Know how hallucinations emerge

Hallucinations arise due to an enhancement of our normal tendency to interpret the world around us by making use of prior knowledge and predictions, says a new study.

The tendency to see, feel, smell and taste things that are not actually there -- so-called hallucinations - may be more common among people with the mental condition psychosis, but it may be present in milder form across the entire population, the study 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," said senior author Paul Fletcher, professor at the University of Cambridge.

"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," Fletcher noted.

In order to address the question of whether such predictive processes contribute to the emergence of psychosis, the researchers worked with 18 individuals who had been referred to a mental health service.

They 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 these 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 was very difficult at first.

Participants were then shown a series of full colour 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 experiment showed a larger performance improvement in people with very early signs of psychosis in comparison to the healthy control group.

This suggested that people 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 journal Proceedings of National Academy of Sciences (PNAS).