A study of the way people process inner and outer speech is the first tangible evidence of a key theory about schizophrenia's drivers.
A new study led by psychologists from UNSW Sydney has provided the strongest evidence yet that auditory verbal hallucinations – or hearing voices – in schizophrenia may stem from a disruption in the brain's ability to recognise its own inner voice.
In a paper published today in the journal Schizophrenia Bulletin, the researchers say the finding could also be an important step towards finding biological indicators that point to the presence of schizophrenia. This is significant as there are currently no blood tests, brain scans, or lab-based biomarkers – signs in the body that can tell us something about our health – that are uniquely characteristic of schizophrenia.
Professor Thomas Whitford, with the UNSW School of Psychology, has been examining the role of inner speech in the cognition of healthy people and people living with schizophrenia spectrum disorders for some time.
"Inner speech is the voice in your head that silently narrates your thoughts – what you're doing, planning, or noticing," he says.
"Most people experience inner speech regularly, often without realising it, though there are some who don't experience it at all.
"Our research shows that when we speak – even just in our heads – the part of the brain that processes sounds from the outside world becomes less active. 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."
Brainwave analysis
Prof. Whitford says this confirms what mental health researchers have long theorised: that auditory hallucinations in schizophrenia may be due to the person's own inner speech being misattributed as external speech.
"This idea's been around for 50 years, but it's been very difficult to test because inner speech is inherently private," he says.
"How do you measure it? One way is by using an EEG, which records the brain's electrical activity. Even though we can't hear inner speech, the brain still reacts to it – and in healthy people, using inner speech produces the same kind of reduction in brain activity as when they speak out loud.
"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."
Sound choices
The researchers divided participants into three groups. The first group included 55 people living with schizophrenia-spectrum disorders who had experienced auditory verbal hallucinations (AVH) in the past week. The second group of 44 participants also had schizophrenia, but either had no history of AVH or hadn't experienced them recently. The third group was a control group of 43 healthy people with no history of schizophrenia.
Each participant was connected to an EEG (electroencephalography) device to measure brainwaves as they listened to audio over headphones. They were asked to imagine saying either 'bah' or 'bih' in their minds at the exact moment they heard recordings of one of those two sounds played through headphones. The participants had no way of knowing whether the sound they heard in the headphones would match the sound they made in their imagination.
In the healthy participants, when the sound that played in the headphones matched the syllable they imagined saying in their minds, the EEG showed reduced activity in the auditory cortex – the part of the brain that processes sound and speech. This suggests the brain was predicting the sound and dampening its response – similar to what happens when we speak out loud.
However, in the group of participants who had recently experienced AVH, the results were the reverse. In these individuals, instead of the expected suppression of brain activity when the imagined speech matched the sound heard, the EEG showed an enhanced response.
"Their brains reacted more strongly to inner speech that matched the external sound, which was the exact opposite of what we found in the healthy participants," Prof. Whitford says.
"This reversal of the normal suppression effect suggests that the brain's prediction mechanism may be disrupted in people currently experiencing auditory hallucinations, which may cause their own inner voice to be misinterpreted as external speech."
Participants in the second group – people with a schizophrenia-spectrum disorder who hadn't experienced AVH recently or at all – showed a pattern that was intermediate between the healthy participants and the hallucinating participants.
What this means
The researchers say this is the strongest confirmation to date that the brains of people living with schizophrenia are misperceiving imagined speech as speech that is produced externally.
"It was always a plausible theory – that people were hearing their own thoughts spoken out loud – but this new approach has provided the strongest and most direct test of this theory to date," Prof. Whitford says.
He says the next thing he and his fellow researchers want to look at is to see if this measure can be used to predict who might transition to psychosis, with the potential to identify people with high risk of developing psychosis, which would allow for early intervention.
"This sort of measure has great potential to be a biomarker for the development of psychosis," Prof. Whitford says.
"Ultimately I think that understanding the biological causes of the symptoms of schizophrenia is a necessary first step if we hope to develop new and effective treatments."
