One-dimensional quantification of the cortical representation area of lower lip mimetic muscles by transcranial magnetic stimulation (TMS)

Background: Recent reports as well as results from animal studies indicate changes of cortical organization in patients with facial palsy with a decrease in size of the face-associated cortical representation area. With regard to these alterations, there are no detailed reports in the literature about the possibility of a cortical mapping of mimetic muscles by TMS. Method: In 21 healthy volunteers we investigated the amplitude and the onset latency of motor evoked potentials (MEPs) from the depressor anguli oris and depressor labii inferioris muscles during cortical transcranial magnetic stimulation by use of a figure-8-shaped coil (double 70 mm coil) as a function of the scalp positions stimulated in order to establish a reproducible representation area along the interaural line. Results: Cortical evoked MEPs characterized by onset latencies of 6-30 ms were elicited by contralateral and ipsilateral stimulation from coil positions between 4 and 13 cm lateral to the vertex. Maximal responses (mean amplitude 1.5 +/- 0.8 mV contralateral, 0.8+/-0.5 mV ipsilateral) with shortest mean onset latencies (11.5+/-1.5 ms contralateral, 12.2+/-2.8 ms ipsilatral) were observed at a coil position of 10 cm latral to the vertex. Conclusion: TMS offers a noninvasive method of one dimensional quantification of the cortical representation area of lower lip mimetic muscles.