CONTROL OF REMEMBERED REACHING SEQUENCES IN MONKEY .1. ACTIVITY DURING MOVEMENT IN MOTOR AND PREMOTOR CORTEX

Motor and premotor cortex firing patterns from 307 single neurons were recorded while monkeys made rapid sequences of three reaching movemen ts to remembered target buttons arrayed in two-dimensional space. A pr imary goal was to study and compare directionally tuned responses for each of three movement periods during 12 movement sequences that unifo rmly sampled the directional space in front of the monkey. The majorit y of neurons showed maximal responses during movements in a preferred direction with smaller increases during movements close to the preferr ed direction. These responses showed a statistically significant regre ssion fit to a cosine function for 72% of the neurons examined. Compar isons among tuning directions computed separately for the first, secon d, and third movement periods suggested the near constancy of preferre d direction across a rapidly executed series of movements even though these movements began at different starting points in space. Although directionally tuned neurons were only broadly tuned for a specific dir ection of movement, the neuronal ensemble carried accurate directional information. A population vector computed by summing vector contribut ions from the entire population of tuned neurons predicted movement di rection with a mean accuracy of 20 degrees. This population code made consistent predictions for each of the 36 movements that were studied using a single set of population parameters. Most of the remaining neu rons (24%) that were not tuned during movement did show significant ch anges in activity during other aspects of task performance. Some nontu ned neurons had nondirectional increases that were sustained during mo vement, while others showed identical phasic bursts during the three m ovement periods. These nontuned neurons may control stabilizations of the shoulder, trunk, and forearm during movement, or forearm movements during button pushing.