Control of Cognition (Naltrexone, Methylphenidate, and ADHD Study (NMAS))

The goal of this research is to understand the functioning of \'regulatory circuits\' in the brain. We are also interested in how the medications methylphenidate and naltrexone affect these regulatory circuits. This research study will use a scanning technique known as functional magnetic resonance imaging (fMRI) which takes a special kind of picture of the brain that enables us to \"see the brain at work\". Adult participants will be given a one-time dose of either methylphenidate, naltrexone,or placebo before each fMRI scan session. The main purpose for this research is to better understand the functioning of regulatory circuits in healthy individuals and individuals with psychiatric disorders. This research may one day help us develop better treatments for these disorders.