Silent periods after electromagnetic stimulation of the motor cortex

Electromagnetic stimulation has been used to excite tracts in the human motor cortex, but little has been done to study inhibitory effects. In three normal subjects we have noted that an electromagnetic stimulation applied to the motor cortex during maximum voluntary contraction produces silent periods lasting up to 300 ms in the contralateral abductor pollicis brevis. Duration of the silent period is directly proportional to the intensity of the stimulus. Abductor hallucis has similar silent periods lasting 150 to 200 ms, although proximal upper and lower limb muscles have shorter silent periods. F and H waves are easily elicited during this silent period suggesting that the alpha motoneuron is still excitable. This silent period may result, at least in part, from a synchronous volley of inhibitory postsynaptic potentials induced by the electromagnetic stimulator at the cortical level. This technique may prove useful in further delineating cortical inhibitory function.