Topography of excitatory and inhibitory muscle responses evoked by transcranial magnetic stimulation in the human motor cortex.
Transcranial magnetic stimulation of the motor cortex produces a motor-evoked potential (MEP) in the electromyogram followed by a silent period (SP) which is thought to be due to cortical inhibition. In this study, the topography and size of the cortical areas from which an MEP and SP are evoked in the abductor pollicis brevis muscle (APB) of the hand were compared. The SP area was found to be large, encompassing and surrounding the MEP area. These findings infer the existence of an inhibitory surround limiting the excitatory area for APB in the human motor cortex.