FEATURE ARTICLE
The Design and Function of Cochlear Implants
Fusing medicine, neural science and engineering, these devices transform human speech into an electrical code that deafened ears can understand
Michael Dorman, Blake Wilson
Hardware For Hearing
In a deafened ear, hair–cell failure severs the connection
between the peripheral and central auditory systems. Cochlear
implants restore the link, bypassing hair cells to stimulate
directly the cell bodies in the spiral ganglion.
A cochlear implant has five main components, only two of which are
inside the body. Above the outer ear, an external microphone picks
up sounds in the environment and directs them to a sound processor,
which sits inside a case behind the ear. The processed signals are
conveyed to a high–bandwidth radio–frequency
transmitter, which beams the information through a few millimeters
of skin to a receiver/stimulator that has been surgically implanted
in the temporal bone above the ear. The signals then pass to an
array of electrodes inside the cochlea. Target cells in the spiral
ganglion are separated from the electrodes by a bony partition.
Scott N.'s device uses the continuous interleaved sampling, or CIS,
strategy to convert acoustic signals into a code for stimulating the
auditory nerve. One of us (Wilson), along with colleagues at the
Research Triangle Institute and Duke University, developed the CIS
strategy. It starts by filtering a signal into frequency bands (16
for Scott N.). For each band, the CIS algorithm converts the slow
changes of the sound envelope into amplitude–modulated trains
of biphasic (having negative and positive components) pulses at the
electrodes. The processor sends information from low–frequency
channels to electrodes in the apex and information from
high–frequency channels to electrodes in the base of the
cochlea. This organization maintains the logic of the frequency map
in a normal cochlea.
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