In the primate cerebral cortex, visual information processing begins in the striate cortex and continues into extrastriate areas. The middle temporal areas (MT) is one of these extrastriate visual areas, and it is distinguished by having a high proportion of cells that respond to stimuli moving within a restricted range of directions and velocities. Our work on this area in previous years has concentrated both on the relation of cells in this area to the perception of visual motion and to the initiation of pursuit eye movements in response to moving stimuli. This year we have investigated the relation of area MT to the maintenance of pursuit eye movements once a visual target is acquired and the target falls on the fovea. To do this, we trained monkeys to pursue a moving visual target and measured with the magnetic search coil technique their ability to do so. We then located the foveal area of MT with microelectrodes and injected into this and adjacent areas a neurotoxin, ibotenic acid, which kills cells but leaves fibers unaffected. We first observed a retinotopic deficit that we had seen previously following injections related to the more peripheral visual field: the monkey was unable to match his eye speed to the target speed when the target was moving in the contralateral visual field near the fovea, regardless of its direction. In addition, we observed a directional deficit: once the monkey had acquired the target, he was unable to match its speed with a pursuit movement when the target moved toward the side of the brain with the lesion. The deficit was present with stimulus motion in any part of the visual field. This deficit in the monkey following a tiny localized cortical lesion is similar to the classical neurologic deficit in man following damage to large cortical areas: a deficit in pursuit toward the damaged side of the brain.
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