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.

According to the researchers, 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. The researchers found that elevated dopamine could make some patients rely more on expectations, which could then result in hallucinations.

"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, Assistant Professor at Columbia University Medical Centre. "In individuals with schizophrenia, this process appears to be altered, leading to extreme perceptual distortions, such as hearing voices that are not there," Horga added.

For the study, published in the journal Current Biology, 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. They 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, the researcher said. 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, and 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," said Horga.