A role for stereoscopic depth cues in the rapid visual stabilization of the eyes

Primates have visual tracking systems that help stabilize the eyes on the surroundings by responding to retinal image motion at ultra-short latencies. However, as the observer moves through the environment, the image motion on the retina depends on the three-dimensional structure of the scene. We report here that the very earliest of these tracking responses is elicited only by objects moving in the immediate vicinity of the plane of fixation: objects nearer or farther are ignored. This selectivity is achieved by means of a stereoscopic depth mechanism which uses the fact that the two eyes have differing viewpoints, so only objects in the plane of fixation have images that occupy corresponding positions on the two retinae. Such behaviour is readily explained by the known binocular properties of some motion-selective neurons in the visual cortex. Some (stereoanomalous) subjects showed highly specific tracking deficits as though lacking one subtype of these neurons.