More focal is not always better: effects of conventional versus high-definition transcranial direct-current stimulation on implicit motor sequence learning

Conventional transcranial direct-current stimulation (tDCS) delivered to the primary motor cortex (M1) has been shown to enhance implicit motor sequence learning (IMSL). Conventional tDCS targets M1 but also the motor association cortices (MAC), making the precise contribution of M1 to IMSL presently unclear. We aimed to address the roles of these areas by comparing conventional tDCS of M1 and MAC to High-Definition (HD) tDCS, which more focally targets M1. In this sham-controlled, crossover study in 89 healthy adults, we used mixed-effects models to analyze sequence-specific and general learning effects in the acquisition, short- and long-term consolidation phases of IMSL, as measured by the serial reaction time task. Conventional tDCS did not influence general learning, improved sequence-specific learning during acquisition (anodal: M=42.64 ms, sham: M=32.87 ms, p=.041) and deteriorated it at long-term consolidation (anodal: M=75.37 ms, sham: M=86.63 ms, p=.019). HD tDCS did not influence general learning, slowed performance specifically in sequential blocks across all learning phases (all p’s<.050), and consequently deteriorated sequence-specific learning during acquisition (anodal: M=24.13 ms, sham: M=35.67 ms, p=.014) and long-term consolidation (anodal: M=60.03 ms, sham: M=75.01 ms, p=.002). Our findings indicate that generalized stimulation of M1 and MAC enhanced acquisition, but hindered consolidation of IMSL. In contrast, focal M1 stimulation by HD tDCS worsened overall performance, likely due to cathodal inhibition of MAC as induced by the return electrodes. Consequently, this disruption of performance supports the notion that these areas fundamentally contribute to IMSL as an integral part in the cortico-basal ganglia-thalamo-cortical network.