Hallucinations are often distressing—a suggestion that something is amiss in our brains. But new research suggests we’re all susceptible to hallucinations, and that may not be such a bad thing.

In a paper released last week in Science , a team from Yale University set out to understand how we interpret the world around us—in short, how we determine what’s real and what’s not. They suspected that people who regularly hallucinate perceive the world based on what they expect to happen, while others, who don’t hallucinate, would rely more what their senses are telling them is happening in the world.

To determine that, authors Phil Corlett and Al Powers began by conditioning participants to hear a tone when they were shown a checkerboard pattern. Then they slowly removed the actual sound and asked people when they heard it. Participants who regularly heard voices were five times more likely to say they heard a tone when there wasn’t one, and they were 25-30% more confident in their choice. But they weren’t alone in hearing things. In fact, all of the participants experienced some induced hallucinations during the experiment.

“I did not expect that people who did not have a psychotic illness would perform so similarly to people who did hear voices,” Powers says. “They were very, very alike.”

“Really healthy people that don’t hallucinate in their everyday lives—graduate students, and postdocs, and highly accomplished people—were very, very susceptible to this effect, even the ones who didn’t hear voices. They were more susceptible to it than I expected,” Corlett says. The study contributes to the idea that schizophrenia, much like autism, may exist as a spectrum, the authors say.

Hearing Things

Powers personally experienced the phenomenon when testing the software for the experiment. He was surprised to find that, despite his deep knowledge of the process and mechanisms, the effect still worked on him. “At the beginning, it was quite exciting,” he says, though he admitted that “it’s concerning, to be honest.”

He and Corlett compared four groups: those with diagnosed psychoses who either did or did not hallucinate and those not diagnosed with psychoses who either did or did not hallucinate. Each of the groups contained 14-15 people. Most of these participants were relatively easy to recruit. But they needed to find people who experience hallucinations as part of daily life yet haven’t been diagnosed with a psychiatric disorder. So they turned to the Connecticut Psychics’ Association, specifically to psychics who regularly hear voices.

Vincent van Gogh frequently suffered from psychotic episodes of hallucinations and delusions.

Many of the study participants who hallucinate but haven’t been diagnosed as psychotic tend to lead normal lives. The hallucinations don’t seem to interfere, like for a police officer in her 50s who heard daily narrations of her activities as she worked.

Powers and Corlett wanted to know if people diagnosed with psychoses had brains that operated differently than the other subjects. To do that, they had to make people who don’t typically hallucinate…hallucinate. And that’s why they turned to a famous technique originally used on dogs—Pavlovian conditioning. The team conditioned participants to associate a checkerboard pattern with a 1-kHz tone, which sounds like the bleep over a swear on late-night TV. Initially, the scientists sprinkled in lots of 1-kHz tones that they were certain the participants could hear. Then, as the task went on, they changed their intensity and often removed them entirely. Like in a hearing test at the doctor, participants were asked to press a button when they heard the tone. In an added twist, participants could indicate how confident they were about their response. Meanwhile, their brains were examined using magnetic resonance imaging (MRI).