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 o n motor-evoked potentials (MEPs) of the frontalis muscle due to cortical tr anscranial magnetic stimulation was investigated in 16 healthy subjects. Bi lateral reproducible responses could be observed in all subjects investigat ed at coil positions varying from 2 to 12 cm lateral to the vertex. MEP amp litudes of the frontalis muscle offered no significant side differences nei ther in amplitude nor in onset latency. Despite a considerable overlap, a s tatistically significant separation (p < 0.0001) of the two areas from whic h reproducible MEPs of upper and lower mimetic muscles could be elicited wa s evident for the calculated mediolateral center with the frontalis muscle area being placed more medially, Our findings are in accordance with anatom ical studies indicating a predominance of corticonuclear descending fibers to lower but not to upper facial motoneurons. Copyright (C) 2001 S. Karger AG, Basel.