Our brains come equipped with a reality-check system that keeps permanently questioning past expectations and beliefs, a new study reports. When this system fails, we hallucinate.


Image credits Soffie Hicks / Flickr.

It’s actually not very hard to make your brain perceive something that isn’t there. Back in the 1980s, for example, researchers at Yale University repeatedly showed volunteers and image, paired with a tone. If they did this for long enough, they found, participants would still ‘hear’ the sound when presented with the image cards — even though the scientists weren’t playing the tone back to them. And of course, there’s a kind of hallucination (both tactile and auditory) that most of us experience disturbingly often and yet simply dismiss as a nuisance — ringxiety.

“People come to expect the sound so much that the brain hears it for them,” says Albert Powers, a psychiatrist at Yale University and an author of the new study.

The fact that it can do that, frankly, is downright scary. I mean, how can I trust my brain ever again when it obviously has no qualms in making me hear what I want to hear? I use it to do my taxes, which has to be the ultimate conflict of interests ever.

Brain autocorrect

These somewhat usual examples suggest that hallucinations form when our brains rely more on beliefs and expectations than the input sensory organs supply it with, says study author and Yale psychiatrist Philip Corlett. To explore this theory, the team used a variation of the 1980s experiment, this time involving four groups: healthy people, people with psychosis who don’t hear voices, people with schizophrenia (a subtype of psychosis) who do, and people who regularly hear voices but don’t find them disturbing.– such as self-described ‘psychics’.

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Participants were trained to associate a checkerboard image with a 1-kilohertz, 1-second-long tune. Its intensity could be modulated during the trial, or it may sometimes be turned off entirely, so the participants were given a button to press when they heard the tune. They were also asked to apply more or less pressure to indicate how confident they were about hearing the sound. During the trials, the team monitored participants’ brain activity using magnetic resonance imaging to see what was going on up there as they made their choices.

The team’s theory was that people who hear voices would be more inclined to trust their auditory hallucinations as genuine. And that’s exactly what happened. Both the self-described psychics and the schizophrenics were almost five times more likely to hear the tone (when there wasn’t one) than the control group. They were also around 28% more confident on average that they heard the tone, the team reports.

Both groups showed abnormal neuronal activity in several brain regions involved in monitoring “internal representations of reality”, he team notes. The more severe a person’s hallucinations were, the less activity the team saw in their cerebellum — the small bit of the brain in the back of your head. The cerebellum plays a key role in planning and carrying out future movements, a role that requires it to keep tabs on what the rest of the brain perceives of the outside world at all times.

The findings suggest that the cerebellum is a key watchdog against our brain’s potential distortion of reality, the team reports. It also goes to show how powerful our ideas or beliefs can be, having the potential to overpower our senses for the right to shape the world we perceive.

An exciting implication of this research is that future clinicians might be able to predict who’s at risk of developing schizophrenia, allowing for treatment much earlier than possible today.

The paper “Pavlovian conditioning–induced hallucinations result from overweighting of perceptual priors” has been published in the journal Science.