Participants will undergo two PET scans with [11C]raclopride using a bolus infusion paradigm to assess baseline D2R availability followed by an infusion to assess changes in DA as measured by [11C]raclopride s displacement following an intravenous injection of methylphenidate (MP). They will also undergo a total of 4 MRI scans, two after placebo and two after MP to assess the circadian variations in the sensitivity of the brain reward network. The two [11C]raclopride scans will be done on separate days at least one week apart, one in the morning starting between (7-9 AM) and one in the early evening (5-7 PM) and they will be followed by an MRI scan to assess the sensitivity of the brain reward network under the influence of MP in the morning (9-11AM) and in the evening (7-9 PM). For the Placebo MRI scans participants will be tested either on separate days or on the morning of the evening session or on the evening prior to the morning session. We will use the MRI scans to assess the sensitivity of the brain reward circuit to visual presentation of drug or food cues and to evaluate resting functional connectivity.