fNIRS : fNIRS will be used to investigate the role of the frontal lobe in DT walking and how it is affected by tDCS [Mirelman et al. 2014]. The fNIRS system (Oxymon MKIII; Artinis Medical Systems) consists of flexible circuit board that carries the near-infrared light sources and detectors. The fNIRS sources and detectors pairs will be placed over the left (Fp1) and right (Fp2) frontal cortex regions of the forehead, as previously reported.