A new study led by UNSW Sydney psychologists provides the clearest evidence yet that hearing voices in schizophrenia may result from a breakdown in how the brain recognizes its own inner voices. This study suggests that the brain may misinterpret internally generated thoughts as sounds coming from the outside world.
Published in a magazine schizophrenia bulletinthis study also points to a possible avenue for identifying biological markers of schizophrenia. This is important because there are currently no blood tests, brain scans, or lab-based biomarkers (signs inside the body that tell us something about your health) that uniquely identify this condition.
Professor Thomas Whitford from the NSW School of Psychology has spent years researching how inner language works in both healthy people and people living with schizophrenia spectrum disorders.
“Inner speech is the voice in your head that silently tells you what you’re thinking: what you’re doing, what you’re planning, what you’re noticing,” he says.
“Most people experience inner speech on a regular basis, often without realizing it, but some never experience it.
“Our research shows that even when you’re just talking in your head, activity in the parts of the brain that process sounds from the outside world decreases, because the brain predicts the sound of your own voice. However, for people who hear voices, this prediction appears to be off, and the brain responds as if the voice were coming from another person.”
Brain waves reveal a long-held theory
Professor Whitford said these findings strongly support a theory that has existed in mental health research for decades: that auditory hallucinations in schizophrenia may result from a person’s own internal speech being mistaken for external speech.
“This idea has been around for 50 years, but it has been very difficult to test because inner conversations are inherently private,” he says.
“How do we measure it? One way is to use EEG, which records electrical activity in the brain. Even if we can’t hear our inner speech, the brain still responds to it. And in healthy people, using inner speech causes the same kind of reduction in brain activity as when speaking out loud.”
“However, such a reduction in activity does not occur in people who hear voices. In fact, their brains respond even more strongly to internal words as if they were coming from another person. That may help explain why voices feel so real.”
Test how the brain predicts sound
To investigate this effect, the researchers divided participants into three groups. The first group included 55 people with schizophrenia spectrum disorders who had experienced auditory hallucinations (AVH) within the past week. The second group included 44 patients with schizophrenia who had no history of AVH or had not recently experienced AVH. The third group consisted of 43 healthy individuals with no history of schizophrenia.
Each participant wore an EEG (electroencephalography) cap and listened to sounds through headphones. At a given moment, they were asked to imagine silently saying either “bar” or “bee” in their minds while hearing one of the same sounds played out loud. Participants did not know in advance whether the sounds they imagined matched the sounds they heard.
In healthy participants, brain activity decreased when an imagined syllable matched a sound played through headphones. This reduced response appeared in the auditory cortex, the area responsible for processing sounds and speech. This pattern suggests that the brain correctly predicted the sound and reduced its response, similar to what happens during normal conversation.
The opposite pattern emerged for participants who had recently experienced auditory hallucinations. Instead of showing reduced activity, their brains responded more strongly when the imagined sounds matched the ones they heard.
“Their brains responded more strongly to internal words that matched external sounds, which was the exact opposite of what we observed in healthy participants,” Professor Whitford says.
“This reversal of the normal inhibitory effect suggests that the predictive mechanisms in the brains of people currently experiencing auditory hallucinations may be disrupted, resulting in their own internal voices being misinterpreted as external voices.”
Participants in the second schizophrenia group, those who had not had any recent hallucinations, had brain responses that fell somewhere between the healthy and hallucinating groups.
What this means for schizophrenia research
Researchers say the results provide the strongest evidence yet that people with schizophrenia may experience imaginary sounds that appear to be coming from outside themselves.
“The theory that people hear their thoughts out loud has always been plausible, but this new approach provides the strongest and most direct test of this theory to date,” Professor Whitford says.
Looking ahead, the research team plans to investigate whether this pattern of brain responses can help predict who is likely to develop psychosis later in life. If successful, high-risk individuals could be identified earlier and treated sooner.
“This type of measurement has great potential as a biomarker for the development of psychosis,” Professor Whitford said.
“Ultimately, I think understanding the biological causes of schizophrenia symptoms is a necessary first step if we want to develop new and effective treatments.”
