Cochlear implants
Cochlear implants can help people with certain kinds of hearing impairment or who are entirely deaf. The implant works by using the tonotopic organisation of the basilar membrane of the inner ear.
"Tonotopic organisation" is the way the ear sorts out different frequencies so that our brain can process that information. In a normal ear, sound vibrations in the air lead to resonant vibrations of the basilar membrane inside the cochlea. High-frequency sounds (i.e. high pitched sounds) do not pass very far along the membrane, but low frequency sounds pass farther in. The movement of hair cells, located all along the basilar membrane, creates an electrical disturbance that can be picked up by the surrounding nerve cells. The brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being heard.
If hearing loss is caused by a problem in the hair cells, which are frequently damaged over time by excessively loud noises, or any other problem related to the changing of sound energy to nerve pulses in the ear, then hearing may be improved by a cochlear implant. The implant analyses the impinging sound and creates an electrical pulse at a certain point along the cochlea, allowing the brain to interpret the frequency of sound as it would if the hair cells of the basilar membrane were functioning properly (see above).
Who can use a Cochlear Implant?
Once a cochlear implant is implanted, any residual hearing a person has may be destroyed. Therefore, cochlear implants are generally only recommended for adults who have very severe or profound hearing loss, or children with profound hearing loss that is not helped sufficiently by hearing aids. After the implant is put into place, sound will no longer be conducted to the inner ear, but will have to be picked up by a microphone and sent through the device's speech processor to the implant's electrodes inside the cochlea. For this reason, people with mild or moderate sensoneural hearing loss or Filters
The internal part of the device contains:
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 divide 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 Receiver
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. The transmitter attaches to the receiver using a magnet that holds through the skin.
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.
When used for children who are born deaf (as opposed to those who lose their hearing later in life), 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.
