Ear Implant

The Past, Present, and Future of Cochlear Prostheses

Accomplishments and Challenges in Treating Sensorineural Deafness Through Electrical Stimulation

deafness affects a Sensorineural peopleof throughoutlarge number of the world. An estimate the number of sensorineural deaf within the U.S. alone who cannot perceive sound without aid is at least 900,000 [ 11; other estimates range as high as 2,000,000. The treatment of choice for the sensorineural deaf is the cochlear prosthesis, or cochlear implant. Why is this the case? Sensorineural deafness can be caused either by cochlear damage or by damage within the auditory nerve or the neurons of the central auditory system [ 11. The hair cells are the sensory cells that transduce mechanical motion into signals that can be recognized by auditory neurons. The auditory neurons carry information from the hair cells to the cochlear nucleus in the brainstem and, via the cochlear nucleus, to higher nuclei in the brain [2]. The normal cochlea and the associated neurons of the central auditory system provide information about both the frequency content and intensity of the auditory signal. Information is conveyed to the acoustic nerve about frequency content by the mechanically tuned properties of the basilar membrane [ 2 ] . The inner hair cells, which connect to the vast majority of afferent neurons, are thought to be the sensory cells of the cochlea; whereas the role of the outer hair cells is still under vigorous investigation [2]. For the purpose of this article, it is sufficient to consider that frequency information is conveyed by cochlear position. The location of hair cells along the cochlea determines their optimal response to frequency: hair cells at the apex are responsive to low frequencies, while hair cells at the base are responsive to high frequencies. The distribution of frequencies along the spiral is logarithmic [2]. If the damage to the auditory system is peripheral, in the inner ear, then a cochlear

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