Differential effects of conventional and high-definition transcranial direct-current stimulation of the motor cortex 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 these areas to IMSL presently unclear. We aimed to address this issue by comparing conventional tDCS of M1 and MAC to 4 * 1 high-definition (HD) tDCS, which more focally targets M1. In this mixed-factorial, sham-controlled, crossover study in 89 healthy young adults, we used mixed-effects models to analyse sequence-specific and general learning effects in the acquisition and 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 seemingly 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 the observed superior conventional tDCS effects on IMSL are possibly attributable to a generalized stimulation of M1 and/or adjacent MAC, rather than M1 alone. Alternatively, the differential effects can be attributed to cathodal inhibition of other cortical areas involved in IMSL by the 4 * 1 HD tDCS return electrodes, and/or more variable electric field strengths induced by HD tDCS, compared with conventional tDCS.