Human motor plasticity induced by mirror visual feedback

Abstract

The clinical use of mirror visual feedback (MVF) was initially introduced to alleviate phantom pain, and has since been applied to the improvement of hemiparesis following stroke. However, it is not known whether MVF can restore motor function by producing plastic changes in the human primary motor cortex (M1). Here, we used transcranial magnetic stimulation to test whether M1 plasticity is a physiological substrate of MVF-induced motor behavioral improvement. MVF intervention in normal volunteers using a mirror box improved motor behavior and enhanced excitatory functions of the M1. Moreover, behavioral and physiological measures of MVF-induced changes were positively correlated with each other. Improved motor performance occurred after observation of a simple action, but not after repetitive motor training of the nontarget hand without MVF, suggesting the crucial importance of visual feedback. The beneficial effects of MVF were disrupted by continuous theta burst stimulation (cTBS) over the M1, but not the control site in the occipital cortex. However, MVF following cTBS could further improve the motor functions. Our findings indicate that M1 plasticity, especially in its excitatory connections, is an essential component of MVF-based therapies.

Figure 1.

Task design. In Experiment 1, behavioral and electrophysiological measurements were examined before and after each motor-training intervention (mirror and nonmirror tasks). In Experiment 2, behavioral measurements in both hands were examined before and after the AO task as well as mirror and nonmirror tasks. In Experiment 3, functional interference using cTBS was applied to the right M1 or OC just after the mirror task. Both groups then performed another set of mirror tasks. The motor function of the left hand was measured before and after the first mirror task, after cTBS, and after the second mirror task (Pre, Post1, Post2, and Post3, respectively).

Figure 2.

Effects of mirror and nonmirror tasks on right M1 function. A, The mean number of ball rotations performed by the left hand increased significantly after the mirror task. B, The average MEP of the left FDI (single subject) increased after the mirror task. C, The mean MEP amplitude was significantly enlarged after the mirror task. **p < 0.001.

Figure 3.

MEP amplitudes as a function of TMS intensity preintervention and postintervention. The MEP amplitude of the left FDI significantly increased after the mirror task at 120% of the resting MT in the mirror group. *p < 0.05.

Figure 4.

Effects of AO, mirror, and nonmirror tasks on right and left M1 functions. The mean number of ball rotations increased significantly after the mirror and AO tasks in the left hand, and after the mirror and nonmirror tasks in the right hand. *p < 0.05, **p < 0.001.

Figure 5.

Effects of cTBS on MVF-induced changes. A, For the cTBS at the M1, the mean number of ball rotations performed by the left hand increased significantly after the first and second mirror tasks (Post1 and Post3), but not after cTBS (Post2). For the cTBS at OC, it increased monotonously (Post1, Post2, and Post3 > Pre). B, The mean MEP amplitude showed a similar pattern of behavior, with significant increases at Post1 and Post3 compared with Pre for the M1 group, and at Post1, Post2, and Post3 for the OC group. *p < 0.001.

Figure 6.

Correlation between M1 plasticity and behavioral improvements. The MEP ratio and the change in number of ball rotations were significantly correlated (Pearson's correlation, r = 0.603, p < 0.001).