Most people who try psychoactive drugs never become addicted. Theoretically, hypersensitivity to drug rewards and hyposensitivity to non-drug rewards may contribute to the development of drug addiction. In chapter 1, I review this literature, focusing on the psychology and neuroscience of reward processing, in nicotine and cannabis addictions. In chapter 2, using a novel task (the DReaM-Choice), I demonstrate that dependent (n=20), compared with occasional smokers (n=20), had greater motivation for and liking of cigarettes, but displayed little evidence of a difference in non-drug reward processing. Surprisingly, I also show the effects of 12 hour abstinence on reward processing were similar in dependent and occasional smokers. I then report a functional magnetic-resonance-imaging (fMRI) experiment (chapter 3), in which dependent smokers (n=22) had greater behavioural motivation for cigarettes and a stronger neural response to winning cigarettes than occasional smokers (n=20). However, there were no differences between the groups in behavioural or neural processing of the non-drug reward (music). I attempted to lessen the motivation to smoke cigarettes in the study reported in chapter 4, by administering a dopamine D2/3 receptor agonist (0.5mg pramipexole) to both dependent (n=20) and occasional (n=20) smokers. Pramipexole had no impact on motivation to smoke cigarettes, though it did impair reward learning and effort-related decision-making for monetary reward. In chapter 5, I found that, in non-dependent cannabis users (n=17), acutely administered cannabis reduced motivation for monetary reward; an effect which was moderated by the presence of cannabidiol in the cannabis. In a separate study, I demonstrate that dependent cannabis users (n=20) had impaired reward learning, but were not amotivated, relative to non-dependent, drug-using controls (n=20). Finally, in chapter 6, I summarise my findings, discuss their theoretical and clinical implications, consider their limitations and suggest future research directions for the field of reward processing in addiction
