Interruption of motor programmes by electrical or magnetic brain stimulation in man.

These experiments describe the effect on voluntary movement of an electrical or magnetic stimulus delivered to the brain through the scalp. Subjects were trained to flex or extend their wrists as rapidly as possible in response to an auditory tone. A single brain stimulus delivered after the tone, and before the usual time of onset of the voluntary reaction, could delay the movement for up to 150 ms, without affecting the pattern of agonist and antagonist EMG bursts. Movement was not delayed when similar experiments were performed with supramaximal stimulation of the median nerve instead of the brain stimulus. The delay following a cortical shock was not due to spinal motoneurones being inaccessible to input during the delay, since H-reflexes given in the middle of the delay period were capable of activating the muscle. Neither could the delay be explained by the brain stimulus altering the time of the subject's intention to respond, since a stimulus delivered to one hemisphere prior to an attempted simultaneous bilateral wrist movement produced a far greater delay on the contralateral than the ipsilateral side. We suggest that the brain stimulus delayed movement by inhibiting a group of strategically placed neurones in the brain (probably in the motor cortex) which made them unresponsive for a brief period to the command signals which initiate agonist and antagonist muscle activity.