respect to the critical stimulus dimensions that determine whether or not interference occurs. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1064 7 Y Murayama DA Leopold NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) The functional properties of neurons in the inferotemporal cortex (IT) have been studied extensively in both awake and anesthetized monkeys. While neurons in these areas show high selectivity for complex patterns, faces, and objects, their role in perception remains unclear. A previous electrophysiological study demonstrated that during binocular rivalry, when excitatory and non-excitatory stimuli were continually presented, most neurons in the inferotemporal cortex responded only when the excitatory stimulus was perceived (Sheinberg and Logothetis, 1997), suggesting that the activity of IT neurons is tightly linked with conscious perception. Recent evidence from our laboratory has suggested that active visual mechanisms of perceptual organization, including those related to binocular rivalry, can continue to operate under diminished states of consciousness that follow the administration of low doses of the dissociative anesthetic ketamine. In the present study, we examined the perception-related responses of IT neurons during rivalry in the monkey under a variety of conscious states, including wakefulness, ketamine dissociation, and general anesthesia. Single unit, multiunit, and local field activity was measured during extended periods of dichoptic stimulation, as well as during Flash suppression stimulus sequences that strongly bias subject's perception in the normal condition. Our analysis will focus on the extent of perception-related modulation of neural signals in the different conditions and characterize the functional significance of IT to perceptual organization. Supported by Max Planck Society. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1198 7 M Wilke A Maier DA Leopold AA Ghazanfar NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) A fundamental property of ambiguous visual patterns is the inevitability of perceptual reversal. The rate of alternation generally depends both upon the physical structure of the stimulus as well as the inherent rate of an individual. Here we present evidence that critical to the reversal process is the prolonged physical presence of the inducing stimulus. We demonstrate that when stimuli are shown only intermittently, the rate of perceptual alternation sharply declines to nearly zero in some subjects. In particular, we found that rotating three-dimensional bistable patterns with mean dominance times less than 10 seconds could be stabilized for periods lasting over ten minutes by correctly adjusting the duration of on and off periods. Since either perceptual state could be maintained in this way, this effect did not simply reflect a perceptual bias on the part of the observer. This trend was also present for other bistable patterns, including those involving geometric depth reversals and apparent motion correspondence. We systematically investigated the influence of stimulus on and off durations on the stability of such patterns, and found that minimizing the duration of the stimulus on time was the predominant factor in perceptual stabilization. We suggest that upon removal of the inducing stimulus a visual memory process retains and holds the last perceptual state, which persists and governs the perceptual interpretation during the subsequent presentation. Electrophysiological studies are currently underway to better elucidate the neural mechanisms contributing to this phenomenon. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1069 7 M Silver DA Leopold NK Logothetis San Diego, CA, USA2001-11-00 31st Annual Meeting of the Society for Neuroscience (Neuroscience 2001) Presentation of different visual stimuli to the two eyes results in alternation between the perception of one stimulus and the other, a phenomenon known as binocular rivalry. While these perceptual shifts clearly can arise from endogenous mechanisms, they can be also be caused by external stimulus changes, such as in the paradigm of flash suppression (Wolfe, 1984). We have previously shown that rivalry can emerge between dichoptic patterns that are rapidly switched between the eyes. In this interocular switching (IOS) paradigm, perception of each stimulus can endure for several seconds and span many physical stimulus swaps. Interestingly, the amount of stable unitary perception experienced with these stimuli is very sensitive to the temporal frequency of the stimulus switches as well as the contrast and similarity of the rivaling stimuli. The current study aims to better understand the determinants of perception in IOS rivalry by examining several aspects of its temporal characteristics. In particular, we investigated whether global stimulus perturbations, such as stimulus blanking, flicker, and flash suppression, have similar perceptual consequences for IOS rivalry as for normal rivalry. Several parameters of flicker are under study, including interocular and interstimulus differences in flicker rate. In addition, we are examining the detectability of sudden stimulus alterations in IOS rivalry over a range of switch rates and contrast values. