Mapping the brain to find out more about schizophrenia
Brain mapping reveals the circuitry involved in distinguishing self-generated sounds from external noises, advancing our understanding of human speech and auditory hallucinations.
Have you ever wondered how we distinguish self-generated sounds from external sounds? A team, led by researchers from NYU Langone Health, its Neuroscience Institute and NYU Tandon School of Engineering (all NY, USA), was curious, which led them to devise a study to investigate the underlying neural circuitry. Their work aims to advance our understanding of human speech and auditory hallucinations in schizophrenia, potentially leading to the development of noninvasive tests for individuals with schizophrenia.
Evolutionarily, the ability of animals and humans to tell the difference between their own calls or voices and those of others is considered a key component of threat perception and enhanced survival. In humans, individuals with the impaired ability to distinguish self-generated from external sounds are thought to experience auditory hallucinations, a common symptom of schizophrenia.
Previous research has demonstrated that the motor cortex – known to be involved in coordinating speech – is involved in this process in other mammals, sending a corollary discharge signal, essentially a copy of the motor command, to sensory cortices. However, the origin of the auditory corollary discharge signal within the motor cortex in humans is unknown. The current team set out to understand more about this signal using both neurosurgical recordings and connectivity techniques.
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They conducted voice experiments with eight adults undergoing routine surgery for epilepsy-related seizures as this provided a unique opportunity to record brain activity invasively; more than 200 probes were inserted in their brains to monitor seizure-related activity as well as capture the auditory corollary discharge signal. They collected 3200 electrical recordings during planned surgical breaks where participants completed voice experiments, which required participants to respond out loud to statements being read aloud by others. Finally, they used a computer model to assess the active regions during the voice experiments.
The researchers mapped the auditory corollary discharge signals’ origin to a subregion of the ventral motor cortex called the precentral gyrus. These signals then migrated across the precentral gyrus to the superior temporal gyrus, a neighboring auditory cortical subregion.
“We believe our study solves a long-standing puzzle in our understanding of human speech, offering the first direct evidence of the motor cortex brain circuits involved in corollary discharge that allow us to stay alert to our surroundings even while we are speaking,” commented the study’s lead investigator, Amirhossein Khalilian-Gourtani (NYU Tandon School of Engineering).
The study offers a possible explanation for the source of auditory hallucinations in individuals with schizophrenia, which may be a result of disruption in this signal between the motor and auditory cortices, causing them to be unable to dissociate self-generated sounds from external sounds.
Further research is needed to determine how and whether the corollary discharge circuit is active directly prior to auditory hallucinations. The team also plans to collaborate with psychiatrists to develop noninvasive testing for this signal in individuals with schizophrenia, to be able to characterize hallucinations appropriately.
