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Cochlear implant

Cochlear implants can help people with certain kinds hearing impairment or
who are deaf to hear. The implant works by using the tonotopic organisation
of the cochlear.

"Tonotopic organisation" is the way the ear sorts out the different
frequencies so that our brain can process that information. High frequency
sounds (which are high pitched sounds) do not pass very far along the
cochlear, but low frequency sounds pass further in. In a normal ear, the
sound causes vibration of hair cells which are located all along the
cochlear. The movement of the hair cells creates an electrical disturbance
that can be picked up by the surrounding nerve cells.

If hearing loss is caused by a problem in the hair cells or any other
problem related to the changing of sound energy to nerve pulses, then the
problem may be able to be rectified by a cochlear implant.

The implant is capable of creating an electrical pulse. It does this at a
certain point along the cochlear. This electrical pulse is interpreted by
the brain as a sound at a certain frequency.

Who can use a Cochlear Implant?

When a cochlear implant is used, the conduction of sound via the normal
processes can not be used again. Sound will no longer be able to be
conducted to the inner ear, but will have to be picked up by the microphone
used with the implant. For this reason, people with mild or conductive
hearing loss cannot use a cochlear implant. Patients must have a sensoneural
hearing loss in order to benifit from the implant.

The brain is adaptive. If someone has not had hearing for a long period of
time the brain will adapt and use the area of their brain normally used for
hearing for other functions. If a person has lost their hearing for a long
time and then regains it using an implant, the sounds can be very
disorientating as the brain is reading these signals not as sounds but as
other things.

The Device

The device can be devided into two parts, the part which is implanted, and
the part worn outside the body. The external part of the device contains:

   * Microphone
   * Processing
        o Speech Processing
        o Filters
   * Transmitter

The internal part of the device contains:

   * Receiever
   * Electrode Array of up to 22 electrodes wound through the cochlea

Processing

The sound wave received by the microphone must be processed to determine
which electrodes should be stimulated. The simplest way of processing would
be to devide the sound into however many electrodes there are, and apply the
resulting voltage to the appropriate electrode. More sophisticated
processing algorithms are used in practice because applying voltage to each
of the electrodes at the same time would cause currents to flow between the
electrodes, which would stimulate the nerves in undesirable ways.

Waveform processing strategies use bandpass filters to divide the signal
into different frequency bands. The algorithm chooses a number of the
strongest outputs from the filters. The number depends on the algorithm, and
can also depend on whether the sound is determined to be a consonant or a
vowel sound.

Feature extraction strategies use features which are common to all vowels.
Each vowel has a fundamental frequency (the lowest frequency peak) and
formants (peaks with higher frequencies). The pattern of the fundamental and
formant frequencies is specific for different vowel sounds. These algorithms
try to recognise the vowel and then emphasise its features.

Transmitter and Reciever

These are used to transmit the processed sound information over a radio
frequency link. This is so that no physical connection is needed, which
reduces the chance of infection.

Electrode array

The electrode array is made from a type of sillicone rubber, while the
electrodes are platinum or a similarly highly conductive material.

Objections to use

The use of cochlear implants is objected to by some, particularly in the
signing community (people who use sign language to communicate). They do not
want deafness to be considered a disability to be "fixed" by cochlear
implants, but rather just a different way of living. Cochlear implants work
best in those who are implanted at a pre-speaking age, and hence are
implanted before the recipient can decide for themselves.

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