Conclusive data concerning hemisphere dominance in facial muscle control are not available yet and the role of intracortical circuits in the modulation of resting and active facial muscles is still debated. We proposed to contribute to these issues investigating, in relaxed and active muscle states, bilaterality and symmetry of cortico-facial command and intracortical excitability of the facial motor cortex (FC).

MEPs induced by TMS of the left and right FC were recorded bilaterally from relaxed and active depressor angulis oris muscle (DAO) of 19 right-handed volunteers. Classic paired-pulse TMS paradigm was used to test short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). ANOVA and post hoc t-test were done.

In both relaxed and active muscle states, the contralateral MEP had a significantly shorter latency (p=0.005) and larger amplitude (p<0.02) than the ipsilateral MEP, with no significant differences according to the side of the stimulated hemisphere. Post hoc t test showed, in both muscle states and for left and right FC stimulation, a significant SICI (p<0.01) and ICF (p<0.05).

Data show that cortical command to lower facial muscles is bilateral and asymmetric, with predominance of the contralateral output, irrespective of the stimulated hemisphere. Furthermore, inhibitory and facilitatory intracortical circuits operate similarly in the left and right FC when muscles are relaxed as well as active.