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(1,2). However, as the observer moves through the environment , the image motion on the retina depends on the three-dimensional stru cture of the scene(3,4). We report here that the very earliest of thes e tracking responses is elicited only by objects moving in the immedia te vicinity of the plane of fixation: objects nearer or farther are ig nored. This selectivity is achieved by means of a stereoscopic depth m echanism which uses the fact that the two eyes have differing viewpoin ts, so only objects in the plane of fixation have images that occupy c orresponding positions on the two retinae. Such behaviour is readily e xplained by the known binocular properties of some motion-selective ne urons in the visual cortex(5). Some (stereoanomalous) subjects showed highly specific tracking deficits as though lacking one subtype of the se neurons.