1 Probing for hemispheric specialization for motor skill learning : a transcranial direct 2 current stimulation study 3 4

27 Convergent findings point to a left-sided specialization for the representation of learned actions in right28 handed humans, but it is unknown whether analogous hemispheric specialization exists for motor skill 29 learning. Here, we explored this question by comparing the effects of anodal transcranial direct current 30 stimulation (tDCS) over either left or right motor cortex (M1) on motor skill learning in either hand, using a 31 tDCS montage to better isolate stimulation to one hemisphere. Results were compared to those previously 32 found with a montage more commonly used in the field. 6 groups trained for 3 sessions on a visually-guided 33 sequential pinch force modulation task with their right or left hand, and received right M1, left M1, or sham 34 tDCS. A linear mixed model analysis for motor skill showed a significant main effect for stimulation group 35 (left M1, right M1, sham), but not for hand (right, left) or their interaction. Left M1 tDCS induced significantly 36 greater skill learning than sham when hand data were combined, a result consistent with the hypothesized 37 left hemisphere specialization for motor skill learning, but also with possible increased left M1 38 responsiveness to tDCS. The unihemispheric montage effect size was half that of the more common 39 montage, and subsequent power analysis indicated that 75 subjects per group would be needed to detect 40 differences seen with only 12 subjects with the customary bihemispheric montage. 41 42