Primates have excellent binocular vision and use vergence eye movements to align the two eyes on a common object, thereby facilitating stereopsis, the visual perception of depth. Binocular disparity, which refers to the slight difference in the locations of the images on the two retinas resulting from the slight difference in the viewpoint of the two eyes, is known to provide an important drive for these vergence eye movements. Disparity-induced vergence eye movements have traditionally been studied using an optical arrangement that allows identical stimuli to be presented independently to the two eyes, thus allowing the application of pure disparity stimuli. In these experiments we studied disparity-induced vergence eye movements in monkeys by placing a red filter over one eye and a green one over the other. The animal faced a tangent screen onto which were projected two large, textured patterns that were identical except in color~one being red (therefore seen only by the eye with the red filter) and the other green (seen only by the eye with the green filter). Sudden changes in the horizontal alignment of the two images (disparity steps) induced vigorous, machine-like vergence eye movements with a roughly exponential time course (time constant, 100 to 150 msec), despite the fact that the animals were neither trained to make such responses nor reinforced for doing so. The latency of such vergence eye movements was typically about 60 msec, which is about 100 msec less than previous values reported in the literature for experiments using smaller visual targets. One possibility was that these ultra-short-latency vergence responses were actually akin to the short-latency ocular-following responses that we have previously reported in response to the motion of large, textured scenes. However, this possibility is unlikely because the short-latency vergence responses did not show postsaccadic enhancement, one of the characteristic features of short-latency ocular following. Whereas neural encoding of disparity is known to occur in the cortex, these vergence responses must be cortically mediated, despite their ultra-short latency.
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