Concurrent action observation but not motor imagery modulates interhemispheric inhibition during physical execution

Motor control relies on an inhibitory connection between the motor cortices of the brain, known as interhemispheric inhibition (IHI). This phenomenon is well established during the execution of unilateral motor tasks. It is unknown if the neurophysiological effects associated with IHI during physical execution (PE) also occur during action observation (AO) and motor imagery (MI) and/or if the addition of these covert processes to PE moderates IHI; speaking to differences in neurophysiology and functional equivalence. Participants (N=23) performed unilateral concentric wrist contractions (50% maximum voluntary contraction) under three conditions: PE alone, concurrent PE+AO, and concurrent PE+MI. To index IHI, we induced an ipsilateral silent period (iSP) and assessed iSP duration during each condition via neuro-navigated single-pulse transcranial magnetic stimulation (TMS) over the ipsilateral motor cortex. Relative to PE alone, iSP decreased during PE+AO, yet only when this condition preceded PE+MI. iSP duration was not modulated during PE+MI. Together, these data suggest that PE+AO promotes bilateral recruitment and interhemispheric cooperation rather than inhibition. AO and MI differentially impact interhemispheric coordination, serving to suppress inhibition only when AO is primed by MI.