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 SilverLL2001 7 MA Silver DA Leopold NK Logothetis Kusadasi, Turkey2001-08-00 76 Twenty-fourth European Conference on Visual Perception Dichoptic presentation of square arrays of dots that have no local interocular conflict can produce binocular rivalry if the two arrays are of opposite contrast relative to a gray background. Subjects reported when they experienced unitary percepts of only one array (and total perceptual suppression of the other array). The percentage of time of unitary perception was higher than would be predicted if rivalry occurred only locally between small independent retinotopic zones. Since the retinal location of each dot did not overlap the corresponding retinal locations of any dots in the other stimulus, unitary perception must have occurred when each dot in the nonvisible array was phenomenologically suppressed by a portion of the perceived array that contained only pixels of background luminance. This suggests that these stimuli were able to rival as textured surfaces rather than as sets of individual elements. Introduction of small (< 0.05 deg) random displacements of each dot increased unitary perception of the arrays. These displacements may have disrupted segmentation mechanisms that were activated by collinearity in the regularly spaced arrays, thereby enhancing the perception of the arrays as surfaces. Experiments are in progress to determine whether Gestalt grouping principles influence the amount of unitary perception. no notspecified http://www.kyb.tuebingen.mpg.de/ published -76 15017 15421 1044 7 DA Leopold Y Murayama NK Logothetis New Orleans, LA, USA2000-11-00 30th Annual Meeting of the Society for Neuroscience (Neuroscience 2000) Previous neurophysiological recordings in monkeys used the paradigm of binocular rivalry to investigate activity of single neurons in the visual cortex during multistable perception. These studies revealed that a subset of cells throughout visual cortex, including the striate and extrastriate areas, is directly involved in spontaneous perceptual reorganizations of an unchanging sensory stimulus. In the present study, the role of perception-related networks is further examined using simultaneous multiple electrode measurements in several visual cortical areas. As in previous studies, monkeys were trained to report their changing percept during rivalry by pressing levers, with optokinetic responses to moving stimuli serving as an additional behavioral control. Rivalry-inducing stimuli consisted of a variety of relatively large (>5 deg) patterns, including images, gratings and moving random dots. Multiple electrodes measured extracellular activity simultaneously in striate and extrastriate visual cortex. In addition to isolated neural responses, local field potential (LFP) as well as multiunit activity (MUA) was measured and analyzed. During rivalry, perceptual changes were reflected not only in the spiking of individual cells, but also in MUA as well as several frequency ranges of the LFP at individual sites. In an effort to better understand the overall flow and organization of perception-related activity in the visual cortex, the temporal covariation between the various neural signals was evaluated with respect to the animal’s changing percept, its state of arousal, as well as its pattern of spontaneous eye movements. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 998 7 Y Murayama DA Leopold NK Logothetis New Orleans, LA, USA2000-11-00 30th Annual Meeting of the Society for Neuroscience (Neuroscience 2000) When humans or monkeys are stereoscopically shown a pair of dissimilar images they experience a sequence of spontaneous perceptual fluctuations known as binocular rivalry. This phenomenon has been studied extensively, and has recently been used as a means to investigate neural mechanisms of perceptual organization. While it is now known that several areas in visual cortex participate in rivalry, the fundamental basis and origin of the spontaneous alternations remain poorly understood. Some evidence suggests that conscious perception is a critical component, since multistable perception is intimately related to attention and awareness. Yet, it is also possible that the brain, given a pair of discordant visual stimuli, automatically generates a sequence of state-changes, alternately processing each conflicting pattern in turn. Such automatic changes could be initiated by passive, free-running sensory networks, or alternatively by more complex mechanisms involving a variety of brain areas. In either case, the subjective perception experienced during waking would simply reflect these automatic processes. The present study addressed these issues by measuring neural activity (single unit responses, multiunit activity, and local field potentials) in the cortex of anesthetized monkeys under visual stimulus conditions that would normally give rise to binocular rivalry. Multiple electrodes afforded the simultaneous monitoring of sites in several striate and extrastriate visual areas. Analysis focused on the identification and characterization of spontaneous state-changes related to the conflicting stimuli that might serve as a basis for perceptual alternation. