Hearing voices that aren’t there, known as auditory hallucinations, is a common yet puzzling symptom experienced by many people diagnosed with schizophrenia. Up to 80 percent of individuals with this mental health condition report hearing voices or sounds that do not exist in their external environment. Despite its prevalence, the precise brain mechanisms underlying this phenomenon have remained largely elusive. However, new scientific research is shedding light on how and why some people with schizophrenia experience these auditory hallucinations while others do not.
For decades, experts have theorized that auditory hallucinations result from a person mistakenly perceiving their own inner thoughts or internal speech as external voices. Normally, when individuals speak or prepare to speak, the brain’s motor planning areas send signals that suppress activity in the auditory cortex—the region responsible for processing sounds. This suppression helps the brain distinguish between sounds generated by oneself and those coming from the outside world. Researchers have hypothesized that a similar mechanism might operate during inner speech—the silent voice inside our heads—but verifying this has been challenging due to the private, internal nature of inner speech.
A recent study published in *Schizophrenia Bulletin* has provided new evidence supporting this theory. The researchers found that in adults without schizophrenia, inner speech indeed suppresses activity in the auditory cortex. Conversely, in people with schizophrenia who experience auditory hallucinations, inner speech actually amplifies the auditory cortex’s response. This contrasting brain activity pattern offers a compelling explanation for why some individuals hear voices.
Thomas Whitford, a cognitive neuroscientist at the University of New South Wales in Australia and co-lead author of the study, explains the difficulty in studying inner speech: “The hard thing with studying inner speech is that it’s inherently private.” To overcome this obstacle, Whitford and his team used electroencephalography (EEG) to monitor brain activity in participants across three groups: people with schizophrenia who currently hear voices, people with schizophrenia who do not currently experience hallucinations (though some may have in the past), and people without any schizophrenia spectrum conditions.
The experimental design was elegant yet straightforward. Participants were asked to imagine saying a simple syllable—either “bah” or “bee”—without moving their mouths. At the same time, they listened through headphones to sounds that either matched or mismatched the syllable they were imagining. In some trials, participants were instructed not to imagine speaking and just listen to the sounds as a control.
In participants without schizophrenia, hearing a sound while simultaneously imagining saying that same sound resulted in a dampened response in the auditory cortex, particularly when the sound matched the imagined syllable. This suppression aligns with the brain’s typical process of distinguishing self-generated speech from external sounds. However, the opposite was true for participants with schizophrenia who experienced auditory hallucinations: when the imagined and heard sounds matched, their auditory cortex response was stronger than when they simply listened passively. Participants with schizophrenia who were not currently hearing voices showed brain activity levels between these two extremes, suggesting a potential predisposition to hallucinations.
This study builds upon prior work by neuroscientist Xing Tian and colleagues at New York University Shanghai, who have extensively investigated the interactions between the brain’s motor and auditory areas. Tian’s research has mapped abnormal signals that could contribute to confusion between inner speech and external sounds, providing a foundation for the new findings.
Experts not involved in the study have praised its contributions. Albert Powers, a psychiatrist at Yale School of Medicine, noted that the research clarifies one possible neural mechanism behind auditory hallucinations but cautioned that further work is needed to determine if this pattern explains all types of sound-based hallucinations seen in schizophrenia. Mahesh Menon, a psychologist and co-head of the Schizophrenia Program at the University of British Columbia, also commended the study for its clever approach to investigating internal brain processes that are notoriously difficult to measure. He emphasized that these insights could have broader implications for understanding similar psychotic symptoms beyond schizophrenia.
An important point highlighted by Powers is that auditory hallucinations do not necessarily indicate severe schizophrenia, nor do all severe cases involve hallucinations. The relationship between symptom severity and hallucination presence is complex. Understanding the diverse brain pathways that lead to hallucinations may pave the way for novel treatments tailored to individuals’ specific neural profiles.
Whitford hopes that the EEG testing method used in this research could one day serve as a tool to assess individuals’ risk of developing psychotic symptoms and auditory hallucinations before they manifest clinically. Such predictive capability would be a significant breakthrough, potentially