“When we go about the world, we’re not just passively perceiving sensory inputs through our eyes and ears,” Corlett says. By their way of thinking, the brain works by “predictive coding”: integrating new information based on the beliefs built on old information. People who regularly hallucinate held the button-that is, they hallucinated-significantly longer than those who don’t.Ĭorlett and Powers see this experiment as evidence for their perspective on how people understand the world around them.

They were told to hold down the button longer to rate their confidence in what they heard. The participants were taught to expect to hear a tone after being shown a flashing light, and then were made to press a button when they thought they heard a tone. The pair recounts a 2017 study they conducted, in which their group tried to induce hallucinations both in people who commonly report hallucinations across the psychotic spectrum and in people who don’t normally hallucinate. In a new paper in Science, they explore how the mysterious experiences fit into a larger, speculative idea about how the brain works-and, in a sense, what the brain is. Why is it so common for people to perceive what isn’t there, and how does the brain allow this to happen in the first place? To find answers, researchers have turned to the mechanics of how we perceive reality itself.įor Corlett and Powers, both from the Yale School of Medicine, hallucinations have everything to do with expectations. And scientists have begun to take seriously the idea that voice hearing and other forms of auditory hallucination can be benign or “nonclinical.” This newfound ubiquity has come with a host of questions. A survey in the early 1990s found that 10 to 15 percent of the population of the United States experienced vivid sensory hallucinations at some point in their lives. This hierarchy perspective represents an ongoing revelation in how widespread and varied hallucinations can be.

At the highest level, according to Corlett’s collaborator Albert Powers, they would be something like hearing “whole sentences of clearly spoken speech of a being who seems quite real.” But, moving further down the line, hallucinations can be far more banal: an imagined text message, a phantom raindrop, a new parent’s mistaken sense of her child by her bedside. But Corlett operates on the idea that hallucinations exist within a hierarchy. Hallucinations were long considered the stuff of psychoses or drug trips, not a regular and inconsequential part of life. To many, this definition may seem shockingly broad. And that, according to the psychologist Philip Corlett, is what makes a hallucination. If you’ve ever felt the buzz of your phone against your thigh only to realize the sensation was entirely in your head, you’ve had a sensory perception of something that isn’t real. There’s a good chance you’ve hallucinated before.

Auditory hallucination