The Loudness Of Vowels Helps The Brain Break Down Speech Into Syl-La-Bles

Nov 20, 2019
Originally published on November 20, 2019 6:52 pm

When we hear a sentence, or a line of poetry, our brains automatically transform the stream of sound into a sequence of syllables.

But scientists haven't been sure exactly how the brain does this.

Now, researchers from the University of California, San Francisco, think they've figured it out. The key is detecting a rapid increase in volume that occurs at the beginning of a vowel sound, they report Wednesday in Science Advances.

"Our brain is basically listening for these time points and responding whenever they occur," says Yulia Oganian, a postdoctoral scholar at UCSF.

The finding challenges a popular idea that the brain monitors speech volume continuously to detect syllables. Instead, it suggests that the brain periodically "samples" spoken language looking for specific changes in volume.

The finding is "in line" with a computer model designed to simulate the way a human brain decodes speech, says Oded Ghitza, a research professor in the biomedical engineering department at Boston University who was not involved in the study.

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Detecting each rapid increase in volume associated with a syllable gives the brain, or a computer, an efficient way to deal with the "stream" of sound that is human speech, Ghitza says. And syllables, he adds, are "the basic Lego blocks of language."

Oganian's study focused on a part of the brain called the superior temporal gyrus.

"It's an area that has been known for about 150 years to be really important for speech comprehension," Oganian says. "So we knew if you can find syllables somewhere, it should be there."

The team studied a dozen patients preparing for brain surgery to treat severe epilepsy. As part of the preparation, surgeons had placed electrodes over the area of the brain involved in speech.

"So then, we asked our patients to lay back, relax and listen," Oganian says.

They heard lots of sentences, including the first line of a sonnet by Shakespeare: "Shall I compare thee to a summer's day."

An analysis of the patients' brain activity confirmed earlier research suggesting that changes in volume were important to detecting syllables.

But the changes happened so fast that there was no way to tell precisely when the brain was responding to volume changes. Was the trigger the quietest point, the loudest point or somewhere in between?

So the team used computer software to slow down each sentence without changing the pitch or other characteristics.

"What we saw with the slow speech is that the neural response went up every time the speech intensity started to rise fast," Oganian says.

Those rapid increases in volume were occurring at the beginning of each vowel sound, she says. And the brain could tell whether vowel was stressed or unstressed.

So when patients heard the word summer, their brains recognized that the stress fell on the first vowel sound not the second.

Detecting this difference is important because stressed and unstressed syllables help create the rhythm of human speech, Oganian says, including poetry.

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ARI SHAPIRO, HOST:

As you listen to this sentence, your brain is breaking down a stream of words into syllables. That process is critical to understanding speech. And now scientists think they know how it works. NPR's Jon Hamilton has more.

JON HAMILTON, BYLINE: A familiar sonnet by Shakespeare...

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED PERSON: Shall I compare thee to a summer's day?

HAMILTON: ...One line, 10 syllables. And our brains detect each one of those syllables. But how? That's what Yulia Oganian and a colleague at the University of California, San Francisco wanted to know. So they looked at a part of the brain called the superior temporal gyrus.

YULIA OGANIAN: That's an area that has been known for over 150 almost years to be really important for speech comprehension. So we knew if we can find syllables somewhere, it should be there.

HAMILTON: The team studied a dozen patients preparing for brain surgery. Oganian says that meant they already had electrodes in place monitoring the brain area involved in speech.

OGANIAN: So then we asked our patients to lay back and relax and listen.

HAMILTON: They heard lots of sentences, including that bit of Shakespeare.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED PERSON: Shall I compare thee to a summer's day?

HAMILTON: And their brain activity suggested that changes in volume were the key to detecting syllables. But it all happened so fast that there was no way to tell exactly when the brain was responding. So they had a computer slow down each sentence.

(SOUNDBITE OF ARCHIVED RECORDING)

UNIDENTIFIED PERSON: Shall I compare thee to a summer's day?

HAMILTON: Oganian says that revealed the precise timing.

OGANIAN: What we saw with the slow speech is that the neural response went up every time the speech intensity started to rise fast.

HAMILTON: The brain was using an abrupt increase in volume as a marker for each syllable. And that increase, at least in English, was occurring at the beginning of each vowel sound. Oganian says the team also noticed something else. The brain could tell whether a vowel was stressed or unstressed.

OGANIAN: So, for example, in summers, there's suh-mmers (ph).

HAMILTON: The uh (ph) sound in summer gets stressed while the er (ph) sound gets less emphasis. Oganian says detecting this difference is critical because stressed and unstressed syllables help create the rhythm of human speech. The idea that the brain might use just one type of volume change to detect syllables has been debated for many years. But Oded Ghitza of Boston University says the system suggested by Oganian's team makes sense.

ODED GHITZA: It is in line with the model that we propose.

HAMILTON: The model Ghitza is talking about is a computer model meant to simulate the way a human brain decodes speech. And Ghitza says detecting syllables is critical.

GHITZA: We are talking one to another until there's a stream going on, right? So how the brain deals with the stream?

HAMILTON: Ghitza says one way is to break the stream into separate parts called syllables.

GHITZA: That's the basic Lego block of language.

HAMILTON: Including poetry. The new research appears in the journal Science Advances. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.