Participants in this study will undergo a medical history, physical examination, blood tests, and an electrocardiogram (EKG). Women of reproductive potential will undergo a pregnancy test. Participants will be given either dextroamphetamine or placebo (an inactive solution) on two occasions separated by at least 3 to 7 days. Participants will then perform neuropsychological tests that will measure attention, problem solving, memory, and ability to complete simple motor tasks.