While resting quietly, subjects receive 20 minutes of weak electrical current stimulation or sham stimulation with no current. For the stimulation, two gauze pads soaked with a conducting salt solution are placed on the head-one on the left side and one above the right eye. The current is passed between the pads and may cause an itching or tingling sensation under the electrodes. Before and after the stimulation, the participant's reaction time-tested by moving a finger as fast as possible at the sound of a tone-and mood are evaluated. Some participants also have an electroencephalogram, or EEG (brain wave recording) during the experiment. After the stimulation, participants take two brief tests of thinking speed, and the mood and reaction time tests are repeated.