New evidence confirms a long-held theory that people with schizophrenia hear 'voices' in their heads by misattributing inner speech as external.
"This idea's been around for 50 years, but it's been very difficult to test because inner speech is inherently private," says Thomas Whitford, a psychology researcher at the University of New South Wales.
Using EEG (electroencephalography) to measure brainwaves, Whitford and his colleagues tested the way that the brain reacts to inner speech, and, in people with schizophrenia, the way their brains react to auditory hallucinations.
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"When we speak – even just in our heads – the part of the brain that processes sounds from the outside world becomes less active," Whitford explains. "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."
Participants included 55 people with schizophrenia and recent auditory hallucinations, 44 people with schizophrenia but no recent auditory hallucinations, and 43 people with no history of schizophrenia.
The cohort was asked to listen to audio over headphones and to say either 'bah' or 'bih' in their minds at the same time that they heard a sound played. This sound was also either 'bah' or 'bih', but participants did not know if their choice of internally voiced word would match what they heard in their headphones.
When the inner speech matched the external sound, the brains of the participants who had experienced recent auditory hallucinations reacted far more strongly.
"In healthy people, using inner speech produces the same kind of reduction in brain activity as when they speak out loud," Whitford says. "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."
This suggests that the 'voices' people with schizophrenia hear are their own inner speech, which their brain wrongly perceives as external. These findings may help doctors identify who is at risk of psychosis even before it occurs, which could allow for early intervention.
The research was published in Schizophrenia Bulletin.