Psychotic-like symptomatology and reward responsivity in chronic ketamine and cannabis users

This thesis assesses psychotic-like symptomatology and reward responsivity in chronic users of two illicit drugs, cannabis and ketamine. As use of these drugs is steadily increasing, with cannabis being the most widely used drug worldwide (following alcohol, caffeine and tobacco) and the recent proliferation of ketamine misuse in parts of Asia, Europe and the United States, it is important to examine the effects of their habitual use. While research has linked cannabis use to sub-clinical psychotic-like symptoms, longitudinal studies examining the association between cannabis use, psychotic-like symptoms and transition to psychosis have revealed mixed findings. Additionally, although acute ketamine administration has been shown to produce psychotic-like symptoms in drug-naive volunteers, there has been less research on the effects of chronic ketamine use. Part 1 of the thesis is a literature review investigating the assessment of cannabis use in studies of individuals meeting clinical ‘high risk’ criteria for transition to psychosis. It examines measures of cannabis use, as well as findings regarding the association between cannabis and subsequent conversion to psychosis. It also examines whether such studies measured further significant outcome variables, such as social and role functioning. Finally, the literature review considers the limitations in how cannabis use has been assessed and the implications of this for future research on the extent to which cannabis influences the development of psychotic-like symptomatology and risk of conversion to frank psychosis. Part 2 of the thesis comprises an investigation of symptoms of prodromal psychosis and reward responsiveness in three groups – chronic users of cannabis, ketamine, and healthy controls. This investigation formed part of a joint project conducted with one other trainee clinical psychologist examining the chronic effects of cannabis and ketamine use on psychosis proneness and cognitive functioning. The empirical paper reports a between subjects study, comparing 20 cannabis users, 20 ketamine users and 20 healthy controls on a number of self-report measures indexing depression (BDI-II), psychosis-like symptoms and schizotypy (PQ-B and O-LIFE), and trait anhedonia (TEPS), and on two laboratory-based tasks assessing reward sensitivity (the ‘Probabilistic Reward Task’) and effort-based decision making (the ‘Effort-Expenditure for Rewards Task’). Both drug using groups were found to have higher levels of schizotypy (O-LIFE) and positive psychosis symptomatology (PQ-B) than controls, while group differences were found on the probabilistic reward task, with controls demonstrating greater response bias than cannabis users and greater discriminability than ketamine users. No group differences were found on the effort-based decision-making task. A critical appraisal of the research forms Part 3 of the thesis. It describes the process of working collaboratively on the project rationale and design, reflections on recruiting and working with drug using participants, and thoughts on clinical implications of the project.