Mirror illusion reduces motor cortical inhibition in the ipsilateral primary motor cortex 1 during forceful unilateral muscle contractions

26 Forceful, unilateral contractions modulate corticomotor paths targeting the resting, 27 contralateral hand. However, it is unknown if mirror-viewing of a slowly moving but 28 forcefully contracting hand would additionally affect these paths. Here we examined 29 corticospinal excitability and short-interval intracortical inhibition (SICI) of the right30 ipsilateral primary motor cortex (M1) in healthy young adults under a no-mirror and mirror 31 condition at rest and during right wrist flexion at 60% maximal voluntary contraction (MVC). 32 During the no-mirror conditions, neither hand was visible, whereas in the mirror conditions, 33 participants looked at the right hand’s reflection in the mirror. Corticospinal excitability 34 increased during contractions in the left flexor carpi radialis (FCR) (contraction: 0.41 mV vs. 35 rest: 0.21 mV) and extensor carpi radialis (ECR) (contraction: 0.56 mV vs. rest: 0.39 mV) but 36 there was no mirror effect (FCR: P=0.743; 2 =0.005, ECR: P=0.712; 2 =0.005). However, 37 mirror-viewing of the contracting and moving wrist attenuated SICI relative to test pulse in 38 the left FCR by ~9% compared with the other conditions (P<0.05; d≥0.62). 39 Electromyographic activity in the resting left hand prior to stimulation was not affected by 40 the mirror (FCR: P=0.255; 2 =0.049; ECR: P=0.343; 2 =0.035), but increased two-fold 41 during contractions. Thus, viewing the moving hand in the mirror and not just the mirror 42 image of the non-moving hand seems to affect motor cortical inhibitory networks in the M1 43 associated with the mirror image. Future studies should determine if the use of a mirror could 44 increase inter-limb transfer produced by cross-education, especially in patients groups with 45 unilateral orthopaedic and neurological conditions. 46