A new study from UNSW Sydney provides some of the clearest evidence yet that hearing voices in schizophrenia may come from the brain confusing its own thoughts as external sounds.
This means that people who hear voices might actually be hearing their own inner speech but their brains think it is coming from someone else.
The research, published in the journal Schizophrenia Bulletin, could also help scientists find biological markers for schizophrenia—clear signs in the body or brain that doctors can use to identify the condition. At the moment, there are no blood tests or brain scans that can diagnose schizophrenia.
Professor Thomas Whitford, a psychologist at UNSW, has been studying how inner speech works in both healthy people and people with schizophrenia.
He explains that inner speech is that quiet voice in your head that talks to you throughout the day, helping you plan, reflect, or think things through. Most people experience this voice all the time without thinking much about it.
Normally, when someone talks out loud, or even just silently in their head, the brain predicts the sound of their own voice. This prediction causes a drop in activity in the part of the brain that processes sound. But in people who hear voices, this prediction seems to go wrong. Instead of calming down, the brain acts as though the sound is coming from outside.
This idea—that people with schizophrenia may be hearing their own inner voice as if it were someone else’s—has been around for about 50 years. But because inner speech is invisible and private, it has been very hard to prove.
The researchers used a brain monitoring tool called EEG, which measures the brain’s electrical activity. Even though we can’t hear inner speech, EEG can pick up how the brain responds to it.
In healthy people, inner speech causes the same kind of drop in activity as speaking out loud. But in people who hear voices, this drop does not happen. In fact, their brains respond even more, as if someone else is speaking to them.
To test this idea, the researchers studied three groups. The first group included 55 people with schizophrenia who had heard voices recently. The second group had 44 people with schizophrenia who hadn’t heard voices lately or ever. The third group included 43 healthy people without schizophrenia.
Each person wore an EEG cap and listened to sounds through headphones. Sometimes they were asked to imagine saying “bah” or “bih” in their minds. Then they would hear one of those sounds. The participants did not know which sound they would hear in advance.
In the healthy group, brain activity dropped when the imagined sound matched the one played in the headphones. This shows the brain predicted the sound and didn’t react as much.
But in the group that had recently heard voices, the opposite happened. Their brains became more active when the imagined and heard sounds matched. This suggests that their brains treated the inner voice as if it were coming from outside.
The second group, people with schizophrenia but no recent voices, had results in between the two other groups.
This research is important because it gives strong support to the theory that schizophrenia might involve problems in how the brain predicts and understands its own thoughts. This misunderstanding may cause a person to hear their inner voice as if it belongs to someone else.
The team hopes to use this brain activity pattern to predict who might develop psychosis in the future. This would help doctors identify and treat people earlier.
Professor Whitford says that this is an exciting step toward understanding the biology behind schizophrenia symptoms. If we can understand what’s going wrong in the brain, we may be able to find better treatments.
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