Neural mechanisms for encoding binocular disparity: receptive field position vs. phase

The visual system utilizes binocular disparity to discriminate the relative depth of objects in space. Since the striate cortex is the first site along the central visual pathways at which signals from the left and right eyes converge onto a single neuron, encoding of binocular disparity is thought to begin in this region. There are two possible mechanisms for encoding binocular disparity through simple cells in the striate cortex: a difference in receptive field (RF) position between the two eyes (RF position disparity) and a difference in RF profiles between the two eyes (RF phase disparity). Although there is evidence which supports each of these schemes, both mechanisms have not been examined in a single study to determine their relative roles. In this study, we have measured RF position and phase disparities of individual simple cells in the cat’s striate cortex to address this issue. Using a sophisticated RF mapping technique that employs binary msequences, we have obtained left and right eye RF profiles of two or more cells recorded simultaneously. A version of the reference-cell method was used to estimate RF position disparity. We find that RF position disparities are generally limited to values which are not sufficient to encode large binocular disparities. In contrast, RF phase disparities cover a wide range of binocular disparities, and exhibit dependencies on RF orientation and spatial frequency in a manner expected for a mechanism that encodes binocular disparity. These results suggest that binocular disparity is encoded mainly through RF phase disparity. However, RF position disparity may play a significant role for cells with high spatial frequency selectivity, which are constrained to have only small RF phase disparities.