NEURAL MECHANISMS UNDERLYING BINOCULAR FUSION AND STEREOPSIS - POSITION VS PHASE

The visual system utilizes binocular disparity to discriminate the rel ative 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 d isparity is thought to begin in this region, There are two possible me chanisms 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 profile be tween the two eyes (RF phase disparity), Although there have been stud ies supporting each of the two encoding mechanisms, both mechanisms ha ve not been examined in a single study, Therefore, the relative roles of the two mechanisms have not been determined, To address this issue, we have mapped left and right eye RFs of simple cells in the cat's st riate cortex using binary m-sequence noise, and then we have estimated RF position and phase disparities, We find that RF position dispariti es are generally limited to small values that are not sufficient to en code large binocular disparities, In contrast, RF phase disparities co ver a wide range of binocular disparities and exhibit dependencies on orientation and spatial frequency in a manner expected for a mechanism that encodes binocular disparity, These results indicate that binocul ar disparity is mainly encoded through RF phase disparity, However, RF position disparity may play a significant role for cells with high sp atial frequency selectivity, which are constrained to small RF phase d isparities.