Neurons in cat visual cortex tuned to the direction of motion in depth: effect of positional disparity.

We investigated sensitivity to the direction of stimulus motion in depth in neurons of cat visual cortex by using bar stimuli with different image velocities on the two retinae. These stimuli were presented at seven different retinal disparities (i.e. with different locations in depth). Approximately one-fourth of the neurons examined were sensitive to the direction of stimulus motion in depth. In general, the motion-in-depth tuning of these neurons was either unaffected by disparity or changed simply and systematically as a function of disparity, even when disparity was varied over the broad range of 12 degrees. In their relative indifference to disparity, the motion-in-depth neurons contrast with the units that are very selective to disparity and that respond best to sideways motion. Human equivalents to these two classes of units might provide a physiological basis for the distinction between binocularly-driven channels for motion in depth and for disparity (i.e. relative position in depth) that have been proposed on psychophysical grounds.