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

Abstract

ObjectiveTo investigate interhemispheric inhibition (IHI) between the facial primary motor cortices (fM1s). MethodsIHI 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. ResultsIHI 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. ConclusionIn 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. SignificanceData suggest that integrated bilateral control of facial muscles occurs mainly at the level of brainstem circuits engaged by corticobulbar output from fM1.