Characterize The Modulatory Effects Of Dopamine D2/D3 Receptor Agonist And Antagonist Drugs On Compulsive Behaviors

Inclusion Criteria: * Male or female, between 18 - 55 years of age; the groups will be matched for either handedness. * Participants must have the ability to comprehend the key components of the consent form and provide informed consent. * Participants must lead and write (in English) at a level sufficient to complete study related assessments. * Assessment by a psychiatrist or psychologist, which includes a face-to-face evaluation of the individual using the DSM-VI diagnosis. * No history of neurological disorder, head/brain injury, hepatitis, or visual impairment. * No MRI contra-indications (metal in body, claustrophobia) and able to provide blood samples (venous accessibility, especially relevant for drug users). * Patients with obsessive-compulsive disorder will have a minimum 2-year history of compulsive behaviours satisfying DSM-IV-TR criteria for OCD. * Participants with chronic stimulant use will have a minimum 2-year history of dependence on class A stimulants, with age of drug abuse onset before 20 years, and will satisfy DSM-IV-TR criteria for dependence on stimulant drugs. * Control volunteers have to be in good mental and physical health. Exclusion Criteria: * A personal history of psychiatric or neurological disorders, as defined by the DSMIV (except OCD in patients with OCD and substance dependence in drug users) * A history or presence of alcohol / substance abuse or dependence (other than nicotine), as defined by the DSM-IV-TR (except drug dependence group). * A BDI-II total score greater than 14 will lead to exclusion from the study. * Treatment with methadone or buprenorphine may interfere with the experimental tasks, and therefore, will lead to exclusion from the study. * Participants who have any laboratory abnormality that in the investigator's judgement is considered to be clinically significant and could potentially affect subject safety or study outcome. * History of clinically significant or current renal dysfunction. * Clinically significant abnormalities in hematology, blood chemistry, MRI, urinalysis or physical examination not resolved by baseline visit. * Impaired liver function at baseline or history of liver dysfunction. * Female participant is pregnant or currently breastfeeding. * Any serious medical disorder or condition that would in the Investigator's opinion, preclude the administration of study medication and or a history of clinically significant hepatic, cardiac, renal, neurologic, cerebrovascular, metabolic or pulmonary disease.