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) of90–130% resting motor threshold (RMT) preceded test motor evoked potentials (MEPs) by 7 interstimulusintervals (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, at8–10 ms ISIs;p= 0.004), but not in DAO at any ISI, instead, there was facilitation at 1–4 ms ISIs and110–130% RMT CS. In the DAO, conditioned responses at 1–4 ms ISIs were significantly larger than bothtest 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 axialmuscles. Control experiments excluded a transcallosal origin of the facilitation observed at the shortestintervals.Significance:Data suggest that integrated bilateral control of facial muscles occurs mainly at the level ofbrainstem circuits engaged by corticobulbar output from fM1.