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 decre ase in size of the face-associated cortical representation area. With regar d to these alterations, there are no detailed reports in the literature abo ut the possibility of a cortical mapping of mimetic muscles by TMS. Method: In 21 healthy volunteers we investigated the amplitude and the onset laten cy of motor evoked potentials (MEPs) from the depressor anguli oris and dep ressor labii inferioris muscles during cortical transcranial magnetic stimu lation by use of a figure-8-shaped coil (double 70 mm coil) as a function o f the scalp positions stimulated in order to establish a reproducible repre sentation area along the interaural line. Results: Cortical evoked MEPs cha racterized 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 positio n of 10 cm latral to the vertex. Conclusion: TMS offers a noninvasive metho d of one dimensional quantification of the cortical representation area of lower lip mimetic muscles.