Neuroscientists have long theorized that the “voices” or auditory hallucinations experienced by those with schizophrenia stem from the brain mistaking its own inner dialogue for external sounds. A study led by Thomas J. Whitford of the University of New South Wales, published on 21 October 2025, uncovered compelling neurophysiological evidence that auditory verbal hallucinations stem from failures in the brain’s self-monitoring system.
When the Brain Mistakes Thought for Sound: Scientists Found the Mechanism Behind Auditory Hallucinations in Schizophrenia
The experience of hearing voices may not come from outside the mind at all. New research identifies a breakdown in corollary discharge, the predictive system of the brain, that prevents inner speech from being recognized as self-generated.
Background
The study is founded on the corollary discharge theory. This theory explains and describes how the brain predicts and moderates the sensory effects of its own actions. Accordingly, when a person speaks or moves, a copy of the motor command of the brain, called an efference copy, is sent to sensory regions, thus suppressing the expected sensory response.
In healthy individuals, this internal signaling allows distinction between self-generated and external stimuli. However, previous research demonstrated that patients with schizophrenia often show weaker suppression when speaking aloud, indicating potential dysfunction in this predictive mechanism. The researchers extended this concept to inner speech.
The inner speech is the silent voice people hear inside their minds when thinking in words. This is also sometimes referred to as the inner dialogue of the brain. The team hypothesized that if inner speech also relies on corollary discharge, then a disruption in this predictive system could cause the brain to misidentify self-generated thoughts as external voices.
Nevertheless, to test their hypothesis, an electroencephalography paradigm capable of precisely timing imagined speech and auditory stimuli was developed. A total of 142 participants participated in the investigation. These include 55 schizophrenia-spectrum patients experiencing hallucinations, 44 without recent hallucinations, and 43 healthy controls.
Each participant wore an EEG cap and listened to syllables such as “bah” or “bih” while imagining pronouncing one at specific intervals. The trials also included three specific conditions to enable the researchers to measure the N1 brain auditory evoked response and determine sensory prediction accuracy. These were match, mismatch, and passive conditions.
Key Findings
The research revealed a striking pattern of neural differences among participant groups. The results have collectively and specifically provided strong evidence linking hallucinations to a failure of inner-speech monitoring. This supports a long-standing theoretical framework in schizophrenia research. The following are the main findings.
• Normal Suppression in Healthy Individuals
Control participants showed reduced N1 brain auditory evoked response amplitude during match trial conditions. This confirmed effective sensory prediction. Their brains accurately anticipated and dampened the sound they imagined, thereby demonstrating proper functioning of corollary discharge during inner speech.
• Reversal in Hallucinating Patients
Patients currently hearing voices exhibited the opposite pattern. Instead of suppression, their N1 amplitude increased when the imagined and heard syllables matched. This reversal indicates a complete failure of predictive self-monitoring and causes internally-generated sounds to be processed as external auditory events.
• Intermediate Responses in Non-Hallucinating Patients
Participants diagnosed with schizophrenia but without hallucinations showed neither full suppression nor reversal. Their EEG results fell between healthy controls and hallucinating patients. This indicates that corollary discharge dysfunction exists along a continuum of severity across schizophrenia-spectrum conditions.
• Correlation with Symptom Severity
The extent of N1 reversal was correlated with the intensity and frequency of auditory hallucinations. Greater disruption of predictive suppression specifically corresponded with stronger auditory experiences. This suggests that this neurophysiological marker may quantify vulnerability to auditory verbal hallucinations.
Implications
The findings validate the corollary discharge dysfunction hypothesis by demonstrating that hallucinations can emerge when the brain fails to recognize self-generated inner speech. The study also transforms understanding of schizophrenia by noting that the voices people hear are not external phenomena but misattributed products of internal cognitive activity.
N1 response was also established as a potential biomarker for psychosis risk. To be specific, because this signal can be objectively measured using EEG, it could enable earlier identification of individuals vulnerable to hallucinations and allow clinicians to monitor therapeutic progress based on measurable neural changes rather than subjective reports.
Moreover, by linking inner speech to auditory hallucinations, the research bridges psychology, neuroscience, and psychiatry. It connects everyday mental processes with severe psychiatric symptoms, underscoring that the same inner voice used for thought and reflection can become distorted through a subtle breakdown in predictive signaling.
The findings invite new treatment approaches. Future therapies might focus on strengthening corollary discharge mechanisms through neurofeedback, targeted brain stimulation, or cognitive retraining. The study opens treatment pathways for developing interventions that restore the capacity of the brain to distinguish self-generated thoughts from external sounds.
FURTHER READING AND REFERENCE
- Whitford, T. J., Chung, L. K., Griffiths, O., Jack, B. N., Le Pelley, M. E., Spencer, K. M., Barreiros, A. R., Harrison, A. W., Han, N. T., Libesman, S., Pearson, D., Elijah, R. B., Godwin, M., Haroutonian, C., Nickerson, A., Chan, S. S., Chong, G. H., Lau, G. K., Wong, Y., … So, S. H. 2025. “Corollary Discharge Dysfunction to Inner Speech and Its Relationship to Auditory Verbal Hallucinations in Patients with Schizophrenia Spectrum Disorders.” Schizophrenia Bulletin. DOI: 1093/schbul/sbaf167