Hallucinations 'help make sense of world'
Hallucinations, often associated with psychotic disorders, may result from a natural process used by the brain to make sense of the world, say scientists.
The results of a new study support the theory that many people are close to the edge of this aspect of madness at least.
Visions and sounds that do not exist can be generated by the brain's habit of predicting what it expects to experience, filling in missing gaps in reality, the search shows.
It is this ability that allows you to recognise a fast-moving black shape in your living room as the cat, even though it was little more than a blur.
Professor Paul Fletcher, one of the scientists from the Department of Psychiatry at Cambridge University, said: "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 aren't actually there, which is the definition of a hallucination.
"In fact, in recent years we've come to realise that such altered perceptual experiences are by no means restricted to people with mental illness. They are relatively common, in a milder form, across the entire population. Many of us will have heard or seen things that aren't there."
The Cambridge team together with colleagues from the University of Cardiff set up an experiment to see if people with psychotic tendencies are better at mentally filling in missing parts of pictures.
Participants were shown black and white images that looked little more than a collection of lines and blotches until the full colour originals were seen.
Once the complete picture was recorded in the brain it became possible to recognise what the crude black and white outlines were meant to represent.
But not everyone found the task equally easy. People with very early signs of psychosis performed better than volunteers with no symptoms of mental illness.
When the same experiment was conducted with a larger group of 40 members of the general public, a range of ability was seen. Individuals who had higher scores in tests for psychosis-proneness - but no psychotic symptoms as such - stood out.
The research is published in the journal Proceedings of the National Academy of Sciences.
Naresh Subramaniam, also from Cambridge University, said: "These findings are important because they tell us that the emergence of key symptoms of mental illness can be understood in terms of an altered balance in normal brain functions.
"Importantly, they also suggest that these symptoms and experiences do not reflect a 'broken' brain but rather one that is striving - in a very natural way - to make sense of incoming data that are ambiguous."