What’s the latest theory on what causes people with schizophrenia to think the “voices” they hear are real?
Is there a non-chemical explanation for what causes auditory hallucinations? A new study from psychologists at UNSW Sydney offers some of the clearest evidence yet for why people with schizophrenia sometimes “hear voices” — a form of auditory hallucination.
The Freudian explanation is that the part of the brain that causes dreams when we sleep seeps into our consciousness while we’re awake; dreaming while awake.
That sounds plausible, but it doesn’t explain why the voices someone with schizoprenia hears are quite different from the voices of characters in their sleep dreams.
The UNSW Sydney research suggests the problem may come down to how the brain recognizes its own inner voice.
Instead of identifying certain thoughts as self-generated, the brain may mistake them for sounds coming from the outside world.
The study, published in Schizophrenia Bulletin, also offers a possible way to identify biological markers for schizophrenia, something that doesn’t currently exist in clinical practice.
The Challenge of Diagnosing Schizophrenia
Unlike many medical conditions, schizophrenia can’t be diagnosed with a blood test, brain scan or lab result.
Doctors rely on observed symptoms and patient reports — which is why there are times a person is misdiagnosed with schizophrenia; they actually have, for instance, bipolar disorder or even autism.
What Inner Speech Actually Is
Professor Thomas Whitford, a senior author on the study, has spent years studying inner speech in both healthy people and schizophrenics.
Inner speech is the silent voice most people hear in their heads — for instance, when they read, re-enact an argument earlier that day or imagine what they’re going to say when they meet their new partner’s parents.
It’s the running commentary behind planning, problem solving or noticing what’s going on around you.
When most people speak, even silently in their heads, the brain predicts the sound of its own voice. Because of that prediction, the part of the brain that processes external sounds becomes less active.
- It’s a built-in filtering system.
- The brain knows the sound is self-generated, so it turns down its response.
According to the researchers, this system appears to malfunction in people who hear voices that don’t exist objectively.
For decades, scientists have suspected that auditory hallucinations might come from a person’s own inner speech being misidentified as someone else’s voice.
The problem has always been testing the idea. Words inside your head are private. You can’t hear it, record it or directly observe it.
EEG technology offered a workaround. Even though talking inside your head is silent, the brain still reacts to it via electro-chemical neuron signals.
In healthy people, those reactions look very similar to what happens during spoken speech.
What Brainwaves Revealed
Using EEG recordings, the team found that healthy individuals show reduced brain activity in the auditory cortex when engaging in inner speech.
This reduction mirrors what happens when people talk out loud. But in people who hear voices, that reduction doesn’t happen.
Instead, their brains respond more strongly, as if the words inside their head are coming from an external source.
That heightened response may explain why the voices feel so vivid and real.
How the Study Was Set Up
The researchers divided participants into three groups. One group included people with schizophrenia who had experienced auditory verbal hallucinations within the past week.
A second group included schizophrenics who had no history of hallucinations.
Hallucinations are the secondary symptom of this disorder; schizophrenia may present with only its primary symptoms, characterized by disorganized thinking.
Schizophrenia is NOT the same as multiple personality disorder, even though the prefix “schiz” is a Greek-derived prefix that means split or division.
The “split” in schizophrenia refers to the disorganization or splitting up of the various components of the human mind — which in healthy people all work together.
The third group consisted of healthy participants with no history of schizophrenia.
Each person wore an EEG cap while listening to sounds through headphones.
Testing Sound Prediction in Real Time
At specific moments, participants were asked to silently imagine saying either “bah” or “bih.” At the same time, one of those sounds was played through the headphones.
Participants didn’t know whether the sound they imagined would match the sound they heard. In healthy subjects, brain activity dropped when the imagined sound matched the real one.
This showed that the brain correctly predicted the sound and suppressed its response.
What Happened in People Who Hear Voices
The pattern was reversed in participants who had recently experienced auditory hallucinations. Instead of reduced activity, their brains showed a stronger response when the imagined sound matched the external sound.
The brain reacted as though the sound were unexpected and coming from someone else. This reversal suggests the brain’s prediction system isn’t working properly during hallucinations.
Participants with schizophrenia who hadn’t experienced recent hallucinations showed brain responses that fell between the healthy group and the hallucinating group.
This suggests that the brain’s ability to distinguish inner speech from external sound may shift over time, rather than being permanently fixed.
The researchers now want to explore whether this brain response pattern could help identify people at risk of developing psychosis (mental illness characterized by loss of touch with reality) before symptoms become severe.
