Optical illusions that fool most people don't seem to trick those who suffer from schizophrenia, concludes a study published in the latest issue of Current Biology. The success may actually be linked to a weakness in a brain mechanism called contextual processing, which is responsible for picking out relevant sensory information from the barrage of stimuli a person constantly experiences. If that's the case, it may explain why some schizophrenics misunderstand other people's actions in the context of a situation or feel paranoia or persecution.

Because vision depends on low-level contextual processing, the researchers, led by Steven Dakin of University College London, devised an experiment to test a person's ability to discriminate one contrasting pattern from another. A disc filled in with a medium-contrast pattern was placed in the center of a larger disc that had a high-contrast pattern. When placed one on top of the other, the difference in contrast appears negligible, when it is really 40 percent. The researchers hypothesized that schizophrenics would not judge the center disc in context of the larger one and therefore not recognize the visual distraction that creates the illusion. In fact, 12 out 15 schizophrenics more accurately judged the contrast of the center disc than did a group of 20 participants who do not suffer from the illness.

"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," Dakin observes. The mechanism has more to do with vision than with cognition, such as attention span, the researchers report. The next step, they say, is to determine whether the low-level contextual processing pervades other sensory responses as well as language and memory.

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