Subjects are stimulated 3 times in a day. Each 20 minutes stimulation are separated by a period of at least 30 minutes. Before and after each stimulation, inhibition is evaluated by cognitive tests (Go Nogo test, Stroop test, Hayling test) and by the BIS 10 scale. During the stimulation, subjects compleat the STAXI and STAI scales. Cognitive tests are repeated 24 hours and 8 days after the stimulation to evaluate duration of the effect. Possible side effects will be notified throughout the protocol.