Profoundly-impaired Hearing Moderately-im Paired Hearing Normal Hearing Cochlear Implants

The top panel of Figure 1 shows a schematic diagram of the normal peripheral auditory system. The ear canal and ossicles (small bones) of the middle ear transmit acoustic signals to the cochlea where they produce a travelling wave moving from base to apex along the basilar membrane. Displacement of a segment of the basilar membrane increases the likelihood that the nerve fibers innervating hair cells coupled to that segment will elicit spikes. The structural properties of the basilar membrane result in a maximal displacement for high frequencies at the cochlea’s base and for low frequencies at the apex. As a result, the spectral content of an acoustic stimulus is represented by an array of nerve-fiber responses where the highest-frequency components are coded by fibers innervating hair cells at the base and the lowest frequency components at the apex.