Hearing imaginary voices is a common but mysterious feature of schizophrenia spectrum disorders. Up to 80 percent of people with these conditions experience auditory hallucinations, hearing speech or other sounds when there are none. Now research has gotten us closer to unraveling the brain mechanisms behind this phenomenon.
Experts have long thought auditory hallucinations arise from a person’s perception of their inner thoughts as real voices coming from the outside world. When people without schizophrenia speak or prepare to speak, the brain region that plans movements suppresses signals in the auditory cortex, the part of the brain that processes sound. This action helps people distinguish their own speech from external noise. Researchers theorized this mechanism could apply to healthy people’s inner speech as well—although that has been difficult to study and verify. Dysfunction in the activity between these brain regions might lead to hearing voices.
In a study published in Schizophrenia Bulletin, researchers demonstrated that inner speech indeed suppressed the brain’s auditory cortex in adults without schizophrenia. But in people with the condition who experienced auditory hallucinations, they found, inner speech boosted the auditory cortex’s response.
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“The hard thing with studying inner speech is that it’s inherently private,” says Thomas Whitford, a cognitive neuroscientist at the University of New South Wales in Australia and co-lead author of the study. To eavesdrop on that inner speech, Whitford and his colleagues used electroencephalography (EEG) to measure brain activity in individuals with conditions on the schizophrenia spectrum, including participants who heard voices and those who did not (but might have in the past), and in those who didn’t have such conditions. The researchers prompted the participants to imagine saying a specific syllable, either “bah” or “bee,” without actually moving their mouth. At the same time, a sound played through the subjects’ headphones that either matched or mismatched the sound they were told to imagine speaking. As a control condition, participants were sometimes told not to imagine anything and to simply listen to the sounds with their headphones.
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In adults without schizophrenia, simultaneously hearing and mentally producing a sound dampened the auditory cortex’s response compared with brain activity when they only listened without thinking of saying anything. The effect was strongest when the sound they heard matched the one they imagined. But participants with schizophrenia who had auditory hallucinations experienced the opposite effect: when the two sounds matched, their brain response was even stronger. People with schizophrenia who did not currently hear voices showed strong dampening in the auditory cortex with mismatched sounds but no dampening when the sounds matched. Whitford suggests this finding may be a sign that these participants had the potential to hallucinate.
This paper builds on previous research by neuroscientist Xing Tian of New York University Shanghai and his colleagues. Tian’s team has conducted numerous studies teasing apart mechanisms in the brain’s motor and auditory regions, including mapping abnormal signals that could lead to confusion between inner and external sounds in people with schizophrenia.
Whitford and his colleagues’ new study helps to clarify one possible mechanism for schizophrenia’s auditory hallucinations, says Albert Powers, a psychiatrist at the Yale School of Medicine, who wasn’t involved in the work. But further investigation is needed to see whether this pattern of brain activity contributes to all the different sound-based hallucinations people with schizophrenia might experience, he says, and not just voices.
Nevertheless, according to Mahesh Menon, a psychologist and co-head of the Schizophrenia Program at the University of British Columbia, who also wasn’t involved in the study, this research is “quite clever,” especially because these internal mechanisms are difficult to test experimentally. Menon adds that the new findings could be valuable for understanding how similar psychotic symptoms occur.
Powers emphasizes that experiencing auditory hallucinations doesn’t always indicate severe schizophrenia and that a person with severe schizophrenia won’t necessarily experience hallucinations. Disentangling the various pathways in the brain that could drive these hallucinations may lead to new treatment options, and “this paper helps to get us there,” he says. Whitford hopes his team’s EEG test could eventually be used to assess someone’s risk of developing psychotic symptoms and hallucinations. That predictive ability, he says, would be the “holy grail” that could help direct people toward early preventive treatment.