Binocular stereopsis is the ability to use differences between the images presented to the two eyes (binocular disparities) to perceive the three dimensional structure of the outside world. Much is now known about how neurons early in the visual cortex combine signals from the two eyes to measure binocular disparity in a small part of the visual field. Little is known about how these signals are then used to represent three dimensional surfaces. ? ? We explored this representation in neurons of area V2 in awake fixating monkeys,using stimuli containing edges that are defined only by disparity (cyclopean edges). We investigated the extent to which processing of purely local visual information (in the vicinity of the receptive field) might explain the results, using the most impoverished stimulus possible containing a cyclopean edge (a circular patch of random dots divided into two regions by a single edge). Many V2 cells responded better to the cyclopean edge than to uniform disparities, and most of these were at least broadly selective for the orientation of the cyclopean edge.? ? The preferred orientation was often similar for cyclopean and luminance contours, suggesting that these cells may support a cue-invariant representation of contours. However,two characteristics argued against a cue-invariant contour representation: (1) the cyclopean edge response was frequently abolished by small changes to the component disparities; and (2) although V2 cells frequently responded to both signs of a cyclopean edge (defined by which side of the edge is in front), they did so at different edge locations. These characteristics are consistent with a simple feedforward scheme in which V2 neurons receive inputs from several V1 subunits with different disparity selectivity. In many ways this suggests that the processing of signals going from V1 to V2 mirrors the long established processing of signals going from the LGN to V1.
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