How your mind protects you against hallucinations

More than 300 years ago, the philosopher René Descartes asked a disturbing question: If our senses can’t always be trusted, how can we separate illusion from reality? We’re able to do so, a new study suggests, because our brain keeps tabs on reality by constantly questioning its own past expectations and beliefs. Hallucinations occur when this internal fact-checking fails, a finding that could point toward better treatments for schizophrenia and other psychiatric disorders.

The study is “very elegant,” and an important step toward identifying the brain regions that produce hallucinations—and keep them in check, says Georg Northoff, a neuroscientist at the University of Ottawa who was not involved with the work.

We don’t always perceive the world as we see—or hear—it. In an experiment devised at Yale University in the 1890s, for example, researchers repeatedly showed volunteers an image paired with a tone. When the scientists stopped playing the tone, participants still “heard” it when the image appeared. A similar auditory hallucination occurs in daily life: when you think you hear your cellphone ring or buzz, only to find it’s turned off. “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.

These examples suggest hallucinations arise when the brain gives more weight to its expectations and beliefs about the world than to the sensory evidence it receives, says study author and Yale psychiatrist Philip Corlett. To test that idea, he, Powers, and colleagues decided to apply a version of the 1890s experiment to four different groups: healthy people, people with psychosis who don’t hear voices, people with schizophrenia (a subtype of psychosis) who do, and people—such as self-described psychics—who regularly hear voices but don’t find them disturbing.

The researchers trained everyone to associate a checkerboard image with a 1-kilohertz, 1-second-long tone. As the team changed the intensity of the tone and sometimes tuned it out entirely, participants were asked to press a button when they heard it, increasing or decreasing pressure to indicate their level of confidence. Using magnetic resonance imaging scanners, the researchers captured a snapshot of the participants’ brain activity as they made their choices.

The team hypothesized that people who hear voices would be more likely to “believe” in auditory hallucinations. That’s precisely what they found: Both the schizophrenics and self-described psychics were nearly five times more likely to say they heard the nonexistent tone than healthy controls. They were also about 28% more confident that they had heard the tone when none was there, the researchers report today in Science .

Both self-described psychics and people with schizophrenia also showed abnormal neuronal activity in several brain regions responsible for monitoring our internal representations of reality. The more severe a person’s hallucinations were, for example, the less activity they displayed in the cerebellum, a wrinkled nodule at the back of the brain. The cerebellum plays a critical role in planning and coordinating future movements, a process that requires constantly updating one’s perception of the outside world.

The findings confirm that, when it comes to how we perceive the world, our ideas and beliefs can easily overpower our senses, Powers says. The work also suggests that the cerebellum is a key checkpoint against this distortion, he adds.

Northoff says future experiments should investigate whether there are any telling differences between the psychotic and healthy brain at rest. Such studies could guide still-experimental therapies such as transcranial magnetic stimulation, which aims to suppress or boost brain activity in targeted areas through electrical currents. Even more promising, says Corlett, is the prospect that studies like this one will help clinicians predict who is likely to develop schizophrenia, allowing them to seek early treatment.