MOTION PERCEPTION FOLLOWING LESIONS OF THE SUPERIOR TEMPORAL SULCUS IN THE MONKEY

We examined the effect of bilateral ibotenic acid lesions, aimed at ar eas MT/MST in three macaques, on their perception of motion. The media l boundary of the lesions in the three monkeys was near the dorsal end of the STS, but the lesions extended different lengths ventrally alon g the STS. The lesions extended the shortest distance ventrally in mon keys 1 and 2, covering most of MST but possibly sparing a portion of l ateral MT. That in monkey 3 damaged all of MT and MST bilaterally and extended through most of FST. All three lesions caused a temporary dis ruption, followed by at least partial recovery, of most motion thresho lds. Permanent effects of the lesions on visual sensitivity were grade d with lesion extent. Contrast sensitivity for detecting low-spatial-f requency (1 cycle/degree) drifting gratings over a wide range of drift rates, as well as for identifying their direction of motion, was slig htly affected only in monkey 3, Only monkeys 2 and 3 showed a deficit in discriminating stimulus speed, and the size of the loss was two- to fourfold, Discrimination of opposite directions of dot pattern motion , which required integration of local motion signals, was mildly affec ted in monkeys 2 and 3, and not affected in monkey 1. However, additio n of directional noise to this discrimination caused the performance o f all monkeys to be permanently disrupted, especially that of monkeys 2 and 3. Finally, direction difference thresholds were elevated by a f actor of 2-4 after the lesions in all three monkeys, Many of these def icits were more pronounced during the first 2 months of testing follow ing the lesion. Thus, our results demonstrate that areas within dorsal STS make an important contribution to the performance of various moti on perception tasks including the discrimination of small differences in direction and speed, and the perception of global motion in the pre sence of directional noise. The residual motion perception, even in th e monkey with virtually complete removal of areas MT/MST, may suggest either that these tasks are normally mediated in part by cortical area s outside of areas MT and MST, or that the disrupted functions were pa rtially assumed by other cortical areas after lesions.