If you hear a voice when no one is speaking, you may have an auditory verbal hallucination (AVH). These hallucinations are common among patients with schizophrenia, although explaining why and how these hallucinations occur has long puzzled scientists.
This week an article published in the magazine Schizophrenia Bulletin suggested that scientists have finally found the answer, however, by amassing clear evidence that AVH may arise from the mind's misinterpretation of its own internal monologue or voice—the silent stream of internal thoughts that participate in its ability to solve problems, make plans, reflect, and self-regulate.
“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,” said Thomas Whitford, study author and professor at the School of Psychology at the University of New South Wales in Sydney, Australia. statement released with the study. “But in people who hear voices, this prediction seems to be untrue, and the brain reacts as if the voice is coming from someone else.”
Read more: You may be hallucinating right now
Reaction to inner voices
Historically, patients with schizophrenia and AVH experience these hallucinations because their brains try to recognize an internal voice, mistaking it for the sound of an external speaker.
“This idea has been around for 50 years,” Whitford said in a statement, “but it has been very difficult to test because inner speech is inherently private.”
To solve this problem, Whitford and his team turned to electroencephalography (EEG) as a method of measuring people's reactions to their own internal monologues.
“Even though we can't hear inner speech, the brain still responds to it,” he added in a statement, suggesting the technology could clarify whether people with AVH regularly respond to their inner voices.
Dimension of a monologue
We are collecting a total of 142 participants, including participants without schizophrenia and AVH and With schizophrenia and AVH, among others, Whitford and his team decided to measure their internal vocal responses. After connecting each person to an EEG machine, the scientists asked participants to imagine making the monosyllabic sound “ba” or “bi” in their minds. Every time participants imagined making one of two sounds, they heard one of them through the headphones, although it was not always the sound they imagined.
It is important to note that when the imagined and heard sounds matched, participants without schizophrenia and AVH showed decreased activity in the auditory cortex, suggesting that they were anticipating the sound (much like what would happen if they said something out loud), while participants With schizophrenia and AVH showed increased activity in the same area, suggesting they were surprised by the sound.
“Their brains responded more strongly to internal speech that matched the external sound,” Whitford said of the participants with hallucinations in the release. “This change in the normal suppression effect suggests that the brain's prediction mechanism may be impaired in people who currently experience auditory hallucinations, which may result in their own internal voice being misinterpreted as external speech.”
Read more: Scientists aren't sure how the inner voice works
A clear and convincing test?
Taken together, the results suggest that people with AVH perceive their internal voice as an external voice. “It has always been a plausible theory that people hear their own thoughts spoken out loud,” Whitford said in a statement. “But this new approach provided the strongest and most direct test of this theory to date.”
In the future, Whitford and his team hope to explore whether this feedback to internal monologue could be a clear biomarker of schizophrenia, a condition for which there is currently no convincing biological indicator that can be measured with a simple brain or blood test.
“A measure like this has great potential to become a biomarker,” Whitford added in a statement. “Ultimately, I think understanding the biological causes of schizophrenia symptoms is a necessary first step if we hope to develop new and effective treatments.”
Read more: Do you have an inner voice? Science can't agree if everyone does it
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