Cochlear implants are hearing devices that can help people with certain kinds of hearing impairment or who have a severe to profound hearing loss. The implant works by using the tonotopic organization of the basilar membrane of the inner ear.
"Tonotopic organization" 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.
In individuals with sensorineural hearing loss, hair cells are often fewer in number and damaged. Hair cell loss or absence may be caused by a genetic mutation or an illness such as meningitis. Hair cells may also be destroyed chemically by an ototoxic medication, or simply damaged over time by excessively loud noises. The cochlear implant by-passes the hair cells and stimulates the cochlear nerves directly using electrical impulses. This allows 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?
Cochlear implants are usually recommended for individuals who have severe to profound sensorineural hearing loss and who do not benefit sufficiently from hearing aids. Individuals with auditory neuropathy may also benefit from cochlear implants. Cochlear implant centers determine implant candidacy on an individual basis and take into account a person's hearing history, cause of hearing loss, amount of residual hearing, speech recognition ability, health status, and family commitment to aural habilitation/rehabilitation.
Once a cochlear implant is put in place, any residual hearing a person has in that ear may be destroyed. For this reason, people with mild or moderate sensorineural hearing loss or conductive hearing loss are generally not candidates for cochlear implantation. After the implant is put into place, sound no longer travels via the ear canal and middle ear but will be picked up by a microphone and sent through the device's speech processor to the implant's electrodes inside the cochlea.
If an individual has been deaf for a long period of time, the brain may begin using the area of the brain normally used for hearing for other functions. If such a person receives a cochlear implant, the sounds can be very disorienting, and the brain often will struggle to readapt to sound.
Children who receive cochlear implants at a young age can usually adapt to them very well. Many are able to learn to comprehend sounds and speech without having to rely on speechreading (lipreading) and to speak very intelligibly with excellent voice quality and using age-appropriate language. Generally, the earlier a child receives a cochlear implant, the better the outcome will be in terms of using the implant to its full capacity. However, how well the child functions with the implant also depends on other factors such as the child's prior use of hearing aids, cause of deafness, and response to therapy after surgery. The FDA recommended age to implant is not earlier than 12 months although some infants as young as 4 months have received cochlear implants. In the United States, more than 100 infants under 12 months have been implanted, and studies show that the younger infants begin to match hearing peers with language skills very quickly due to their brain's plasticity.
A critical component of success in using a cochlear implant is appropriate therapy. Implantation will not "fix" a child's hearing, though today's models function as an excellent prosthetic substitute to a biological cure. Appropriate speech/language therapy is vital to the development of speech and language. If a child receives a cochlear implant, their brain will have to learn how to process the auditory information. Most children with cochlear implants should receive speech/language therapy for at least several years post-implant, though implanted infants are often age-appropriate prior to preschool. The participation of the child's family in working on speech and language development is even more important than therapy.
The device can be divided into two parts, the part which is implanted, and the part worn outside the body. The external part of the device contains:
- Speech Processing
The internal part of the device contains:
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. These strategies emphasize transmission of the temporal aspects of speech.
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. These strategies emphasize the transmission of spectral aspects of speech. Feature extraction strategies are no longer widely used.
This is used to transmit the processed sound information over a radio frequency link to the internal portion of the device. Radio frequency is used 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.
This component receives directions from the speech processor by way of radio waves sent from the transmitter. (It also receives its power through the transmission.) The receiver is also a sophisticated mini computer that translates the processed sound information and controls the electrical current sent to the electrodes in the cochlea. It is embedded in the skull behind the ear.
The electrode array is made from a type of silicone rubber, while the electrodes are platinum or a similarly highly conductive material. It is connected to the internal receiver on one end and inserted into the cochlea deeper in the skull. (The cochlea winds its way around the auditory nerve, which is tonotopically organized just as the basilar membrane is). When an electrical current is routed to an intracochlea electrode, an electrical field is generated and auditory nerve fibers are stimulated.
Programming the Speech Processor
The cochlear implant must be programmed individually for each user. This is performed by an audiologist trained to work with cochlear implants. The audiologist sets the minimum and maximum current level outputs for each electrode in the array based on the user's reports of loudness. The audiologist also selects the appropriate speech processing strategy and program parameters for the user.
In 2005, the top three cochlear implant devices are manufacted by Cochlear Corporation, Australia, Advanced Bionics, United States, and MED-EL, Austria. These are similar devices. There is no clear-cut consensus that any one of these implants is superior to the others. Users of all three devices display a wide range of performance after implantation.
Of the three companies, Cochlear Corporation has the longest history in cochlear implantation. Cochlear implants have very low failure rates. Each of the companies frequently adapt ideas of the competitive companies. Advanced Bionics is distinguished by the fact that they have the most powerful and fastest implant and speech processing capabilities. MED-EL has the longest electrode array of the three companies enabling insertion into the deepest part of the cochlea.
Since the devices have a similar range of outcomes, other criteria are often considered when choosing a cochlear implant: usability of external components, cosmetic factors, battery life, reliability of the internal and external components, customer service from the manufacturer, the familiarity of the user's surgeon and audiologist with the particular device, and anatomical concerns.
Objections to use
The use of cochlear implants is objected to by some, particularly in the Deaf signing community (people who use sign language to communicate). Many members do not view deafness as a disability that has to be "fixed" by cochlear implants, but rather just a different way of living. Cochlear implants for children work best when implanted at a young age, when the brain is still learning to interpret sound, and hence are implanted before the recipients can decide for themselves. There is a debate about the age that would be the best or safest for children to receive cochlear implants. However, as cochlear implants have improved and proven themselves to work well, the objections are getting less tenable. Proponents of cochlear implants believe that, since mammals are meant to have a hearing sense, deafness is a disability to be corrected. To them, objecting to a cochlear implant is akin to objecting to a heart transplant, prosthetic arm or glasses.
Opponents of cochlear implants often compare implantation to cultural genocide, as the common ground for the signing community is deafness, either through personal experience or by personal relationships with others who have hearing impairments. Opponents of implantation tend to object not to the medical benefits of implantation, but the perceived loss of the sense of community that has defined deaf history. Statistics show that the majority of deaf infants are born into hearing families. For such children becoming a member of the deaf community is not automatic, nor is it initially a decision for them to make; rather it is a choice made by their parents, in what they perceive to be the child's best interests as they understand them.
- NPR Story about improvements on Cochlear implants
- My Bionic Quest for Boléro (Wired, November 2005): Story about the author's pursuit to have music directly input to his cochlear implant
- Cochlear implant case study