Do We All Hallucinate?

Every year around this time, mice find their way into our 110-year-old home. They’re sneaky, of course, so a pretty rare event, perhaps once a season, to clearly see one skitter toward the basement.

Every day, though, I whip my head around at every shadow and creak, convinced I’m seeing a mouse out of the corner of my eye.

Am I hallucinating? According to a study published today in the journal Proceedings of National Academy of Sciences (PNAS), my brain is, in fact, acting in much the same manner as someone with psychosis, using my prior experience of actually seeing a mouse to imagine one into my pantry.

“To my knowledge this is the first direct demonstration (within one study) that there is a continuity between perception in the healthy and in psychopathology,” said Dr. Predrag Petrovic, an associate professor at the Karolinska Institutet in Sweden who wasn’t involved in the study.

Researchers were able to demonstrate how we all rely on prior experience to shape our perceptions by showing images to two groups of people.

First, they asked a group of healthy individuals and a group of who showed early signs of psychosis whether they could see anything in the following black and white image:

Most people saw black and white blobs. Then, they showed this image to both groups:

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.”

Read more at Discovery News