THE FUNCTIONS OF THE MEDIAL PREMOTOR CORTEX .2. THE TIMING AND SELECTION OF LEARNED MOVEMENTS

Monkeys with medial premotor cortex (MPC) lesions are impaired on a si mple learned task that requires them to raise their arm at their own p ace. However, they can succeed on this task if they are given tones to guide performance. In the externally paced task the tones could aid p erformance in several ways. They tell the animal when to act (trigger) , they remind the animal that food is available and so motivate (predi ctor), and they remind the animal of what to do (instruction). Monkeys with MPC lesions can respond quickly to visual cues (experiment 1), a nd they can respond as well as normal monkeys when there is no immedia te trigger (experiment 2). They are also quick to relearn a task in wh ich external cues tell them what to do (experiment 5). However, they a re poor at selecting between movements on a simple motor sequence task (experiment 3), and they are poor at changing between two movements ( experiment 4). On these tasks there were cues to act as triggers and p redictors, but there were no external instructions. We conclude that t he reason why animals with MPC lesions perform better with external cu es is that these cues act as instructions. The cues prompt retrieval o f the appropriate action. This is true whether the task requires the a nimal to perform one action (experiments 1 and 2) or to select between actions (experiments 3 and 4).