Voluntary rhythmical movement is reset by stimulating the motor cortex

Using six normal subjects, we mapped the best location for magnetic cortical stimulation to "reset" the phase of a voluntary alternating movement 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' preferred rate. We used "nett resetting" as a measure of phase resetting, based upon the relative amplitudes of the averages of the stimulated and a phase-locked control position record. Nine sites covering a 5 cm square region of the contralateral cortex were systematically stimulated. All the subjects showed evidence of resetting in response to magnetic stimulation over one or more cortical sites during movements made against the extension torque and all subjects demonstrated higher levels of nett resetting under these conditions than in response to similar cortical stimulation during unloaded movements. The best cortical sites for inducing resetting were the same as those from which the largest short-latency responses were evoked in the contralateral forearm flexor and extensor muscles, i.e. the motor cortex. At the cortical sites where magnetic stimulation did induce resetting, the initial electromyographic (EMG) effects consisted of a short-latency excitation followed by a period of inhibition. This silent period was followed by a short burst of excitation often occurring simultaneously in the wrist flexor and extensor muscles, and only thereafter by the return of rhythmical alternating EMG activity characteristic of the wrist movement. The latency to the first rhythmical EMG peak following the stimulus was closely related to the period of the subject's prestimulus movement.