Effects of MT lesions on visuomotor performance in macaques

Monkeys with selective bilateral lesions of area MT were trained on tasks designed to examine visuomotor function. They were required to: 1- retrieve a small food pellet from a narrow slot; 2- locate and retrieve a loose peanut mounted on a background of fixed peanuts; and 3- retrieve an erratically moving food pellet from a spinning bowl. After the lesions, these monkeys were behaviorally impaired relative to their own preoperative performances and also relative to the postoperative performances of the control monkeys with lesions in optic radiation fibers (OR) under MT or lesions in the posterior parietal cortex (PP). Although their performance improved with practice and time, the MT-lesioned monkeys showed long-term impairments twenty weeks after surgery. Control monkeys performed no worse on the tasks after their lesions. Another task which required the monkeys to retrieve a food pellet without visual guidance revealed that all the animals performed equally poorly when visual cues were unavailable, but that only the control monkeys benefited when visual cues were available. None of the monkeys were impaired on a pattern discrimination learning task. Besides that, direct observations revealed that the MT-lesioned animals grasped peanuts in a manner different from the control animals.