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An observer moving forwards through the environment experiences a radial pattern of image motion on each retina. Such patterns of optic flow are a potential source of information about the observer's rate of progress, direction of heading and time to reach objects that lie ahead. As the viewing distance changes there must be changes in the vergence angle between the two eyes so that both foveas remain aligned on the object of interest in the scene ahead. Here we show that radial optic flow can elicit appropriately directed (horizontal) vergence eye movements with ultra-short latencies (roughly 80 ms) in human subjects. Centrifugal flow, signalling forwards motion, increases the vergence angle, whereas centripetal flow decreases the vergence angle. These vergence eye movements are still evident when the observer's view of the flow pattern is restricted to the temporal hemifield of one eye, indicating that these responses do not result from anisotropies in motion processing but from a mechanism that senses the radial pattern of flow. We hypothesize that flow-induced vergence is but one of a family of rapid ocular reflexes, mediated by the medial superior temporal cortex, compensating for translational disturbance of the observer.

Primates use vergence eye movements to align their two eyes on the same object and can correct misalignments by sensing the difference in the positions of the two retinal images of the object (binocular disparity). When large random-dot patterns are viewed dichoptically and small binocular misalignments are suddenly imposed (disparity steps), corrective vergence eye movements are elicited at ultrashort latencies. Here we show that the same steps applied to dense anticorrelated patterns, in which each black dot in one eye is matched to a white dot in the other eye, initiate vergence responses that are very similar, except that they are in the opposite direction. This sensitivity to the disparity of anticorrelated patterns is shared by many disparity-selective neurons in cortical area V1, despite the fact that human subjects fail to perceive depth in such stimuli. These data indicate that the vergence eye movements initiated at ultrashort latencies result solely from locally matched binocular features, and derive their visual input from an early stage of cortical processing before the level at which depth percepts are elaborated.

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.