The goal of this study is to investigate the effects of stimulant drugs such as methylphenidate on the effectiveness of sensory reinforcers. We have previously hypothesized that stimulant drugs prevent normally occurring decreases (habituation) in the reinforcing effectiveness of sensory stimuli. We believe that the ability of stimulant drugs to make the effects of sensory reinforcers longer lasting may play and important role in stimulant addiction. This study will be the first to examine the ability of stimulant drugs to make sensory reinforcers longer lasting. Individuals with Attention Deficit Hyperactivity Disorder (ADHD) who are prescribed stimulant drugs to increase attention will be studied. This project is designed to test the hypothesis that stimulant drugs increase attention in ADHD individuals by making the effects of sensory/social reinforcers longer lasting. Funded by RIA's Howard T. Blane Director’s Award for Development of Innovative Research in the Addictions (BDAA), 2014-2015.