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The Times (London)

June 26, 2006, Monday

One in five witnesses sees imagined events as reality

By Sam Lister, Health Correspondent

MORE than one in five people would make unreliable witnesses to crime because of the brain's tendency to fill in gaps in memory with imagined scenarios, researchers have found.

A team from University College London has shown that more than 20 per cent of people questioned about events they have seen are unable to distinguish between what they have actually witnessed and other elements invented by the mind.

The research, led by Jon Simons and Paul Burgess at the UCL Institute of Cognitive Neuroscience, suggests the neurological basis for poor witness statements and hallucinations - and why many people have a weaker grip on reality than they think.

The paper, which appears today in the publication NeuroImage, also shows that the areas activated in healthy people while remembering whether an event happened or was imagined are the same as those in dysfunctional people who experience hallucinations.

Dr Burgess, of the Department of Psychology at UCL, said that the work had clear implications for the validity of witness statements.

"In our tests, volunteers either thought they had imagined words which they had actually been shown or said they had seen words which in fact they had just imagined - in over 20 per cent of cases," Dr Burgess said. "That is quite a lot of mistakes to be making, and shows how fallible our memory is - or, perhaps, how slim our grip on reality is."

However, he added that, for the majority of people, a "reality monitoring function" acted as a filter for imagined scenarios. "People confuse what we imagined occurred in a situation - which is related to what we expect to happen or what usually happens - with what actually happened," he said.

"Most of us, though, have a critical reality monitoring function so that we are able to distinguish well enough between what is real and what is imagined and our imagination does not have too great an impact on our lives - unless the reality check system breaks down such as after stroke or in cases of schizophrenia."

Hallucinations are believed to be caused by a difficulty in discriminating information present in the outside world from information that is imagined. In schizophrenia, the difficulty in separating reality from imagined events becomes exaggerated so some people have hallucinations and hear voices that simply do not exist.

These results indicate a link between the brain areas implicated in schizophrenia and the regions that support the ability to discriminate between perceived and imagined information.

In the tests, healthy subjects were shown 96 well-known word pairs such as Laurel and Hardy, bacon and eggs, and rock and roll. The participants were asked to count the number of letters in the second word. Often, the word was not actually shown and the subject had to imagine the word - such as Laurel and ?. Participants were then asked which of the second words they had actually seen on screen and which ones they had only imagined. The subjects' brain activity was observed using fMRI scans while they remembered whether words had been imagined or seen on screen.

When people accurately remembered whether they had actually seen a word or just imagined it, brain activity in the key areas increased - many of which are found in brain area 10, which is involved in imagination and reality checking, develops last in the brain and is twice as big in humans as in other animals. In the people who did not remember correctly, activation in brain area 10 was reduced.