The functions of the medial premotor cortex. II. The timing and selection of learned movements.

Monkeys with medial premotor cortex (MPC) lesions are impaired on a simple learned task that requires them to raise their arm at their own pace. However, they can succeed on this task if they are given tones to guide performance. In the externally paced task the tones could aid performance in several ways. They tell the animal when to act (trigger), they remind the animal that food is available and so motivate (predictor), and they remind the animal of what to do (instruction). Monkeys with MPC lesions can respond quickly to visual cues (experiment 1), and they can respond as well as normal monkeys when there is no immediate trigger (experiment 2). They are also quick to relearn a task in which external cues tell them what to do (experiment 5). However, they are 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 predictors, but there were no external instructions. We conclude that the reason why animals with MPC lesions perform better with external cues is that these cues act as instructions. The cues prompt retrieval of the appropriate action. This is true whether the task requires the animal to perform one action (experiments 1 and 2) or to select between actions (experiments 3 and 4).