Comparing the effects of anodal and cathodal transcranial direct current stimulation of primary motor cortex at varying intensities on motor learning in healthy young adults

Inconsistent results are observed in the effects of transcranial direct current stimulation (tDCS) with different montages on motor learning. This study aimed to compare the effects of anodal and cathodal tDCS (c-tDCS) over primary motor cortex (M1) at different intensities on motor learning in healthy young adults. The participants were randomly divided into: (1) 1 mA M1 c-tDCS, (2) 1 mA M1 anodal tDCS (a-tDCS), (3) 2 mA M1 c-tDCS, (4) 2 mA M1 a-tDCS and (5) M1 sham tDCS groups. The groups received 20-min stimulation with serial reaction time task (SRTT) incidentally, while the tDCS was turned off after 30 s in the sham tDCS group. Response time (RT) and error rate (ER) during SRTT were assessed prior, during and 72 h after the intervention. The results of the paired t-test indicated that online learning occurred in all groups (p < 0.05), except in M1 c-tDCS (1 mA) (p > 0.05). One-way ANOVA analysis also indicated that there were differences in offline learning (RT (F(DF) = 5.19(4); p < 0.001; and ER (F(DF) = 9(4), p < 0.0001) among groups, with more offline learning in 1 mA M1 a-tDCS, 2 mA M1 c-tDCS and 2 mA M1 a-tDCS groups (p < 0.05). On the other hand, the 1 mA M1 c-tDCS group did not indicate any consolidation effect or even a trend toward negative offline learning. M1 a-tDCS with different intensities and also 2 mA M1 c-tDCS may be helpful for the enhancement of motor learning in young healthy adults. This study enhances our understanding of tDCS intensity and polarity effects on motor learning, with potential for optimizing therapeutic protocols.