Understanding how Cochlear Implants Work and What They Do.

To gain a clear understanding of how cochlear implants work, a sensible starting point will be to also take a closer look at the physiology of the hearing process. The ear consists of three distinct compartments. The first and the only one that is visible to others is the outer ear. It consists of a roughly shell-shaped external portion, termed the pinna, which surrounds the entrance to a narrow channel known as the ear canal. Together they form a funnel-like structure that focusses incoming soundwaves onto a membrane known as tympanum or, more commonly, the eardrum. Beyond the membrane is a chain of three tiny bones, the first of which is in contact with the eardrum, while the last impinges on the window to the inner ear.

Up to this point, how cochlear implants work is different from the way in which a healthy human ear operates. That is to say that each depends upon different means to conduct sound signals to the inner ear. Where there may be problems with the sensory organs of the middle, the processes are also markedly different. Implants relay radio signals from an external microphone and transmitter to an embedded receiver and thence via an electrode array to the inner ear. In a healthy ear, the tympanic membrane vibrates in response to sounds and those vibrations are then relayed via the three tiny bones mentioned earlier, completing the conduction process by agitating the cochlear window.

Beyond this window lies a fluid filled structure that is lined by delicate hair cells. If damaged or absent, how the cochlear implant will work is to stimulate the auditory nerve directly by means of a pulsed electrical signal generated by a speech processor and delivered by means of the electrode array. By contrast, in the healthy ear, movements produced in the cochlear fluid as a result of the vibrating window serve to agitate the hair cells. These, in turn, respond not with an electrical signal but a nerve impulse which will also be delivered to the brain by the auditory nerve.

Given the different nature of the natural and artificial signals transmitted to the auditory region of the brain, it is not too surprising that these provoke differing responses. While the electrical signals, like the neural impulses, match the amplitude and frequency of the original external sounds, they result in a different perception of sound that at first may seem unfamiliar. Just how well and how quickly cochlear implants will work for any given individual will therefore depend upon the ease with which he or she is able to adapt to the new sound experience.

The fact that tens of thousands of these devices have been implanted successfully since the technology became viable is testament to the fact that, for any candidate who has been screened effectively and deemed as eligible, the benefits can be life-changing. Children deaf from birth with no experience of sound, as well as those whose hearing has become impaired later in life, will need a period of rehabilitation but will soon be able to recognise speech and interact effectively both at home and in the workplace.

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