Neural mechanisms for encoding binocular disparity: Receptive field position versus phase

The visual system uses binocular disparity to discriminate the relative dep th of objects in space. Because the striate cortex is the first site along the central visual pathways at which signals from the left and right eyes c onverge onto a single neuron, encoding of binocular disparity is thought to begin in this region. There are two possible mechanisms for encoding binoc ular disparity through simple cells in the striate cortex: a difference in receptive held (RF) position between the two eyes (RF position disparity) a nd a difference in RF profiles between the two eyes (RF phase disparity). A lthough there is evidence that supports each of these schemes, both mechani sms have not been examined in a single study to determine their relative ro les. Zn this study, we have measured RF position and phase disparities of i ndividual simple cells in the cat's striate cortex to address this issue. U sing a sophisticated RF mapping technique that employs binary m-sequences, we have obtained left and right eye RF profiles of two or more cells record ed simultaneously. A version of the reference-cell method was used to estim ate RF position disparity. We find that RF position disparities generally a re limited to values that are not sufficient to encode large binocular disp arities. In contrast, RF phase disparities cover a wide range of binocular disparities and exhibit dependencies on RF orientation and spatial frequenc y in a manner expected for a mechanism that encodes binocular disparity. Th ese results suggest that binocular disparity is encoded mainly through RF p hase disparity. However, RF position disparity may play a significant role for cells with high spatial frequency selectivity that are constrained to h ave only small RF phase disparities.