The tendency to see, feel, smell and taste things that are not actually there is known as hallucinations. Although hallucinations may be more common among people with the mental condition psychosis, it may be present in milder form across the entire population. A new study has revealed that hallucinations arise due to an enhancement of our normal tendency to interpret the world around us by making use of prior knowledge and predictions.
Senior author Paul Fletcher, professor at the University of Cambridge, said, "In order to make sense of and interact with our physical and social environment, we need appropriate information about the world around us, for example the size or location of a nearby object. Having a predictive brain is very useful - it makes us efficient and adept at creating a coherent picture of an ambiguous and complex world. But it also means that we are not very far away from perceiving things that are not actually there, which is the definition of a hallucination."
In order to address the question of whether such predictive processes contribute to the emergence of psychosis, the research team worked with 18 individuals who had been referred to a mental health service. The researchers examined how these individuals, as well as a group of 16 healthy volunteers, were able to use predictions in order to make sense of ambiguous, incomplete black and white images. The volunteers were asked to look at a series of black and white images, some of which contained a person, and then to say for a given image whether or not it contained a person.
Because of the ambiguous nature of the images, the task appeared to be very difficult at first. Study participants were then shown a series of full color original images, including those from which the black and white images had been derived: this information could be used to improve the brain's ability to make sense of the ambiguous image.
The researchers observed a larger performance improvement in people with very early signs of psychosis in comparison to the healthy control group. This suggested that study participants from the clinical group were indeed relying more strongly on the information that they had been given to make sense of the ambiguous pictures.
The study was published in the Proceedings of National Academy of Sciences (PNAS).