The activity of supplementary motor area neurons during a maintained precision grip

Two monkeys were trained to exert a precision grip of the thumb and forefinger and to maintain constant near-isometric force for a one-second duration. Both animals were trained to perform the task with about equal proficiency with either hand. A total of 134 neurons were recorded from the supplementary motor area (SMA) of the hemisphere contralateral to the performing hand. SMA neurons were identified by either the presence of peripheral fields on the contralateral arm or by consistent changes in discharge frequency during contralateral arm movement. Sixty-one cells demonstrated reliable changes in firing frequency during performance of the maintained precision grip. SMA neurons showed little tendency to discharge at higher frequency during force change rather than during maintained force. Only two neurons significantly increased firing frequency with increased finger force and no modulation of discharge related to rate of force change could be shown. The changes in spike frequency among SMA cells related to the arm were, on the average, about 100 msec after the onset of contraction in the forearm flexors and extensors of the wrist and fingers, although a contingent of cells discharging consistently before the onset of muscular activity was found.