Researchers in Montreal have made inroads into understanding why people who've lost their hearing maintain the ability to speak after they can no longer hear themselves.

The ability to hear yourself speak is key to pronouncing words properly. But speech involves more than just sounds. It can be regarded as an entirely mechanical process of movements of the jaw, tongue and vocal cords.

For example, adults who become deaf are able to continue speaking for years after hearing loss.

Jaw movements were recorded during speech

"This sort of suggested to us that there are inputs that guide the speech systems that have absolutely nothing whatsoever to do with sound," said psychology Prof. David Ostry of McGill University.

He and his colleagues developed a robotic device that pushed or pulled slightly on the jaw during speech. The device was wired up to adults with no hearing loss. It changed the mechanical component of speech while the acoustics remained the same.

Ostry recorded 20 volunteers as they repeated three tasks: speaking out loud, silent speech (speech without vocalization) and non-speech jaw movements.

Volunteers were asked to keep their volume constant by monitoring a level meter

He also played the audio tape back to the subjects and concluded their mouths, throats and jaws were compensating for the effect of the robotic pressure. They couldn't tell when the device was working or was shut off.

"Independent of acoustics, there is a particular path of the jaw that the nervous system wishes to follow," said Ostry. "Presumably this is what permits people who become deaf as adults to produce intelligible speech following deafness."

He said the automatic balance is achieved by muscle spindle receptors that monitor the length and movement of muscles in the vocal tract, and by touch and pressure sensors in tissues of the mouth and face.

Now that the team has shown how the body makes speech in the absence of hearing, Ostry hopes researchers will determine how to apply the finding. He believes robotics may play an important role in maintaining speech or restoring it for the deaf.

The study appears in Thursday's issue of the journal Nature. The National Institute on Deafness and Other Communication Disorders, the Natural Sciences and Engineering Research Council of Canada, and Le Fonds pour La Formation de Chercheurs et l'Aide à la Recherche, Quebec sponsored the research.