Elton Ngan

Recent cognitive research has made important contributions to our understanding of delusions, but there have been few attempts to generate a general model that address both delusion formation and maintenance, with a focus on the mechanisms that may underlie the cognitive characteristics of delusions. This thesis describes studies replicating and extending our understanding of two previously identified cognitive biases, and then outlines the development and initial testing of a new model of delusions.The Dual-Stream Modulation Failure (DSMF) model suggests that delusions are the result of an imbalance between two streams of reasoning: Stream 1, which is automatic and intuitive, and Stream 2, which is slower and more deliberative. The degree to which each stream is favoured in a given situation may depend on two modulators: conflict and emotion. Cognitive conflict may cause an individual to consider the available evidence more carefully, while emotional salience may favour reflexive, non-deliberative processing. In schizophrenia, conflict modulation failure (CMF) and/or accentuated emotional modulation (AEM) may result in an under-recruitment of Stream 2 processing and over-reliance on Stream 1, increasing the likelihood that erroneous interpretations will form and be maintained.The first study employs a variation of a classic probabilistic reasoning paradigm to provide new insights into to the jumping-to-conclusions and over-adjustment biases. The second study describes the first multivariate analysis of bias against disconfirmatory evidence (BADE) data using all available information from the task to aid interpretation of the cognitive underpinnings of the evidence integration necessary in the BADE task. The third and fourth studies investigate the DSMF model of delusions using a conditional reasoning paradigm that places content believability in conflict with logical validity. Support is provided for CMF, with the schizophrenia group showing a greater drop in performance and a significantly smaller increase in activity in areas of the brain associated with conflict and deliberative processing for conflict compared to non-conflict stimuli. No evidence was found for AEM, suggesting that future DSMF research may either need to make adjustments to the experimental paradigm or to the model itself.

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