Subjects will have two fMRI sessions on different days. During each fMRI session, they will perform both the Stop Signal Task and the Emotional Conflict Resolution Task . On the morning of the scan, placebo and Concerta will be administered in a double-blind, crossover design, with order of medication and placebo randomized. The scan sessions will take place between 3 and 7 hours after medication administration. After each scan, the subject and a research assistant will go to a quiet room in the imaging center; the subjects will be given arithmetic level set at a level of difficulty that the subject has mastered. The subject will be given 15 minutes to complete as many problems as possible.