Neural selectivity for binocular disparity is well documented for various cortical areas, but little is known about the processing of 3D shape. We recorded activity from neurons in areas V1 and V2 of the monkey visual cortex during behaviorally induced fixation. 3D shape selectivity was tested with computer generated displays of 3D surfaces and block objects rendered either as random-dot stereograms or as uniform reflecting surfaces under oblique illumination. A step- or roof-edge was placed in the receptive field at optimal orientation. The uniform objects were designed so that different types of 3D edges produced identical contrast borders over the receptive field. Results. (1) Random-dot stereograms. We found cells that were highly selective for 3D shape features. A cell might respond to step edges of certain orientation and step direction, but not to roof edges or plane surfaces, another cell to convex (or concave) roof-edges, but not to step edges. These cells showed invariant selectivity over a certain range of distances (relative to fixation distance) and were unresponsive outside that range. (2) Cells that were selective for 3D edge shape in random-dot stereograms showed similar selectivity also for uniform reflecting objects despite the fact that in this case the displays for different shapes of edges were identical over the receptive field. We conclude that early stages of visual cortex contain a rich representation of 3D shape primitives. Since the signals resolve the inherent ambiguity of visual displays, the underlying mechanisms must be integrating the image context beyond the conventional receptive field.
ASJC Scopus subject areas
- Sensory Systems