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How your mind protects you against hallucinations | Science

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…

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