Dopaminergic Modulation of Brain Activation Using Simultaneous PET/Pharmacological MRI

* INCLUSION CRITERIA: Healthy Volunteer Participants * Males or females between 18 and 55 years of age. * Ability to provide written informed consent. * Willing to abstain from drug use on scheduled testing days. * Have or had any prior experience with alcohol use or stimulant drugs including cocaine, methylphenidate, amphetamine or methamphetamine, diet pills (prescription or over the counter), caffeine, and others but did not have a substance use disorder. EXCLUSION CRITERIA: * Pregnant or breastfeeding. Females of childbearing potential must have negative urine pregnancy test and not be currently breastfeeding. Postmenopausal or surgically sterile (tubal ligation or hysterectomy) females satisfy these criteria. * Unwilling or unable to refrain from use within 24 hours of scheduled study procedures: psychoactive medications or medication that may affect study results (e.g., antibiotics (must finish course at least 24 hours prior to a scheduled procedure), antidiarrheal preparations, anti-inflammatory drugs \[systemic corticosteroids are exclusionary\], anti-nausea, cough/cold preparations) (self-report, medical history). The following medications are allowable for entry on this study: analgesics (non-narcotic); antacids; antiasthma agents that are not systemic corticosteroids; antifungal agents for topical use; antihistamines (non-sedating); H2-Blockers/proton pump inhibitors (PPI); laxatives. The use of antihyperlipidemics and/or diuretics are permitted if they have been taken for at least 1 month before procedure visits and dose has been stabilized. The episodic use of benzodiazepines such as alprazolam (Xanax), diazepam (Valium) and lorazepam (Ativan), will not exclude participants from this study unless they have been taken within the last 24 hours prior to the study. * The following current chronically used medications are exclusionary from the study: stimulant or stimulant-like medications (amphetamine, methylphenidate, modafinil); analgesics containing narcotics; anorexics (sibutramine); antianginal agents; antiarrhythmics; antiasthma agents that are systemic corticosteroids; antibiotics; anticholinergics; anticoagulants; anticonvulsants; antidepressants; antidiarrheal preparations; antifungal agents (systemic); antihistamines (sedating); antihypertensives; anti-inflammatory drugs (systemic); antineoplastics; antiobesity; antipsychotics; antivirals (except for treatment of HSV with agents without CNS activity, e.g. acyclovir, ganciclovir, famciclovir, valacyclovir); anxiolytics (benzodiazepine or barbiturates); hormones (exceptions: thyroid hormone replacement, oral contraceptives, and estrogen replacement therapy); insulin; lithium; muscle relaxants; psychotropic drugs not otherwise specified (nos) including herbal products (no drugs with psychomotor effects or with anxiolytics, stimulant, antipsychotic, or sedative properties); sedatives/hypnotics. Note that nicotine and/or caffeine use will not exclude participants. * Current or past Diagnostic and Statistical Manual (DSM)-IV or DSM-5 diagnosis of a psychiatric disorder as determined by history and clinical exam including substance use disorder (except for nicotine/caffeine), alcohol use disorder (or alcohol dependence, if assessed using DSM-IV) anxiety disorder or panic attacks. Past history of a mental disorder as defined by DSM-IV or DSM-5 will be excluded only if it required hospitalization (any length), or chronic medication management (more than 4 weeks), and that could impact brain function at the time of the study. * Those with a binge drinking history every month continuously for the last 10 years will also be excluded. Binge drinkers are those who being female consume 4 or more drinks and males consume 5 or more drinks in one occasion at least once a month. * Major medical problems that can impact brain function at the time of the scan (including but not limited to HIV; glaucoma, central nervous system including seizures and psychosis; cardiovascular including hypertension and arrhythmias; metabolic, autoimmune, endocrine) as determined by history and clinical exam. * Any clinically significant laboratory finding as determined during the screening procedures. * Have had previous radiation exposure (from X-rays, PET scans, or other exposure) that, with the exposure from this study, would exceed NIH annual research limits. * Head trauma with loss of consciousness for more than 30 minutes. * Presence of ferromagnetic objects in the body that are contraindicated for PET/phMRI of the head (pacemakers or other implanted electrical devices, brain stimulators, some types of dental implants, aneurysm clips, metallic prostheses, permanent eyeliner, implanted delivery pump, or shrapnel fragments), fear of enclosed spaces, or other standard contraindication to MRI/magnetic resonance spectroscopy (MRS) (self-report checklist). * Cannot lie comfortably flat on back for up to 2 hours in the PET/phMRI scanner. * Body weight \> 204 kg (\> 450 lbs). * Allergy to methylphenidate. * Clinically significant EKG abnormalities. Clinically significant findings on EKG will be assessed by the Medical Advisory Investigator on the study or through a cardiology consult. EKG reviews are documented in Clinical Research Information System (CRIS). * History of glaucoma as determined by medical history. * NIH employees who are study investigators, as well as their superiors, subordinates and immediate family members (adult children, spouses, parents, siblings). * Non-English speakers (must also be able to read and comprehend English). Subjects will not be excluded from enrollment in this study if their urine test is positive for drugs. However, if they test positive on scheduled study procedure days involving study imaging and neuropsychological testing, the procedures will be postponed and rescheduled. We will allow for up to 3 rescheduled study days that were the result of positive urine drug screens. If the drug test is positive on the third rescheduled visit, the participant will be withdrawn from the study. The intent of the research has no prospect of direct benefit to the subject. Therefore, we are excluding non-English speakers in this research study since it includes the administration of questionnaires, surveys and assessments that are validated for English, although some are available in Spanish. In addition, our fMRI paradigms (particularly the Delay Discounting task) require that the subject be able to speak, read and comprehend English.