Cochlear Implants

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Why were cochlear implants developed?

Bilateral hearing loss is present in 12.7% of U.S. adults, and severe to profound loss in 0.26% to 0.7% of adults, leading to significant difficulty with verbal communication. The enormous problem of irreversible sensorineural hearing loss prompted multiple researchers to develop, collaborate, and compete to create the world’s most successful neural implant.

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Who created cochlear implants?

In 1957 a surgeon in France temporarily implanted an electrode near cranial nerve VIII within the internal auditory canal (IAC) while operating on a patient deafened by temporal bone resection. The first implantation of an electrode into the cochlea was performed by Dr. William House (neurotologist) and Dr. John Doyle (neurosurgeon) in 1961. Over the course of the 1960s and the 1970s, the development of single- and multi-channel implants was pioneered by teams in the United States, France, and Australia. By the late 1980s, cochlear implantation was a scientifically and commercially accepted method of enabling sound awareness and verbal communication in THE profoundly deaf.

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What are the components of a cochlear implant?

A cochlear implant consists of an internally implanted portion and an externally worn portion that is taken off and on like a hearing aid ( Fig. 38.1 ). The microphone, which detects the ambient sound waves intended to be transmitted to the user, and the speech processor, a computer that deconstructs the sound waves into electrical signals, sit behind the ear. These components are coupled, either within the same component or via a cord, to the transmitter which is a coiled wire that transmits electrical signals to the internal device via radiofrequency waves.

Implanted under the scalp is the receiver-stimulator, which has a matching coiled wire to receive electrical impulses from the transmitter. The transmitter and the receiver-stimulator both contain a magnet to facilitate colocalization (and accurate transmission of impulses) when the external portion is placed on the scalp. The receiver-stimulator is connected to an electrode array that is placed within the cochlea and transmits electrical signals to the appropriate portions of the acoustic nerve within Rosenthal’s canal. Some cochlear implants feature a separate ground electrode, while others contain the ground within the electrode array.

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How do cochlear implants enable the perception of sound in users?

The speech processor is designed to encode sound in the same way that the normal-hearing cochlea does: signal location and signal timing. As the basilar membrane ascends from the base to the apex, the hair cells are tuned to gradually decreasing characteristic frequencies. Cochlear implants take advantage of the physical layout of this tonotopic map to deliver electrical impulses derived from high-frequency sounds to the basal electrodes and those from low-frequency sounds to the apical electrodes. As hair cells normally transmit an increasing frequency of actional potentials with increasing frequency of tones, the processor encodes the electrical impulses in the same way.

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Do cochlear implant users experience sound the same way as normal hearers?

In a normally functioning cochlea, the 3000 to 3500 inner hair cells with unique characteristic frequencies allow fine-tuning of the place coding provided to the auditory nerve. The lesser number of electrodes (up to 22) in a cochlear implant require the same electrical information to be bundled in larger packets, reducing the dynamic range of sounds perceived by the brain. Due to this and other factors, implants are unable to convey the full tonal richness of language and music. In addition, bypassing the hair cells to directly stimulate the spiral ganglion neurons sometimes results in users experiencing sound perceptions such as hissing, buzzing, or an electronic or robotic quality, especially when the implant is first activated. However, with time, cochlear implant users become adept at utilizing this electrical signal to enable greatly improved speech and environmental sound perception.

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How well do cochlear implants work?

Many factors impact how well an implant helps a particular user to achieve verbal communication. Younger patients with post-lingual deafness and a shorter duration of severe to profound hearing loss have a greater likelihood of detecting speech after implantation. The surgical technique also plays a role in outcomes. Patients in whom the implant’s final location lies entirely within the scala tympani demonstrate better performance, on average, than those whose electrode translocates through the basilar membrane into the scala vestibuli. However, even when taking all of the above factors into account, there is still significant variability in individual outcomes. In other words, despite overall favorable prospects, we cannot accurately predict whether a given candidate will achieve open-set word recognition after cochlear implantation, although, on the whole, most implantees will.

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