Motor potentials of inferior orbicularis oculi muscle to transcranial magnetic stimulation. Comparison with responses to electrical peripheral stimulation of facial nerve.

Magnetic stimulation at the vertex evoked a motor potential (MP) in the inferior orbicularis oculi muscle of 10 healthy subjects with an onset latency of 8-13 msec. Its amplitude increased and its latency decreased when the muscle was contracted: the latency measured 9.5 +/- 1.3 msec with an intensity of stimulation 10-15% above threshold in the contracted muscle. This MP is secondary to excitation of the motor cortex. With the coil placed over the occipital scalp and the same stimulation intensity, an MP was recorded with an onset latency at 4.5 +/- 0.6 msec. This response reflects the activation of the facial nerve root. The peripheral electrical stimulation of the facial nerve at the mandible angle elicited an MP with an onset latency at 3.5 +/- 0.4 msec. Most records showed the presence of late components at about 30 msec for all types of stimulation.