Evidence for elongated receptive field structure for mechanisms subserving stereopsis

To study the spatial extent and shape of the binocular disparity mechanisms subserving depth perception, we employ the spatial summation paradigm of contrast threshold for front/back depth discrimination at a fixed binocular disparity. The stimuli were Gabor patches with disparity set at either 4 or 8 arcmin and spatial frequency set at an optimal value of 4 cy/deg. Contrast threshold was measured as a function of length and width of the Gabor patches to determine the aspect ratio of greatest efficiency. The space constant of the Gaussian envelope varied between 0.0375 degrees and 0.9 degrees in either vertical or horizontal directions, or both simultaneously. For vertical elongation of the Gabor patches, discrimination sensitivity improved by 4-6 dB for a doubling of the length of the Gabor patches, then reduced more slowly as the length further increased. However, extending the Gabor patches horizontally across cycles produced little or no sensitivity improvement. Instead, discrimination performance collapsed in a fashion that is incompatible with many models of disparity processing. The results imply that the main mechanisms subserving stereoscopic depth discrimination are vertically elongated for vertical-bar Gabors and encounter special difficulties integrating horizontal disparity information. Disparity discrimination sensitivity for very small targets was also much greater than predicted by the single-mechanism fit, implying the presence of a second, independent mechanism with a very small summation field, which may underlie the fine stereoscopic processing system.