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 1045 7 HK Plettenberg SM Smirnakis DA Leopold NK Logothetis New Orleans, LA, USA2000-11-00 30th Annual Meeting of the Society for Neuroscience (Neuroscience 2000) The perceptual organization of visual patterns is thought to draw upon hard-wired constraints of the sensory apparatus as well as active extraretinal influences such as attention. Elaborate automatic or ‘passive’ processing of a retinal pattern is perhaps best illustrated by the selective responses of neurons to complex objects in animals that are fully unconscious due to anesthesia. More ‘active’ elements of vision might be represented by phenomena such as multistability, where the impression of a constant sensory pattern continually changes. The present study examined the expression of active and passive mechanisms of perceptual organization in monkeys that were in a state of ‘dissociation’ following the administration of ketamine. Monkeys were trained to sit in a primate chair and accept both intravenous catheters as well as intramuscular injections. By administrating low doses of ketamine, the animals could be brought from wakefulness into a state in which they remained motionless with their eyes wide open. Visual processing was assessed by measuring eye movements generated in response to a variety of visual stimuli. Within a range of drug concentrations, strong but irregular optokinetic nystagmus (OKN) was elicited by moving patterns. When stimuli were presented that normally give rise to multistable perception, OKN alternately reflected each of the competing patterns, suggesting that perceptual mechanisms are active even during diminished states of consciousness. When moving stimuli were presented monocularly, OKN became asymmetric, suggesting that ketamine decreases the cortical contribution to OKN generation in the brainstem. no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 15017 15421 127 7 DA Leopold Tübingen, Germany2000-02-00 30 3. Tübinger Wahrnehmungskonferenz (TWK 2000) The conscious perception of visual patterns and objects is thought to derive from their impact upon specialized neurons in the brain’s visual areas. Physiological recordings in animals have demonstrated that a given image striking the retina leads to a characteristic activity pattern among neurons in many subcortical and cortical brain structures. Such sensory ‘encoding’ of a stimulus is sometimes considered to lead automatically to its perception. Yet such a simple view does not account for the fact that conditions exist in which perception can be entirely dissociated from sensory stimulation. A visual pattern can, for example, impact the brain without leading to any perception. Conversely, a vivid visual percept can arise in the absence of any sensory stimulation whatsoever. Investigating such conditions may be important for understanding how sensory and perceptual signals are integrated in the brain, and may provide clues as to the neural underpinnings of conscious visual awareness. We have approached such issues with a series of neurophysiological studies in monkeys viewing multistable visual patterns. Such patterns, when continually present, lead to an unstable perceptual experience, in which the visual impression oscillates between two or more alternatives many times each minute. I will describe neurophysiological experiments in which monkeys viewing such patterns. The monkeys were trained to report their subjective experience while neurons in several visual cortical areas were continually monitored. The results suggest that the activity of a subset of neurons throughout the visual cortex is directly related to the subjective perception of a visual stimulus, while others in the same areas are more dedicated to scrutinizing the sensory pattern. I will discuss the sensory vs. perceptual nature of the neural representations in these areas. In addition I will speculate on possible origins of perceptual alternation, focusing upon what such past and future studies might reveal about mechanisms of normal conscious vision. no notspecified http://www.kyb.tuebingen.mpg.de/ published -30 15017 15421 993 7 DA Leopold NK Logothetis Washington, DC, USA1995-11-00 19 26th Annual Meeting of the Society for Neuroscience (Neuroscience 1996) no notspecified http://www.kyb.tuebingen.mpg.de/ published -19 992 7 DL Sheinberg DA Leopold NK Logothetis Washington, DC, USA1995-11-00 19 26th Annual Meeting of the Society for Neuroscience (Neuroscience 1996) no notspecified http://www.kyb.tuebingen.mpg.de/ published -19 990 7 DA Leopold NK Logothetis Fort Lauderdale, FL, USA1995-05-00 S813 Annual Meeting of the Association for Research in Vision and Ophthalmology (ARVO 1995) no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 989 7 DL Sheinberg DA Leopold NK Logothetis Fort Lauderdale, FL, USA1995-05-00 S668 Annual Meeting of the Association for Research in Vision and Ophthalmology (ARVO 1995) no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 987 7 DA Leopold NK Logothetis Houston, TX, USA1994-00-00 Neuroscience Forum 1994 no notspecified http://www.kyb.tuebingen.mpg.de/ published 0 2078 15 DA Leopold 1997-00-00 no notspecified published BondarLL2006 10 IV Bondar D Leopold NK Logothetis