Implants for Deaf People
by Duncan Graham-Rowe,
New Scientist, January
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Two deaf women
in the US have become the first people to undergo the risky
procedure of having implants in their brainstems.
The devices are designed to restore hearing by directly
stimulating nerves. Some deaf people have been given implants
that sit just outside the brainstem, but these do not work very
Feeding auditory signals directly into the brainstem should work
better, but because the brainstem carries signals from the
entire body to the brain, any damage caused by an implant could
The procedure is far more risky than, say, placing implants in
the cortex to try to restore some vision. "If you damage the
cortex it's not that big a deal. But at the brainstem level
every neuron you damage could damage function," says Bob Shannon
of the House Ear Institute in Los Angeles, the surgeon who
pioneered the procedure. "We took 15 years to convince ourselves
that this could be done safely."
Most deafness is caused by problems with the sound-detecting
hair cells in the cochlea in the ear. Cochlear implants bypass
the hair cells and stimulate the auditory nerve directly. But
they cannot help people with a damaged cochlea or auditory
This often happens as a result of type II neurofibromatosis
(NF2), a rare disease that causes benign tumours in the inner
At the moment, the only way to restore hearing to people with
NF2 is to stimulate the brainstem using a non-penetrating device
called an auditory brainstem implant.
ABIs enable the person to hear, but usually not well enough to
understand speech because the implant cannot separately
stimulate different groups of nerves corresponding to distinct
frequency ranges, or "channels".
Cochlear implants do not have this problem because nerves
corresponding to audible frequencies are spread along the length
of the cochlea.
By stimulating different points on the cochlea it is possible to
activate eight or more channels- enough to understand speech
over the phone.
ABIs, on the other hand, tend to stimulate only a single
channel. But Shannon hopes that his implant, in which eight
electrodes of different lengths are inserted into the brainstem,
will be able to stimulate several bundles of nerves individually
and produce different frequencies.
The key to his design is the shape of the electrodes. Too sharp
and they cut cells, too blunt and they crush them. After
experimenting with different shapes on animals and cadavers,
Shannon came up with a design resembling the tip of a pencil
that glides past neurons without harming them and proved safe
enough to test on people.
In the first patient, a 19-year-old woman who received an
implant earlier last year, only one of the eight electrodes
seems to have worked. It is still too early to evaluate the
second patient, a 42-year-old woman given an implant in
Although one channel is no better than ABIs, Shannon hopes that
in future implants he can get at least four electrodes working.
Having a single channel working improves lip-reading by 30 per
cent, but four channels would be enough to understand speech.
That could really improve the quality of people's lives, says
Stuart Rosen, a speech and hearing specialist at University
If this procedure proves successful, it might also help
congenitally deaf children who are born without a cochlear
nerve, adds Richard Ramsden at the Manchester Royal Infirmary,
who has performed most of the ABI implants in the UK.
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