Each PET scan session will last approximately 3 hours. Each subject will receive a total of four [15O]H20 injections to measure regional cerebral blood flow (rCBF), and two [11C]AA infusions to measure incorporation k* for AA during a single PET scan session. He/she will have an arterial catheter and venous line inserted during the entire session, and one transmission scan at the beginning of the session. The order of the scans will be randomized. The order of 4 blood flow scans will be: Rest-Photic Activation-Photic Activation-Rest OR Photic Activation-Rest-Rest-Photic Activation. The order of two [C11]AA scans will be Rest-Photic Stimulation Or Photic Stimulation-Rest.