It takes sound waves in the environment and transforms them into nerve impulses to the auditory nerve.

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Cochlear implants can deliver sound collected from an external microphone directly to the auditory nerve and into the brain.

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Several weeks later, although hearing sensitivity had recovered to normal levels, activity in the auditory nerve connecting the inner ear to the brain remained impaired.

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Today's electrodes stimulate the auditory nerve electrically, but they do so indirectly because they are separated from the auditory-nerve fibres by fluid and a bony wall.

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The measurements, translated to electrical impulses, would be delivered to the three vestibular nerves that emanate from the three semicircular canals, much like audio signals are delivered to the auditory nerve in cochlear implants.

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Because each semicircular canal has its own nerve, a vestibular implant may actually be easier to perfect than a cochlear implant, which has to deliver many different sound frequencies to a single, bundled auditory nerve.

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So John Middlebrooks, also at the University of Michigan, and Russell Snyder, at the University of California, San Francisco, have gone back to an idea first proposed in the 1970s: to penetrate the auditory nerve and stimulate the nerve fibres directly.

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One of the hardest tasks facing those who fit these devices is working out the precise choreography of the voltages and timings that need to be applied to the 20 or so electrodes embedded in the auditory nerve, in order to make them work properly.

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