Children learn to talk by listening to others speak, but what happens when that line of communication is severed? Surprisingly, people who go deaf as adults can chat intelligibly for years afterward. Now, thanks to a robot that tweaks jaw movements, scientists may have figured out why.
Neuroscientists David Ostry and Sazzad Nasir of McGill University in Montréal, Canada, suspected that in addition to listening, people pay attention to muscles in their own faces, tongues, and vocal tracts to judge whether they are saying words correctly. To test the theory, the two recruited 11 adult volunteers, some with normal hearing and some who had gone deaf within the past 20 years but who wore cochlear implants.
The crux of the experiment was distorting the muscular sensations of speaking. The researchers connected the subjects to a robotic device that tugged their lower jaw outward a tiny amount. The two groups were then asked to say words like "sass" and "saw"--words that start with the same mouth position and use vowels that open the mouth wide--as the robot pulled their jaw outward by a few millimeters. The tiny shifts in mouth position didn't change the way words sounded, Ostry says. But what if the participants could not rely on any sound cues to adjust their speech?
The team had the deaf subjects turn their implants off during the experiment. After repeating hundreds of words, all five of the deaf participants began to pull their jaws in slightly to partially correct for the manipulation, the researchers report online this week in Nature Neuroscience. "These are such small changes, but the nervous system cares when you get the movements wrong by tiny amounts," Ostry says. The results indicate that sensations from the jaw and vocal tract alone can be used for speech learning.
"It's beautiful work," says James Lackner, a neuroscientist at Brandeis University in Waltham, Massachusetts. The findings may one day inspire speech therapy approaches that teach people to notice changes in mouth position rather than listening for changes in sounds, says Shari Baum, a neurolinguist at McGill University who was not involved in the research. But it's important not to forget the significance of hearing, especially for children learning to speak, she says.