Our observations point to two different cortical motor representation areas, with the cricothyroid muscle-related area being located more medially.

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.

The effect of the coil position along the interaural line on motor evoked potentials of lower-lip muscles to cortical transcranial magnetic stimulation was investigated in 17 healthy subjects. Using a figure-8-shaped coil, we observed ipsi- and contralateral middle-latency motor evoked potentials in all subjects, when the coil was centered within an area between 4 and 13 cm lateral to the vertex. Maximal responses (mean amplitude 1.4 ± 0.8 mV contralateral, 0.7 ± 0.5 mV ipsilateral) with shortest mean onset latencies (11.3 ± 1.6 ms contralateral, 12.1 ± 2.8 ms ipsilateral) were obtained at a stimulus position of 10 cm lateral to the vertex. Cortical maps of mean amplitude and mean response duration showed inverse U-shaped configuration. Furthermore, we observed an additional polyphasic and mostly bilateral response in 16 of the 17 subjects.

The cortical representation of the abductor pollicis brevis muscle (APB) was investigated by transcranial magnetic stimulation (TMS) along the interaural line on the head surface in 17 healthy subjects. Comparing amplitudes of motor evoked potentials (MEPs) with those obtained by cortical TMS of lower-lip muscles, we found a considerable overlap between the areas from which reproducible MEPs of APB and lower-lip muscles could be elicited. However, there was a statistically significant separation of the two areas, the APB area being placed more medially. With regard to short examination procedures it can be concluded that a separation between distal arm and facial muscles is possible not only by two-dimensional cortical mapping procedures, but also by one-dimensional cortical TMS.

The effect of coil position on the head surface along the interaural line on motor-evoked potentials (MEPs) of the frontalis muscle due to cortical transcranial magnetic stimulation was investigated in 16 healthy subjects. Bilateral reproducible responses could be observed in all subjects investigated at coil positions varying from 2 to 12 cm lateral to the vertex. MEP amplitudes of the frontalis muscle offered no significant side differences neither in amplitude nor in onset latency. Despite a considerable overlap, a statistically significant separation (p < 0.0001) of the two areas from which reproducible MEPs of upper and lower mimetic muscles could be elicited was evident for the calculated mediolateral center with the frontalis muscle area being placed more medially. Our findings are in accordance with anatomical studies indicating a predominance of corticonuclear descending fibers to lower but not to upper facial motoneurons.