ANTICIPATORY ACTIVITY OF MOTOR CORTEX NEURONS IN RELATION TO DIRECTION OF AN INTENDED MOVEMENT

Monkeys were trained to hold a handle in a central zone midway between push and pull while awaiting an instruction to push or pull in response to a subsequent perturbation, triggering the instructed movement and followed by a reward if the movement was correct. Recordings in pre- and postcentral sensorimotor cortex revealed instruction-induced changes of neuronal activity during the period intervening between the instruction and the perturbation-triggered movement. Effects of the instruction were differential, depending on which of the 2 instructions were given; such differential responses to the instruction were detected in 61% of precentral pyramidal tract neurons (PTN), 44% of precentral non-PTN and 11% of postcentral neurons. Since motor cortex PTN axons end on .alpha. and .gamma. motoneurons and on interneurons of the spinal cord, changes of PTN activity with intention or motor set provide a mechanism for suprasegmental control and presetting of spinal cord reflex excitability specific to the nature of an impending movement.