A new study led by psychologists at UNSW Sydney has provided the clearest evidence so far that hearing voices in schizophrenia may stem from a breakdown in how the brain recognises its own inner speech, causing internally generated thoughts to be experienced as if they were coming from outside.
Published in Schizophrenia Bulletin, the research supports a 50-year theory that auditory hallucinations can occur when the brain fails to correctly predict and “tone down” its response to a person’s own inner voice.
Professor Thomas Whitford of the UNSW School of Psychology described inner speech as “the voice in your head that silently narrates your thoughts,” adding that in most people, the brain responds less to this self-generated speech.
“Our research shows that when we speak – even just in our heads – the part of the brain that processes sounds from the outside world becomes less active,” Whitford said. “But in people who hear voices, this prediction seems to go wrong, and the brain reacts as if the voice is coming from someone else.”
To test the effect, the researchers studied three groups: 55 people with schizophrenia spectrum disorders who had experienced auditory verbal hallucinations in the past week, 44 people with schizophrenia without recent hallucinations, and 43 healthy individuals.
Participants wore an EEG cap while listening to sounds through headphones and were asked to imagine saying “bah” or “bih” silently while hearing one of the same sounds played aloud.
In healthy participants, brain activity fell when the imagined syllable matched the sound they heard, suggesting the brain correctly predicted the sound and reduced its response. In contrast, participants who had recently experienced hallucinations showed the opposite pattern, with brain responses becoming stronger when the imagined and heard sounds matched.
“Their brains reacted more strongly to inner speech that matched the external sound, which was the exact opposite of what we found in the healthy participants,” Whitford said.
Researchers said the findings could help guide future work on biological markers for psychosis risk, noting there are currently no lab-based biomarkers that uniquely identify schizophrenia. The team plans to explore whether the brain response pattern could help predict who may later develop psychosis and enable earlier intervention.
“This sort of measure has great potential to be a biomarker for the development of psychosis,” Whitford said.
