Functional Plasticity Induced by Mirror Training: The Mirror as the Element Connecting Both Hands to One Hemisphere

Background. Mirror therapy (MT) is a promising therapeutic approach in stroke patients with severe hand paresis. Objective. The ipsilateral (contralesional) primary sensorimotor cortex (SMC) and the mirror neuron system have been suggested to play decisive roles in the MT network. The present study investigated its underlying neural plasticity. Methods. Two groups of healthy participants (n = 13 in each group) performed standardized fine motor tasks moving pegs and marbles (20 min/d for 4 days) with their right hand with either a mirror (mirror training group, MG) or a nonreflective board (control training group, CG) positioned orthogonally in front of them. The number of items moved by each hand was tested after each training session. Functional MRI (fMRI) was acquired before and after the training procedure to investigate the mirror training (MTr)-specific network by the analysis of the factors Time and Group. Results. The hand performance test of the trained right hand did not differ between the 2 groups. The untrained left hand improved significantly more in the MG compared with the CG. fMRI analysis of action observation and imitation of grasping tasks demonstrated MTr-specific activation changes within the right dorsal and left ventral premotor cortex as well as in the left SMC (SMCleft). Analysis of functional and effective connectivity showed a MTr-specific increase of functional coupling between each premotor region and the left supplementary motor area, which in turn showed an increased functional interaction with the ipsilateral SMCleft. Conclusions. MTr remodels the motor system by functionally connecting hand movement to the ipsilateral SMC. On a system level, it leads to interference of the neural circuit related to motor programming and observation of the trained hand with the illusionary movement of the untrained hand.