EP 129. The effect of tDCS over M1 prior to and during implicit motor sequence learning

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Motor learning is an essential skill allowing the acquisition of new movement patterns. Transcranial direct current stimulation (tDCS) has been shown to modulate neuronal excitability along with behavioural performance in a polarity-dependent manner. Besides polarity, the effects of tDCS on motor learning also vary with the timing of stimulation. In order to gain a better understanding regarding the optimal timing of tDCS application to modulate implicit motor sequence learning, motor-cortical tDCS was applied prior to and during training on a serial reaction time task (SRTT). The SRTT employs a fixed sequential pattern of button presses allowing the assessment of implicit motor sequence learning. A random pattern served as control condition. 36 healthy subjects were assigned to one of two experiments: tDCS was applied prior to (experiment 1) or during (experiment 2) SRTT training. Anodal vs. cathodal vs. sham tDCS was applied to the left primary motor cortex (M1) for ten minutes in a counterbalanced order. Reaction times of the right hand were measured at Baseline and after SRTT training (End of Acquisition (EoA)). Anodal tDCS prior to SRTT training yielded a beneficial effect on the acquisition of the motor sequence, i.e. reaction times decreased significantly from Baseline to EoA in the sequential pattern. Moreover, at EoA, reaction times were significantly faster following anodal as compared to cathodal tDCS. Reaction times of the random pattern were not differentially modulated by stimulation polarity indicating a sequence-specific effect. In contrast, tDCS applied during SRTT training did not differentially modulate the acquisition of the motor sequence. These results indicate that the timing of tDCS is a crucial parameter yielding distinct effects on implicit motor sequence learning. Anodal tDCS prior to SRTT training resulted in a facilitation emphasising a beneficial effect of increased left M1 excitability for the subsequent acquisition of a new motor sequence. In contrast, modulation of motor-cortical excitability by tDCS during SRTT training yielded no differential effects. This finding implies that tDCS effects may vary with the activation level during stimulation (rest vs. movement).