Improving motor performance without training: the effect of combining mirror visual feedback with transcranial direct current stimulation

Abstract

Mirror visual feedback (MVF) during motor training has been shown to improve motor performance of the untrained hand. Here we thought to determine if MVF-induced performance improvements of the left hand can be augmented by upregulating plasticity in right primary motor cortex (M1) by means of anodal transcranial direct current stimulation (a-tDCS) while subjects trained with the right hand. Participants performed a ball-rotation task with either their left (untrained) or right (trained) hand on two consecutive days (days 1 and 2). During training with the right hand, MVF was provided concurrent with two tDCS conditions: group 1 received a-tDCS over right M1 (n = 10), whereas group 2 received sham tDCS (s-tDCS, n = 10). On day 2, performance was reevaluated under the same experimental conditions compared with day 1 but without tDCS. While baseline performance of the left hand (day 1) was not different between groups, a-tDCS exhibited stronger MVF-induced performance improvements compared with s-tDCS. Similar results were observed for day 2 (without tDCS application). A control experiment (n = 8) with a-tDCS over right M1 as outlined above but without MVF revealed that left hand improvement was significantly less pronounced than that induced by combined a-tDCS and MVF. Based on these results, we provide novel evidence that upregulating activity in the untrained M1 by means of a-tDCS is capable of augmenting MVF-induced performance improvements in young normal volunteers. Our findings suggest that concurrent MVF and tDCS might have synergistic and additive effects on motor performance of the untrained hand, a result of relevance for clinical approaches in neurorehabilitation and/or exercise science.

Keywords: mirror visual feedback; motor learning; primary motor cortex; transcranial direct current stimulation (tDCS).

Fig. 1.

Experimental setup and design. A: participants performed a complex ball-rotation task on two consecutive days (day 1 and day 2). On day 1, participants performed the task first with the left (untrained) hand (L pre) in counterclockwise orientation. Subsequently, the right (trained) hand performed the task for 10 consecutive trials (R1–10) in clockwise orientation while watching the movements of the performing hand in a mirror [mirror visual feedback (MVF), see C]. Total duration of MVF training was 20 min. During MVF, participants either received anodal transcranial direct current stimulation (a-tDCS) or sham stimulation (s-tDCS) over right primary motor cortex (M1). At the end of the training period, the left (untrained) hand was reevaluated in counterclockwise orientation (L post). On day 2 the ball-rotation task was performed again under the exact experimental conditions as on day 1 but without tDCS. B: control experiment. The experimental setup and design was comparable to A. However, participants received no MVF (without MVF) during 20 min of a-tDCS while the right hand was trained for 10 consecutive trials (R1–10). Here, participants were instructed to watch the left stationary hand while the right hand was covered with a box. C: illustration of the ball-rotation task. The left hand (L) was supposed to rotate the balls in counterclockwise orientation. Training of the right hand (R) was performed in clockwise orientation. During training, the right hand was always covered with a box. Groups 1 and 2 received MVF and a-tDCS or s-tDCS during training of the right hand, whereas the control group had to perform the task without MVF but with a-tDCS. See text for details.

Fig. 2.

Behavioral data for the ball-rotation task. Performance of the left (untrained) hand before (L pre) and after (L post) MVF-training with the right hand (R1–10). Results are shown for day 1 and day 2, separated by 24 h (broken line). The gray bar (day 1) indicates the period where either 20 min of a-tDCS + MVF or sham (s-tDCS + MVF) was applied. Please note that tDCS was applied on day 1 only. Application of a-tDCS over right M1 during MVF resulted in superior performance gains for the left (untrained) hand (L pre vs. L post) compared with s-tDCS. The induced effects were specific for the left hand since no such changes were observed for the trained hand. The plot of mean performance and performance range shows mean ± SE values.

Fig. 3.

Absolute improvements in the ball-rotation task for the left (untrained) hand. Please note that a-tDCS + MVF augmented behavioral gains compared with s-tDCS + MVF not only on day 1 but also on day 2. These results indicate that superior performance gains were still present on day 2, 24 h after termination of a-tDCS. Participants that received a-tDCS without MVF showed similar behavioral gains compared with s-tDCS + MVF. n/s, Not significant. Bars represent mean values, and whiskers represent SE values. *Significant between-group differences; #significant within-group comparisons (L pre vs. L post). See text for details.