Neurophysiology of Cochlear Implants

The cochlear implant is the clinically most successful neuroprosthetic device (for review of other used neuroprosthesis, click here). Cochlear implants are „electronic ears“ that replace non-functional inner ears. They electrically stimulate the auditory nerve in deaf subjects. Using these devices the majority of subjects with a non-functional inner ear can learn to recognize speech and communicate acoustically without any additional non-acoustic help. Prelingually deaf children can learn their mother language in the acoustic form and can learn to communicate with hearing friends acoustically. Currently, approx. 600,000 subjects use this neuroprosthetic device on an everyday basis. Our group is working on the understanding of the neurophysiological processes in the electrical stimulation of the auditory nerve.
Deafness itself induces a massive change in the functional properties of auditory nerve fibres: not only they can not be „driven“ by sound, they also lose spontaneous activity. When auditory nerve fibres are stimulated electrically, the temporal jitter of the responses is very small compared to the acoustical stimulation of a hearing cochlea. As a consequence, the dynamic range of responses is reduced noticeably: the fibres can “code” only changes of intensity in the range of ~ 3-10 dB, compared to 40-80 dB in a “hearing” auditory nerve (review in Hartmann and Kral, 2004). Another problem cochlear implant stimulation has to cope with is the extensive spread of the electrical field in the cochlea. A hearing auditory system is very sensitive to changes in the position of the most excited region in the cochlea: a change in position of this region in the order of few hair cells (order of tenths of µm) induces a perceptual change! Auditory nerve fibers have therefore sharp threshold curves. Such curves can by no means be replicated with electrical stimulation, although the electrical field can be sharpened substantially with “beam forming” (current focusing) by means of a tripolar configuration of the stimulation electrodes (Kral et al., 1998). With such an approach we have been able to limit the spread of activation to one critical band, although we could not reach the place resolution of the hearing ear. Fortunately, the temporal code is more important than the place code in the spectral range of speech (up to 5 kHz). This accounts for the remarkable success of cochlear implants despite the reduced resolution of the “electrical“ place code.
Furthermore, we investigate the interaction of electric and acoustic stimulation in residually-hearing ears. Electroacoustic stimulation has since subjects with residual hearing in low frequencies could be implanted due to hearing preservation surgery. Combined electroacoustic stimulation improves understanding of speech in noise substantially, but the reasons for this spectacular success are not clear. We identified that most interactions of electric and acoustic responses in a hearing auditory nerve fibers are suppressive in nature, not introducing distortion to the evoked activity (Tillein et al., 2015). Follow-up studies on electrophonic responses has shown that they are generated at the cochlear places corresponding to the frequency components in the electric stimulus (Sato et al., 2016; 2017). Recently, for these subjects we initiated a project on using the cochlear implants as a diagnostic probe to determine the residual auditory function following implantation (details see here). We currently study effects of cochlear health (Konerding et al., 2020) and of pulse shape on cochlear implant stimulation (Quass et al., 2020).

Illustration of a cochlear implant device on the head of a child (left), the path of the implant in the head (middle, implant electrode carrier shown in brown) and its optimal placement in the scala tympani (right). Figure from Kral et al., 2016, Lancet Neurol.

Receptive fields of an auditory nerve fiber to acoustic (AS), electric (ES) and combined (EAS) stimulation. The data demonstrate that in the EAS mode, the responses are often combinations of the acoustic and the electric responses with no significant distortion of the frequency response area. FRA = frequency response area (acoustic stimulation). PSTH = poststimulus time histogram. Figure from Tillein et al. 2015.

We are one of the few labs internationally that can record auditory nerve activity with cochlear implants.

Links to Cochlear Implant Companies:
Advanced Bionics
Cochlear
MedEl
Oticon