Sample size: Based on the effects of tDCS on DT walking outcomes in other cohorts [Leite et al. 2014;Zhou et al. 2014], the research group consider a conservative change of 15% in HbO2 levels after tDCS, as compared to sham, 18 subjects per group will provide >80% power. In order to allow for potential inter-subject variability and to address secondary questions (e.g., effect of disease severity), would be to assess 30 participants in each group.