By: Inas Essa

Taking turns on time in a conversation without interrupting each other is key in keeping it smooth and ending it well. While it seems to come naturally, complicated work is done in our brains to make it happen correctly. 

This hard work requires a remarkable degree of coordination to make the gap between turns about 200 milliseconds—just an eyeblink. That indicates that speakers usually plan their responses while listening to their conversation partner(s). Although the network of brain regions has been involved in this process of speech planning, the neural part underlying the initial processes that enable rapid turn-taking remained uncovered.

The Brain Circuit Involved in Conversations

In a recent study from the NYU Grossman School of Medicine and University of Iowa published in the journal Nature, researchers have identified the distinct brain circuit that becomes active as we plan our spoken replies during a conversation. The new study sheds light on the circuit that enables such quick replies, which are planned before the end of the partner’s turn.

“Our study pinpoints brain networks behind the planning that makes this back and forth possible, which have been elusive until now,” says senior author Michael Long, Ph.D., the Thomas and Suzanne Murphy Professor of Neuroscience and Physiology at NYU Grossman School of Medicine.

Highlighting a Previously Blind Spot

Researchers tried previously to link speech functions to brain networks using electroencephalograms (EEG); a test or a record of brain activity through attaching small sensors to the scalp to pick up the electrical signals produced by the brain. However, it could not pinpoint the location of nerve circuits with enough resolution or capture activity patterns related to the conversational planning of replies.

Another technology that was used, yet did not come up with distinct results, was electrocorticography (ECoG), which involves placing electrodes directly on the surface of the brain. This method revealed that the brain achieves natural conversation by combining perception of what is heard, the planning of a reply, and the production of the sounds that make up words.

This new study is, therefore, the first to capture brain activity during the reply planning phase between them, which has been the hardest part to study.

The Experiment

“Researchers can talk to patients and watch the activity of brain circuits as they talk or listen, but planning has no physical correlate,” says Long. “When we combined ECoG measures with a technique that asks patients structured questions, we exposed an underlying planning network.”

During the study, the team placed electrodes on the brain surfaces of patients during surgeries intended to remove either a tumor or brain tissue causing epileptic seizures. Surgeons placed patients under local anesthesia initially to be able to determine the brain regions that are active while the patient talks to avoid damage to the patient’s speech centers. Then, the researchers used a series of motor tasks to localize the planning network that becomes active when preparing the speech.

The findings demonstrate that separate cortical modules perform specific functions within the language network during interactive speech production. Also, they indicate that planning-related responses during the interaction are largely restricted to a frontotemporal circuit centered on the cIFG, the caudal inferior frontal gyrus, and the cMFG, the caudal middle frontal gyrus.

Helping People with Speech Problems

“This study provides a first description of the specific brain mechanisms that generate language as we speak in natural, every day contexts,” says Gregg Castellucci, Ph.D., a postdoctoral fellow in Long’s lab. “Crucially, the brain mapping we found using simple, controlled tasks held up in tests of natural human behavior,” he added.

The researchers find this work promising to guide the design of new therapies for people with speech problems, including those with apraxia, trouble planning speech movements, and aphasia, difficulty processing language, which can accompany conditions like autism or result from trauma caused by a stroke.

References

nature.com
nyulangone.org/news/study-reveals-brain-networks-enabling-human-conversation