Thomas Leuthard / Flickr

What you see at any given moment is only partially based on what’s actually in front of you. Sensory information is part of visual perception, but the other part is the knowledge you already hold about your environment—what you expect to see.

“When we open our eyes, typically most of us have this feeling of a very clear picture of the world that’s out there,” says Christoph Teufel, a lecturer in neuroscience at Cardiff University. “That's actually not really the case … We use what we already know about the world in order to generate an unambiguous perception of the world.”

Teufel is the lead author on a new study published in Proceedings of the National Academy of Sciences that suggests visual hallucinations experienced by people with schizophrenia and other mental disorders might just be a result of the way humans process what they see. A hallucination could come from relying more heavily on prior knowledge than on the actual sensory information coming into the brain.

The researchers, from Cardiff and the University of Cambridge, tested this idea using two-tone images, which are created by taking normal images and flattening them out into just black and white, with no in-between shades of grey. The researchers asked participants if they could see a person or object in the picture. After, they exposed them to the original images the two-tones had been made from (without telling them which went with which). Then, the participants looked at the two-tones again, and once again looked for the objects or people within. (There were also some control two-tones in there that didn’t have any particular pattern.)

Here’s an example: On the left, the two-tone image; on the right, the original image it came from.

Christoph Teufel

The researchers ran this experiment twice—once with people who had “low-level but measurable psychotic experiences” that were still below diagnosable levels, and a second time with only healthy people. They found that people in the first group, those with psychotic symptoms, did much better on the task than those in the second; they were able to spot the picture in the image more often after seeing the template. Because both groups had seem the same two-tone images each time, Teufel says it was having the knowledge of the template image that made the difference, allowing some people to see a picture where before they saw only splotches.

“What we think we might be able to show here is that it’s not a qualitative change in how the brain processes visual information that puts somebody at risk of developing hallucinations. It’s really a slight change in this balance between the sensory input that a system gets and the amount of prior knowledge that an observer uses,” Teufel says.

The second experiment, with the mentally healthy sample, provides more evidence for a spectrum of the ways people combine the sensory information with knowledge. The participants in that group who did better at using their prior knowledge to spot the pictures in the two-tone images also tended to have personality traits that put them more at risk for developing psychosis. That doesn’t mean they’ll necessarily ever have hallucinations, but that people use their prior knowledge in vision to varying degrees.

“People who are at risk of hallucinations ... might at some point use so much prior knowledge that they see things that are not really out there in the environment,” Teufel says. “They only have that stored knowledge, that prior knowledge of these things.”

We want to hear what you think about this article. Submit a letter to the editor or write to