Abstract
Introduction Cognitive models of psychosis highlight the role of underlying differences in cognitive function and information processing in the development and maintenance of psychotic symptoms. As a result there is now an interest in developing a greater understanding of these cognitive changes, in order to guide the development of evidence-based therapeutic interventions. An influential cognitive model of psychosis suggests that the core underlying cognitive difference in psychosis may be one of altered contextual processing. Recent work has suggested that this may be reflected in differential perception of visual illusions. However, it is not clear if such differences are present early in the development of psychosis. Such differences have also been reported to be associated with cannabis use. Further, it has been suggested that, in addition to being risk factors for the development of psychosis, psychoactive substances such as cannabis may provide a useful model for understanding psychosis. The current thesis thus investigated, in two separate studies: (1) the consequences of cannabis use on contextual visual processing and (2), whether reported contextual processing differences in psychosis are present at illness onset. Study One Two main hypotheses were tested. A. That THC, a key cannabis compound would reduce contextual visual suppression as measured using the Chubb illusion, and that this effect would be reduced via pre-treatment with another cannabis compound, cannabidiol (CBD). B. That THC would transiently induce symptoms of psychosis and that this increase would be reduced via CBD pre-treatment. No evidence was found to support the primary hypothesis. However, the secondary hypothesis was supported by the data. Study Two The primary hypothesis was that contextual visual suppression, again measured with the Chubb illusion, would be reduced in patients with first episode psychosis relative to a control group.Although not significant, the data supported this hypothesis. Discussion The results of Study One indicate that THC does not reduce visual contextual suppression as measured by the Chubb illusion. This is in contrast to evidence from other illusions, and may reflect different neural mechanisms underlying contextual visual processing. However, the study provided clear evidence that THC can induce psychotic symptoms and that this effect can be reduced by CBD pre-treatment. Study Two replicates previous findings of reduced contextual processing in psychosis and provides evidence that this may be present from the onset of illness. These findings are discussed and interpreted with regards to study limitations, clinical implications and future work.
| Date of Award | 2012 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Robin Murray (Supervisor) & Philip McGuire (Supervisor) |