Mechanisms for binocular depth sensitivity along the vertical meridian of the visual field.

By removing the visual cortex unilaterally, and recording along the intact 17/18 border, we have investigated the influence of the corpus callosum on the tuning curves for stimulus disparity of cat cortical neurons. Responses to binocular stimulation were examined to movement in the same (in-phase) and in the opposite direction (antiphase) across the two retinae. In lesioned cats, as in normal cats, units were encountered which showed high sensitivity and narrow tuning for stimulus disparity. In contrast to normal cats, however, lesioned cats showed a reduced proportion of units displaying moderate binocular interactions, as well as a substantial increase in disparity-insensitive cells. The loss of disparity sensitivity after decortication was associated with a reduced incidence of both selectivity for the direction of stimulus motion and binocular activation. No large differences between the preparations were seen, however, in the ocular dominance of the total populations of cells. Differences between the normal and lesioned cats were found in binocular responses to in-phase but not to antiphase stimulation motion. Tuning curves in lesioned cats showed reduced binocular inhibition but no changes in binocular facilitation. Our findings indicate that callosal input contributes to unit disparity sensitivity by enhancing direction-selective binocular inhibition. The corpus callosum generates disparity sensitivity in a population of units in the superficial cortical layers which may play a particular role in the perception of stereoscopic depth.