Stereoscopic mechanisms in monkey visual cortex: binocular correlation and disparity selectivity.

The neural signals in visual cortex associated with positional disparity and contrast texture correlation of binocular images are the subject of this study. We have analyzed the effects of stereoscopically presented luminous bars and of dynamic random-dot patterns on the activity of single neurons in cortical visual areas V1, V2, and V3-V3A of the alert, visually trained rhesus macaque. The interpretation of the results and considerations of possible neural mechanisms led us to recognize 2 functional sets of stereoscopic neurons. (1) A set of neurons, tuned excitatory (T0) or tuned inhibitory (TI), which respond sharply to images of zero or near-zero disparity. Objects at or about the horopter drive the T0 neurons and suppress the TI, while objects nearer and farther have the opposite effects on each type, inhibition of the T0 and excitation of the TI. The activity of these neurons may provide, in a reciprocal way, the definition of the plane of fixation, and the basic reference for binocular single vision and depth discrimination. (2) A second set of neurons includes tuned excitatory at larger crossed or uncrossed disparities (TN/TF) and neurons with reciprocal excitatory and inhibitory disparity sensitivity with cross-over at the horopter (NE/FA). Binocularly uncorrelated image contrast drives these neurons to a maintained level of activity, which shifts, in response to correlated images, toward facilitation or suppression as a function of positional disparity. These neurons may operate in the neural processing leading to stereopsis, both coarse and fine, and also provide signals for the system controlling binocular vergence. These results indicate that cortical visual neurons are binocularly linked to respond to the relative position and contrast of the images over their receptive fields, and also that both these aspects of binocular stimulation may be utilized by the brain as a source of stereoscopic information.