A new study led by psychologists at UNSW Sydney offers the clearest evidence so far that hearing voices in schizophrenia may arise from a breakdown in how the brain recognizes its own inner voice. The research suggests that the brain may be misidentifying internally generated thoughts as sounds coming from the outside world.
Published in the journal Schizophrenia Bulletin, the study also points toward a possible path for identifying biological markers of schizophrenia. This is important because there are currently no blood tests, brain scans, or lab based biomarkers (signs in the body that can tell us something about our health) that uniquely identify the condition.
Professor Thomas Whitford of the UNSW School of Psychology has spent years studying how inner speech works in both healthy individuals and people living with schizophrenia spectrum disorders.
"Inner speech is the voice in your head that silently narrates your thoughts - what you’re doing, planning, or noticing," he says.
"Most people experience inner speech regularly, often without realizing it, though there are some who don’t experience it at all.
"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. 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."
Brainwaves Reveal a Longstanding Theory
According to Prof. Whitford, these findings strongly support a theory that has existed in mental health research for decades: that auditory hallucinations in schizophrenia may result from a person’s own inner speech being mistaken for external speech.
"This idea’s been around for 50 years, but it’s been very difficult to test because inner speech is inherently private," he says.
"How do you measure it? One way is by using an EEG, which records the brain’s electrical activity. Even though we can’t hear inner speech, the brain still reacts to it - and in healthy people, using inner speech produces the same kind of reduction in brain activity as when they speak out loud.
"But in people who hear voices, that reduction of activity doesn’t happen. In fact, their brains react even more strongly to inner speech, as if it’s coming from someone else. That might help explain why the voices feel so real."
Testing How the Brain Predicts Sound
To explore this effect, the researchers divided participants into three groups. The first included 55 people with schizophrenia spectrum disorders who had experienced auditory verbal hallucinations (AVH) within the past week. The second group included 44 people with schizophrenia who either had no history of AVH or had not experienced them recently. The third group consisted of 43 healthy individuals with no history of schizophrenia.