Optimization of mirror therapy to excite ipsilateral primary motor cortex

Mirror illusion therapy (mirror therapy) is based on the experimental substrate of a visual illusion of active hand movement to excite ipsilateral primary motor cortex (iM1).

We tested whether iM1 excitability could be modulated by enabling or disabling vision of the active hand during mirror therapy.

Motor cortical activations of healthy right-handed participants were identified by magnetoencephalography. Participants performed voluntary index finger extension of their dominant and non-dominant hands, separately, while viewing mirror reflection images of their active hand superimposed upon their hidden inactive hand. This was performed either with vision of the active hand (uncovered viewing condition) or without vision of the active hand (covered viewing condition).

In the covered viewing condition, the iM1could be excited in all participants (n = 10) and this excitation did not differ whether the active hand was the dominant or non-dominant hand. However, in the uncovered viewing condition, dominant and non-dominant hands were able to excite iM1 only in some participants (n = 4 and n = 7, respectively). Moreover, the participants' responses to the illusion validation questionnaire revealed that the covered viewing condition could cause clearer visual illusion for the active hand than the uncovered viewing condition.

Disabling vision of the active hand during mirror therapy was more effective to excite iM1 responses by creating more immersive visual illusion of the active hand.