ii) Abstinent tobacco dependent group: history of or current alcohol abuse or dependence, current consumption outside recommended guidelines >21 units/week for males and >14 for females, or a score of =>8 (indicator of hazardous and harmful alcohol use) as measured using the AUDIT; history of dependence for cocaine, cannabis, opiates or other substance of abuse; history of problem gambling; taking varenicline, bupropion or other prescription medications for smoking cessation. Any previous or current psychiatric diagnosis listed in DSM-V Axis I, which in the opinion of the clinical team will compromise conduct and interpretability of the study.