During transcranial Direct Current Stimulation (tDCS), low-intensity DC current is applied across the scalp to enhance specific performance or training efficacy on a range of complex cognitive tasks; moreover tDCS has been suggested to produce minimal side-effects (undesired cognitive changes). The central premise of this proposal is that tDCS achieves task-specific modulation through a cellular mechanism where only neuronal circuits primed during tDCS (for example by training) are modulated by tDCS, while non-primed mechanisms are not modulated. The specific goal of this proposal is thus to establish a cellular substrate for DCS mediated activation-specific changes. This proposal addresses two independent, but not exclusive, cellular mechanisms: Can DCS lead to plastic changes by direct modulation of neuronal membrane excitability, dependent on action potential activity but not necessarily dependent on synaptic function (Aim 1)? How does DCS modulation of synaptic efficacy depend on ongoing synaptic activity (Aim 2)?