Tongue representation in the lateral cortical motor region of the human brain as assessed by transcranial magnetic stimulation

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 I I 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.