Summary: A new study reveals that auditory hallucinations in schizophrenia may arise when the brain fails to recognize its own inner voice as self-generated. Normally, the brain predicts the sound of one’s inner speech and dampens auditory responses accordingly, but this prediction breaks down in people who hear voices.
EEG readings showed that instead of reduced auditory activity, individuals with recent hallucinations had stronger brain responses when imagining speech that matched external sounds. The discovery provides the strongest evidence yet for the misattributed inner speech theory and may pave the way toward early biomarkers for psychosis.
Key Facts:
- Brain Prediction Error: The auditory cortex fails to suppress self-generated inner speech, causing the brain to misinterpret it as external sound.
- EEG Evidence: Participants with recent hallucinations showed reversed brainwave responses, revealing disrupted prediction mechanisms.
- Clinical Potential: Findings may lead to biological markers to identify or predict psychosis before full onset.
Source: University of New South Wales
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 assess is 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.”
Key Questions Answered:
A: The study suggests it occurs when the brain misidentifies its own inner speech as external speech due to a disruption in how it predicts self-generated sounds.
A: Using EEG, they compared brain activity in people imagining sounds with and without schizophrenia, revealing reversed suppression effects in those hearing voices.
A: Understanding the biological origin of hallucinations could lead to early diagnostic tools and more targeted interventions for psychosis.
About this schizophrenia and auditory hallucination research news
Author: Lachlan Gilbert
Source: University of New South Wales
Contact: Lachlan Gilbert – University of New South Wales
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Corollary discharge dysfunction to inner speech and its relationship to auditory verbal hallucinations in patients with schizophrenia spectrum disorders” by Thomas Whitford, et al. Schizophrenia Bulletin
Abstract
Corollary discharge dysfunction to inner speech and its relationship to auditory verbal hallucinations in patients with schizophrenia spectrum disorders
Background and Hypothesis
Auditory-verbal hallucinations (AVH)—the experience of hearing voices in the absence of auditory stimulation—are a cardinal psychotic feature of schizophrenia-spectrum disorders. It has long been suggested that some AVH may reflect the misperception of inner speech as external voices due to a failure of corollary-discharge-related mechanisms. We aimed to test this hypothesis with an electrophysiological marker of inner speech.
Study Design
Participants produced an inner syllable at a precisely specified time, when an audible syllable was concurrently presented. The inner syllable either matched or mismatched the content of the audible syllable. In the passive condition, participants did not produce an inner syllable. We compared the amplitude of the N1, P2, and P3-components of the auditory-evoked potential between: (1) schizophrenia-spectrum patients with current AVH (SZAVH+, n = 55), (2) schizophrenia-spectrum patients without current AVH (SZAVH−, n = 44), (3) healthy controls (HC, n = 43).
Study Results
The HC group showed reduced N1-amplitude in the Match condition (relative to Passive and Mismatch), replicating our previous results. In contrast, the SZAVH+ group showed the opposite effect: enhanced N1-amplitude in the Match condition (relative to Passive and Mismatch). The SZAVH− group showed reductions in the Mismatch condition (relative to Passive and Match).
Conclusions
This study provides empirical support for the theory that AVH are related to abnormalities in the normative suppressive mechanisms associated with inner speech. This phenomenon of “inner speaking-induced suppression” may have utility as a biomarker for schizophrenia-spectrum disorders generally, and may index a tendency for AVH specifically at more extreme levels of abnormality.
