Researchers at UNSW Sydney say they have found some of the clearest evidence yet that auditory verbal hallucinations in schizophrenia may stem from a breakdown in how the brain recognizes its own inner speech. The team argues the brain may mislabel self-generated thoughts as if they were external voices.
The study, published in Schizophrenia Bulletin, focuses on a well-known theory in psychiatry: that hallucinated voices can arise when the brain’s normal system for predicting the sound of one’s own speech fails. If that prediction signal misfires, activity in sound-processing regions may look more like the brain is hearing someone else.
How inner speech is measured
Because inner speech is private, testing it directly has long been difficult. The researchers used EEG recordings to track brain responses while participants silently imagined speaking simple syllables while hearing sounds through headphones.
In everyday speech, the brain typically dampens activity in the auditory cortex because it anticipates the sensory consequences of one’s own voice. The new work examined whether that suppression effect also appears during imagined speech, and whether it differs in people who hear voices.
What the EEG signals showed
The experiment compared 55 people with schizophrenia spectrum disorders who had experienced auditory hallucinations within the previous week, 44 people with schizophrenia without recent hallucinations, and 43 healthy participants. During the task, participants imagined saying “bah” or “bih” while the same sounds were sometimes played aloud, without advance notice of a match.
Healthy participants showed reduced brain responses when the imagined syllable matched the sound they heard, consistent with accurate prediction and sensory suppression. By contrast, people with recent hallucinations showed the opposite pattern, with stronger responses when the imagined and heard sounds matched.
Participants with schizophrenia who had not recently heard voices showed responses between the other two groups. The authors say that gradient could indicate a link between the disrupted prediction mechanism and the current presence of hallucinations, rather than diagnosis alone.
Could this become a biomarker?
The team says the findings strengthen the case that some hallucinated voices feel real because the brain processes inner speech as if it were coming from outside. They also argue the EEG signature could eventually contribute to a biomarker approach, an area where schizophrenia still lacks definitive lab or imaging tests.
Next, the researchers plan to test whether this response pattern can help identify people at elevated risk of psychosis before symptoms fully emerge. Earlier identification could support earlier intervention, although the authors note further validation is needed across broader populations and clinical settings.
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