Robust enhancement of motor sequence learning with 4mA transcranial electric stimulation

AbstractBackground and ObjectivesMotor learning experiments with transcranial direct current stimulation (tDCS) at 2mA have produced mixed results. We hypothesize that tDCS will boost motor learning provided sufficiently high field intensity on the motor cortex.MethodsIn a single-blinded, between-subject design, 72 healthy right-handed participants received either anodal or cathodal tDCS at 4mA while they learned to perform a sequence of key presses using their non-dominant hand for about 12 minutes. Cathodal stimulation served as an active control for sensation. A separate sham-stimulation group established baseline performance. Gains during practice and rest periods were analyzed (called micro-online and -offline learning). The target for stimulation was identified on the motor cortex using fMRI. After optimization with individual current flow models, we selected a single montage for all 108 participants with 4 frontal and 4 parietal electrodes each drawing 1mA.ResultsWe found significant gains in performance with anodal stimulation (Cohen’s d=0.7). The boost in performance persisted for at least one hour. Subsequent learning for a new sequence and the opposite hand also improved. Concurrent tDCS enhanced micro-offline learning, while subsequent learning relied on micro-online gains. Sensation ratings were comparable in the active groups and did not exceed moderate levels. The new electrode montage achieved a better tradeoff between stimulation intensity and sensations on the scalp as compared to alternative montages.ConclusionThe present paradigm shows reliable behavioral effects at 4mA and is well-tolerated. It may serve as a go-to experiment for future studies on motor learning and tDCS.HighlightstDCS resulted in a lasting boost of concurrent learning with effect size of Cohen’s d=0.7.Subsequent learning was also improved, indicating a form of meta-learning.Detailed analysis of behavior suggests an effect of tDCS on sequence consolidation.A novel electrode montage with 1mA through each of 4+4 electrodes was well-tolerated.