the mind

Perfectly Healthy Brains Hallucinate

Human brain model with blue glow, studio shot — Image by © Tetra Images/Corbis Photo-Illustration: Photo: Pink River/Getty Images

Early one morning, University of Queensland psychiatrist John McGrath had just turned off the water and was stepping out of the shower when the new dad heard one of his children calling for him. He poked his head out of the bathroom door and called the kid’s name, but got no response. He started to panic — but then stopped short. That’s right, he remembered. His children weren’t actually home.

McGrath had experienced a hallucination, defined as seeing or hearing something that’s not really there. Historically, hallucinations and delusions (a strongly held false belief that isn’t in accordance to someone’s cultural or religious setting) have been seen as symptoms of psychosis or other serious mental illness. “But these types of things happen to nearly everyone, including me,” McGrath said.

His recent discovery that around 1 in 20 of us will experience a hallucination at some point in our life suggests that, rather than being associated with illness, the phenomenon is actually associated with normal brain function. Now, in a new study in the journal PNAS, neuroscientist Christoph Teufel of the University of Cardiff in Wales and his colleagues, including University of Cambridge psychiatrist Paul Fletcher, show that visual hallucinations may be the result of our brains doing their best to make sense of a chaotic and ambiguous world. Taken together, these studies point to the idea that psychosis isn’t an either-or disorder that you either have or you don’t. Instead, psychosis may exist on a spectrum. “It’s not a broken brain that generates hallucinations, it’s just part of how the brain works,” Teufel said. What varies, it seems, is the degree to which this happens.

Many people think of their visual system working like a camera: The eyes record an image and the brain processes it. The problem with this conceptualization, Teufel says, is that it’s dead wrong. Part of the problem is that the information fed to the brain from the eyes is actually not that detailed, and so the brain fills in the gaps based on its previous knowledge. It’s a system that likely evolved as a result of the dangers inherent in our early environment and the need to make quick decisions. And since our previous knowledge is usually accurate, the system generally works well. The idea that we see or hear things that aren’t there seems incomprehensible.

And yet Teufel, who specializes in the neuroscience of vision, and Fletcher believed that the origins of hallucinations could be found in the visual system itself, rather than in some brain circuit specifically associated with psychosis or schizophrenia. They believed that hallucinations were the result of the brain’s overreliance on previous information, which meant the brain filled in a person’s normal visual gaps with details that weren’t really there. To test this idea, he designed a series of experiments that would measure how much people relied on prior knowledge to identify objects in a series of confusing images. If he was right, people who were prone to psychosis would be more likely to rely on this information.

The first set of experiments recruited a group of 16 healthy adults and a group of 18 adults who had experienced some signs of psychosis but had not yet developed a full-blown psychotic disorder. First, these individuals were asked to look at a series of images that, at first glance, “looked like meaningless black and white blobs,” Teufel said.

After they had a chance to look at the black-and-white images, they were asked whether they could identify a person or object in the image. Then, they were shown the color photographs that were used as templates to create the black-and-white images. Last, the group was shown the same set of black-and-white images and again asked to identify whether they saw a person or object in the image. The people who had experienced symptoms of psychosis improved significantly more in the second test than the healthy adults did, indicating that their brains were better at using prior knowledge to fill in the gaps in their visual system.

In a separate experiment, they recruited 40 healthy adults and tested how prone they were to symptoms of schizophrenia and psychosis. Then these adults, too, completed the same image tests as above. Again, Teufel and colleagues found that those people most prone to symptoms of psychosis also scored the best on the second test.

This finding is important, Teufel says, because it supports the idea that psychosis is a spectrum disorder, much like autism, not a condition that you either do or don’t have. That visual hallucinations, a major symptom of psychosis, arise from normal visual processes supports this idea, as does McGrath’s work looking at the frequency of hallucinations and delusions in the general population. Earlier this year in JAMA Psychiatry, McGrath and colleagues published results from the World Mental Health Study, which surveyed 31,261 adults from 18 countries around the world. They found that around 1 in 20 people had experienced a hallucination at some point in their life. Although most people experienced the hallucination only once, a subset experienced them several times, and an even smaller subset showed signs of psychosis. Most of the people who had hallucinations never had full-blown psychosis, although the hallucinations did seem to be signs of general psychological distress.

Together, these results show that hallucinations seem to be a normal part of the human experience, as long as they don’t take over a person’s life. “There’s a continuum between mental illness and mental health,” Teufel says, and we’re all on it.

Perfectly Healthy Brains Hallucinate