Have you wondered how our brain can separate all the sounds of speech it hears into distinct, syllabic words? Until a recent study, neuroscientists believed the brain recognized speech volume to distinguish between syllables and when they started and ended. Now, with a new study from UC San Francisco, we know that the brain is actually recognizing the stress placed on the beginning of vowel sounds to differentiate the sounds it processes.
The brain uses the speech signal of the rising volume on the vowel sounds in the speech cortex area, positioned in the middle superior temporal gyrus. This signal is unique because the rising volume on the beginning of vowel sounds is universal across all spoken languages in humans. The brain is able to extract information about the syllables and where they are in relation to other words based on this signal.
In order to reach this conclusion, speech neuroscientists monitored the brain with electrodes during playbacks of speech recordings on 11 patients who were in the hospital, connected to electrodes that were attached for seizure-mapping. The data was analyzed against the neural patterns during the syllables of language the patients heard.
By further slowing down the speech by 4x, the researchers could determine that the signals occurred at the rising volume stress at the beginning of the vowel sounds, instead of at the rising stress of the whole syllable as previously thought. The brain is capable of determining the context of the speech it hears where intonation and syllabic emphasis is important (such as homonyms and homographs).
Homographs are words that are spelled exactly the same but have different meanings, such as bass (musical instrument) vs. bass (the fish); live (as in a state of living) vs live (as in a live tv show). The more difficult homographs, such as console, are interpreted by the brain as where the stress is on the vowel: console a friend has an emphasis on the long o sound of the second o, versus a tv console which has a different emphasis on the o. These differences help the brain cue into what is being said.
Oganian, Yulia, and Edward F. Chang. “A Speech Envelope Landmark for Syllable Encoding in Human Superior Temporal Gyrus.” Science Advances, vol. 5, no. 11, 20 Nov. 2019, doi:10.1126/sciadv.aay6279.