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Hallucinations are the apparent perception of something not present, be them a tiny dragon in your cupboard, the smell of burned tires in your coffee, or menacing voices in your head. Hallucinations are reported by millions of people around the world affected by mental conditions such as schizophrenia and depression, but they can also be caused by psychedelics like LSD or ‘magic mushrooms’. In a new study, researchers at the University of Oregon found that mice

given a psychedelic drug had dampened activity in the brain’s visual center. Their results suggest that hallucinations may occur when the brain over-interprets the information in front of it.

What’s going on inside the hallucinating brain

The research team at the University of Oregon in Eugene injected mice with a hallucinogenic drug called 4-iodo-2,5-dimethoxyphenylisopropylamine (DOI). Like other hallucinogenic substances, DOI produces its effects by binding with serotonin 2A receptors. Serotonin is often called ‘the happy chemical’ because it contributes to wellbeing and happiness. However, it is involved in a wide range of functions in the body, including vision. Previous research showed that drugs which block these receptors in the brain prevent hallucinations in people with schizophrenia.

The hallucinating mice were placed in front of a digital screen which flashed various pictures. All the while, researchers examined the brain activity of the mice and compared it to normal conditions. They found that while the mice were hallucinating on the images, brain activity in the visual cortex — the region of the brain responsible for interpreting visual information — was dampened.

This was a very counter-intuitive finding. One would expect that neurons in the visual cortex would fire in over-drive when a person is hallucinating, not the other way around. However, this makes sense if you look at it from the context of visual processing. The most important clue was that the visual signals sent to the visual cortex were almost the same as those sent in the absence of the drug, which shows that the brain still received the same information. What differed was the way that information was processed.

Writing in the journal Cell Reports, the researchers conclude that hallucinations may be the product of over-interpretation. Essentially, the brain is filling the blanks in what it perceives as missing information.

“Understanding what’s happening in the world is a balance of taking in information and your interpretation of that information,” Cristopher Niell, a neuroscientist at the University of Oregon in Eugene, said in a statement. “If you’re putting less weight on what’s going on around you but then over-interpreting it, that could lead to hallucinations.”

The study’s main limitation is that it worked only with mice, but the findings could reasonably translate to humans as well. Hallucinating mice shared many characteristics seen in humans, such as visible movement and behavioral changes.

The authors are also careful to mention that over-interpretation isn’t the only cause for hallucinatory experiences. Instead, there are likely many causes and uncovering each of them may be important in a medical setting for the treatment of schizophrenia and other mental disorders that cause hallucinations.