Mirror illusion reduces motor cortical inhibition in the ipsilateral primary motor cortex during forceful unilateral muscle contractions

Forceful, unilateral contractions modulate corticomotor paths targeting the resting, contralateral hand. However, it is unknown if mirror-viewing of a slowly moving but forcefully contracting hand would additionally affect these paths. Here we examined corticospinal excitability and short-interval intracortical inhibition (SICI) of the right-ipsilateral primary motor cortex (M1) in healthy young adults under a no-mirror and mirror condition at rest and during right wrist flexion at 60% maximal voluntary contraction (MVC). During the no-mirror conditions, neither hand was visible, whereas in the mirror conditions, participants looked at the right hand's reflection in the mirror. Corticospinal excitability increased during contractions in the left flexor carpi radialis (FCR) (contraction: 0.41 mV vs. rest: 0.21 mV) and extensor carpi radialis (ECR) (contraction: 0.56 mV vs. rest: 0.39 mV) but there was no mirror effect (FCR: P=0.743; η_P^2=0.005, ECR: P=0.712; η_P^2=0.005). However, mirror-viewing of the contracting and moving wrist attenuated SICI relative to test pulse in the left FCR by ~9% compared with the other conditions (P<0.05; d≥0.62). Electromyographic activity in the resting left hand prior to stimulation was not affected by the mirror (FCR: P=0.255; η_P^2=0.049; ECR: P=0.343; η_P^2=0.035), but increased two-fold during contractions. Thus, viewing the moving hand in the mirror and not just the mirror image of the non-moving hand seems to affect motor cortical inhibitory networks in the M1 associated with the mirror image. Future studies should determine if the use of a mirror could increase inter-limb transfer produced by cross-education, especially in patients groups with unilateral orthopaedic and neurological conditions.