Demographic and clinical variables for all subjects are included in Table 1 and Table 2. The subject groups did not differ with regard to any demographic or clinical variable, and no subject tested positive on the urine toxicology screen for illicit substances, with the exception of marijuana in the case of the smokers. Marijuana smokers had an average age of onset of 14.9 years, smoked an average of 25.6 joints per week and had a mean urinary cannabinoid concentration, normalized to their creatinine level, of 505.8 ng/ml on the day of scanning. One subject within each of the study groups endorsed using tobacco in an occasional fashion. Subjects did not differ on their past use of illicit substances according to their ASI scores, no subject in either group met diagnostic criteria for past abuse or dependence for any illicit substance (other than marijuana in the case of the marijuana smokers), and no subject in either group reported use of any illicit substance greater than 5 times in their lives. However, the chronic, heavy marijuana smokers reported using alcohol more days
