Effects of concurrent transcranial direct current stimulation on human motor skill learning: A systematic review and Bayesian meta-analysis.

Motor learning is important throughout the human lifespan for the acquisition, refinement, and preservation of motor skills, and for regaining functional abilities through rehabilitation training. Therefore, it is essential to develop strategies and paradigms to facilitate motor learning. Transcranial direct current stimulation (tDCS) can be applied concurrently with motor practice with the aim of augmenting learning-related processes. Here, we meta-analyzed the effects of tDCS applied during motor practice on core features of motor skill learning, namely speed and accuracy, based on 73 eligible studies identified in our systematic search (with quantitative synthesis focusing on tDCS). We used Bayesian hierarchical random-effects modeling to estimate pooled effects and explore potential moderators, such as number of sessions, stimulation intensity, and outcome measure. We found positive effects of tDCS targeting the motor cortex (n = 46), cerebellum (n = 15), or other cortical areas (n = 11; mainly prefrontal or parietal cortex) on practice-related improvements in speed and accuracy, with smaller effects observed for consolidation between sessions. Multiple practice sessions with concurrent tDCS demonstrated more favorable effects. In conclusion, these findings support that applying tDCS during motor practice can enhance practice-related improvements and, to a lesser extent, retention of motor skill, speed, and accuracy. The accumulation of effects with repeated exposure and cross-task generalization supports the continued investigation of tDCS in real-world applications, including neurorehabilitation.