Inhibition Transcranial Random Noise Stimulation

subjects are stimulated 3 times in a day by transcranial random noise stimulation (tRNS) (Starstim). 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.