Introduction
Effective stroke management is important as the treatment of stroke consumes a significant amount of resources.1 Functional recovery can occur even in the chronic stage of stroke with plastic changes induced in the affected primary motor cortex (M1) by intense motor training of the paretic hand.2,3 Additional commitment of noninvasive stimulation over the affected M1 by repetitive transcranial magnetic stimulation (TMS)4 or by transcranial direct current stimulation5 promotes functional recovery in association with plastic changes within the affected M1. However, the effectiveness of such strategies is limited to patients with mild to moderate hand paresis. Thus, an effective therapy regime for patients with severe hand paresis or loss of hand function is still lacking.
Mirror therapy (MT) is a promising therapeutic approach in stroke patients, even in those with complete loss of hand function. By using a mirror that is positioned orthogonally in front of the patient, the reflection of the right arm in the mirror provides an illusion that the left arm is being moved. MT was first described by Ramachandran and colleagues6 in patients with phantom pain after limb amputation and has been suggested to improve hand functional deficit in stroke patients.7 Ramachandran and Altschuler8 suggested that the ipsilateral (contralesional) corticospinal tracts from the M1 play a role while training with a mirror,8 but a recent functional MRI (fMRI) study reported an activation shift toward the ipsilesional sensorimotor cortex (SMC) after MT in chronic stroke patients in contrast with a control group.9 It has also been suggested that the mirror neuron system (MNS) plays an important role while observing one’s hand in the mirror and training via action observation (AO) and imitation (IMI).8,10
The underlying neural plasticity of MT is currently unknown. Therefore, in the present study, fMRI was used to address this issue. We focused on training-induced plasticity by training with a mirror rather than the direct effect of observing one’s hand in the mirror, as was investigated with the use of functional imaging11,12 or TMS.13-15