Schizophrenia: Delusion without illusion

Scientists have discovered that schizophrenia sufferers are not fooled by a visual illusion and are able to judge it more accurately than non-schizophrenic observers. The study by UCL (University College London) and King’s College London suggests that in everyday life, schizophrenics take less account of visual context. If this is part of a more general failure to deal appropriately with context, it could explain why some sufferers might misattribute people’s actions or feel persecuted.

The study, published in the journal Current Biology, used an illusion where an object’s contrast appears reduced by its surroundings. A medium-contrast patterned disc was shown to volunteers, who had to judge its appearance in the presence of a high-contrast background. Of the 15 participants with chronic schizophrenia, 12 were found to make more accurate judgments than the most accurate person in a control group of 33 non-schizophrenic volunteers.

Dr Steven Dakin, of the UCL Institute of Ophthalmology, says: “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.

“The trick here was to use an illusion where a failure to use context worked to the advantage of people with schizophrenia. This was critical because people can perform poorly for lots of reasons, but superior performance tends to be for a specific reason and is more revealing of the underlying cause.

“Our findings may shed some light on the brain mechanisms involved in schizophrenia. 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.”