Control of mirror activity by transcranial direct current stimulation of the supplementary motor area.

Objective: We investigated whether transcranial direct current stimulation (tDCS) applied to the supplementary motor area (SMA) modulates mirror activity induced by transcranial magnetic stimulation (TMS) during a unilateral precision pinch task.

Methods: Healthy subjects performed a left-hand precision pinch task during the delivery of single-pulse TMS to the left primary motor cortex (M1) to evoke mirror activity in the right hand. Anodal, cathodal, and sham tDCS were applied over the SMA via a randomized crossover design. Motor-evoked potentials recorded from the right first dorsal interosseous muscle were used to quantify mirror activity. Pinch performance variability was evaluated.

Results: Although anodal tDCS tended to increase mirror activity, the effect did not differ significantly from that of sham tDCS. In contrast, cathodal tDCS significantly reduced mirror activity compared with the effects of sham and anodal tDCS. Pinch performance variability did not differ among the tDCS conditions, indicating that mirror activity changed independently of motor performance. Resting motor-evoked potential amplitudes elicited by single-pulse TMS were not significantly altered by tDCS targeting the SMA.

Conclusion: Mirror activity is modulated by SMA excitability. The findings indicate that cathodal tDCS applied to the SMA reduces mirror activity, potentially through alterations in cortical network interactions between the SMA and M1.