Lack of evidence for interhemispheric inhibition in the lower face primary motor cortex

Objective: To investigate interhemispheric inhibition (IHI) between the facial primary motor cortices (fM1s). Methods: IHI was investigated in 10 healthy subjects using paired-pulse TMS in the depressor anguli oris (DAO), upper trapezius (UT) and first dorsal interosseous (FDI) muscles. Conditioning stimuli (CS) of 90–130% resting motor threshold (RMT) preceded test motor evoked potentials (MEPs) by 7 interstimulus intervals (ISIs) ranging 4–12 ms. In the DAO, we also examined IHI at 1–2 ms ISIs. Results: IHI was detected in the UT (CS 130% RMT; ISI 8 ms; p = 0.02) and FDI (CS 120% and 130% RMT, at 8–10 ms ISIs; p = 0.004), but not in DAO at any ISI, instead, there was facilitation at 1–4 ms ISIs and 110–130% RMT CS. In the DAO, conditioned responses at 1–4 ms ISIs were significantly larger than both test MEPs and the response induced by the CS alone. Conclusion: In the DAO there was no evidence of IHI even though this was clear in hand and axial muscles. Control experiments excluded a transcallosal origin of the facilitation observed at the shortest intervals. Significance: Data suggest that integrated bilateral control of facial muscles occurs mainly at the level of brainstem circuits engaged by corticobulbar output from fM1.