Cortical transcranial magnetic stimulation of the frontalis muscle: characteristic features and details on motor-evoked potentials of the forehead.

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. Copyright 2001 S. Karger AG, Basel