Processing of shape defined by disparity in monkey inferior temporal cortex.

Neurons in the monkey inferior temporal cortex (IT) have been shown to respond to shapes defined by luminance, texture, or motion. In the present study, we determined whether IT neurons respond to shapes defined solely by binocular disparity, and if so, whether signals of disparity and other visual cues to define shape converge on single IT neurons. We recorded extracellular activity from IT neurons while monkeys performed a fixation task. Among the neurons that responded to at least one of eight random-dot stereograms (RDSs) containing different disparity-defined shapes, 21% varied their responses to different RDSs. Responses of most of the neurons were positively correlated between two sets of RDSs, which consisted of different dot patterns but defined the same set of eight shapes, whereas responses to RDSs and their monocular images were not correlated. This indicates that the response modulation for the eight RDSs reflects selectivity for shapes (or their component contours) defined by disparity, although responses were also affected by dot patterns per se. Among the neurons that showed selectivity for shapes defined by luminance or disparity, 44% were activated by both cues. Responses of these neurons to luminance-defined shapes and those to disparity-defined shapes were often positively correlated to each other. Furthermore the stimulus rank, which was determined by the magnitude of responses to shapes, generally matched between these cues. The same held true between disparity and texture cues. The results suggest that the signals of disparity, luminance, and texture cues to define the shapes converge on a population of single IT neurons to produce the selectivity for shapes.