If you hear a voice when no one is speaking, then you might be in the midst of an auditory verbal hallucination (AVH). These hallucinations are common among patients with schizophrenia, though the explanation of why and how these hallucinations occur has long stumped scientists.
This week, a paper published in the Schizophrenia Bulletin has suggested that scientists have finally found an answer, however, having collected clear evidence that AVH could come from the mind’s misinterpretation of its own inner monologue or voice — the silent stream of internal thoughts that are involved 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, a study author and a professor at the University of New South Wales School of Psychology in Sydney, Australia, in a statement released with the research. “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.”
Read More: You May Be Hallucinating Right Now
Reacting to Internal Voices
Historically, it’s been theorized that schizophrenia patients with AVH have these hallucinations because their brains struggle to identify their internal voice, mistaking it for the sound of an external speaker.
“This idea’s been around for 50 years,” Whitford said in the statement, “but it’s been very difficult to test because inner speech is inherently private.”
To solve that issue, Whitford and his team turned to electroencephalography (EEG) as a method for measuring people’s responses to their own internal monologues.
“Even though we can’t hear inner speech, the brain still reacts to it,” he added in the statement, suggesting that the technology could clarify whether people with AVH react regularly to their internal voices.
Measuring the Monologue
Gathering a total of 142 participants, including participants without schizophrenia and AVH and with schizophrenia and AVH, among others, Whitford and his team set out to measure their inner-voice responses. Hooking each person to an EEG machine, the scientists prompted the participants to imagine making a single-syllable sound, either ‘bah’ or ‘bih,’ in their mind. Each time the participants imagined making one of the two sounds, they heard one of them through a pair of headphones, though it wasn’t always the same sound that they were imagining.
Importantly, when the imagined and heard sounds matched, the participants without schizophrenia and AVH demonstrated a decrease in the activity in their auditory cortices, suggesting that they anticipated the sound (a lot like what would happen if they said something aloud), while the participants with schizophrenia and AVH showed an increase in the activity in that same area, suggesting that the sound surprised them.
“Their brains reacted more strongly to inner speech that matched the external sound,” Whitford said of the participants with hallucinations in the release. “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.”
Read More: Scientists Aren’t Sure How the Inner Voice Works
A Clear and Conclusive Test?
Taken together, the results seem to suggest that people with AVH perceive their internal voice as an external one. “It was always a plausible theory — that people were hearing their own thoughts spoken out loud,” Whitford said in the statement. “But this new approach has provided the strongest and most direct test of this theory to date.”
In the future, Whitford and his team hope to find out whether this reversed reaction to the inner monologue could be a clear biomarker of schizophrenia, a condition that currently lacks a conclusive biological indicator that could be measured with a simple brain or blood test.
“This sort of measure has great potential to be a biomarker,” Whitford added in the statement. “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.”
Read More: Do You Have an Inner Voice? Science Can’t Agree If Everyone Does
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