Hallucinations (false perceptions) and delusions (bizarre beliefs) are characteristic symptoms of schizophrenia and other psychotic illnesses.
In order to understand how disturbances in brain function may give rise to these complex symptoms, we require cognitive neuroscientific models of the normal processes that are involved in perception and belief.
Existing models treat perception and belief separately, leading to a need for a two-factor theory proposing that both are deranged in schizophrenia.
We suggest that recent advances invoking Bayesian theory in cognitive neuroscience offer a way of considering perception and belief as arising from the same process: error-dependent updating in a hierarchical Bayesian structure.
Within the framework of this Bayesian model, one can consider both hallucinations and delusions as emerging owing to disruptions in the same updating mechanism, without the need to posit coincident deficits in two separate systems.
According to this model, disruptions in prediction-error firing from lower-level systems in the hierarchy require higher-level systems to reject and change inferences in order to accommodate this error signal.
At lower levels this may lead to false perceptions but, if it continues, new and more bizarre beliefs will emerge because of a continued sense that the world is not well predicted or modelled by previous beliefs.
Advances in cognitive neuroscience offer us new ways to understand the symptoms of mental illness by uniting basic neurochemical and neurophysiological observations with the conscious experiences that characterize these symptoms. Cognitive theories about the positive symptoms of schizophrenia — hallucinations and delusions — have tended to treat perception and belief formation as distinct processes. However, recent advances in computational neuroscience have led us to consider the unusual perceptual experiences of patients and their sometimes bizarre beliefs as part of the same core abnormality — a disturbance in error-dependent updating of inferences and beliefs about the world. We suggest that it is possible to understand these symptoms in terms of a disturbed hierarchical Bayesian framework, without recourse to separate considerations of experience and belief.
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C.D.F. is supported by the Wellcome Trust and the Danish National Research Foundation. P.C.F. is supported by the Bernard Wolfe Health Neuroscience Fund and by the Wellcome Trust. We are grateful to K. Friston and E. C. Johnstone for their comments on an earlier draft of this paper.
- Cognitive behavioural therapy
A form of psychotherapy in which the patient is encouraged to examine the cognitive processes by which they arrive at a particular state of mind, and to change these processes together with the accompanying behaviours that may reinforce them.
- Corollary discharge
The estimate of sensory feedback that is derived from the internal copy of the motor signal (the efference copy).
- Efference copy
An internal copy of a motor signal that can be used to predict the sensory consequences of the movement.
- Latent inhibition
The phenomenon whereby a stimulus that has been previously presented but has not had any predictive value becomes more difficult to associate with an outcome when presented at a later stage at which it does have predictive value. That is, learning related to the pre-exposed stimulus is slow compared with learning related to new stimuli.
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Fletcher, P., Frith, C. Perceiving is believing: a Bayesian approach to explaining the positive symptoms of schizophrenia. Nat Rev Neurosci 10, 48–58 (2009). https://doi.org/10.1038/nrn2536
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