TONGUE MOTOR-RESPONSES FOLLOWING TRANSCRANIAL MAGNETIC STIMULATION OFTHE MOTOR CORTEX AND PROXIMAL HYPOGLOSSAL NERVE IN MAN

Surface recordings of EMG responses were performed bilaterally from th e tongue following transcranial magnetic cortex (TCS) and nerve stimul ation (TNS) to characterize the activated corticonuclear pathways and to obtain normative data for a diagnostic use. TCS over the face-assoc iated motor cortex with 1.3 times the response threshold for relaxed m uscles produced bilateral tongue responses with similar latencies and amplitudes for ipsi- (8.3 +/- 1.1 ms, 1.3 +/- 0.7 mV) and contralatera l responses (8.5 +/- 1.0 ms, 1.7 +/- 0.8 mV, n = 20, 10 subjects). In individual subjects maximal ipsilateral and contralateral responses we re elicited by stimulation over about the same cortex area which lay 2 -4 cm lateral and 0-2 cm anterior to the center of the hand motor repr esentation area. Magnetic stimulation of the hypoglossal nerve with 70 % of the maximal stimulator output and a circular coil placed over the posterior lateral skull produced a more proximal nerve excitation tha n electrical stimulation at the mandible, as reflected by the response latencies (3.4 +/- 0.9 ms vs. 2.1 +/- 0.7 ms). The effect of magnetic TNS was independent of the direction of the coil currents. Central mo tor latencies as calculated by subtracting the response latencies afte r TNS from the overall latency after TCS were 4.8 +/- 1.2 ms and 5.0 /- 1.1 ms for ipsi- and contralateral responses, respectively. The fin dings suggest the existence of a direct and fast conducting connection between motor cortex and brainstem tongue motor nuclei on both sides in man. (C) 1997 Elsevier Science Ireland Ltd.