A new study led by psychologists at UNSW Sydney offers the clearest evidence so far that hearing voices in schizophrenia may arise from a breakdown in how the brain recognizes its own inner voice. The research suggests that the brain may be misidentifying internally generated thoughts as sounds coming from the outside world.

Published in the journal Schizophrenia Bulletin, the study also points toward a possible path for identifying biological markers of schizophrenia. This is important because there are currently no blood tests, brain scans, or lab based biomarkers (signs in the body that can tell us something about our health) that uniquely identify the condition.

Professor Thomas Whitford of the UNSW School of Psychology has spent years studying how inner speech works in both healthy individuals and people living with schizophrenia spectrum disorders.

"Inner speech is the voice in your head that silently narrates your thoughts - what you’re doing, planning, or noticing," he says.

"Most people experience inner speech regularly, often without realizing it, though there are some who don’t experience it at all.

"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. This is because the brain predicts the sound of our own voice. 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."

Brainwaves Reveal a Longstanding Theory

According to Prof. Whitford, these findings strongly support a theory that has existed in mental health research for decades: that auditory hallucinations in schizophrenia may result from a person’s own inner speech being mistaken for external speech.

"This idea’s been around for 50 years, but it’s been very difficult to test because inner speech is inherently private," he says.

"How do you measure it? One way is by using an EEG, which records the brain’s electrical activity. Even though we can’t hear inner speech, the brain still reacts to it - and in healthy people, using inner speech produces the same kind of reduction in brain activity as when they speak out loud.

"But in people who hear voices, that reduction of activity doesn’t happen. In fact, their brains react even more strongly to inner speech, as if it’s coming from someone else. That might help explain why the voices feel so real."

Testing How the Brain Predicts Sound

To explore this effect, the researchers divided participants into three groups. The first included 55 people with schizophrenia spectrum disorders who had experienced auditory verbal hallucinations (AVH) within the past week. The second group included 44 people with schizophrenia who either had no history of AVH or had not experienced them recently. The third group consisted of 43 healthy individuals with no history of schizophrenia.

Each participant wore an EEG (electroencephalography) cap while listening to sounds through headphones. At specific moments, they were asked to imagine saying either ‘bah’ or ‘bih’ silently in their minds while hearing one of those same sounds played aloud. Participants did not know in advance whether the sound they imagined would match what they heard.

In healthy participants, brain activity dropped when the imagined syllable matched the sound played through the headphones. This reduced response appeared in the auditory cortex, the region responsible for processing sound and speech. The pattern suggests the brain correctly predicted the sound and lowered its response, similar to what happens during normal speech.

The opposite pattern appeared in participants who had recently experienced auditory hallucinations. Instead of showing reduced activity, their brains reacted more strongly when the imagined sound matched what they heard.

"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," Prof. Whitford says.

"This reversal of the normal suppression effect suggests that the brain’s prediction mechanism may be disrupted in people currently experiencing auditory hallucinations, which may cause their own inner voice to be misinterpreted as external speech."

Participants in the second schizophrenia group, those without recent hallucinations, showed brain responses that fell between the healthy group and the hallucinating group.

What This Means for Schizophrenia Research

The researchers say the results provide the strongest confirmation yet that people living with schizophrenia may be experiencing imagined speech as if it were coming from outside themselves.

"It was always a plausible theory - that people were hearing their own thoughts spoken out loud - but this new approach has provided the strongest and most direct test of this theory to date," Prof. Whitford says.

Looking ahead, the research team plans to explore whether this brain response pattern could help predict who may later develop psychosis. If successful, it could help identify individuals at high risk earlier, allowing treatment to begin sooner.

"This sort of measure has great potential to be a biomarker for the development of psychosis," Prof. Whitford says.

"Ultimately, I think that understanding the biological causes of the symptoms of schizophrenia is a necessary first step if we hope to develop new and effective treatments."