CORTICAL MECHANISMS MEDIATING THE INHIBITORY PERIOD AFTER MAGNETIC STIMULATION OF THE FACIAL MOTOR AREA

We studied the silent period (SP) that interrupts voluntary electromyo graphic activity (EMG) in facial muscles, after transcranial magnetic stimulation (TMS), in normal subjects. High-intensity magnetic stimula tion with a 12-cm round coil centered at the vertex induced a long-las ting SP (215 ms), whereas supramaximal stimulation of the facial nerve only induced a short (< 20 ms) and incomplete EMG suppression, and cu taneous stimuli had no inhibitory effect at all. Cutaneous trigeminal stimulation delivered after TMS evoked blink-like reflexes, showing th at facial motoneurons were not inhibited during the SP. Simultaneous r ecordings from perioral muscles (large cortical representation) and fr om orbicularis oculi and masseter muscles (small cortical representati on) showed SPs of identical duration. Focal stimuli with a figure-of-e ight coil showed that positioning of the coil was critical and that th e optimal scalp sites for evoking the largest motor potentials and lon gest SPs coincided. Low-intensity stimulation occasionally elicited sh ort SPs without a preceding motor potential. We conclude that the SP i nduced in facial muscles by TMS results from the excitation of cortica l inhibitory interneurons surrounding the upper motoneurons. (C) 1997 John Wiley & Sons, Inc.