Inhibitory effects of transcranial magnetic motor cortex stimulation in man

Transcranial magnetic stimulation (TMS) of the human hand-associated primary motor cortex of one hemisphere elicits corticospinally mediated excitatory responses followed by a an inhibition of tonic EMG-activity in contralateral muscles and elicits a transcallosally mediated suppression of motor activity in ipsilateral muscles. The inhibitory stimulation effects can easily be visualized in rectified and averaged EMG traces by the inhibition of tonic voluntary EMG activity. The aim of this review is to characterize the neuronal structures involved in the generation of the contralateral postexcitatory inhibition (PI, syn. contralateral silent period) and ipsilateral transcallosal inhibition (TI, syn. interhemispheric or ipsilateral inhibition) of tonic EMG activity in hand muscles as a basis for an evaluation of this parameters in the diagnostic use of TMS.