How Your Brain Detects the Rhythms of Speech Into Syl-la-bles

speech

The lines of a favorite poem, or the closing lines of a favorite book stay with us for a long time. Not only do we memorize the string of words, but we sound them out in our head — we know when a reader does the lines justice, and we’re frustrated when it comes off sounding monotonous or not as eloquent as we’ve grown accustomed to. As we read, our brain decodes the letters of each sentence into a specific set of sounds, moving in rhythm. Until now, we were never quite sure as to how the brain processed this.

Researchers at the University of California, San Francisco, think they may have come across a clue — and published their report in Science Advances. As we pronounce vowel sounds, the volume of our voice increases. The brain may have a way of picking up on these changes in volume, the same way it tries to read social cues on the faces of other people.

As the brain decodes speech, rather than continuously search for syllables as was once believed, it periodically samples bits of spoken language. As it takes in these samples, it looks for changes in volume. Oded Ghitza, a research professor in biomedical engineering at Boston University, who did not participate in the study, noted that the research was in line with a computer model meant to simulate how the brain filters human speech. As the brain heard the words of a sentence, with the volume alternating between vowels and consonants, it had a means of processing language. Think of the alternating sounds in each sentence you construct as the waves of an ocean.

The study, carried out by postdoctoral researcher Yulia Oganian, focused activity in a part of the brain known as the superior temporal gyrus, known to neurobiologists for its role in speech and language development. If any part of the brain recognized syllables, Oganian thought, this would be it.

Oganian and her team of researchers experimented on a dozen patients awaiting brain surgery to treat symptoms of severe epilepsy. Before they underwent surgery, doctors placed electrodes on their heads, over the temporal gyrus region.

“We asked our patients to lay back, relax, and listen,” Oganian recalls.

A number of sentences were then read to the patients while they relaxed, including the memorable (and often misunderstood) first line of Shakespeare’s 18th sonnet: “Shall I compare thee to a summer’s day.”

Oganian’s researchers looked at the brain activity of the patients before reading to them and saw that volume was having an impact on how they were processing sentences — but they weren’t able to isolate where the changes in volume were happening that the brain took its cues from as it all happened very rapidly. Her team had to decide what the brain picked up on first — was it observing the loudest points or the quietest points in the sentence? Maybe it wasn’t even that clear cut and occurred somewhere in between.

They consulted audio recording software and played slowed down recordings of each sentence used in the first trial, but made sure to keep the pitch consistent so they could still understand the words.

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

The volume of the speaker picked up at the beginning of each vowel sound, she noticed. The brain also was able to determine whether the vowel came out stressed or unstressed. And the brain could tell whether vowel was stressed or unstressed. When the patient heard the word summer, brains could differentiate the pronunciation of the u with a long emphasis, and the e, with a much shorter one.

This difference could have implications in how we understand and interpret languages and language learning — even the way we recognize literature from mere words on a page. There’s still much more to uncover, but perhaps we’re just a few vowel sounds away from having phrases in other languages impact us just as much as words in our own.

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