Cortical Representation of the Orbicularis Oculi Muscle as Assessed by Transcranial Magnetic Stimulation (TMS)

Abstract

To analyze characteristic features and details on motor-evoked potentials (MEPs) of the orbicularis oculi muscle resulting from cortical transcranial magnetic stimulation (TMS) in normal subjects as a basis for further investigations on motorcortical representation in patients presenting with facial nerve diseases. MEPs of the orbicularis oculi muscle resulting from focal cortical TMS with a figure-8-shaped coil were investigated in 17 healthy subjects with special regard to amplitude and onset latency as a function of the coil position on the head surface along the interaural line and in the anterior-posterior direction. The results were then compared with our data on lower-lip mimetic muscles and on the frontalis muscle obtained in previous studies. Bilateral reproducible responses could be observed at coil positions varying from 1 to 13 cm lateral to the vertex. During moderate muscle activation, maximum responses (mean amplitude 0.75 +/- 0.44 mV contralateral, 0.74 +/- 0.36 mV ipsilateral) were obtained 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 associated with a further increase in mean amplitudes. At rest, bilateral responses could be elicited in 15 subjects (88.2%). During moderate muscle activation, the shortest mean onset latencies were obtained at the optimum stimulus 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 orbicularis oculi muscle with those obtained on lower-lip muscles and on the frontalis 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 lower-lip motorcortical areas. A statistically significant separation of the cortical representation areas of forehead, orbicularis oris, and lower-lip mimetic muscles is possible by focal cortical TMS reflecting a kind of somatotopic organization of the face-associated motorcortex. Compared with the results on lower-lip and forehead muscles, orbicularis oculi muscle responses show characteristics of both.