For the PET scan, a catheter (thin plastic tube) is inserted into an artery in the subject's wrist or elbow crease to collect blood samples during the procedure, and a second catheter is placed in a vein in the opposite arm to inject radioactive tracers. The subject lies on the scanner bed, wearing a special facemask and goggles. The mask helps hold the head still during the scans, and the goggles either block all light or administer bright flashing lights. Radioactive water is injected into the vein, followed by a 1-minute PET scan to measure brain blood flow. This is repeated three more times. Then, a radioactive fatty acid is injected into the vein, followed by a 1-hour PET scan to measure brain phospholipid metabolism. This is repeated once. The images of blood flow and phospholipid metabolism in the different regions of the brain under the conditions of darkness and during visual stimulation provide information on how and where the brain responds to visual stimulation. The entire procedure takes about 3 hours.