Examining Interhemispheric Inhibition During the Simultaneous Combination of Multiple Action States


Unilateral movement relies on an inhibitory connection between the motor cortices of the brain, referred to as interhemispheric inhibition (IHI). IHI is well established during the execution of unilateral actions. However, it is unknown if and how IHI is moderated when physical execution (PE) is combined with either simultaneous action observation (AO) or motor imagery (MI), informing the neurophysiology of these covert 'action states'. Participants (N=24) performed unilateral concentric wrist contractions (50% MVC) under three conditions: PE, PE+AO, and PE+MI. To index IHI, we induced an ipsilateral silent period (iSP) during each contraction via transcranial magnetic stimulation (TMS) over the ipsilateral motor cortex and with electromyography recorded from extensor carpi radialis. If MI or AO require less lateralization (i.e., bilateral recruitment of the motor cortices), combined conditions should result in a shorter iSP vs. PE. Relative to the mean iSP for PE (30.5, SD=24ms), iSP was reduced only during PE+AO (21.9, SD=17ms, d=.36), not PE+MI (29.0, SD=28ms, d=.06). These data suggest that PE+AO promotes bilateral recruitment (greater 'interhemispheric cooperation'). A preliminary analysis (N=17) of motor evoked potential (MEP) amplitude obtained from electromyography recorded over the ipsilateral hemisphere suggests corticospinal excitability was consistent across the task. Specifically, relative to PE (1535, SD=1370uV), MEP amplitude did not change during PE+MI (1456, SD=313uV, d=.06) nor PE+AO (1492, SD=1216uV, d=.03). Overall, this work expands knowledge of neural mechanisms sub-serving multiple action states and underlying neurophysiology.