Examination of functional interactions between prefrontal and inferotemporal cortex during binocular rivalry
Introduction and Aims
The brain has a remarkable way to resolve the conflict that arises when disparate visual stimuli are presented to corresponding retinal locations. It undergoes stochastic perceptual transitions, wherein only one of the stimuli attains conscious awareness. This phenomenon, which dissociates sensory stimulation from perception is called binocular rivalry (BR) and has been exploited extensively to study the neural correlates of visual awareness . Neurophysiological experiments with non human primates trained to report their percept during BR have demonstrated that the percentage of cells correlated the animals perceptual report increase down the ventral visual pathway . In the inferotemporal cortex (IT), 90 percent of the visually selective cells fire in concurrence with the reported visual percept . Interestingly, ventrolateral prefrontal cortex (VLPFC), which has reciprocal anatomical connections with IT, shows increased BOLD activity in human subjects undergoing rivalry specifically during perceptual switches . Furthermore, work in our lab has shown the existence of cells whose activity changes in accordance with the visual percept in the VLPFC of macaque monkey . However, a mechanistic understanding of how these two areas interact with each other during alternations in perception remains unknown. The aim of this study is to explore with electrophysiology in non human primates, the functional interactions between IT and VLPFC, during a. spontaneous perceptual switches and b. stimulus suppression and dominance during BR. We illustrate below the first steps taken towards this direction, namely training of rhesus macaques in a visual discrimination task.
We have trained two rhesus macaques in a visual categorization task, wherein they discriminate stimuli alternating between three different categories which are presented to them with the help of a stereoscope. The animals maintain stable fixation in a +/-1.5 degree window for up to 25-30 seconds in a row, while reporting their percept by holding a lever pulled in response to two different categories of stimuli, namely a face and checkerboard pattern. They have been additionally trained not to pull a lever or release an already pulled lever when a physical mixture of the two stimuli, mimicking piecemeal perception, is presented.
Results and Conclusions
The animals perform the categorization task with good accuracy. One animal has reached the desired discrimination accuracy of ~95%, while the other has an accuracy of ~80% across sessions. Recently, the animal with higher accuracy has been exposed to single periods of binocular rivalry intermingled between periods of physical alternation. Preliminary observations show that the animal pulls levers to report his percept in a fashion similar to physical alternation. We are now in the process of standardizing stimulus parameters in order to optimize dominance and suppression times suitable for data collection. The next step is to examine their psychophysical behavior, when presented with rivalrous stimuli, and to verify that it complies with temporal parameters that are characteristic of binocular rivalry.
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A. Exemplar stimuli on which the animals have been trained. B. The accuracy of discrimination of the two animals under training, G07 and F06.
Development of a Multi-Tetrode Drive for deep structure electrophysiological recordings in the macaque brain
Introduction and Aims
Understanding the principles that underlie information processing by neuronal networks requires simultaneous recordings from large populations of single units. Twisted wire tetrodes(TWTs), typically made by wounding four very thin wires (diameter 12 microns) together, are ideally suited for such population recordings. They are advantageous over single electrodes; both with respect to reliability as well as the number of single units isolated [1,2] and have therefore been used extensively for superficial cortical recordings. However, their limited tensile strength poses a restriction to their use for recordings in deep brain areas. To our knowledge, TWTs have never been used for acute recordings in deep brain structures. We therefore developed a method to overcome this limitation in order to utilize tetrodes for electrophysiological recordings in the inferotemporal cortex (IT) of rhesus macaque. Our aim is to develop a) a method for making tetrodes stiffer and b) a multi-tetrode driving system for advancing them through a ball and socket chamber to precise locations for electrophysiological investigation of IT in the non human primate brain.
A mechanical technique has been developed for pulling regular TWTs inside 30 mm long stainless steel tubes (outer diameter - ~120 microns, inner diameter - ~50 microns) thus giving them rigidity. After this, the tetrode tip is glued and grinded and these new tube tetrodes (TuTs) are inserted inside a 30 Gauge stainless steel holding tube. The impedance is then reduced by gold electroplating. Finally, they shall be positioned in the multi-tetrode recording drive which is being custom built for holding and independently moving, upto 5 TuTs through a ball and socket chamber.
Results and Conclusions
Initial test recordings made with our TuTs in the prefrontal cortex of an awake macaque reveals that they yield good cluster isolation similar to conventional TWTs. Further, we have implanted one animal with a ball and socket chamber for testing the multi-tetrode driving system. To conclude, we developed special TuTs which are stiffer as compared to TWTs, and have signal quality comparable to them. They are now ready to be integrated in the multi-tetrode driving system for further testing and performing electrophysiological recordings in the inferotemporal cortex.
Eduard Krampe, Axel Klug
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A. Schematic drawing showing the multi-tetrode drive placed on top of a ball and socket chamber sitting on top of the brain. The skull has been removed for demonstration purposes. B. Tube tetrode is shown looking out from the implanted guide tube of the ball and socket chamber. Notice the location of the tetrode above the IT cortex C. Micrograph of the Tube tetrode with four contacts embedded in glue. D. Screenshot taken during electrophysiological recording with a Tube tetrode. On top, are the waveforms recorded on each of the four tetrode channels. Below is shown online sorting of the signals by plotting /clustering the peak to peak amplitude of each waveform from different channels of the tetrode.