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Study reveals brain networks critical for conversation

Scientists have been gaining insights into the brain regions involved in conversation. wavebreakmedia / Shutterstock

Having a conversation requires a high level of coordination between people. Speakers take turns and need to anticipate when others will finish talking. Replies are rapid. The gap between speakers is typically 200 milliseconds, or about the blink of an eye. This means that people are often planning their responses while listening.

Earlier studies identified several brain regions that are likely involved in the speech planning process. But the exact neural circuits responsible for planning replies weren’t well understood. To study these neural networks, a team led by Dr. Michael A. Long of the NYU Grossman School of Medicine conducted a series of experiments to map the brain during speech planning.

The study was funded in part by NIH’s National Institute on Deafness and Other Communication Disorders (NIDCD). Results appeared in Nature on January 5, 2022.

The researchers measured brain activity in eight volunteers undergoing brain surgery to remove tumors or treat epilepsy. Patients undergoing brain surgery are initially kept conscious so that doctors can ask them questions and avoid damaging speech centers in the brain.

To precisely monitor brain activity, the team used a technology called electrocorticography (ECoG). Electrodes are placed directly on the surface of the brain. Hundreds of electrodes were placed on each patient’s left brain hemisphere, which is involved in speech and language.

While using ECoG, the researchers posed a series of structured questions to the volunteers. The key word needed to respond to the question changed position, allowing the researchers to determine when speech planning began. For example: “The opposite of soft is what common word?” vs “What word is the opposite of soft?”

Based on these questions, the team was able to track the brain activity related to planning and distinguish it from brain activity while perceiving questions or producing speech.

They found that brain activity during speech planning was distinct from perception and speech production. The patterns suggested that the brain networks for each function are largely separate.

The team then mapped the location of the circuits involved. They found that most of the electrodes that responded during planning were located in two regions: the caudal inferior frontal gyrus (cIFG) and the caudal middle frontal gyrus (cMFG). The cIFG, or “Broca’s region,” has long been known as an important language processing center. But the cMFG has not generally been considered important for speech planning, so its involvement was unexpected.

Further tests showed that the planning networks identified during the task were also active during natural conversations. When the researchers engaged the patients in casual conversation, they observed the same pattern of brain activity during speech planning.

These results shed light on the brain circuitry that enables quick verbal exchanges. “Our study pinpoints brain networks behind the planning that makes this back and forth possible, which have been elusive until now,” Long says.

The findings may also lead to a better understanding of speech disorders such as stuttering.

—by Erin Bryant