The study of electrical stimulation of the auditory system has a long history, dating back to the preliminary work of Alessandro Volta, inventor of the electric battery. Volta experimented on himself, placing metal rods connected to a 50-volt circuit in each of his own ears. He describes the sounds he heard as sounding “like a thick boiling soup”.
Since these rather crude experiments, electrical stimulation of the auditory nerve has been further refined, ultimately leading to the single channel cochlear implant developed at House Ear Institute in 1969 (House & Urban 1973; House & Berliner, 1982).
In the 1970s, multi-channel cochlear implants were developed where the use of several electrodes took advantage of the tonotopic, or frequency to place, mapping of the cochlea. In order to understand how such a device works, it’s important to consider the auditory system. A hearing aid, which functions by amplifying the sounds coming into the ear, is generally designed to deal with conductive forms of hearing loss, where sound is impeded prior to reaching the inner ear.
However, a slightly more problematic form of hearing loss involves the degeneration of hair cells in the cochlea, which transduce sound from mechanical energy into electric current that the brain can interpret. This form of hearing loss is known as sensorineural hearing loss. Unfortunately, simply turning up the gain on a hearing aid will not alleviate this form of hearing loss.
In order to encode sound in a form that the brain can decipher, a cochlear implant system functions by first picking up sound from a microphone located on the speech processor worn around the ear of a patient. This sound is then digitized by the speech processor and sent to the transmitter, which is magnetically attached to the wireless receiver, part of the implantable portion of a CI device.
The transmitter communicates with the receiver through the skull via radiolink, facilitating bidirectional communication between the internal and external components of the system. The FM radio signal that is picked up by the receiver is then converted into a series of electrical pulses that are sent to the corresponding stimulating electrode in the cochlea.
The position of the active electrode in the cochlea determines which frequency range is being stimulated. As the electrodes go from the base to the apex of the cochlea, different populations of neurons are selectively activated, with progressively lower characteristic frequencies. This electroneural interface between the electrode array and the remaining auditory nerve fibers was inspired by the vocoder work of researchers at Bell Labs concerned with how much information was needed to transmit intelligible speech over telephone lines (Dudley 1939).
Neuroscientists reasoned that if only six to seven channels were needed to transmit speech over the telephone, perhaps the same number of electrodes would be sufficient in a cochlear implant (Finn 1998). It turns out that they were right, as high levels of speech intelligibility in quiet conditions have been achieved with these multichannel devices (Osberger et al., 2000; Garnham et al, 2002; Skinner et al., 1994). For more on cochlear implants, see review by Loizou, 1998.
The advent of the cochlear implant has been hailed as one of mankind’s greatest achievements. Throughout our history, deafness has afflicted many individuals, cut off from their peers due to an inability to communicate through sound. Both speech and music, two of the most central forms of human communication and expression, were irrevocably lost to those who experience profound hearing loss.
This left many disconnected from the world around them. Helen Keller once reflected that “blindness separates us from things, but deafness separates us from people.” While deafness may have separated the hearing impaired from the world of sound, they slowly formed a newfound sense of unity in the unique perspective that they shared. A community was forming, one with a distinct identity and culture. This newborn Deaf culture (with a capital “D”) produced its own set of social beliefs, customs, arts, values, history and even language.
As with every technology, the cochlear implant was introduced, received, and embedded into the cultural tapestry of the times. Eventually the devices were targeted to newly born deaf children in an attempt to facilitate spoken language development within key critical periods. This inevitably led to a great deal of controversy and debate regarding the ethics behind the choice to implant infants, a decision that rested solely in the hands of parents, who would generally give consent after consulting with medical personal strongly rooted in a hearing based society.
Those in the Deaf community have felt a different kind of pressure arising from these scientific discoveries, one based on the need to defend their cultural group. This has led to their widespread protest against the use of cochlear implants. While many in the hearing community view deafness as a disability that must be cured, those in the Deaf community associate themselves as part of a minority cultural group, with deafness being their defining trait.
They view cochlear implants as an attack on their way of life, a tool symbolizing forced cultural assimilation imposed by a largely hearing based society. At one extreme, CI devices can be seen as limiting the diversity and natural variation of the human species, reducing the size of the Deaf community, and risking what could potentially lead to a form of ethnocide.
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