Including preparation, each MRI study will last about 45 minutes and each PET study will last about 3 hours. For each MRI study, participants will be asked to lie on their backs for 15 minutes in the MRI scanner. For each PET study, participants will first be injected with a dose of [11C]raclopride, a radioactive drug used in brain imaging, and will then lie on their backs for 30 minutes in the PET scanner.