Cortical representation of the orbicularis oculi muscle as assessed by transcranial magnetic stimulation (TMS)

Objectives. To analyze characteristic features and details on motor-evoked potentials (MEPs) of the orbicularis oculi muscle resulting from cortical t ranscranial magnetic stimulation (TMS) in normal subjects as a basis for fu rther investigations on motorcortical representation in patients presenting with facial nerve diseases. Study Design: Ps of the orbicularis oculi musc le resulting from focal cortical TMS with a figure-8-shaped coil were inves tigated in 17 healthy subjects with special regard to amplitude and onset l atency as a function of the coil position on the head surface along the int eraural line and in the anterior-posterior direction. The results were then compared with our data on lower-lip mimetic muscles and on the frontalis m uscle obtained in previous studies. Results. Bilateral reproducible respons es could be observed at coil positions varying from 1 to 13 cm lateral to t he vertex. During moderate muscle activation, maximum responses (mean ampli tude 0.75 +/- 0.44 mV contralateral, 0.74 +/- 0.36 mV ipsilateral) were obt ained at a mean stimulus position of 8.6 +/- 1.6 cm. lateral and 2.0 +/- 2. 2 cm anterior to the vertex for contralateral responses, and of 8.6 2.0 cm lateral and 2.8 +/- 2.4 cm anterior to the vertex for ipsilateral responses , respectively. Voluntary muscle activation by forced eye-closure was assoc iated with a further increase in mean amplitudes. At rest, bilateral respon ses could be elicited in 15 subjects (88.2%). During moderate muscle activa tion, the shortest mean onset latencies were obtained at the optimum stimul us position on the interaural line, both for contralateral (10.2 +/- 1.3 ms ) and ipsilateral (10.6 +/- 1.5 ms) MEPs. Comparing our data on the orbicul aris oculi muscle with those obtained on lower-lip muscles and on the front alis muscle, there was a considerable overlap of coil positions from which reproducible MEPs could be elicited in all three groups of mimetic muscles, but with the orbicularis oculi area being placed between forehead and lowe r-lip motorcortical areas. Conclusions: A statistically significant separat ion of the cortical representation areas of forehead, orbicularis oris, and lower-lip mimetic muscles is possible by focal cortical TMS reflecting a k ind of somatotopic organization of the face-associated motorcortex. Compare d with the results on lower-lip and forehead muscles, orbicularis oculi mus cle responses show characteristics of both.