M.A. Stefanovich
Leading Engineer, JSC "Research Institute
of a Long-distance Radio Communication" Scientific-Industrial Complex” (Sankt-Petersburg)
E-mail: marg.stefanovich@yandex.ru
S.V. Levin
Ph.D. (Med.), Senior Research Scientist, Saint-Petersburg Institute of Ear, Nose, Throat, and Speech
E-mail: megalor@gmail.com

• A man loses hearing with pathology of inner hair cells. Cochlear Implant is a device converting frequency and intensity of sound in electric impulses for stimulation of cochlear nerve fibers (CNF).

Thin and elastic tube with platinum electrodes is inserted into Cochlea. The number of electrodes in different types of Cochlear Implant may be 12, 16 or 22. Impulses similar to having been saved are reproduced with the help of cochlear nerve fibers to the higher auditory centers. Frequency range from 150 to 7500 Hz is divided through band-pass filters into a number of ranges according to the amount of being stimulated electrodes. CI listener hears almost all the sounds but aural feelings differ from those he heard before loss of hearing. The main reason of such a dissonance is that Impulses from an output of the CI are reproduced by the other fibers of CN than it was while normal hearing. CI listener hears the sound with almost the same pitch as it was before loss of hearing if the (frequency of tone sound is approximately 700 Hz. CI listener hears the sounds with the lower frequency than 700 Hz as higher-pitched, and he hears the sounds higher than 700 Hz as lower-pitched. For example, while speech perception different CI listeners can hear syllables

“A-TA” as “A-TA”, “A-KA”, “A-PA”. CI listener memorizes new aural feeling of words similar to foreign ones. Another peculiarity of sound perception through CI is in the fact that an electrode is stimulated in the same mode according to any frequency at the input of the corresponding filter (BPF). The information on frequency of a sound signal in a range of the BPF is lost. At perception of music the same electrode is stimulated at sounding several notes. For example, if the range of the BPF is 240-357hz CI listener hears music notes from “B” to “F” as equally pitched. He can’t guess the melody he knew well before profound hearing impairment. CI listener will hear all the music notes as different-pitched if electrodes’ stimulating mode changes in different notes perception.

Using Cochlear Implant with 12 active electrodes while stimulating its neighboring single electrodes aural pitch may change from 1 to 1.5 tones. For example, CI listener hears the sound with pitch Z1 while stimulating E1 electrode and he hears the sound with pitch Z2 while stimulating E2 electrode. CI listener will hear the sound with the pitch (Z1+Z2)/2 at simultaneous stimulation of E1 and E2 impulses of identical amplitude. If the frequency of sound signal is equal to minimum or maximum frequency for one electrode stimulation CI listener will hear the sounds that will approximately differ to 0.5 tones. CI listener can hear 7 various in pitch sounds (modified octave) during electrodes’ sequent stimulation according to Table 1. CI listener will hear the sound with the changes audible pitch approximately in 0.5 tones if two next electrodes are simultaneously stimulated by impulses with identical amplitude.

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