Participants attended an orientation session to familiarize themselves with the procedures of the study, practice the questions, provide informed consent, schedule their study sessions, and provide a 20-ml (2 × 10 ml) blood sample for genotyping. They were instructed to abstain from taking drugs (including alcohol, caffeine, and nicotine) 24 hr before each session and to fast from midnight the night before the sessions. On arrival at the laboratory for each session, urine was tested for recent drug use and pregnancy, and a breath alcohol level was obtained. Participants were tested individually and remained in a comfortably furnished room with TV and reading material for the 4-hr session. They could watch emotionally-neutral movies and read during the sessions when measurements were not being taken, but they were not allowed to study.
