The study of the brain’s ability to differentiate between external sounds and one’s own voice has long fascinated neuroscientists, particularly in relation to how the brain processes auditory feedback during speech. A recent groundbreaking study has offered new insights into the neural circuits involved in this phenomenon, known as auditory corollary discharge signals. These signals are electrical pathways in the brain that allow individuals to distinguish their own voice from external sounds, which plays a critical role in speech production and auditory perception.
The study, conducted by researchers from NYU Langone Health and the NYU Tandon School of Engineering, focused on patients with epilepsy undergoing routine brain surgery. The researchers aimed to map the neural pathways involved in auditory corollary discharge signals and determine their role in speech processing and potential links to conditions like schizophrenia and stuttering. The findings, which have been published in the Proceedings of the National Academy of Sciences, represent a significant advancement in understanding how the brain processes speech and auditory feedback.
At the core of this research is the relationship between two key regions of the brain: the motor cortex, responsible for voluntary muscle movements, and the auditory cortex, which processes sound. These regions communicate with one another through a series of electrical signals that help the brain manage the feedback it receives from speaking. When we speak, the brain receives auditory feedback, allowing us to monitor our speech. The corollary discharge system ensures that the brain can filter out the sound of our own voice from the surrounding environment, helping us focus on external noises or sounds that might be important for communication, safety, or environmental awareness.
In terms of evolutionary biology, the ability to distinguish between one’s own vocalizations and external sounds is thought to have enhanced survival. For example, animals like crickets and songbirds must distinguish their own calls from those of others, which is crucial for tasks like mating, territory defense, or evading predators. Similarly, bats rely on sound reverberations to navigate their surroundings. For humans, this ability plays an equally vital role, enabling us to maintain attention to our environment even while speaking. Disruptions to this system, however, can lead to issues such as auditory hallucinations or the inability to distinguish one’s own voice from external sources, as is commonly seen in schizophrenia.
The study involved eight adult participants with epilepsy, who were all undergoing surgical procedures to locate the source of their seizures. During these procedures, the patients volunteered to take part in experiments that involved speaking specific words and listening to audio recordings of those words. The research team then used an array of over 200 electrodes, which had been inserted into the patients’ brains to monitor seizure activity, to record electrical brain activity during the experiments.
Through these recordings, the researchers were able to trace the pathway of corollary discharge signals. They discovered that these signals originate in the ventral part of the motor cortex, specifically in a subregion called the precentral gyrus. These signals were found to travel across the folds of the motor cortex and into a neighboring region of the auditory cortex, known as the superior temporal gyrus. This connection between the motor and auditory cortices is crucial for filtering out the sound of one’s own voice and focusing on external auditory information.
The discovery of these pathways provides the first direct evidence of the brain circuits involved in corollary discharge during speech. According to Dr. Amirhossein Khalilian-Gourtani, the lead investigator of the study, this research addresses a long-standing puzzle in neuroscience: understanding the exact brain circuits involved in the brain’s ability to filter out self-generated sounds while speaking. This mechanism is essential for maintaining awareness of our environment and processing external sounds, even while we are engaged in a task like speaking.
The implications of these findings extend beyond speech processing and auditory perception. One key application is in understanding mental health conditions such as schizophrenia. Schizophrenia is often characterized by auditory hallucinations, where individuals hear voices that are not present in their environment. According to Dr. Adeen Flinker, the senior investigator of the study, disturbances in the corollary discharge system may contribute to these hallucinations. In some people with schizophrenia, the brain may fail to differentiate between internal and external auditory signals, leading to confusion between self-generated sounds and external noises.
The study provides a potential explanation for how such disruptions occur at the neural level, offering a new perspective on the neurobiological basis of auditory hallucinations. The researchers also believe that disruptions in this same system may contribute to other speech-related disorders, such as stuttering, where the brain’s feedback mechanisms are not properly synchronized.
In the course of their experiments, the researchers asked participants to perform a series of voice-related tasks, such as repeating a word they had just heard or describing a picture using a specific word. These exercises were designed to test the participants’ ability to distinguish between internal and external sounds while speaking. Throughout these experiments, the researchers recorded electrical activity in the brain to identify which regions were involved in processing the auditory feedback related to speech. The team found that the corollary discharge signals were not only involved in speech production but also helped participants filter out background noise and maintain focus on the task at hand.
The research also highlights the potential for using these findings to develop new diagnostic and therapeutic strategies for individuals with mental health disorders. The researchers are already planning further studies to explore how disruptions in the corollary discharge circuit might be linked to the onset of auditory hallucinations in people with schizophrenia. In the future, they hope to develop noninvasive methods for testing these brain circuits in individuals with psychiatric conditions, which could lead to improved diagnostic tools and treatments.
The work of the NYU Langone and NYU Tandon team has opened up new avenues for research in both neuroscience and psychiatry. By identifying the specific neural circuits involved in auditory corollary discharge, this study offers new insights into how the brain processes speech and external auditory signals. It also underscores the importance of understanding how these processes go awry in conditions like schizophrenia, where the ability to distinguish between self-generated sounds and external stimuli is impaired.
The findings of this study may eventually lead to more effective treatments for auditory-related disorders, such as schizophrenia and stuttering. Researchers are already exploring ways to use this knowledge to improve brain stimulation techniques or develop non-invasive interventions that could help individuals with these conditions regain their ability to filter out irrelevant sounds and distinguish their own voice from those of others.
Source: NYU Langone Health