Additional Study Information: In this study, we are trying to learn more about how stimulant medication produces an improvement in emotional lability (frequent mood changes and excessive emotional reactions) in patients with Attention Deficit Hyperactivity Disorder (ADHD.) Although ADHD is primarily associated with inattention, hyperactivity, and impulsivity, emotional ability plays a crucial role in the disorder. The study involves psychiatric evaluations, magnetic resonance imaging (MRI) scans, and treatment with either a medication called Vyvanse (lisdexamfetamine dimesylate) or a placebo (a pill with no active drug ingredient). Vyvanse is approved by the Food and Drug Administration (FDA) for treatment of ADHD. Participants will be compensated for their time.