Have you ever thought how do our brains recognize voices and speech? This question was not answered until a research team at University of California San Francisco (UCSF) the University of California, San Francisco, showed in a study—published in the journal Science—that the shaping of sound by our articulators "an acoustic trail" that the brain follows, exactly like how radars track ships.
Breaking Down Speech into Acoustic Features
To investigate this, UCSF researchers placed neural recording devices directly onto the surface of the brains of six patients who were undergoing epilepsy surgery. This allowed the researchers to capture very rapid changes in the brain.
The researchers collected data as they listened to 500 unique English sentences spoken by 400 different people, through recording from a brain area called the Superior Temporal Gyrus (STG), which previous research has shown to be involved in speech perception.
Many researchers have presumed that brain cells in the STG would respond to individual sound segments known as “phonemes”, such as the /b/ sound in (boy). However, the UCSF team reported that the brain is instead exquisitely tuned to detect simpler elements, which are known to linguists as “features”. The STG is tuned to respond to the acoustic features of the particular way that sounds are generated from the vocal tract.
“As a result of that, when we hear someone talking, and producing the stream of different speech elements different areas in the brain light up”.
How does the Bain recognize Speech?
“Features” as linguists use the term, are distinctive acoustic signatures created when speakers move the lips, tongue or vocal cords.
For example, consonants such as (/p/, /t/, /k/, /b/ and /d/) require speakers to use the lips or tongue to obstruct air flowing from the lungs. When this occlusion is released, there is a brief burst of air, which has led linguists to categorize these sounds as “plosives”. Others, such as (/s/, /z/, /f/ and /v/), are grouped together as “fricatives”, because they are produced by forcing air through a narrow channel made by placing two articulators close together.
The articulation of each plosive creates an acoustic pattern common to the entire class of these consonants, as does the turbulence created by fricatives.
The arrangement which the team discovered in the STG is reminiscent of feature detectors in the visual system for edges and shapes, which allow us to recognize objects, like bottles, no matter which perspective we view them from.