PROCESSING OF SHAPE DEFINED BY DISPARITY IN MONKEY INFERIOR TEMPORAL CORTEX

Hiroki Tanaka, Takanori Uka, Kenji Yoshiyama, Makoto Kato and Ichiro Fujita
Laboratory for Cognitive Neuroscience, Department of Biophysical Engineering,
Graduate School of Engineering Science, Osaka University
Medical School, Osaka University
CREST, Japan Science and Technology Corporation, Toyonaka, Osaka

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 responded to shapes defined solely by binocular disparity, and if so, whether their shape selectivity was invariant for disparity and other visual cues. we recorded extracellular activity from IT neurons, while monkeys performed a fixation task. Among the neurons responding to at least one of the eight random-dot stereograms (RDSs) presented, which contained different disparity-defined shapes, 20% varied their responses to different RDSs. This stimulus-selective response modulation disappeared when monocular images of the RDSs were presented, but was preserved between 2 sets of RDSs, which consisted of different dot patterns but defined the same set of 8 shapes. These results indicate that the response modulation represents neuronal selectivity for the shapes defined by disparity, not for the dot patterns. Among the neurons that showed selectivity for shapes defined by luminance or disparity, 40% were activated by both cues. These cue-convergent types of neurons showed similar shape selectivity for these cues. Similar results were obtained between disparity and texture cues. The IT neurons thus respond to disparity-defined shapes which are not present in the monocular retinal images, but are constructed within the brain. The results support and extend the notion that IT neurons can encode shape regardless of the visual cues that define them.


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