After studying the color photo, most people could make out the shapes in the first image. But those prone to psychosis were able to see it better.
Perhaps, then, relying on prior experience can be advantageous in certain circumstances, said senior author Professor Paul Fletcher from the Department of Psychiatry at the University of Cambridge, but might also make one more prone to developing psychosis under the wrong circumstances, such as stress.
Past research has focused on people who see things that aren't there. For example, people prone to hallucinations are more likely to hear snatches of songs in white noise if you tell them it's there, even when the noise is actually just static. In fact, until this study, there was an idea that suggested prior knowledge actually had a lesser effect on people with psychosis, says Chris Frith, an emeritus professor of neuropsychology at University College London who wasn't involved in the current study.
A famous optical illusion test known as the Hollow-Mask Illusion, in which people tend to see a convex face even though the mask is concave, shows that people with schizophrenia are less likely to see the illusion.
In this case, though, "we say they can better spot patterns that ARE there," Fletcher says.
Because vision is a constructive process, it makes sense that the process could be greatly exaggerated to result in hallucinations (one of the key symptoms of psychosis, Fletcher notes).
"Our normal perceptual system is almost on the verge of hallucination all of the time," Fletcher said. "You have to put a little of what you already know to perceive things. I suspect when you see the mouse, it's just a little noise, a flicker or a shadow, and you have an expectation that gives it a mouse-like shape."
It's important to understand, say the researchers not involved with this study, because it could help determine people who are at risk of psychotic disorders.
"We will never be able to fundamentally help this group of patients if we don't understand what mechanisms mediate psychosis. The present study is one of few studies that is focusing on understanding this," Petrovic said.
The study was small -- a total of 34 people participated -- so the next step is to extend the study to a much larger group, Fletcher said.