NEW YORK: Scientists have found a link between hallucinations and dopamine, an organic chemical that plays several important roles in the brain and body.
The researchers found that people with schizophrenia who experience auditory hallucinations tend to hear what they expect, an exaggerated version of a perceptual distortion that is common among other people without hallucinations.
Those with hallucinations and other psychotic symptoms are known to have elevated dopamine, the main area of focus for available treatments for psychosis, but it was unclear how this could lead to hallucinations.
They found that elevated dopamine could make some patients rely more on expectations, which could then result in hallucinations.
The findings, published in the journal Current Biology, explain why treatments targeting the production of dopamine could help alleviate this condition.
"Our brain uses prior experiences to generate sensory expectations that help fill in the gaps when sounds or images are distorted or unclear," said Guillermo Horga from the Columbia University Medical Center in the US.
"In individuals with schizophrenia, this process appears to be altered, leading to extreme perceptual distortions, such as hearing voices that are not there," said Horga.
"Furthermore, while such hallucinations are often successfully treated by antipsychotic drugs that block the neurotransmitter dopamine in a brain structure known as the striatum, the reason for this has been a mystery since this neurotransmitter and brain region are not typically associated with sensory processing," said Horga.
The researchers designed an experiment that induces an auditory illusion in both healthy participants and participants with schizophrenia.
They examined how building up or breaking down sensory expectations can modify the strength of this illusion.
The scientists also measured dopamine release before and after administering a drug that stimulates the release of dopamine.
Patients with hallucinations tended to perceive sounds in a way that was more similar to what they had been cued to expect, even when sensory expectations were less reliable and illusions weakened in healthy participants.
This tendency to inflexibly hear what was expected was worsened after giving a dopamine-releasing drug, and more pronounced in participants with elevated dopamine release.
It was more apparent in participants with a smaller dorsal anterior cingulate (a brain region previously shown to track reliability of environmental cues).
"All people have some perceptual distortions, but these results suggest that excess dopamine can exacerbate our distorted perceptions," Horga said.
"Novel therapies should aim to improve the processing of contextual information by targeting the dopamine system or downstream pathways associated with modulation of perceptual processing, which likely include the dorsal anterior cingulate cortex," Horga said.