Translated by Rumia Bose
The film “A beautiful mind” ends with the Nobel prize being awarded to John Forbes Nash Jr. (Russell Crowe), the brilliant scientist who, for a large part of his life, suffered from psychoses and hallucinations. As Nash is leaving the venue in Stockholm he sees the Russian agents Charles, Marcee and Parcher. The agents however existed only in his psychotic world as hallucinations. When asked how he could ever believe in their existence he answered: “Because the ideas I had about supernatural beings came to me the same way that my mathematical ideas did.”1. How can this be? How can you see things, whole scenes and events which are not there, and experience them as real?
Is the brain an analysing machine?
Till recently we thought that it worked as follows. You are continuously bombarded with information coming via your eyes, ears, nose, all your sensory organs. From all this information you have to choose what is relevant, what you will react to. All perceptions are – consciously or unconsciously – analysed: what looks good, what was helpful in the past, what do I want to do, what is dangerous, and more. You make choices and react on the basis of this analysis. But this process turns out to be impossible. What should you look at? There is so very much to look at. And even if you knew which way you should be looking, which objects in your field of vision should you be focusing on? Or is it what you hear that is more important? This means that there needs to be a preconceived plan which directs which way you look, what you focus on, what you perceive. What would such a plan look like? It would need to be based on what you like, what you have earlier found to be useful, what you want to do, what you usually consider to be dangerous and much more. Therefore all the above-named aspects of analysing your perceptions need to be included in a master plan beforehand.
No, the brain is a prediction machine!
But what I refer to as a plan is in fact too limited. Within your brain there is a coherent model of the whole world outside you. That model is based on all your previous knowledge, perceptions, needs, feelings, emotions. Based on that model, your brains form a prediction about how the world will look in the following moment. In your brain this prediction is checked against what you perceive. This is very efficient, because if these coincide, then your brains do not have to do anything different. If the perception does not coincide with the prediction then one of two things happens: either your brains adjust the model and the prediction with it, or you change the world in such a way that the perception does coincide with the prediction. Two examples can perhaps help understand this.
It usually takes some effort to identify anything in this illustration2. It is a Dalmatian. Once you know this and have been able to visualise it, on a following occasion you see a dog at once, because you have adjusted your expectation.
Anil Seth demonstrates another example in a TED talk. Without the preceding information on the basis of which you can predict what you are about to hear, it is difficult to isolate words from the noise. Some people do not manage this even after they are told what can be heard. It is not always easy to find this sort of example, because predictions and perceptions are almost always being instantaneously and imperceptibly coordinated.
Hallucinating through sensory deprivation
The correction of predictions by perceptions can go wrong and then you may hallucinate. The simplest example is when you cannot sense anything. If you put someone in a space, wrapped in cotton, in the dark, in absolute silence and free of odour, then it does not take long before the person hallucinates. Your brain always makes predictions, and these begin to lead a life of their own because they can no longer be corrected by perceptions.
Schizophrenia and hallucinations
People with schizophrenia, such as Nash, often suffer from hallucinations. In their case there is nothing wrong with their sensory organs, but if the perception does not coincide with the prediction then their brains attach more significance to the prediction than to the perception. They do not adapt the model of reality in their brains ( sufficiently) on the basis of their perceptions. That starts with seeing a hazy figure or hearing murmurs that are not there. This precursory hallucination then drives the next prediction. And in this way in the course of time entire new plots develop which are indistinguishable for the affected person from real life. The model of reality begins to lead a life of its own which has little or nothing in common with what is actually taking place.
Psychosis proneness and brain development
Psychosis proneness is related to brain development after conception and in childhood3. This usually does not manifest in childhood with hallucinations and other psychotic phenomena. It is however possible to determine psychotic predisposition with subtle tests. These reveal that a psychosis-prone person has a predilection for experiencing normal perceptions as unusual or surprising. This hampers the construction of a stable model of the world with accompanying predictions, because each time something unusual is perceived, the prediction has to be adapted. Later, mostly during or shortly after puberty, this can lead to the perceptions having less and less corrective effect on predictions. As described above, this can lead to hallucinations and other psychotic phenomena. The usual advice to such people is to avoid stress, but I think that they should avoid ( too) surprising sensory perceptions to avoid developing a psychosis. Often this avoidance does not work sufficiently and antipsychotics are required. Antipsychotics work on a brain system that follows the discrepancies between predictions and perceptions and activates mechanisms for correcting these4. This also explains that behavioural adjustment and antipsychotics complement each other. The more a person can avoid surprising sensory perceptions the less antipsychotics he will need.
Do you hallucinate?
Your brains therefore make predictions over what is happening in the world around you, and these are corrected by perceptions of the world entering through your sensory organs. In this way the model in your brain corresponds to reality. When you hallucinate, this correction is missing and what is playing in your head does not match the reality. But because that what is happening in your head does not appear any different from normal, a hallucination is not distinguishable from reality. This means that you can never tell if what is playing in your head is being corrected by your senses. Even if you are not psychosis-prone you never know for sure if you are hallucinating.
Poletti M, Tortorella A, Raballo A (2019): Impaired Corollary Discharge in Psychosis and At-Risk States: Integrating Neurodevelopmental, Phenomenological, and Clinical Perspectives. Biol Psychiatry Cogn Neurosci Neuroimaging DOI: 10.1016/j.bpsc.2019.05.008
Corlett PR, Horga G, Fletcher PC, Alderson-Day B, Schmack K, Powers AR (2019): Hallucinations and Strong Priors. Trends in Cognitive Sciences 23:114–127.
Cassidy CM, Balsam PD, Weinstein JJ, Rosengard RJ, Slifstein M, Daw ND, et al. (2018): A Perceptual Inference Mechanism for Hallucinations Linked to Striatal Dopamine. Curr Biol 28:503-514.e4.
Sterzer P, Adams RA, Fletcher P, Frith C, Lawrie SM, Muckli L, et al. (2018): The Predictive Coding Account of Psychosis. Biol Psychiatry 84:634–643.
Adams RA, Stephan KE, Brown HR, Frith CD, Friston KJ (2013): The computational anatomy of psychosis. Front Psychiatry 4:47.
Hoffman DD (2011): The Construction of Visual Reality; in Blom JD, Sommer IEC (eds): Hallucinations, p. 7-15. Springer Verlag. ISBN 978-1-4614-0959-5
Vaughanbell A (2009): Hallucinations in sensory deprivation after 15 minutes [Internet]Mind Hacks [cited 2019 Jan 28];Available from: https://mindhacks.com/2009/10/…
Corlett PR, Frith CD, Fletcher PC (2009): From drugs to deprivation: a Bayesian framework for understanding models of psychosis. Psychopharmacology (Berl) 206:515–30.
Fletcher PC, Frith CD (2009): Perceiving is believing: a Bayesian approach to explaining the positive symptoms of schizophrenia. Nat Rev Neurosci 10:48–58.
Merckelbach H, van de Ven V (2001): Another White Christmas: fantasy proneness and reports of ‘hallucinatory experiences’ in undergraduate students. Journal of Behavior Therapy and Experimental Psychiatry 32:137–144.
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