Shape processing is an essential visual function that underlies our ability to identify familiar items, interpret facial expressions, and comprehend written language. Understanding the neural mechanisms behind shape processing would constitute a key insight into visual perception. Previous studies have shown that shape is represented at the earliest levels of visual cortex (V1 and V2) in terms of oriented edges, and that cells at the highest levels (CIT and AIT) are selective for behaviorally relevant 3- dimensional (3D) stimuli like hands and faces. Little is known about the intervening stages where information about oriented edges is transformed into selectivity for complex 3D objects. The long term objective of this proposal is to fill this gap by investigating the neural mechanisms of shape processing that lead from oriented edges in 3D objects. The proposed approach to this issue is based on prior experiments in which we tested the responses of macaque area V4 cells to a large set of gradually varying 2D shape features. The systematic tuning profiles observed int these experiments suggest that shape is represented at intermediate levels in terms of simple contour features like angled corners and curve segments. The proposed experiments would distinguish between two hypotheses: (a) That these 2D contour features are indeed the essential shape primitives extracted at intermediate stages, to be combined later at higher stages into complex 3D percepts, and (b) That the observed 2D tuning profiles are actually cross-sections through more specific 3D tuning volumes, so that the true shape primitives are not (for example) 2D angles and curve segments but 3D corners and rounded volumes. These two hypotheses will be explored by extending the shape features stimulus set into the 3rd dimension, to test whether cells respond consistently to a given 2D profile (regardless of 3D information) or instead show more selective tuning for a specific range of 3D shape features. These tests will be carried out at intermediate and higher level stages in the object processing pathway (V2, V4, PIT and CIT) to determine how and what point 3D shape information is generated.
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