All subjects will participate in 5 different randomized sessions on separate days. The first session will be a familiarization with the behavioral tasks. A second experiment will consist of training with no further interventions to obtain baseline UDP changes. In another two sessions, subjects will be premedicated in a blind manner with amphetamine or placebo before administration of somatosensory nerve stimulation followed by motor training to induce UDP. In the last experiment, the participants will be premedicated with amphetamine and will be exposed to sham somatosensory stimulation prior to the motor training to induce UDP.