Marijuana is the most widely used illicit drug in the United States today, with over 5 million individuals reports use within the last 20 days. Decision making is required for behaviors ranging from simple motor movements to complex reasoning about future consequences. A recent report from our lab has shown that marijuana users have impaired decision making as measured by the Iowa Gambling Task, a laboratory- based decision making task. Marijuana users, as compared to controls, made significantly more risky choices. Furthermore, mathematical modeling of these data has suggested that marijuana users perform poorly because they have deficient cognitive and response style processes. The overall goal of this application, then, is to isolate the specific processes underlying the poor performance of marijuana users by investigating performance on tasks specific to each of these two processes. In the proposed studies recent heavy marijuana users will be compared to non-using controls on performance of an N-back task, a test of working memory and a GO/NOGO task, a test of impulsive response style, as well as the Gambling Task. In addition, functional magnetic resonance imaging (fMRI) will be used to examine brain activity during the performance of these three tasks. Discriminant analyses will be used to distinguish the relationship among performance measures in an effort to identify the root causes of deficient decision-making in this population. We will also relate patterns of brain activity to performance variables and to each other to look for common elements across patterns. This will provide information about the underlying neural processes associated with, chronic marijuana use. Because marijuana is often thought to be less risky than other abused drugs, its potential deleterious effects are often ignored or unrecognized. Chronic marijuana use is therefore a major national public health concern. The present studies seek to identify the nature and sources of deficient decision-making by marijuana users.