Schizophrenia is diagnosed on the basis of symptoms rather behavioural or biological markers. The most typical symptoms are hallucinations (false perceptions) and delusions (false beliefs). Furthermore these symptoms can take many different forms so that patients with the same diagnosis may have no symptoms in common. Given this heterogeneity, it may prove more productive to concentrate on understanding symptoms, rather than schizophrenia in general. However, it is possible consider perceptions within the same framework as beliefs. The current, Bayesian, view is that perception involves the use of sensory information (the evidence) to update our model of the world (our prior belief). Within this framework there are three routes to false perceptions and false beliefs. 1) The sensory information is abnormal; 2) The prior belief is abnormal; 3) There is an abnormality in the way in which the two are integrated. This link between hallucinations and delusions with belief formation suggests that treatment with cognitive behaviour therapy might have a role to play since this treatment is concerned with changing beliefs.

A long-standing view has been that hallucinations and delusions are caused by abnormal sensations and experiences (route 1). This approach has lead to a plausible account of delusions of control (e.g. an external agent is causing my actions) and auditory hallucinations (e.g. I can hear my thoughts being spoken aloud). It is well established that the sensory consequences of our own actions are normally suppressed. This is why we can’t tickle ourselves. There is evidence that this suppression is reduced in patients with schizophrenia. As a result their active movements will feel like passive movements. Given this abnormal sensation it would be reasonable to conclude that one’s movements were being controlled by external forces.

However, more recent experiments suggest that such abnormal sensation is not enough. Patients seem to have abnormal prior beliefs about the presence of agents (route 2). This is most clearly the case for symptoms such as delusions of reference (the false belief that people are communicating with you). Such an abnormal prior belief should lead to problems with mentalising tasks (‘Theory of Mind’) in which it is necessary to ‘read’ the mental states of others. There is now considerable evidence that patients with schizophrenia do have problems with such tasks, but the link with specific symptoms is much less clear. From the analysis above patients with symptoms such as delusions of reference should make errors on ToM tasks due to over-mentalising. There is preliminary evidence that patients do indeed detect evidence of agency which normal controls do not see.

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place: Aarhus University Hospital, Risskov, Skovagervej 2, 8240 Risskov, denmark 

time: 1 pm - 2:05 pm

date: november 9th

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