A recent Yale-led study showed that individuals who report hearing voices in their heads — whether or not they have a diagnosed psychotic illness — are more susceptible to induced hallucinations.

Researchers at the Yale School of Medicine and University College London found that auditory hallucinations result from a disconnect between the brain’s expectations and reality. Through a technique called Pavlovian conditioning, subjects were trained to associate a visual sign — a checkerboard — with the sound of a tone. After months of trials, all study participants reported hearing the tone, even when no sound was present. People who hear voices in their heads were much more likely to experience this hallucinatory effect, according to Chris Mathys, a professor at University College London and co-author of the study.

By examining the neural regions that were active during the induced hallucinations, the researchers concluded that these auditory hallucinations occur when the brain places too much confidence in its prior expectations of reality. The study’s results were published in the journal Science on Aug. 11.

“Hallucinations arise because you overtrust or overweigh the information coming from … your prior expectations of the world,” said Albert Powers ’04, a lead author of the study and clinical instructor at the Yale Department of Psychiatry.

The researchers tested four groups of subjects: people with a diagnosed psychotic illness who hear voices, people with an illness who do not hear voices, people without a psychotic illness who hear voices and people without either a diagnosis or voices.

The brain is constantly generating predictions of what sensations it will experience next, according to Mathys, and for the first time, researchers have applied this predictive nature of the brain to the phenomenon of hallucinations. In addition to being more likely to experience induced hallucinations, people who hear voices were also more confident that they heard tones that were not actually there. The study’s results provide a clearer understanding of how hallucinations occur, and Mathys emphasized that the findings may shed light on potential treatments and preventative measures.

According to Philip Corlett, a co-author of the study and a researcher at the Department of Psychiatry, the results may lead to a novel approach for identifying people at risk of developing psychotic illnesses. He said people’s performance on the conditioning task, combined with their brain’s responses to the task, could be used to “stratify their risk of ever developing psychotic illnesses.”

The researchers plan on using methods like transcranial magnetic stimulation to eliminate hallucinations completely in certain individuals, Corlett said. In addition, they expressed interest in developing drugs that can modulate the regions of the brain responsible for overtrusting one’s expectations over reality. This type of treatment would help regulate the effects of strong perceptual expectations, which lead to auditory hallucinations.

This study also provides important insights on “normal hallucinators” — people who hear voices but do not have a psychotic illness — according to Tanya Luhrmann, a professor of anthropology at Stanford University.

“People often say they hear voices but they have a cultural reason for saying that, like they want to show that God is talking to them or they want to say that they really are in contact with the dead,” Luhrmann said.

According to Corlett, many of the normal hallucinators in the study believed that they were psychics, and considered their voice-hearing to be a gift, rather than a concern.

Luhrmann added that the study corroborates her own findings that people can be trained to respond to stimuli and experience hallucinations. In the Yale-led study, even healthy individuals reported tones that were not present, showing that they can be conditioned with visual and auditory stimuli to hallucinate. Luhrmann added that the results reinforce her belief that brain differences exist between “normal” and psychotic hallucinators.

About one in eight adults hear voices in their heads, according to a 2011 study by the National Institutes of Health.

Alice Parkalice.park@yale.edu