Anyone who’s done a bad Elvis impression knows that contorting your mouth makes talking feel wrong – never mind how ridiculous you sound. People who have lost their hearing use the same sense to retain their speech, new research suggests.

When five deaf volunteers were asked to talk while a robot nudged their jaws slightly, they quickly learned to compensate for the perturbation.

“One of the real mysteries of human language is that people who become deaf as adults remain capable of producing intelligible speech for years in the complete absence of any auditory input,” says David Ostry, a neuroscientist at McGill University in Montreal, Canada, who led the new study along with colleague Sazzad Nasir.

Most neuroscientists who study speech focus on how the brain learns from sounds to correct for errors. Yet just as a tennis player learns whether a forehand shot will land in or out just from the feel, people sense whether or not they are speaking correctly, Ostry says.


Robotic hindrance

To separate this ability from learning by hearing, he and Nasir enlisted the help of five deaf people with cochlear implants that allowed them to hear.

With their implants switched off, the volunteers were asked to repeat short words such as “sass” and “sane” that flashed onto a computer screen. At the same time, a robot arm pushed gently on their jaw, moving it a few millimetres. The robot could also measure how much a subject pushed back.

The nudge was too gentle to affect speech, Ostry says. “Subjects will leave the lab and often there’s no conscious awareness that the robot was doing anything.”

After hundreds of words, the subjects corrected for the prodding by moving their jaws several millimetres to oppose the robot’s nudge. Surprisingly, volunteers made the same adjustment when their cochlear implants were turned back on.

When six hearing volunteers were subjected to the same robotic treatment, four of them learned to similarly correct their mouth muscle movements.

The right moves

“When you talk you want to get movement right perhaps as much as you want the speech to sound properly,” Ostry says. “It’s not only making it sound right, it’s making it feel right.”

Deaf people may rely entirely on sensing their muscle movements to speak correctly, but the extent to which people “listen” to their bodies probably depends on the situation, says Jeffery Jones, a cognitive neuroscientist at Wilfrid Laurier University in Waterloo, Canada.

“If I’m in a very noisy environment such as a dance bar, I’m not going to hear myself very well, so I’m going to rely on another form of feedback,” he says.

Harnessing this sixth sense for speech might help people with stutters and other speech problems, Ostry says.

“It’s worth investing in therapies for speech that focus on movements and possibly focus on movement independent of speech sounds.”

Journal reference: Nature Neuroscience (DOI: 10.1038/nn.2193)