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 p otentials of lower-lip muscles to cortical transcranial magnetic stimulatio n 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 c m lateral to the vertex. Maximal responses (mean amplitude 1.4 +/- 0.8 mV c ontralateral, 0.7 +/- 0.5 mV ipsilateral) with shortest mean onset latencie s (11.3 +/- 1.6 ms contralateral, 12.1 +/- 2.8 ms ipsilateral) were obtaine d at a stimulus position of 10 cm lateral to the vertex. Cortical maps of m ean amplitude and mean response duration showed inverse U-shaped configurat ion. Furthermore, we observed an additional polyphasic and mostly bilateral response in 16 of the 17 subjects.