A new study led by psychologists from UNSW Sydney has provided the strongest evidence yet that hearing voices in schizophrenia may happen because the brain struggles to recognize its own inner voice.
The discovery, published in Schizophrenia Bulletin, could help scientists identify biological signs that point to schizophrenia—something that has never been possible before.
Currently, schizophrenia is diagnosed based on behavioral symptoms. There are no blood tests or brain scans that can confirm it. Finding a measurable biological sign, or biomarker, would be a major breakthrough.
Professor Thomas Whitford from UNSW’s School of Psychology has spent years studying how inner speech—our inner voice that narrates our thoughts—works in both healthy people and those with schizophrenia.
“Inner speech is that voice in your head that talks you through what you’re doing or thinking,” Professor Whitford explains.
“Normally, when people ‘speak’ in their minds, the part of the brain that processes external sounds becomes less active because the brain predicts what its own voice should sound like. But in people who hear voices, this prediction seems to fail. Their brain reacts as if the voice came from someone else.”
The research supports a theory that’s been around for decades: that auditory hallucinations in schizophrenia may be caused by the brain mistaking inner speech for external sounds.
Testing this theory has always been challenging because inner speech is silent and private. However, the researchers used EEG technology—which records electrical activity in the brain—to observe what happens when people imagine speaking.
In healthy participants, inner speech caused a reduction in brain activity in the auditory cortex, the area that processes sound. This shows the brain correctly predicted the sound of its own imagined voice.
But in people who recently experienced auditory hallucinations, the opposite occurred—their brain activity increased when they imagined speaking, as though they were hearing someone else’s voice.
“This reversal of normal brain response suggests that the brain’s ability to predict and recognize its own voice is disrupted,” Professor Whitford said. “That disruption could be what makes their own thoughts sound like external voices.”
The study involved three groups: 55 people with schizophrenia-spectrum disorders who had recently experienced auditory hallucinations, 44 with schizophrenia who hadn’t experienced voices recently or ever, and 43 healthy individuals as a control group.
All participants were asked to imagine saying “bah” or “bih” in their minds while listening to recordings of these same sounds through headphones.
The EEG results showed that healthy participants had reduced brain activity when the imagined and heard sounds matched, while those with recent hallucinations had the opposite—enhanced brain responses. Participants with schizophrenia who hadn’t heard voices recently showed results between the two extremes.
This pattern provides strong evidence that auditory hallucinations arise from the brain’s misinterpretation of its own internal speech. It also suggests that this brainwave pattern could serve as a potential biomarker to identify schizophrenia or predict psychosis risk.
Professor Whitford and his team plan to explore whether these findings could help detect who might develop psychosis before symptoms become severe. Early detection could make treatment far more effective.
“Understanding the biological roots of schizophrenia is a vital step toward better treatments,” Professor Whitford said. “If we can identify who is at risk and intervene early, we may one day prevent or significantly reduce the impact of these symptoms.”
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The study is published in Schizophrenia Bulletin.
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