The silent period after transcranial magnetic stimulation is of exclusive cortical origin: evidence from isolated cortical ischemic lesions in man.

In two representative patients suffering from focal isolated ischemic lesions of the arm-associated (patient 1) or leg-associated (patient 2) primary motor cortex excitatory responses (motor evoked potentials, MEPs) and inhibitory phenomena (silent period, SP) following transcranial magnetic motor cortex stimulation (TMS) are demonstrated. Furthermore, supramaximal peripheral nerve stimulations for testing spinal inhibitory actions were performed. Results were compared to a control group of 12 normal subjects. In patient 1, SP induced by TMS in the clinically affected left extensor carpi radialis muscle (ECR) was lacking in the presence of an only marginally reduced MEP and a normal spinal silent period in this muscle. Normal MEPs and SP durations were observed in the right ECR and in the first dorsal interosseus (FDI) and anterior tibial (TA) muscles on both sides. Similarly, in patient 2 a loss of SP induced by TMS in the clinically affected right TA was observed with normal SP durations in the left TA and both FDI muscles and normal MEP amplitudes in all muscles studied. It is concluded that both early and late phases of SP induced by TMS are of cortical origin and generated in the primary motor cortex.