People who become deaf as adults use information from the vocal tract to hone their speech, researchers in Montreal have found.

Many people who are deaf are able to speak coherently years after losing their hearing, but scientists don't understand why that is.

To find out, researchers studied five middle-aged people who lost their hearing in adulthood and wore cochlear implants to pick up sounds. These subjects were compared with six middle-aged people with normal hearing. 

All subjects were asked to say four short words — "saw," "say," "sass" and "sane" — that require precise jaw positioning.

As part of the experiment, jaw motions were disturbed slightly by a robotic device, which initially caused deformed sounds to be emitted from the mouth. 

The experiment was designed to see if the people who were deaf were able to adapt to the jaw disturbances even though they could not hear what sounds they were making.

With training, the volunteers were able to adjust their jaw movements by a few millimetres to compensate for the robotic nudge, whether or not they were wearing the cochlear implants.

What's more, hearing and deaf subjects learned equally fast, David Ostry and Sazzad Nasir of McGill University report in Sunday's online issue of the journal Nature Neuroscience.

The pair attribute the adaptive ability to mechanoreceptors in the muscles, skin and soft tissues of the vocal tract that remember how they should feel when a word is pronounced.

The findings suggest that our brains learn to correct speech based on feedback from muscle movements as well as sounds.

The results may help people to overcome stutters.