Vergence eye movements in response to binocular disparity without depth perception

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 latencies1,2. 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 (ref. 3), despite the fact that human subjects fail to perceive depth in such stimuli4,5. These dataindicate 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.