P 92. Effect of transcranial direct current stimulation (tDCS) on complex motor skill learning

Abstract

Transcranial direct current stimulation (tDCS) applied over primary motor cortex (M1) has been shown to influence motor skill learning in a polarity-dependent manner. Most of the studies used simplified models of motor skill learning with particular emphasis on hand movements with sequential character. However, the effect of tDCS on complex multi-joint learning paradigms has so far not been investigated. The aim of the study was to investigate polarity-specific tDCS effects on motor skill learning and consolidation in a complex whole-body dynamic balance task (DBT). We hypothesized that tDCS over the supplementary motor area (SMA), a region that is known to be involved in the control of multi-joint whole-body movements, will result in polarity specific changes in DBT learning. In a parallel double-blinded design we applied 20 min of either anodal or cathodal tDCS over the supplementary motor area (SMA), while subjects performed a complex whole-body dynamic balancing task (DBT). A reference electrode was attached over the right forehead. For the DBT we used a discovery learning approach in which no information about the performance strategy was provided. Therefore, during the time course of DBT learning, subjects had to discover their optimal strategy to improve task performance. 20 min of anodal tDCS over SMA with a reference electrode (cathode) placed over the right forehead impaired motor skill learning of the DBT compared to sham. This effect was still present on the second day of training. Reversing the polarity (cathode over SMA, anode over right forehead) did not affect motor skill learning neither on the first nor on the second day of training. To better disentangle whether the impaired motor skill learning was due to a modulation of SMA or prefrontal regions, we performed an additional control experiment. Hence, we applied anodal tDCS over SMA together with a larger presumably more ineffective reference electrode (cathode) over the right forehead. Interestingly, this alternative tDCS electrode setup did not affect the outcome of DBT learning. Our results provide novel evidence, that a modulation of the prefrontal cortex by the cathodal (reference) tDCS electrode impairs complex multi-joint motor skill learning. We suggest that inhibition of higher cognitive processes (e.g. strategy finding, feedback processing) rather than excitation of SMA might explain our finding. Hence, future studies should take the positioning of the tDCS reference electrode into account when investigating complex motor skill learning.