Abstract
Unilateral movement relies on an inhibitory connection between the hemispheres of the brain, termed interhemispheric inhibition (IHI), resulting in a contralateral pattern of control. Physical execution (PE) and motor imagery (MI; the mental rehearsal of a movement) cause similar patterns of cortical activity, with more bilateral activation evidenced for MI. However, it is unknown whether IHI is integral to both PE and MI of unilateral actions. Here, we compared PE paired with action congruent or incongruent MI to isolate and inform on the neurophysiology of MI. Participants (N = 9, target N = 20) physically performed unilateral concentric wrist extensions (50% MVC) under three blocked conditions: PE, PE+MIcon (congruent MI; execute and imagine extension), and PE+MIincon (incongruent MI; execute extension, imagine flexion). To index IHI, we induced an ipsilateral silent period (iSP) during each contraction through transcranial magnetic stimulation over the ipsilateral motor cortex with electromyography recorded from wrist extensor muscles. Compared to the mean iSP for PE (34.9, SD=10ms), iSP was reduced in both imagery conditions (PE+MIcon, 30.3, SD=9ms, d=.53; PE+MIincon, 32.9, SD=12ms, d=.26). These data suggest that MI (coupled with PE) promotes bilateral recruitment of the motor cortices. Additionally, motor evoked potentials (used to index corticospinal excitability) recorded from the contralateral wrist muscles decreased across the session (post-pre; d=.26), suggesting repetitive actions caused mental fatigue, in line with past work. This work informs on neurophysiology of MI showing interhemispheric cooperation occurs during the performance of MI.
