Activity of primate precentral neurons during voluntary movements triggered by visual signals

Awake, intact monkeys were trained to perform discrete flexion or extension movements of the hand about the wrist in response to visual signals. The object of the movement was to align a cursor, coupled to a manipulandum, on a target line. Cursor and target lines are displayed on a video monitor placed in front of the monkey. The target line was stepped to the right or left, randomly with regard to direction and timing, with each step implying an instruction for the monkey to make a voluntary movement for alignment. Single unit recording was made in the forelimb area of contralateral precentral cortex. Neurons were classified by their responses to passive sensory stimulation and the effects of local intracortical microstimulation into two populations; wrist flexion-extension (F-E) neurons, and all other forelimb neurons (non-wrist (F-E)). A significantly higher proportion of wrist (F-E) neurons as compared to non-wrist (F-E) neurons were task-related. Moreover the wrist (F-E) neurons exhibited exclusively reciprocal responses to the oppositely directed visual signals, whereas the non-wrist (F-E) neurons showed both reciprocal and bidirectional responses. No significant differences in mean latencies of responses, either in respect to the visual signals or to movement onset, were observed between the two populations of neurons. However the range of latencies in both instances was greater in the non-wrist (F-E) populations. The wrist (F-E) population showed significantly less response variability than the non-wrist (F-E) population with regard to response latencies to visual signals and movement onsets, and the degree of correlation between duration of response and reaction time.