INHIBITORY ACTIONS OF MOTOR CORTEX FOLLOWING UNILATERAL BRAIN-LESIONSAS STUDIED BY MAGNETIC BRAIN-STIMULATION

Noninvasive transcranial magnetic stimulation (TMS) of the motor corte x not only induces short-latency, motor-evoked potentials (MEP) in con tralateral muscles, but also inhibitory phenomena. One type of inhibit ory action appears directly after the MEP in contralateral muscles and can be visualized by blockade of tonic, voluntary electromyographic ( EMG) activity (postexcitatory inhibition, PI). Evidence for a cortical origin of PI, especially in its later part, was derived from double c ortical stimulation in previous studies and is further supported by ex amination of PI in patients with focal hemispheric unilateral brain le sions in the present study. Thirty patients with different sites of va scular or tumour lesions were studied by TMS. In 6 patients with circu mscribed lesions of the primary sensorimotor cortex a significant shor tening of PI to contralateral muscles was observed, In 7 patients with focal lesions of the thalamus or internal capsule, in 6 patients with lesions of the premotor cortex and in 5 patients with lesions restric ted to the parietal or temporal lobe, a significant prolongation of PI to the contralateral muscles was detected. Six patients with transien t ischemic attacks showed either prolongation or shortening of PI. We conclude that PI is predominantly generated in the primary motor corte x, correspondingly its damage causes shortening of PI. In contrast, da mage to brain areas that project to the primary motor cortex is follow ed by prolongation of PI. This remote effect on the primary motor cort ex may result from disinhibition of cortical interneurones.