Perceiving is believing: a Bayesian approach to explaining the positive symptoms of schizophrenia

Hallucinations and delusions are striking features of schizophrenia that have been difficult to explain. Fletcher and Frith discuss cognitive theories of the positive symptoms of schizophrenia and describe how abnormalities in error-dependent learning could underlie both hallucinations and delusions. 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.