Abstract/Summary

The American Psychiatric Association describes anhedonia as the inability to experience pleasure and interest from normally pleasurable activities. Clinical studies, however, suggest that the idea of anhedonia may be more nuanced than the definition indicates. Therefore, this thesis focused on anhedonia as a dysfunction of the reward system and its sub-components. The overall aim of this thesis was to help understand the symptoms of anhedonia in adolescent depression better to reveal new targets for potential treatment development. The thesis begins with reviews of the current literature on reward processing in depression, particularly in young people, and the relationship between reward processing in the brain and anhedonia (papers 1 & 2). Next, the thesis reports on a study whereby a task was used to measure anhedonia in adolescents with depression, the task was adapted from another on reward and effort learning. This task was used to examine the subcomponents; anticipation, motivation (effort), consummation, and reward and effort learning. Given the onset of COVID�19 in 2020 and 2021, the next piece of work described in the thesis is one on the effects of COVID-19 on mood and anhedonia in adolescents with depression symptoms by ethnicity. In this study internet use was also examined in young people with depression to see if they used it more or less during COVID-19 for mental health information (paper 4). This study used both quantitative and qualitative methods. Participants filled out questionnaires and responded to open-ended questions online as part of the study. As COVID-19 persisted, it was decided to complete the next task-based study also online (paper 5). This study was similar to paper 3 but was adapted for online use. Overall, the findings from the papers in this thesis demonstrate the effects of increased symptoms of anhedonia and depression in adolescents on reward and effort learning behaviour and internet use. Results of the review papers (papers 1 and 2) suggest that a better definition of anhedonia is needed which could lead to new behavioural tasks that allow us to examine anhedonia objectively. Papers 3 and 5 are the outcome of seeking to develop/adapt new tasks that include the sub-components of reward and effort learning. These studies suggest that it is feasible to use primary rewards for studies in depression and that they might be more appropriate than money for adolescents to assess anhedonia and reward processing. Our findings show that subjective liking and wanting of rewards decreases as anticipatory anhedonia increases. Furthermore, reward learning was significantly higher than effort learning across all participants and the learning about rewards was significantly lower when anticipatory anhedonia was higher. When measuring physical effort, we found that individuals with depression (MFQ) and anhedonia (ASA) symptoms were slower to begin the task's high-effort trials and that effort perception taken at the end was positively correlated with willingness to exert effort at the beginning and end of the task. As a result, these studies are significant because they provide conceptual insights into the relationship between anhedonia in young individuals and learning about rewards and the effort necessary to attain them. Finally, Paper 4, contributed to these studies by revealing that young people’s mood was affected by COVID-19 and that the internet was useful for seeking mental health help. This is encouraging as it supports our use of the internet to deliver treatments. Also, as we moved our task online, we show that examining youth behaviour in this way is feasible in young people. To our knowledge, the work described in this thesis is the first to examine reward and effort learning simultaneously in young people with depression symptoms. The findings suggest a differentiation between anticipatory and consummatory responses due to our task designs. It was also shown that reward (p>0.05) and effort (p<0.05) learning differ between depression vs non-depression groups. Moreover, it was shown that subjective anticipatory anhedonia is related to “wanting high reward” and “willingness to exert effort for high reward” which are novel findings of this research. Therefore, in terms of understanding depression better, both our online (studies) and face-to-face reward and effort tasks seem promising ways to increase knowledge. Finally, understanding the link between objective reward/effort processing and anhedonia could provide new targets for treatment development.