Why Do We Hallucinate?

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Justine Alford

Guest Author

2903 Why Do We Hallucinate?

For those with certain mental illnesses, like schizophrenia, perceiving things – sights, smells and tastes – that aren’t there can be a common, and often terrifying, experience that comes with psychosis, a loss of contact with external reality. But what underlies these psychotic experiences? A new study is offering us some insight, suggesting that people prone to psychosis display visual perception that favors prior knowledge, rather than the available incoming sensory evidence.

It’s actually perfectly normal for us to use our experiences and knowledge to interpret the world around us – we are not always afforded the luxury of information about our surroundings, like the distance of certain objects. Our visual system overcomes this hurdle by combining ambiguous sensory information with knowledge we have gleaned throughout our lives, helping us to generate a more robust representation of the complex world in which we live.


“Vision is a constructive process – in other words, our brain makes up the world that we ‘see’,” first author Christoph Teufel from Cardiff University said in a statement. “It fills in the blanks, ignoring the things that don’t quite fit, and presents to us an image of the world that has been edited and made to fit with what we expect.”

Of course, such a predictive system is not error-proof, and hallucinations aren’t an entirely uncommon phenomenon. But since they are more prevalent in those suffering psychosis, researchers from the universities of Cardiff and Cambridge set out to examine the idea that these predictive processes could possibly contribute to the emergence of the disorder.

As described in Proceedings of the National Academy of Sciences, the team began their study by looking at how individuals use predictions to interpret incomplete images. Comparing 16 healthy volunteers with 18 individuals who experience early psychotic symptoms, participants were presented with a series of two-tone images containing a person and similar-looking control pictures without an embedded object. They were then asked to state whether or not the image contained a person, easier said than done at first given the ambiguity of the pictures.

Next, they gave the same volunteers a series of complete, full-color images, some of which had been used as templates to generate the black and white pictures. The first task was then repeated, offering participants the opportunity to draw on their experience to make predictions about the images.


As anticipated, the team witnessed a greater improvement in performance the second time around in the group prone to psychosis compared to the control group. According to the researchers, this indicates that those in the clinical group relied more heavily on prior information to interpret the images. 

“It is surprising to find better task performance in people with mental health difficulties since there are many things that can make psychological tasks more challenging for them,” senior author Paul Fletcher told IFLScience. 

When they repeated these experiments with a larger group of 40 healthy individuals, they observed a positive correlation between task performance and their scores on a test of psychosis proneness. Taken together, these findings suggest that visual perception in people prone to psychosis shows a shift towards the use of prior knowledge rather than incoming sensory evidence.

But importantly, Fletcher argues, “the process of psychosis may be understandable not as some major derangement of function but rather as an exaggeration of the normal process, with all of us having the mental apparatus that could lead us to see, hear, feel, taste and smell things that are not actually there.” Why some people are more prone to this than others, he adds, is likely a very complex story, probably due to a whole host of genetic and environmental reasons. 


It would be interesting to follow up this research by looking into possible underlying neurobiological mechanisms for these observed differences, and although Fletcher tells IFLScience his group has already conducted some brain imaging studies, it's too early to draw any conclusions yet. 


  • tag
  • schizophrenia,

  • vision,

  • hallucinations,

  • perception,

  • sensory information,

  • psychosis