Does anodal tDCS over M1 really enhance motor sequence learning? A non-replication of earlier findings in a double-blind, pre-registered large-sample study in humans.

Background: Transcranial direct current stimulation (tDCS) is one of the most widely used noninvasive neuromodulation methods. Despite its popularity, some recent studies highlighted issues about the reproducibility of earlier reported tDCS results. Until recently, it was assumed that tDCS elicits its neuromodulatory effects by increasing cortical excitability through direct polarization of cortical neurons. However, recent studies have shown that the electric field that reaches the cortex is relatively weak, whereas the electric field in the scalp underneath the stimulation electrodes is sufficiently strong to stimulate peripheral nerves, thereby potentially indirectly affecting cortical excitability and plasticity. Objective: In this study, we aimed to replicate the effect of anodal tDCS in enhancing motor sequence learning and investigate if the effect is caused by the polarization of cortical neurons, or more indirectly through stimulation of peripheral nerve in the scalp, or a combination of both mechanisms. Methods: In a double-blind, pre-registered study including 99 healthy young adults, we investigated the effect of 1mA anodal tDCS over the left primary motor cortex (M1) on motor sequence learning in three serial reaction time task (SRTT) sessions using a between-subjects design. In addition to the standard sham condition, we introduced an additional control condition in which the peripheral input was blocked using the BL10 topical anesthetic gel to investigate the potential contribution of peripheral nerve stimulation in mediating tDCS effects in motor sequence learning. Results: Our results provided evidence of motor sequence learning in all three stimulation conditions (p<0.0001). However, no significant differences were observed among the three stimulation conditions (p = 0.94). Conclusion: We were unable to replicate previous findings indicating significant beneficial effects of tDCS on motor sequence learning. Consequently, we were unable to address our main research question of whether tDCS effects are driven by the resulting electric field in the cortex or by stimulation of peripheral nerves in the scalp. This non-replication of one of the presumably most reliable tDCS effects in a much larger sample size than the original studies, among the findings of comparable studies reporting similar outcomes, should prompt a renewed discussion regarding the efficacy of tDCS as a neuromodulation technique, particularly given the earlier reported concerns about its reproducibility and reliability.