Scientists have long puzzled over the origins of auditory hallucinations, and the “hearing of voices”, a symptom affecting 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.
The authors concluded that auditory hallucinations may result from a combination of two distinct impairments in the brain’s ability to process and predict sensory information, reports StudyFind.
Their findings, published in 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), signals 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 hypothesised that in people with auditory hallucinations, these systems might be impaired in different ways. To test this, they designed 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.
The study not only advances scientists’ understanding of auditory hallucinations but also highlights the importance of considering the interaction between motor and sensory systems in mental health. It suggests that what we often think of as purely sensory phenomena may have deep roots in the brain’s motor planning and prediction mechanisms.
While this study focuses on schizophrenia, its implications could extend to other conditions where reality perception is altered, such as in certain forms of dementia or drug-induced states. Moreover, it underscores the complexity of the human brain and the intricate balance required for normal perception and cognition.
Limitations
The study had a relatively small sample size of 40 schizophrenia patients, which may limit its generalisability, noted the authors. All patients were on medication, which could have affected the results. The study also focused on a specific type of hallucination (auditory verbal) and may not apply to other types of hallucinations or sensory disturbances.
Additionally, the research relied on EEG measurements, which, while informative, don’t provide the spatial resolution of other brain imaging techniques.
Study details
Impaired motor-to-sensory transformation mediates auditory hallucinations
Fuyin Yang, Hao Zhu, Xinyi Cao et al.
Published in PLOS Biology on 3 October 2024
Abstract
Distinguishing reality from hallucinations requires efficient monitoring of agency. It has been hypothesised that a copy of motor signals, termed efference copy (EC) or corollary discharge (CD), suppresses sensory responses to yield a sense of agency; impairment of the inhibitory function leads to hallucinations. However, how can the sole absence of inhibition yield positive symptoms of hallucinations? We hypothesise that selective impairments in functionally distinct signals of CD and EC during motor-to-sensory transformation cause the positive symptoms of hallucinations. In an electroencephalography (EEG) experiment with a delayed articulation paradigm in schizophrenic patients with (AVHs) and without auditory verbal hallucinations (non-AVHs), we found that preparing to speak without knowing the contents (general preparation) did not suppress auditory responses in both patient groups, suggesting the absent of inhibitory function of CD. Whereas, preparing to speak a syllable (specific preparation) enhanced the auditory responses to the prepared syllable in non-AVHs, whereas AVHs showed enhancement in responses to unprepared syllables, opposite to the observations in the normal population, suggesting that the enhancement function of EC is not precise in AVHs. A computational model with a virtual lesion of an inhibitory inter-neuron and disproportional sensitization of auditory cortices fitted the empirical data and further quantified the distinct impairments in motor-to-sensory transformation in AVHs. These results suggest that “broken” CD plus “noisy” EC causes erroneous monitoring of the imprecise generation of internal auditory representation and yields auditory hallucinations. Specific impairments in functional granularity of motor-to-sensory transformation mediate positivity symptoms of agency abnormality in mental disorders.
StudyFinds article – 40% of schizophrenia patients hear voices – New study reveals why (Open access)
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