Long lasting mystery solved – People with Schizophrenia “Hear Voices”.

Oct 2025 : A team of psychologists at the University of New South Wales (UNSW) Sydney has uncovered the most compelling evidence so far that hearing voices in schizophrenia may result from the brain’s inability to recognise its own internal speech.

UNSW is known for its research strengths in a variety of fields, including engineering, medicine, science, law, and business. It is home to numerous research centers and institutes, including the Kirby Institute for Infection and Immunity, the Australian Centre for Space Engineering Research, and the Centre for Energy and Environmental Markets.

The study suggests this discovery could help identify biological clues linked to schizophrenia. This is a crucial development, as there are currently no blood tests, brain scans, or laboratory-based biomarkers (physical indicators that reflect a person’s health) that are specific to the condition; hearing voices in schizophrenia may arise from the brain’s failure to recognise its own inner speech.

Professor Thomas Whitford from the UNSW School of Psychology has long investigated how inner speech influences thinking in both healthy individuals and those 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. Most people experience inner speech regularly, often without realising it, though there are some who don’t experience it at all. Our research shows that when we speak, even just in our heads (within ourselves), the part of the brain that processes sound 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”.

While referring to Brainwave analysis, Proffessor Whitford says this confirms what mental health researchers have long theorised: that auditory hallucinations in schizophrenia may be due to the person’s own inner speech being misattributed as external speech. “This idea’s been around for 50 years, but it’s been very difficult to test because inner speech is inherently private. 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”.

Auditory-verbal hallucinations (AVH), a cardinal feature of schizophrenia-spectrum disorders, are characterized by the perception of voices without corresponding external stimulation. AVH have long been posited to reflect self-generated thoughts being misperceived as external voices. This abnormality may be caused by failures of the corollary discharge mechanisms which typically use a copy of the motor signal to predict, and often suppress, the sensory consequences of self-generated actions, and potentially also self-generated thoughts, as per Hughlings Jackson’s conceptualisation of thoughts as our “most complex motor act”. An abnormality in these mechanisms may obscure the distinction between “self”, and “world”, and hence lead to the misattribution of self-generated thoughts and actions to external sources.

Electroencephalography (EEG) studies have consistently shown that healthy individuals exhibit speaking-induced suppression (SIS), which is the phenomenon that self-generated speech elicits a smaller neurophysiological response in the auditory cortex than the same sounds externally-generated. SIS has commonly been operationalised as a reduction in the amplitude of the N1-component of the auditory-evoked potential to self-generated speech sounds. Patients with schizophrenia have consistently been found to exhibit subnormal levels of SIS, that is, they show lower levels of N1-suppression to their own speech sounds compared to matched healthy controls for a review. These SIS deficits in schizophrenia have been argued to reflect a failure to suppress the activity of the auditory cortex to self-generated speech due to a corollary-discharge-related dysfunction.

Sound choices –

The researchers divided participants into three groups:

The first group included 55 people living with schizophrenia-spectrum disorders who had experienced auditory verbal hallucinations (AVH) in the past week.

The second group of 44 participants also had schizophrenia, but either had no history of AVH or hadn’t experienced them recently.

The third group was a control group of 43 healthy people with no history of schizophrenia.

Each participant was connected to an EEG (electroencephalography) device to measure brainwaves as they listened to audio over headphones. They were asked to imagine saying either ‘bah’ or ‘bih’ in their minds at the exact moment they heard recordings of one of those two sounds played through headphones. The participants had no way of knowing whether the sound they heard in the headphones would match the sound they made in their imagination.

In the healthy participants, when the sound that played in the headphones matched the syllable they imagined saying in their minds, the EEG showed reduced activity in the auditory cortex, the part of the brain that processes sound and speech. This suggests the brain was predicting the sound and dampening its response, similar to what happens when we speak out loud.

However, in the group of participants who had recently experienced AVH, the results were the reverse. In these individuals, instead of the expected suppression of brain activity when the imagined speech matched the sound heard, the EEG showed an enhanced response.

“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”.

While, participants in the second group, those people with a schizophrenia-spectrum disorder who hadn’t experienced AVH recently or at all, showed a pattern that was intermediate between the healthy participants and the hallucinating participants.

The researchers say this is the strongest confirmation to date that the brains of people living with schizophrenia are misperceiving imagined speech as speech that is produced externally.

“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”, Proffessor Whitford says. He says the next thing he and his fellow researchers want to assess is if this measure can be used to predict who might transition to psychosis, with the potential to identify people with high risk of developing psychosis, which would allow for early intervention.

“This sort of measure has great potential to be a biomarker for the development of psychosis. 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”.

Team Maverick