Scientists Link Schizophrenia Voices to Brain Misreading Its Own Inner Speech

A study led by psychologists at UNSW Sydney provides some of the clearest evidence so far that hearing voices in schizophrenia may be linked to a disruption in how the brain recognizes its own inner voice. The findings suggest that the brain may sometimes misidentify internally generated thoughts as sounds coming from the external world.

Published in Schizophrenia Bulletin, the research also points to a potential pathway for identifying biological markers of schizophrenia. This is important because there are currently no blood tests, brain scans, or other laboratory biomarkers capable of uniquely diagnosing the condition.

How Inner Speech Normally Works

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

Inner speech refers to the silent “voice in your head” that narrates thoughts, plans, or observations. Many people experience it constantly without consciously noticing it, while others report little or no inner speech at all.

According to Whitford, the brain normally reduces its response to sounds when a person speaks — even when the speech is only imagined. This happens because the brain predicts what its own voice should sound like, causing the auditory system to become less reactive.

In people who hear voices, however, this prediction system appears to malfunction. Instead of recognizing the voice as self-generated, the brain may respond as though it belongs to someone else.

Brainwave Data Support a Long-Standing Theory

The findings strongly support a theory that has circulated in psychiatric research for decades: auditory hallucinations in schizophrenia may occur when inner speech is mistakenly perceived as external speech.

Testing this theory has traditionally been difficult because inner speech is private and cannot be directly observed.

To overcome this challenge, researchers used electroencephalography (EEG), a method that measures electrical brain activity, to examine how the brain responds during imagined speech.

In healthy individuals, generating inner speech produces reduced activity in brain regions responsible for processing sound — a pattern similar to what occurs during actual speech.

Among people who experience auditory hallucinations, however, this reduction appears to be missing. Instead, the brain may respond more strongly during inner speech, as if the sound were coming from an outside source.

Researchers believe this heightened response may help explain why hallucinated voices can feel vivid and emotionally real.

How the Study Tested Auditory Prediction

Participants were divided into three groups:

• 55 individuals with schizophrenia spectrum disorders who had experienced auditory verbal hallucinations within the previous week;
• 44 individuals with schizophrenia who either had no history of hallucinations or had not experienced them recently;
• and 43 healthy participants with no history of schizophrenia.

While wearing EEG caps, participants listened to sounds through headphones. At certain moments, they were asked to silently imagine saying either “bah” or “bih” while hearing one of those syllables played aloud.

Participants did not know in advance whether the imagined sound would match the sound they were about to hear.

In healthy participants, activity in the auditory cortex decreased when the imagined syllable matched the sound played through the headphones. Researchers say this suggests the brain successfully predicted the sound and reduced its response accordingly.

Participants who had recently experienced hallucinations showed the opposite pattern. Instead of reduced activity, their brains reacted more strongly when the imagined and external sounds matched.

Whitford explained that this “reversal” of the normal suppression effect suggests the brain’s prediction system may become disrupted during hallucinations, potentially causing a person’s own inner voice to be interpreted as external speech.

The schizophrenia group without recent hallucinations showed responses that fell between the healthy participants and those with recent hallucinations.

Why the Findings Matter

Researchers say the results provide some of the strongest evidence yet that imagined speech in schizophrenia may sometimes be experienced as though it originates outside the self.

The team also plans to investigate whether this distinctive brain-response pattern could help predict who may later develop psychosis. If successful, such findings could help identify high-risk individuals earlier and allow treatment to begin sooner.

Whitford noted that measures like these may eventually become useful biomarkers for psychosis development. He added that understanding the biological mechanisms underlying schizophrenia symptoms is an important step toward developing more effective treatments.