What is a Cochlear Implant?

What is a Cochlear Implant and How Does It Work?

When considering this question, some may be inclined to suggest that it is simply a more sophisticated form of hearing aid. In practice, however, this is far from the truth. Not only are these devices not designed to amplify ambient sounds to render them audible to the hearing impaired, the function they actually perform is largely irrelevant to the needs of those who are able to manage their hearing loss with a conventional hearing aid. To understand what a cochlear implant actually is, we need to look a little more closely at how a healthy ear works and at the way in which its normal functioning may sometimes become impaired.

What are sounds? They are the effects we perceive due to mechanical disturbances in the air arising from some external source and which act initially on the eardrum. Eventually, those sounds may present as birdsong, a gunshot, a piano concerto, or the voice of a loved one, the difference being determined by the precise make-up of the soundwaves originally generated. Before these waves can be perceived and interpreted, they must complete a journey from the outer portion of the ear to its innermost recess – a journey that, in some cases, can only be completed because of what a cochlear implant is able to do.

In a hearing individual, sound waves striking the eardrum cause it to vibrate at the same frequencies. The eardrum impinges on a trio of tiny bones or ossicles named the malleus, incus, and stapes, or the hammer, anvil, and stirrup. The vibrations are then conducted between the ossicles in succession, until the last of the three vibrates against the cochlear window and causes the fluid beyond it, in the portion of the cochlea known as the organ of Conti, to reproduce the conducted wave patterns. It is only at this point, in fact, that what a cochlear implant is designed to do becomes important.

In order to provide an intelligible signal for onward transmission to the sound centres in the brain where it can be interpreted, those wave patterns must be converted to nerve impulses. This occurs when the motions of the cochlear fluid disturb the delicate hair-like projections of the cells lining the organ of Conti, transforming a mechanical representation of the incoming sounds into a neural stimulus that travels via the auditory nerves to the auditory cortices in the brain where the nature, volume, and direction of external sounds are determined with the aid of another portion of the brain termed the prefrontal cortex.

So, what exactly is a cochlear implant? Many of those who are affected by severe to profound hearing loss can attribute their condition to extensive damage to, or the absence of the hair cells mentioned above. This is known as sensorineural deafness. What the implanted device does is generate electrical rather than neural impulses in response to external sounds. It then applies these signals directly to the auditory nerve, bypassing the conductive pathway of the middle ear entirely.

Unlike a conventional hearing aid, it does not amplify the incoming sounds, and the effect of the electrical impulses on the brain is somewhat different to that experienced by a hearing individual. What is important, however, is that the signals from a cochlear implant vary with the wavelength and volume of the incoming sounds in the same way as naturally conducted sound, so the sound centres in the brain are able to adapt to the new type of signals delivered by the implanted device and interpret them as the corresponding sounds.

The device consists of four main components, beginning with a microphone to detect external sounds. Located behind the ear, a speech processor selects and digitises incoming sounds then sent to an integrated transmitter. Beneath the skin, a receiver converts the incoming digital signals to electrical impulses delivered via an electrode array to different regions of the auditory nerve.

What needs to be understood is that a cochlear implant may not be suitable or even necessary for everyone. The surgery is only recommended for those with sensorineural deafness if they are unable to benefit from conventional hearing aids. Recommended candidates must then undergo an in-depth evaluation to eliminate any possibility that implantation may prove ineffective. Once approved, following the surgery, what is normally necessary for recipients of a cochlear implant will be some minor adjustments to the device to optimise its performance.
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