The paranoia, or sense of persecution, experienced by some schizophrenics could be due to a problem they have processing contextual information, according to researchers at University College London (UCL).

Researchers at the London university found that schizophrenics are not fooled by visual illusions that easily trick non-schizophrenics.

Volunteers were shown high-contrast black and white patterned images, with sections altered so that the level of contrast is much lower. They were then asked effectively to match the contrast of the altered section to its twin in a line up of otherwise identical shapes.

Schizophrenics find this task relatively easy, because their brain takes no account of the surrounding information when judging the level of contrast in the altered section of the pattern. Non-schizophrenic brains, however, make relative judgments about the altered section, because of the surrounding higher contrast pattern.

Dr Steven Dakin, of the UCL Institute of Ophthalmology, commented: "We often think of people with schizophrenia as not seeing the world the way it really is - for example, during hallucinations - but we have shown that sometimes their vision can be more accurate than non-sufferers."

He explained that people who do well at this kind of task, tend to do so for a very specific reason, while poor performance can be due to a number of different factors.

"Our findings may shed some light on the brain mechanisms involved in schizophrenia," he added.

"Normally, contextual processes in the brain help us to focus on what’s relevant and stop our brains being overwhelmed with information. This process seems to be less effective in the schizophrenic brain, possibly due to insufficient inhibition - that is, the process by which cells in the brain switch each other off."

He suggests that if this is part of a more general problem in dealing with information about context, it could explain why many schizophrenics misinterpret people's actions, and can feel persecuted.

The research is reported in the journal Biology. ®

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