Scientists at UNSW Sydney have identified a brain “glitch” that may explain why some people with schizophrenia hear voices, offering the strongest evidence yet for a decades-old theory about auditory hallucinations.
The study, published in Schizophrenia Bulletin, suggests that hallucinations occur when the brain fails to correctly recognise its own inner speech, the silent voice people use to think, causing internally generated thoughts to be experienced as external voices.
Professor Thomas Whitford from the UNSW School of Psychology explained that in most people, the brain automatically predicts when speech is self-generated and reduces its response accordingly. “Inner speech is the voice in your head that silently narrates your thoughts,” he said. “Our research shows that when we speak, even just in our heads, the part of the brain that processes external sounds becomes less active.”
However, in people who experience auditory hallucinations, this predictive process appears to malfunction. “The brain reacts as if the voice is coming from someone else,” Whitford said.
To investigate this, researchers studied three groups: 55 people with schizophrenia spectrum disorders who had experienced auditory hallucinations in the past week, 44 people with schizophrenia without recent hallucinations, and 43 healthy participants.
Participants wore EEG caps while listening to sounds through headphones and were asked to silently imagine saying syllables such as “bah” or “bih” as the same sounds were played aloud. In healthy participants, brain activity decreased when the imagined syllable matched the sound, indicating accurate prediction and a dampened response.
In contrast, participants who had recently heard voices showed the opposite effect. Their brain activity increased 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 healthy participants,” Whitford said.
Researchers say the findings could pave the way for identifying biological markers of psychosis risk. Currently, there are no lab-based biomarkers that uniquely identify schizophrenia.
The team hopes future research will determine whether this brain response pattern could help predict who may later develop psychosis, enabling earlier diagnosis and intervention.
