Exclusion Criteria:  claustrophobia history of neurological disease, heart attack, stroke, kidney disease, or myxedema chronic usage of alcohol current usage of psychotropic medication Type 1 diabetes mellitus Regular consumption of insulin, Metformin® or other medications (statins, NSAIDs, beta-blockers, glucocorticoids) that affect glucose and/or insulin utilization. difficulty swallowing pregnancy breastfeeding For PET: research imaging-related radiation exposure that exceeds current MGH Radiology Radiation Safety Commitee guidelines.  Inclusion Criteria:  BMI < 30 20/20 vision or correctable to 20/20 with contact lenses MRI compatible For PET with Optional 150 ml Blood Sampling Only: Must weigh at least 110 lbs to minimize risks per PHRC guidelines.