The purpose of this study is to evaluate the effects of Δ-9-tetrahydrocannabinol (THC) on dopamine function in the brain. THC is the active ingredient of marijuana, cannabis, ganja, or “pot.” Dopamine is a chemical found in the human brain. We will measure the effects of THC on dopamine function by taking brain scans (pictures of the brain) that show chemical activity. These brain scans are taken using Positron Emission Tomography (PET). Pictures of brain structure will be taken by using a Magnetic Resonance Imaging (MRI) scan. The MRI and PET scans will take place at North Shore University Hospital.