In tDCS, static weak polarizing electrical currents applied to the scalp penetrate cortical regions of the brain. These currents are believed to preferentially modulate the activity of neurons with axons that are oriented longitudinally in the plane of the applied electric field, producing changes in the activity of individual cortical neurons (Creutzfeldt et al., 1962; Bindman et al., 1962; Purpura & McMurtry, 1965). The induced changes in excitability occur primarily via modulation of voltage-sensitive cation channels (Lopez et al., 1991). Unlike TMS, tDCS does not directly induce cell firing, but rather modulates neuronal activity. Anodal stimulation of the cortex generally increases the excitability of underlying neurons by depolarizing cell membranes, while cathodal stimulation decreases cortical excitability via hyperpolarization (although this is not always the case (Creutzfeldt et al., 1962)). More recent studies have combined tDCS with single-pulse TMS to assess 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
