Paranoid Illusions

Look closely. Schizophrenic observers correctly matched the central circle to a reference disk, but control patients were fooled by the visual illusion.

Schizophrenics can't always distinguish fact from fiction, but a new study shows that they can see right through some visual illusions. And that surprising finding helps bolster one interpretation of the mental illness: that it may be due to a general inability to interpret sensory information in its proper context.

Hallucinations, delusions, and paranoia are several of the well-known symptoms of schizophrenia. But what those diverse cognitive problems may indicate is a general breakdown in the brain's ability to correctly filter a cacophony of incoming sensory data. That may also go for vision: To interpret visual data correctly, the primary visual cortex needs to filter information. When the retina sends an image to the visual cortex, neighboring neurons in the cortex clamor for attention; some inhibit and some enhance each other. The overall balance helps the brain make sense of the flood of incoming information.

So if schizophrenia is a breakdown in an innate ability to process information, neurons in schizophrenics' brains might not correctly suppress each other, says psychologist Steven Dakin of University College London's Institute of Ophthalmology. To test this, Dakin and his team used certain visual illusions that take advantage of the brain's instinct to enhance contrast. The researchers showed 15 schizophrenic subjects and 20 control subjects a shaded patterned disk against a high-contrast background. To normal eyes, that background makes the central disk appear slightly greyer than it actually is. The researchers then assessed the subjects' perception, showing them a series of disks of increasing grayness. For each disk, the observer had to guess whether the patch was of more or less contrast than the original image.

The results were startling: 12 of the 15 schizophrenic observers were more accurate than the most-accurate member of the control group. "The illusion is pretty substantial," Dakin says, but "the schizophrenics were almost completely immune to its effect." Though it's still early, the authors hope their study might have some diagnostic value. Existing criteria for schizophrenia diagnosis tend to be subjective, based on interviews, Dakin says. "We're hoping (this study) might be a step towards more objective diagnostics."

"(These) are very important results with wide implications," says psychiatrist William Phillips of the University of Stirling in the U.K. The findings support the theory that schizophrenics' problems with filtering information may apply to many domains of cognition and perception, he says.

Related sites
Dakin's site
Phillips' site


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