Not Everyone with Schizophrenia Hears Voices. Here’s Why

Why do only some people with schizophrenia hear voices?

By Hannah Seo edited by Lauren J. Young

Hearing imaginary voices is a common but mysterious feature of schizophrenia. Up to 80 percent of people with the condition experience auditory hallucinations, meaning they hear voices or other sounds when there are none. Today, new research brings us closer to discovering the brain mechanisms behind this phenomenon.

Experts have long believed that auditory hallucinations occur when a person perceives their inner thoughts or speech as real voices coming from the outside world. When people without schizophrenia speak or prepare to speak, the region of the brain that plans movements suppresses signals in the auditory cortex. This helps people distinguish their own speech from outside sounds. The researchers thought this mechanism might also apply to healthy people’s self-talk, although it has been difficult to study and verify. Dysfunction in activity between these brain regions could lead to hearing voices.

In a study published last month in Schizophrenia Bulletin, Researchers demonstrated that inner speech effectively suppressed the brain’s auditory cortex in non-schizophrenic adults. But for people with this condition and those like it who have had auditory hallucinations, inner speech boosted the response of the auditory cortex.

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“The difficulty with studying inner speech is that it is 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 listen to this inner speech, Whitford and his team used electroencephalography (EEG) to measure the brain activity of people with schizophrenia spectrum disorders, including participants who heard voices and those who did not (but may have done so in the past), as well as participants who did not suffer from such disorders. The researchers prompted participants to imagine saying a specific syllable, “bah” or “bee,” without actually moving their mouth. At the same time, a sound played through headphones worn by participants either matched or did not match the sound they were asked to imagine they were speaking. As a control condition, participants were sometimes asked not to imagine anything and simply listen to the sounds through their headphones.

Simultaneously hearing and mentally producing a sound attenuated the auditory cortex response in adults without schizophrenia, compared to simply listening to it without thinking about saying anything. The effect was stronger when the sound these participants heard in their headphones matched the sound they imagined. In contrast, participants with schizophrenia who had auditory hallucinations experienced the opposite effect: When the two sounds matched, their brain response was even stronger. The results for people with schizophrenia who were not currently hearing voices fell between the other two groups. Whitford suggests that this could be a sign that these participants had the potential to hallucinate.

This article builds on previous research by neuroscientist Xing Tian of New York University in Shanghai and colleagues. Tian’s team has conducted numerous studies on understanding mechanisms in motor and auditory regions of the brain, including mapping abnormal signals that can lead to confusion between internal and external sounds.

The new research from Whitford and colleagues helps clarify a possible mechanism for the auditory hallucinations of schizophrenia, says Albert Powers, a psychiatrist at the Yale School of Medicine, who was not involved in the study. But further research is needed to see whether this pattern of brain activity contributes to all the different auditory hallucinations that people with schizophrenia might experience, he says.

Still, this research is “really, really smart,” not least because these internal mechanisms are difficult to test experimentally, says Mahesh Menon, a psychologist and co-director of the schizophrenia program at the University of British Columbia, who was also not involved in the paper. Menon adds that the new findings could be helpful in understanding how similar psychotic symptoms occur.

Powers points out that having auditory hallucinations does not always indicate severe schizophrenia and that having a severe case of the illness does not necessarily mean a person will have hallucinations. Untangling the different brain pathways that might lead to these hallucinations could lead to new treatment options, and “this paper helps us do that,” he says. Whitford hopes his team’s EEG test can eventually be used to assess the risk of developing psychotic symptoms and hallucinations. This predictive ability, he says, would be the “holy grail” that could help direct people toward early preventive treatment.

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