Abstract
Motor evoked potentials (MEPs) of the lingual muscles elicited by focal cortical transcranial magnetic stimulation (TMS) with a figure 8-shaped coil were investigated in 17 healthy subjects with special regard to amplitude and onset latency as a function of the coil position on the head surface. Bilateral reproducible responses could be observed at coil positions mostly varying from approximately 6 to 13 cm lateral to the vertex. During moderate muscle activation, maximum responses were obtained at a mean stimulus position of about 11 cm lateral and 3 cm anterior to the vertex with similar onset latencies, but with significantly higher amplitudes on the contralateral side (3.5 ± 1.9 mV, 9.5 ± 1.1 ms contralateral; 2.6 ± 1.5 mV, 9.7 ± 1.0 ms ipsilateral). Comparing our data on the orbicularis oculi muscle with the results obtained on lower lip muscles, we found a considerable overlap of those coil positions from which reproducible MEPs could be elicited in both groups of target muscles, but the lingual area was placed more laterally. Thus, a statistically significant separation of the cortical representation areas of lingual and lower lip mimetic muscles is possible by focal cortical TMS, reflecting somatotopic organization of the face-associated motor cortex.
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