Transcranial cortical magnetic stimulation of lower-lip mimetic muscles: effect of coil position on motor evoked potentials.

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.