 Participants perform several tasks described below in a practice session and then during five more sessions on separate days. They perform the tasks before and after undergoing transcranial direct current stimulation tDCS plus electrical stimulation ES, and during a procedure that involves sham stimulation. For tDCS, small rubber electrodes are soaked with water and taped to the subjects head, one above the eye and the other on the back of the head. The current passes between the two electrodes. For ES, two pairs of electrodes are attached to the subjects wrist with a paste. A very short pulse of current is passed between the electrodes, creating an electrical field that stimulates the brain. For the sham stimulation, the electrodes are similarly placed, but there is no stimulation. The tasks are: 