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

Motor learning is important throughout human lifespan for 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 processes of motor learning. Weak-current transcranial electrical stimulation (TES) can be applied with the aim of augmenting effects of concurrent motor practice. Two dominant application modalities of TES, transcranial direct or alternating current stimulation (tDCS and tACS), can modulate and entrain endogenous neural activity, respectively. Here, we investigate effects of tDCS and tACS applied during motor practice on core features of motor skill learning, namely speed and accuracy, based on seventy-three studies employing a broad variety of motor tasks. 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 medium positive effects of tDCS applied to target the motor cortex (n=48), cerebellum (n=16), or other cortical areas (n=17, mainly the prefrontal cortex or parietal cortex) on increases in speed and accuracy during motor practice, with smaller effects observed for consolidation between sessions. Multiple practice sessions with concurrent tDCS demonstrated favorable effects. In contrast, tACS yielded negligible effects. In conclusion, we provide evidence of positive effects of applying tDCS but not tACS during motor practice to enhance practice-related improvements and retention of motor skill, speed and accuracy. The accumulation of positive effects with repeated exposure, as well as cross-domain effects, supports the use of tDCS in real-world applications, including neurorehabilitation.