A new study from researchers at the University of New South Wales (UNSW) Sydney has provided some of the strongest evidence so far about why some people with schizophrenia hear voices.
The findings suggest that these voices may not come from outside the person at all. Instead, the brain may be mistakenly treating a person’s own inner thoughts as if they are coming from someone else.
The research was published in the journal Schizophrenia Bulletin and could help scientists move closer to finding biological signs of schizophrenia. At present, schizophrenia is diagnosed based on symptoms and clinical assessments because there are no blood tests, brain scans, or other medical tests that can definitively identify the condition.
Schizophrenia is a serious mental health disorder that affects how a person thinks, feels, and experiences reality. One of its best-known symptoms is hearing voices that other people cannot hear.
These experiences, known as auditory hallucinations, can be distressing and may interfere with daily life. Scientists have spent decades trying to understand exactly why they occur.
The new study focuses on something most people experience every day without noticing: inner speech. Inner speech is the silent voice inside your head that helps you think, plan, solve problems, remember things, or talk yourself through situations. Most people have this internal conversation throughout the day.
Professor Thomas Whitford, a psychologist at UNSW Sydney, explains that when people speak out loud, their brains automatically predict the sound of their own voice before they hear it. Because the brain expects the sound, the part of the brain that processes hearing becomes less active. This helps people recognize that the voice belongs to them.
The same process appears to happen during inner speech. Even though no sound is produced, the brain still predicts what the inner voice would sound like. As a result, activity in the brain’s sound-processing regions is reduced.
For many years, researchers have suggested that people who hear voices may experience a problem with this prediction system. According to this theory, the brain fails to recognize inner speech as self-generated. Instead, it treats the inner voice as though it is coming from an outside source.
Although this idea has existed for around 50 years, proving it has been difficult because inner speech cannot be directly observed. Researchers can ask people about their thoughts, but they cannot hear another person’s inner voice.
To investigate the question, the research team used electroencephalography, commonly known as EEG. This technology measures the brain’s electrical activity using sensors placed on the scalp. EEG allows scientists to observe how the brain responds during different mental tasks, including inner speech.
The study included three groups of participants. The first group consisted of 55 people with schizophrenia who had recently experienced hearing voices. The second group included 44 people with schizophrenia who had not recently heard voices or had never experienced them. The third group included 43 healthy adults without schizophrenia.
During the experiment, participants wore an EEG cap and listened to sounds through headphones. At certain times, they were asked to silently imagine saying simple sounds such as “bah” or “bih.” Afterward, one of those sounds was played through the headphones. Participants did not know in advance which sound they would hear.
The researchers then examined how the participants’ brains reacted.
Among healthy participants, brain activity decreased when the imagined sound matched the sound they later heard. This showed that the brain had successfully predicted the sound and therefore responded less strongly when it occurred.
However, the results were very different for participants who had recently experienced hearing voices. Instead of showing reduced activity, their brains became more active when the imagined and heard sounds matched. This suggests that their brains treated the internally generated speech as though it came from an external source.
Participants with schizophrenia who had not recently heard voices showed results that fell between the other two groups. Their brain responses were neither as strongly reduced as those of healthy participants nor as strongly increased as those who frequently heard voices.
The findings provide strong support for the idea that auditory hallucinations may arise when the brain has difficulty recognizing its own internally generated thoughts. Rather than identifying inner speech as self-produced, the brain may mistakenly interpret it as coming from another person.
Researchers believe this discovery could have important clinical benefits. If this unusual brain activity pattern can be reliably measured, it may eventually help identify people who are at high risk of developing psychosis before severe symptoms appear.
Earlier identification could allow doctors to provide support and treatment sooner, potentially improving outcomes.
The study also moves scientists closer to understanding the biological processes behind one of schizophrenia’s most puzzling symptoms. By learning more about how the brain processes thoughts and inner speech, researchers hope to develop better treatments and new ways to monitor the condition.
While more research is needed, the study offers an important step forward in understanding schizophrenia. It suggests that the voices some people hear may be closely connected to their own inner speech, and that a breakdown in the brain’s ability to recognize those thoughts may be at the heart of the experience.
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