The passage of weak DC currents across the head (DC polarization) has been done for centuries with various effects reported. Many of the reports and studies of DC effects in humans are, however, archaic, anecdotal, uncontrolled, or scientifically baseless. Recently, it has been shown by objective means, in controlled experiments, that this type of treatment has robust and lasting effects on the excitability of the motor cortex in healthy humans. In vitro and animal experiments have revealed little about the mechanism of this effect, but neither has convincing evidence of toxicity been found at moderate doses. Therefore, we are becoming interested in whether this type of treatment can be used to improve cognitive processing in individuals with brain lesions, particularly of the prefrontal cortex. The purpose of this project is to prepare the way for clinical trials by beginning to establish the safety of this technique in humans and then to look for preliminary evidence of potentially useful effects on cognition when moderate doses are applied to the prefrontal cortex. First, we propose to test the safety of 20 min of anodal and cathodal DC applied the left prefrontal area, at 1 mV, by looking for clinical-level effects on verbal fluency, cognitive processing speed, EEG, mood, or motor reaction speed after exposure. The duration of exposure in this study will match the time required to gather preliminary efficacy data in healthy controls and patients. Then, in a dose-finding study, we propose to find the anodal DC amperage necessary for producing a clear effect on excitability, as measured by an increase in the amplitude of the motor evoked potential to transcranial magnetic stimulation. Then, if no significant adverse effects are in the first two studies, a preliminary efficacy study will look for differences in these measures between groups receiving anodal, cathodal, and sham DC of the left prefrontal cortex during stimulation.