Electrodes on auditory nerve
When hearing, sound moves tiny hairs in the inner ear, which then translate the mechanical stimulus into an electrical one and transmit it to the processing centers of the auditory sense. Cochlear implants are increasingly able to do all of this.
The hearing of he althy people responds at sound levels between 0 and 20 decibels (dB), with slight hearing loss the thresholds are already 20 to 30 dB. For comparison: We typically speak at 40 to 60 dB. In the case of severe hearing loss, sound is only perceived above 60 to 90 dB - the upper value corresponds to the noise level directly on a motorway. If, despite hearing aids, you still have limited understanding of speech, a cochlear implant is appropriate.
In order to separate noise and speech from each other, in addition to tricky processing, a method close to natural conditions is also suitable: fitting both ears with two implants. This enables directional hearing and makes optimal use of the best of all speech processors, the brain. However, this procedure is expensive, so that only about 1000 people worldwide are equipped with two cochlear implants, about half of whom live in Europe.
When we hear, tiny hairs in the inner ear are deflected by the sound. They belong to cells that, in response to this mechanical stimulus, deliver electrical impulses to the auditory nerve, stimulating it to transmit a signal to the auditory processing centers. So-called cochlear implants should at least approximately replace this function if the hair cells are defective in the case of severe hearing loss. They also convert sound into electrical signals, but electrodes now transmit these to the nerve. The first patients received such implants in the mid-1970s. They are now used by around 100,000 children and adults worldwide.
In order for the brain to recognize speech, for example, a processor has to analyze the frequency spectrum of the sound signal. In he althy people, this is due to the structure of the cochlea: hair cells at the base react to high frequencies, while those at the top react to low frequencies. This "scale" arrangement is also used by the implants; they stimulate the auditory nerve via a series of electrodes strung on a silicone thread. Depending on the device manufacturer, the frequency spectrum is divided into 8 to 22 channels; each one corresponds to an electrode.
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Deaf associations point out that adults who were born deaf or who were deaf before the language learner would no longer learn spoken language even with a cochlear implant. Only sign language enables them to communicate on a par with spoken language. Deaf people must be provided with an interpreter for sign language if required, for example at work or when visiting authorities. That number seems small compared to the 5,000 inner hair cells of a he althy ear, but unfortunately, having more channels does little to improve speech understanding. Because the comparatively large electrodes always stimulate several nerve cells, the division according to frequencies becomes blurred. If it were possible to bring these stimulators closer to the cells, the perception of pitches could certainly be improved. The auditory nerve has a smaller dynamic range of around 10 to 20 decibels (dB) than the he althy ear, which can register sound pressure levels from 0 to a maximum of 120 dB. The processor still transmits a range of 80 dB by compressing the input signal to the dynamic range of the auditory nerve.
The earlier an implant is placed in a patient after deafness, the better the prognosis. Even children born deaf have a very good chance of learning to hear and speak if they receive the device early. Hearing matures particularly at the age of one to two years, and speech acquisition develops particularly strongly during this time. After the implantation, hearing training and speech therapy are particularly important. People who have been deaf for years typically benefit less because the auditory nerve degenerates over time if not stimulated. The majority of patients understand speech after a few weeks or months of training; about every second person can then manage without reading lips and can make phone calls.
Of course, cochlear implants do not provide a complete replacement for the complex biological system. They do not yet convey a normal hearing impression. In the future, however, special algorithms should optimize speech understanding and even music enjoyment should be possible.© Spectrum of Science
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