VOLUNTARY RHYTHMICAL MOVEMENT IS RESET BY STIMULATING THE MOTOR CORTEX

Using six normal subjects, we mapped the best location for magnetic co rtical stimulation to ''reset'' the phase of a voluntary alternating m ovement of the right wrist made against three different torques (0.26 N m extension, 0 and 0.09 N m flexion torque) at the subjects' preferr ed rate. We used ''nett resetting'' as a measure of phase resetting, b ased upon the relative amplitudes of the averages of the stimulated an d a phase-locked control position record. Nine sites covering a 5 cm s quare region of the contralateral cortex were systematically stimulate d. All the subjects showed evidence of resetting in response to magnet ic stimulation over one or more cortical sites during movements made a gainst the extension torque and all subjects demonstrated higher level s of nett resetting under these conditions than in response to similar cortical stimulation during unloaded movements. The best cortical sit es for inducing resetting were the same as those from which the larges t short-latency responses were evoked in the contralateral forearm fle xor and extensor muscles, i.e. the motor cortex. At the cortical sites where magnetic stimulation did induce resetting, the initial electrom yographic (EMG) effects consisted of a short-latency excitation follow ed by a period of inhibition. This silent period was followed by a sho rt burst of excitation often occurring simultaneously in the wrist fle xor and extensor muscles, and only thereafter by the return of rhythmi cal alternating EMG activity characteristic of the wrist movement. The latency to the first rhythmical EMG peal; following the stimulus was closely related to the period of the subject's prestimulus movement.