the excitability changes produced by tDCS (Nitsche & Paulus, 2000, 2001; Nitsche et al., 2003, 2005). These studies demonstrated that anodal tDCS significantly increases the size of the TMS evoked MEP, while cathodal tDCS decreases MEP size. Furthermore, these excitability changes persisted after the end of the tDCS stimulation, with the duration and magnitude of the effects varying as a function of the current intensity and duration of tDCS (Nitsche & Paulus, 2000). A subsequent study demonstrated that if tDCS is applied at 1 milli-ampere for at least 9 minutes, the induced excitability changes after cessation of stimulation were long-lasting (90 minutes when anodal tDCS was applied for 13 minutes) (Nitsche & Paulus, 2001). These long-lasting changes are believed to occur at an intracortical level, perhaps mediated through NMDA receptor activity (Liebetanz et al., 2002; Nitsche et al., 2003, 2004b, 2004a, 2005).
