Hallucinations and delusions: the price we pay for our creative brain
Chris Frith, Wellcome Trust Centre for Neuroimaging at University College London
Abstract
Hallucinations (false perceptions) and delusions (false beliefs) are characteristic symptoms of schizophrenia, but can also be reported by neurological patients (e.g. anosognosia, Capgras syndrome). These abnormalities arise as a result of brain malfunction. It is easy to understand how brain damage can give rise to negative disorders like cortical blindness or agnosia. In these cases the signals from the senses no longer arrive in the higher order association areas for further analysis. However, it is much more difficult to understand how brain damage can create novel experiences. This difficulty is removed when we realise that we have no direct contact with the world and that our perception of the world depends upon our creative brain.
Our picture of the world is built from the crude signals provided by our senses. These signals are not enough on their own. We also need prior expectations. The brain uses a Bayesian approach to make models of the world. On the basis of prior expectations about what we might be confronting, our brain predicts what signals should impinge on our senses. On the basis of the errors in these predictions our brain can improve our model of the world until it becomes very accurate. All this happens rapidly and unconsciously, but the final perception that results from these mechanisms can often be conscious. This final perception is the model of the world created by our brain. The mechanism operates in the same way for beliefs as well as perceptions. Confronted with the new information our brain decides whether this new information requires that our current beliefs about the world should be changed and then, if necessary, changes these beliefs.
This mechanism, through which we can infer hidden states of the world from crude signals, can also be applied to the mental worlds of other minds. We can infer other peoples' intentions and goals by watching their movements and these inferences enable us to predict what people will do next. Of particular importance is our ability to infer when someone is trying to communicate with us. Communication is possible because we can share our models of the world. We can infer what other peoples' models are and compare them with our own. This is neural hermenautics.
But when this mechanism goes wrong we experience hallucinations and delusions. Random activity in primary visual or auditory areas can be turned by the brain into the experience of faces or music. Failure to monitor the errors in our predictions can lead to the false belief that we have carried out actions correctly when we may not have moved at all. The hallucinations and delusions associated with schizophrenia apply to the mental world. In particular, these patients often believe that communication is occurring when this is not the case. Furhermore, communication often fails because of their inability to share models with others and their failure to update beliefs on the basis of errors of prediction. Hallucinations and delusions are the price we pay for our creative brains.