Relation of activity in precentral cortical neurons to force and rate of force change during isometric contractions of finger muscles.

The activity of single neurons within the hand area of the precentral motor cortex of primates was recorded during the performance of a maintained precision grip between the thumb and forefinger. The finger opposition forces were exerted against a strain gauge which allowed force changes to be studied under near isometric conditions. Task performance required the generation of a force ramp (the dynamic phase) and thereafter the maintenance of a stable force for one second (the static phase). Intracortical stimulation through the recording electrode was used to verify that the recordings were made from the appropriate somatotopographic area of the motor cortex. From a total of 221 recorded neurons, 76 were found to be either activated or deactivated during performance of the task. Among the 51 activated neurons, most discharged at higher frequencies during the dynamic phase, than during the static phase. The discharge of some of these neurons could be related to both force (F) and rate of force change (dF-dt) whereas certain others could only be correlated with dF-dt. The change in discharge frequency for these neurons generally occurred prior to the onset of EMG activity. Eight neurons were more active during maintained force than during the force ramp. The discharge frequency could not be correlated with dF-dt and only one showed a significant positive relation to force. The change in discharge frequency for these neurons either coincided or occurred after the onset of EMG activity.