40% of schizophrenia patients hear voices

New study reveals why

SHANGHAI — In the silent world of thought, some hear voices. Scientists have long puzzled over the origins of auditory hallucinations, a symptom that affects many with schizophrenia. A recent study from researchers in China and the United States may have cracked a crucial part of this longstanding enigma, potentially paving the way for better treatments and understanding of this often-misunderstood condition.

Study authors conclude that auditory hallucinations may result from a combination of two distinct impairments in the brain’s ability to process and predict sensory information. Their findings, published in the journal PLOS Biology, suggest that these hallucinations arise from a complex interplay between motor and sensory systems in the brain rather than simply being a product of overactive imagination or sensory processing gone awry.

The study focused on two groups of schizophrenia patients: those who experienced auditory verbal hallucinations (AVHs) and those who did not. By comparing these groups to each other and to healthy individuals, the researchers were able to pinpoint specific differences in brain function that may contribute to the experience of hearing phantom voices.

At the heart of the study are two key concepts in neuroscience: corollary discharge (CD) and efference copy (EC). These are signals that the brain generates when planning or executing a movement, including speech. In a healthy brain, CD acts as a general inhibitory signal, dampening sensory responses to self-generated actions. This helps us distinguish between our own actions and external events. EC, on the other hand, is more specific, enhancing sensory responses related to the particular action being performed.

The researchers hypothesized that in people with auditory hallucinations, these systems might be impaired in different ways. To test this, they designed clever experiments that allowed them to measure brain responses during various stages of speech preparation and execution.

Participants were asked to prepare to speak in two different scenarios. In one, they knew what they were going to say (specific preparation), while in the other, they only knew they would speak but not what (general preparation). During these preparation phases, the researchers played sounds and measured the brain’s responses using electroencephalography (EEG).

The results were striking. In healthy individuals, general speech preparation suppressed overall auditory responses – a sign of normal CD function. However, this suppression was absent in both groups of schizophrenia patients, suggesting a fundamental impairment in this inhibitory mechanism.

When it came to specific speech preparation, the differences became even more apparent. In healthy individuals and schizophrenia patients without hallucinations, preparing to say a specific syllable enhanced brain responses to that same syllable when heard. In patients with auditory hallucinations, however, the opposite occurred – their brains showed enhanced responses to syllables different from the one they were preparing to say.

These findings paint a picture of a “broken” CD system combined with a “noisy” EC system in individuals who experience auditory hallucinations. The lack of proper inhibition (broken CD) may explain why these individuals have difficulty distinguishing between internal thoughts and external voices. Meanwhile, the imprecise enhancement (noisy EC) could account for the varied and often nonsensical content of hallucinations.

To further support their findings, the researchers developed a computer model that simulated these impairments. The model successfully replicated the patterns observed in the real-world data, providing additional evidence for their theory.