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 well.
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 be disastrous.
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 nerve.
This often happens as a result of type II neurofibromatosis (NF2), a rare disease that causes benign tumours in the inner ear.
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 November.
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 College London.
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.
New Scientist issue: 10th January 2004
Author: Duncan Graham-Rowe
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