NEURONAL MECHANISMS UNDERLYING STEREOPSIS - HOW DO SIMPLE CELLS IN THE VISUAL-CORTEX ENCODE BINOCULAR DISPARITY

Binocular neurons in the visual cortex are thought to form the neural substrate for stereoscopic depth perception. How are the receptive fie lds of these binocular neurons organized to encode the retinal positio n disparities that arise from binocular parallax? The conventional not ion is that the two receptive fields of a binocular neuron have identi cal shapes, but are spatially offset from the point of retinal corresp ondence (zero disparity). We consider an alternative disparity-encodin g scheme, in which the two receptive fields may differ in shape (or ph ase), but are centered at corresponding retinal locations. Using a rev erse-correlation technique to obtain detailed spatiotemporal receptive -field maps, we provide support for the latter scheme. Specifically, w e show that receptive-field profiles for the left and right eyes are m atched for cells that are tuned to horizontal orientations of image co ntours. However, for neurons tuned to vertical orientations, the left and right receptive fields are predominantly dissimilar in shape. Thes e results show that the striate cortex possesses a specialized mechani sm for processing vertical contours, which carry the horizontal-dispar ity information needed for stereopsis. Thus, in a major modification t o the traditional notion of the neural basis of stereopsis, we propose that binocular simple cells encode horizontal disparities in terms of phase at multiple spatial scales. Implications of this scheme are dis cussed with respect to the size-disparity correlation observed in psyc hophysical studies.