Intracortical inhibition and facilitation in human facial motor area: Difference between upper and lower facial area

Objective: To investigate the intracortical inhibitory and excitatory systems in the motor cortical representation of upper and lower facial muscles. Methods: Paired-pulse transcranial magnetic stimulation (TMS) was applied to 7 healthy volunteers, with the interstimulus interval (ISI) between the conditioning stimulus (CS) and test stimulus, varied from 1 to 20 ms. CS was set at 90% of motor threshold. Muscle evoked potentials (MEPs) were recorded from first dorsal interosseus (FDI), orbicularis oculi (o. oculi) and mentalis muscles. Result: TMS evoked MEPs in o. oculi on both ipsi- and contralateral sides in all subjects. In the paired-pulse study, MEP amplitude in the mentalis decreased at short ISIs of 1-3 ms, followed by increases at 12-20 ms. These effects were similar to those in the FDI. O. oculi did not show a distinct inhibitory period at short ISIs and facilitation at long ISIs was detected but was significantly less than in FDI and mentalis. In o. oculi, there was no significant difference between the effects of ipsilateral and contralateral CS on the MEPs. Conclusion: The bi-hemispheric control of volitional movement and the modulation from brainstem projections appear to markedly influence intracortical inhibitory and excitatory systems in the motor cortical representation of o. oculi. © 2001 Elsevier Science Ireland Ltd. All rights reserved.