Monday, May 08, 2006

Cochlear implant

[Inspired by the wonderful documentary "Sound and Fury"...]
A cochlear implant is a surgically implanted hearing aid that can help provide a sense of sound to a person who is profoundly deaf or severely hard of hearing. The cochlear implant is often referred to as a bionic ear. Unlike other kinds of hearing aids, the cochlear implant doesn't amplify sound, but works by directly stimulating any functioning auditory nerves inside the cochlea with electrical impulses. External components of the cochlear implant include a microphone, speech processor and transmitter.

An implant does not restore or create normal hearing. Instead, under the appropriate conditions, it can give a deaf person a useful auditory understanding of the environment and help them to understand speech when coupled with post-implantation therapy. According to researchers at the University of Michigan [1], approximately 100,000 people worldwide have received cochlear implants; roughly half are children and half adults. The vast majority are in developed countries due to the prohibitive cost of the device, surgery and post-implantation therapy — Mexico had performed only 55 cochlear implant operations by the year 2000 (Berruecos 2000).

Cochlear implants are controversial, and their introduction has seen the renewal of a century-old debate about models of deafness that often has the medical profession on one side and the Deaf community on the other. While cochlear implants have been welcomed by late-deafened adults, hearing parents of deaf children, audiologists, speech pathologists, and surgeons, the implantation of deaf children has been vigorously opposed by many from the signing Deaf community.

Ethical issues
Cochlear implants for congentially deaf children are most effective when implanted at a young age, during the critical period in which the brain is still learning to interpret sound; hence they are implanted before the recipients can decide for themselves. Deaf culture advocates question the ethics of such invasive elective surgery on healthy children — pointing out that manufacturers and specialists have exaggerated the efficacy and downplayed the risks of a procedure that they stand to gain from. Parents and audiologists paint a much brighter picture.

Much of the strongest objection to cochlear implants has come from the Deaf community, which consists largely of pre-lingually deaf people who use a sign language as their preferred language. Very distinct from adults who have lost their hearing, many do not share the pathological view of deafness held by the medical profession that deafness as a disability to be "fixed", but instead celebrate being Deaf and value their membership of the visual culture that they have grown up in (see Deaf culture).

The confict over these opposing models of deafness has raged for hundreds of years, and cochlear implants are the latest in a history of medical interventions promising to turn a deaf child into a hearing child — or, more accurately, a child with a mild hearing impairment. Parents with implanted children equate this to refusing to treat any other handicap or disease which has an effective treatment.

Critics argue that the cochlear implant and the subsequent therapy often become the focus of the child's identity, at the expense of a positive Deaf identity and the ease of communication in sign language. Measuring the child's success by their success in hearing and speech will lead to a poor self image as "disabled" (because the implants do not produce normal hearing) rather than having the healthy self-concept of a proud Deaf person. Proponents of cochlear implants counter that the child's life proceeds normally once the initial adjustments in audiological mapping are completed. The older child goes for a "checkup" to tune up their map once or twice a year, and the implanted infant is often finished with speech therapy by preschool.

Some of the more extreme responses from Deaf activists have labelled the widespread implantation of children as "cultural genocide". As cochlear implants began to be implanted into deaf children in the mid to late 1980s, the Deaf community responded with protests in the US, UK, Germany, Finland, France and Australia. Opposition continues today but in many cases has softened, and as the trend for cochlear implants in children grows, deaf community advocates have tried to counter the "either or" formulation of oralism vs manualism with a "both and" approach; some schools now are successfully integrating cochlear implants with sign language in their educational programs. However, some opponents of sign language education argue that the most successfully implanted children are those who are encouraged to listen and speak rather than overemphasize their visual sense.

How the cochlear implant works
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....


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