Does Anodal Transcranial Direct Current Stimulation Over the Primary Motor Cortex Really Enhance Motor Sequence Learning?

Background: Transcranial direct current stimulation (tDCS) has traditionally been assumed to modulate the brain by directly polarizing cortical neurons. However, recent work has suggested that the resulting electric field reaching the cortex is weak, whereas the field in the scalp is strong enough to stimulate peripheral nerves, potentially influencing cortical excitability indirectly. In this study, we aimed to replicate earlier findings that anodal tDCS enhances motor sequence learning (MSL) and to investigate whether the effects are primarily driven by cortical polarization, peripheral nerve stimulation, or a combination of both. Methods: In a double-blind, preregistered study including 99 adults, we examined the effect of 1 mA anodal tDCS over the primary motor cortex (M1) on MSL using a 3-session serial reaction time task (SRTT). Alongside sham, we added an additional control condition to block peripheral nerve input using BL10 gel thereby isolating peripheral from cortical contributions to tDCS effects. Results: We observed significant within- and across-session improvements in MSL in all conditions ( Conclusions: Given the lack of a significant effect of tDCS on MSL, we could not address our main research question regarding the underlying neurophysiological mechanisms. Nonetheless, our findings contribute to the evidence that tDCS motor effects are unlikely to be as large as previously suggested and add to the body of literature that questions the reliability of tDCS as a neuromodulation technique.