Spoken language may seem effortless, but it requires a complex dance of the muscles in the lips, jaw, tongue and voice box. New research reveals the patterns of brain activity that control these finely wrought movements, essentially keeping people from being perpetually tongue-tied.
The wide variety of sounds in American English are produced by combining just a small set of movements, researchers reported online today (Feb. 20) in the journal Nature. In the study, scientists recorded the activity of neurons in a part of the brain called the sensorimotor cortex, which coordinates muscle movements, while people pronounced various syllables. Researchers measured brain activity using electrodes placed on the surface of the brain as part of a clinical treatment for epilepsy.
The results showed that the activity patterns of large populations of cells corresponded in space and time to certain phonetic features.
In essence, the scientists have charted a map of the brain's somatosensory cortex for specific facial and oral body parts, computational neuroscientist Nicho Hatsopoulos of the University of Chicago, who was not involved in the study, told LiveScience. Furthermore, the timing of activity in different brain areas is important, Hatsopoulos said. "You see activity in one part that might represent the lips or tongue, and then later in time, another area gets activated."
The resulting brain activity is like a carefully tuned orchestra; each instrument section generates a specific sound, and those sounds are coordinated to produce the overall symphony.
The researchers' map of different vocal regions in the brain mirrored the layout of the vocal tract. They found an additional region, representing the voice box (the larynx), which is not present in non-human primates. The area could be a unique human specialization for speech, the researchers say.
Previous studies have shown that merely stimulating one spot in the brain cannot produce speech sounds. Rather, speech requires the concerted activity of many different neurons, as the new study confirms.
The speech features found in the study aren't unique to English, but define many other languages as well, hinting that humans evolved to have these characteristics. Further fine-tuning of the speech features may give different languages their unique sounds.
Beyond helping in understanding how the brain works, this research could ultimately lead to the development of brain-machine interfaces that could decode what a person with facial paralysis is trying to say, Hatsopoulos said.