Subjects will be excluded if they are unable to undergo MRI scanning due to pregnancy, implanted metallic devices (cardiac pacemaker or neurostimulator, some artificial joints, metal pins, surgical clips or other implanted metal parts) or claustrophobia. Subjects will also be excluded if they have 1) any major medical illnesses to include, but not limited to, uncontrolled high blood pressure or high blood sugar, cardiovascular disease, asthma, peripheral vascular diseases, coagulopathies, history of superficial or deep vein thrombosis, HIV, or other infectious diseases (e.g. Hepatitis B), 2) mood or anxiety disorders, or substance-induced psychiatric disorders, 3) neurological illnesses to include, but not limited to, seizure disorders, multiple sclerosis, movement disorders, or history of head trauma, CVA, CNS tumor, and other conditions that cause neurocognitive sequelae, 4) significant alcohol or other drug use, other than nicotine dependence, 5) if their T1 weighted images reveal gross structural abnormalities and/or, 6)have a history of syncope. Urine pregnancy tests will be performed on all female volunteers of child-bearing age before each experimental session. Subjects who are positive for any drug other than nicotine or marijuana will be excluded. Subjects in nicotine protocols may not be actively seeking or engaged in smoking cessation treatment. If a subject requires change of antipsychotic medications because of exacerbation of psychotic symptoms, the subject will be considered clinically unstable and his/her participation will be discontinued. Patients with Axis-I mood disorders or substance-induced psychiatric disorders are excluded. We also exclude patients with significant alcohol or other illicit drug use, other than nicotine dependence. This is operationally defined as no SCID/DSM-IV Axis-I substance dependence in the past 6 months, and no current substance abuse. Exceptions are occasional use of marijuana.