Anhedonia is a key symptom of major depressive disorder (MDD) and comprises behavioural deficits in three reward processing subtypes: reward liking, reward wanting, and reward learning. However, neuroimaging findings regarding the neural abnormalities underpinning these deficits are complex. We have conducted a systematic review to update, reframe and summarize neuroimaging findings across the three subtypes of anhedonia in MDD. Using PubMed, The Cochrane Library, PsycINFO, and Web of Science databases, we identified 59 fMRI studies comparing participants with current or remitted MDD with controls, using reward processing tasks. For reward liking and wanting, striatal hypoactivation was observed, alongside hypoactivation and hyperactivation across frontal regions. For reward learning, blunted frontostriatal sensitivity to positive feedback was observed. These findings highlight the importance of studying anhedonia not only as a clinical manifestation but also as a neurobiological mechanism underlying depressive disorder and other broader psychiatric conditions. Keywords Anhedonia. Depression. Neuroimaging. fMRI. Reward processing Major depressive disorder (MDD) is both common, with a lifetime prevalence of 16.6% in the USA (Kessler, Petukhova, Sampson, Zaslavsky, & Wittchen, 2012), and consequential, being the second leading contributor to global years lived with disability (YLDs) worldwide (Ferrari et al., 2013). Anhedonia is one of two key symptoms required for a diagnosis of MDD in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5; American Psychiatric Association, 2013), and is defined as 'markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day' (American Psychiatric Association, 2013), and so represents a deficit in reward processing. In a study examining the factor structure for DSM-IV, MDD symptoms in a sample 2,615 army recruits, the best fit for the data indicated that MDD consisted of both a somatic and nonsomatic component (Elhai et al., 2012), and anhedonia had the second highest factor weighting (Beta = 0.76) for the nonsomatic component (after depressed mood), as well as the second highest factor weighting of all symptoms (Elhai et al., 2012). This suggests that anhedonia is a core feature of depression. Anhedonia is a symptom which warrants attention; indeed, reward processing deficits are associated with increased risk of new onset MDD (Rawal, Collishaw, Thapar, & Rice, 2013), anhedonia may precede illness onset, and, moreover, it can often persist past the remission of other depressive symptoms (Schrader, 1997), as do deficits on reward processing tasks (Pechtel, Dutra, Goetz, & Pizzagalli, 2013). Three subtypes of anhedonia In DSM-5, anhedonia comprises deficits in hedonic experience of rewards and motivation for rewards (American Psychiatric Association, 2013). However, reviews have called for research to conceptualize anhedonia as comprising deficits across three partially separable subtypes of reward processing:

ObjectivesChildren exposed to electronic cigarette (e-cigarette) adverts may perceive occasional tobacco smoking as less harmful than children not exposed to e-cigarette adverts. Given the potential cross-cueing effects of e-cigarette adverts on tobacco smoking, there is an urgent need to establish whether the effect found in prior research is robust and replicable using a larger sample and a stronger control condition.DesignA between-subjects experiment with one independent factor of two levels corresponding to the advertisements to which participants were exposed: glamorous adverts for e-cigarettes, or adverts for objects unrelated to smoking or vaping.ParticipantsEnglish school children aged 11–16 (n=1449).OutcomesPerceived harm of occasional smoking of one or two tobacco cigarettes was the primary outcome. Secondary outcomes included: perceived harm of regular tobacco smoking, susceptibility to tobacco smoking and perceived prevalence of tobacco smoking in young people. Perceptions of using e-cigarettes were gauged by adapting all the outcome measures used to assess perceptions of tobacco smoking.ResultsTobacco smokers and e-cigarette users were excluded from analyses (final sample n=1057). Children exposed to glamorous e-cigarette adverts perceived the harms of occasional smoking of one or two tobacco cigarettes to be lower than those in the control group (Z=−2.13, p=0.033). An updated meta-analysis comprising three studies with 1935 children confirmed that exposure to different types of e-cigarette adverts (glamorous, healthful, flavoured, non-flavoured) lowers the perceived harm of occasional smoking of one or two tobacco cigarettes (Z=3.21, p=0.001).ConclusionsThis study adds to existing evidence that exposure to e-cigarette adverts reduces children’s perceptions of the harm of occasional tobacco smoking.