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People with schizophrenia may hear voices because their brains fail to recognise their own inner speech, new research has found.
The study, published in Schizophrenia Bulletin and led by scientists at the University of New South Wales (UNSW Sydney), provides evidence that auditory hallucinations arise when the brain mistakes self-generated thoughts for external sounds.
According to the researchers, the findings, part of the study titled Corollary discharge dysfunction to inner speech and its relationship to auditory verbal hallucinations in patients with schizophrenia spectrum disorders, could help identify biological markers for schizophrenia—a disorder that currently has no definitive lab test or brain scan for diagnosis.
Hearing voices, or auditory verbal hallucinations (AVH), is one of the hallmark symptoms of schizophrenia-spectrum disorders. It involves perceiving speech without any actual sound. For decades, researchers had believed that these voices might be the brain’s own inner speech being misinterpreted as coming from an external source. The new study validates this long-standing theory.
What happens in the brain during inner speech?
The UNSW team conducted experiments with 142 participants divided into three groups:
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- People with schizophrenia currently hearing voices
- People with schizophrenia who were not hearing voices
- Healthy participants with no history of schizophrenia
Using EEG (electroencephalography) to record brain activity, participants were asked to imagine saying simple syllables like “bah” or “bih” in their heads while listening to those same sounds played through headphones.
In most people, when we speak silently in our minds, the brain predicts what our voice should sound like. This “prediction” dampens activity in the auditory cortex, the brain region responsible for processing sound. It is a way for the brain to recognise, “That’s me talking.”
In the study, healthy participants showed this expected reduction in brain response when the sound they imagined matched what they heard, confirming that the brain correctly identifies self-generated speech.
How does schizophrenia change this brain response?
In participants who were hearing voices, the opposite happened. Instead of the brain’s auditory region quieting down when their inner speech matched the sound, it actually became more active, as if the voice were coming from someone else.
This abnormal reaction, called corollary discharge dysfunction, could explain why people with schizophrenia experience their thoughts as external voices.
The researchers believe that understanding this brain mechanism is the first step toward targeted therapies. By identifying people whose brains show these prediction errors, clinicians might one day predict who is at high risk of developing psychosis and intervene earlier.
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This content is for informational purposes only and is not a substitute for professional medical advice.
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