When a word is uttered, muscles and receptors present in the mouth and throat retain a memory of their positioning and feeling when the word was uttered. Scientists suggest that these signals provide key inputs to the brain as they sharpen the power of speech.
An unusual experiment was conducted by researchers David Ostry and Sazzad Nasir at McGill University, Montreal to unravel the enduring enigma: why are many deaf people still able to speak coherently, even years after losing their hearing power?
Five middle-aged people, who had lost their hearing ability in adulthood and were presently deaf, were recruited for the study. But they had a cochlear implant to pick up sounds.
With the implant turned off, they were told to repeat 4 specific words while the front position of their lower jaw was gently pulled forward by a miniscule device joined to the bottom row of their front teeth.
Though tiny, the movement was enough to deform the sounds, emitted from the participant’s mouth.
The focus of the experiment was to observe if the volunteers could adapt to the sudden speech deformation, even though they could not hear the sounds they were producing.
The four words chosen were ‘saw’, ‘say’, ‘sass’ and ‘sane’. The choice was made on the basis of the way they pronounced their vowel, diphthong and fricative- the hiss of ‘s’- which demand a very precise jaw position to be pronounced clearly.
As the volunteers ran through a programme of 300 utterances, they gradually learnt to rectify errors in their pronunciation even though they could not hear the deformed sounds that they made. Moreover, they learnt at the same pace as a group of people with similar age and normal hearing ability who also performed the same experiment.
Ostry told, “The deformation (by the machine) is in the orders of millimetres. Even when these individuals can't hear what they're saying, when the motion path of the jaw is changed just a tiny amount, it's enough to prompt a corrective response.”
The study has been published online by the ‘Nature Neuroscience’ journal.
Ostry was quoted as saying, “When a child learns to talk, it gets two kinds of information. One is the auditory information, being the sound of its own voice. At the same time, it also gets information from receptors that are in the skin and in the muscles. These receptors develop not only in expectation of what words should sound like, they also develop an expectation of what the word should feel like.”
He added, “When you're deaf, you've lost one of the two systems that basically support speech, but you still have the other one. And the other one accounts for an enormous proportion for the total sensory inflow that's associated with speech. It’s like having a flat [tyre] and discovering that you have a spare after all.”
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