How the brain turns our intended words into the sounds of speech

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How the brain turns our intended words into the sounds of speech
A new study from UC San Francisco challenges the traditional view of how the brain strings sounds together to form words and orchestrates the movements to pronounce them.

Speaking is one of the most complicated things a human can do. Before you even say a word, your brain has to translate what you want to say into a perfectly sequenced set of instructions to the dozens of muscles you use to speak.

For more than a century, scientists thought all this planning and coordination — called speech-motor sequencing — happened in a part of the frontal lobe called Broca’s area. 

Now, a new study from UC San Francisco shows that it relies on a much wider network of neurons across many brain areas. This network is centered in an area called the middle precentral gyrus, or mPrCG, which scientists thought might only control the larynx, a part of the vocal tract that helps us make high- or low-pitched sounds.

“It turns out that this part of the brain has a much more interesting and important role,” said Edward Chang, MD, Chair of Neurosurgery, and senior author of the study. “It strings together the sounds of speech to form words, which is crucial to being able pronounce them.”

The study, which appears July 16 in Nature Human Behaviour, could inspire new ways of looking at speech disorders, aid in the development of devices that allow paralyzed people to communicate, and help preserve a patient’s ability to speak after brain surgery. 

Beyond Broca’s area

Broca’s area, named for physiologist Pierre Paul Broca, who discovered it in 1860, has been believed to handle most of our language processing. That encompasses both how we make sense of language we hear or read, and how we produce the words we intend to say. 

But several years ago, Chang, who is a member of the UCSF Weill Institute for Neurosciences and has spent over a decade exploring the question of how the brain produces speech, began to suspect that it involves areas beyond Broca’s. 

In a rare case study, he’d seen that when a patient had a tumor removed from their mPrCG, they developed apraxia of speech, a condition in which people know what they want to say but struggle to coordinate the movements needed to say it clearly. The same condition didn’t result from similar surgeries in Broca’s area.

Chang and then-graduate student Jessie Liu, PhD also noticed activity associated with speech planning in the mPrCG while developing a device to allow people with paralysis to communicate.

To investigate what was happening, Chang, Liu, and postdoctoral scholar Lingyun Zhao, PhD, worked with 14 volunteers undergoing brain surgery as part of their treatment for epilepsy. Each patient had a thin mesh of electrodes placed on the surface of their brain, which could record brain signals happening just before they spoke their words. 

Neurosurgeons like Chang routinely use these electrodes to help them map where in the patient’s brain the seizures are happening. If there are speech areas nearby, the surgeon will map those as well, to avoid damaging them during surgery.

Liu and Zhou were able to piggyback on the technology to see what was happening in the mPrCG when patients were talking.

They showed the volunteers sets of syllables and words on a screen and then asked them to make the sounds out loud. Some sets were simple repeated syllables, like “ba-ba-ba,” while others included more complex sequences, like “ba-da-ga,” that contain a variety of sounds.

The researchers saw that when they gave participants more complex sequences, the mPrCG was more active than when participants were given simple ones. The team also found that the increase in activity in that region predicted how quickly the participants would begin speaking after they read the words. 

“Seeing this combination — working harder to plan more complex sequences and then signaling muscles to put the plan into action — tells us that even though the mPrCG is outside of Broca’s area, it’s critical to orchestrating how we speak,” Liu said.

Connecting intention to action

The team also used the electrodes to stimulate the mPrCG in five of the study participants while they were uttering set sequences of syllables.

If the sequences were fairly simple, the participants had no problem. But when they were given more complex sequences, the stimulation caused participants to make errors resembling the apraxia of speech Chang saw in his case study.

That adds more evidence that the mPrCG is central to coordinating multiple different speech sounds and acts as a bridge connecting what a person wants to say with the actions that are required to say it.

“It’s playing this vital role that had been thought to belong to Broca's area but didn't quite fit there,” Liu said. “This points us in a new research direction, where learning how the mPrCG does this will lead us to a new understanding of how we speak.”  


Funding: This work was funded by the NIH (R01-DC012379) and philanthropy.

Authors: Patrick Hullett, MD, PhD, of UCSF is also an author of the study. 

About UCSF: The University of California, San Francisco (UCSF) is exclusively focused on the health sciences and is dedicated to promoting health worldwide through advanced biomedical research, graduate-level education in the life sciences and health professions, and excellence in patient care. UCSF Health, which serves as UCSF's primary academic medical center, includes top-ranked specialty hospitals and other clinical programs, and has affiliations throughout the Bay Area. UCSF School of Medicine also has a regional campus in Fresno. Learn more at ucsf.edu, or see our Fact Sheet.

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