by DR. CHINTA SIDHARTHAN
New research reveals that the brain’s failure to self-monitor motor signals plays a key role in schizophrenia-related hallucinations, offering fresh insights into the mechanisms behind these perceptual distortions.
In a recent study published in the journal PLOS Biology, researchers investigate how impaired self-monitoring linked to dysfunctions in motor signal copies contributes to auditory hallucinations in schizophrenia.
Perception vs. reality
Our perceptions about the surrounding environment originate from external sensory stimuli like sights, sounds, imagination, and recalling memories. Monitoring the different sources of these perceptions is vital, with hallucinations arising when these fail, and the brain is unable to separate the source from the perception.
This self-monitoring of different sources of perception is achieved through internal forward models, in which copies of motor signals such as corollary discharge (CD) or efference copy (EC) are involved in inhibiting or enhancing sensory processing. These copies of motor signals either enhance or suppress sensory processing, which allows internally generated sensations to be distinguished from external sensory stimuli.
In mental health diseases like schizophrenia, patients often experience auditory hallucinations. Recent evidence suggests that distinct dysfunctions in CD and EC, rather than a single inhibitory failure, might be disrupted and, as a result, cause auditory hallucinations in these patients.
About the study
In the present study, researchers examine specific impairments in the inhibitory function of CD and the enhancement function of EC that contribute to auditory hallucinations experienced by individuals with schizophrenia.
A total of 40 schizophrenia patients were divided into two groups based on whether the patient did or did not experience auditory verbal hallucinations. General preparation and specific preparation tasks were utilized to determine how speech preparation affects perception. These tasks were designed to explore how CD and EC influence auditory responses during different stages of speech preparation.
In both tasks, study participants were shown visual cues and were required to speak while hearing auditory probes in the form of tones or syllables.
The general preparation task involved a visual cue with no linguistic information; therefore, the participants were not aware of what they would say. Auditory probes, which were introduced in half of the trials, consisted of one of four auditory syllables ‘ba,’ ‘ka,’ ‘ga,’ or ‘pa’ or a pure tone of one kHz.
In the specific presentation task, the visual cue consisted of a specific syllable that the participants prepared to speak with auditory probes that either matched or were different from the syllable in the visual cue.
Baseline trials included speaking without preparation and passive listening to the probes without speaking. The reaction times for speaking were recorded for each participant and analyzed to determine how different task conditions impacted the response times.
Brain activity was also recorded using an electroencephalogram (EEG), with all data filtered to remove artifacts such as eye movements. Brain responses to the auditory probes were then analyzed to explore the influence of motor signals on sensory processing.
Clinical and demographic data were also analyzed using various statistical tools.
Study findings
Dysregulation of motor signal copies involving the inhibition of CD and enhancement of EC caused errors in self-monitoring, leading to auditory hallucinations in schizophrenia patients. The study’s computational modeling revealed that the combination of a “broken” CD and a “noisy” EC in patients with auditory hallucinations could explain their inability to accurately distinguish between internal and external auditory signals.
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