Effects of Caffeine on Reinforcement Learning in Healthy Adults Using PET/MRI

Inclusion Criteria: * Age ≥ 18 and ≤ 45. * Habitual caffeine intake ≥ 100 mg and ≤ 450 mg daily. * Non-smokers. * Clinically healthy. * Have normal vision or corrected to normal vision. Exclusion Criteria: * Pregnant or lactating women. * Women using hormonal contraceptives. * BMI \< 18.5 or \> 29.9 * Sleep disturbance or extreme chronotype. * Urine test positive on one of the following substances: benzoylecgonine, morphine, d-Methamphetamine, d-Amphetamine, Benzodiazepines, Secobarbital, Methadone, Buprenorphine Glucuronide, Nortriptyline, MDMA, Oxycodone, PCP, Propoxyphene, and Cannabis/THC * Diagnosis of depression, anxiety, psychosis, or neurologic disorders in the last 5 years. * Heart or cardiovascular diseases. * Diabetes or other metabolic diseases. * Under chronic medications, for instance, painkiller and steroid. * Allergy to lactose (main ingredient of blank control dose) * Incapable to operate the tasks or comprehend the study information in English. General MRI and PET safety exclusion criteria for all subjects: * Metallic foreign bodies such as cardiac pacemakers, perfusion pumps, aneurysm clips, metallic tattoos anywhere on the body, tattoos near the eye. * Pre-existing medical conditions including a likelihood of developing seizures or claustrophobic reactions * Inability to lie flat on scanner bed for about 90 min as assessed by physical examination and medical history (e.g. arthritis) * Recent exposure to radiation (i.e., PET from other research studies) that, when combined with this study, would be above the allowable limits * Pregnancy or breastfeeding: A negative serum or urine pregnancy test is required on the day of the PET procedure * Body weight of \> 300 lbs (weight limit of the MRI scanner table)