Timing of the responses in the motor cortex of monkeys to an unexpected disturbance of finger position.

Monkeys were trained with food rewards to hold the wrist and fingers of their right hand in a flexed posture and maintain force with the finger tips against an isometric lever for a number of seconds. Once the animal had learned to produce a reliable performance of the task an assembly was attached to the skull through which microelectrodes could be introduced into the precentral cortex to record the activity of single neurones. Neurones whose activity was correlated with the force of finger flexion were studied; some of these could be identified as pyramidal tract neurones by their response to electrical stimulation in the medullary pyramids. While the monkey was flexing against it, the lever was sometimes suddenly released so that the fingers flexed without resistance. This unexpected disturbance was often followed by a change in the discharge of precentral neurones, although the monkey had not been trained to respond to the release in any particular way. On release of the lever the discharge of a given cortical neurone might either increase or decrease, and the direction of this change could not be predicted from the behaviour of the neurone during the isometric task. The most common response was an increase in cortical cell firing in neurones whose natural discharge was associated with the active development of force. The discharge of pyramidal tract neurones changed 25-50 msec after the sudden unexpected peripheral disturbance. Earlier changes were seen in some other neurones situated within the precentral gyrus and in the anterior bank of the central sulcus.