Discussion
upper-limb motor learning
CORTEX
This prospective, hypothesis-driven, double-blind, randomized controlled trial demonstrated that tDCS over the left DLPFC significantly enhanced early-phase upper-limb motor learning in both the left-handed simple peg and assembly tasks, which was the result anticipated by our hypothesis. Importantly, this study fulfilled the suggested criteria in a recent consensus and critical position article on tDCS [In the offline assessment, the post-hoc test revealed that active tDCS significantly improved learning in the assembly task but did not have an effect in the simple peg task. A meta-analysis reported that tDCS over the DLPFC significantly improved offline working memory performance in healthy cohorts; however, no effect was observed in online performance [Regarding the effect size, our results of two PPT tasks were far above the size reported for anodal tDCS on the primary motor cortex comparing sham tDCS reported in a previous meta-analysis in upper limb dexterity motor learning (0.04 (95% CI 0.01–0.07) [In addition, normalized Z-scores were calculated to elucidate whether tDCS elicited a larger learning gain in tasks with a high cognitive demand. Larger learning gain in the active tDCS group was observed in the assembly task compared to that in the simple peg task at both online and offline sessions, as hypothesized. Successful completion of the assembly task requires higher cognitive demand, action planning [In this study, there was no significant relationship between baseline performance and tDCS-induced learning gain in either of the two tasks. Previous studies [