Imma Perfetto
Cosmos science journalist
Researchers trying to understand how people with schizophrenia hear voices, have focussed on a breakdown in the brain to tell the difference between self-generated thoughts and external voices.
New research suggests abnormalities in 2 key brain processes likely contribute to what are referred to as: ”auditory verbal hallucinations,” (AVH) or “hearing voices”. This is the most common symptom of schizophrenia, which affects 1 in 300 people worldwide.
The new research is written up in PLOS Biology. The researchers thought that in patients with schizophrenia who experience AVH, processes in the brain’s ability to “self-monitor” malfunction.
To investigate this, they used electroencephalogram (EEG) experiments to measure the brain waves of 40 patients diagnosed with schizophrenia – 20 of whom experience AVHs and 20 who do not – while they were preparing to speak a syllable.
Throughout the entire action of speaking (including preparing to speak) the brain sends a signal known as a “corollary discharge” to suppress the perception of these sounds. This is important for labelling sounds as either the result of internal or external sources.
In addition, when a person prepares to speak, a duplicate of the instructions are sent as an “efference copy” to the region of the brain that processes sound – the auditory cortex. This is done so the brain can make a prediction of what sound it is about to hear, which also helps the brain distinguish between self-made sounds and the less predictable ones made by other people.
It is thought that efference copy is the origin of inner speech.
The findings of the study indicate that when patients with AVH prepared to speak, a “noisy” or imprecisely activated efference copy resulted in an enhanced auditory cortex response to internal sounds, different to the syllable they planned to say.
In addition, a “broken” corollary discharge meant there was no inhibition of auditory responses and impaired ability to distinguish between internally generated and externally generated sounds.
The authors conclude that impairments in these two processes likely contribute to auditory hallucinations and that targeting them in the future could lead to new treatments.
