Modeling of Auditory Neuron Response Thresholds with Cochlear Implants

Abstract

The quality of the prosthetic-neural interface is a critical point for cochlear implant efficiency. It depends not only on technical and anatomical factors such as electrode position into the cochlea (depth and scalar placement), electrode impedance, and distance between the electrode and the stimulated auditory neurons, but also on the number of functional auditory neurons. The efficiency of electrical stimulation can be assessed by the measurement of e-CAP in cochlear implant users. In the present study, we modeled the activation of auditory neurons in cochlear implant recipients (nucleus device). The electrical response, measured using auto-NRT (neural responses telemetry) algorithm, has been analyzed using multivariate regression with cubic splines in order to take into account the variations of insertion depth of electrodes amongst subjects as well as the other technical and anatomical factors listed above. NRT thresholds depend on the electrode squared impedance (β = −0.11 ± 0.02, P < 0.01), the scalar placement of the electrodes (β = −8.50 ± 1.97, P < 0.01), and the depth of insertion calculated as the characteristic frequency of auditory neurons (CNF). Distribution of NRT residues according to CNF could provide a proxy of auditory neurons functioning in implanted cochleas.

Towards a Complete In Silico Assessment of the Outcome of Cochlear Implantation Surgery

“…The perimodiolar electrodes were originally designed to obtain the same performance with a lower stimulus threshold -compared to the threshold needed for straight electrodes inserted in a lateral wall position. However, the perimodiolar approach uses an electrode design that may cause severe internal trauma due to the penetration of the array and translocation to scala vestibuli, while having the same efficacy as lateral electrodes in terms of eCAP measurements [28]. There is still controversy about whether the electrode-modiolus distance has an influence on the CI outcome [20,24,26,27,29,45].…”

“…This insertion was done by simulating the real scenario of the surgery using our surgical planning software, thus obtaining a realistic trajectory line of insertion, and then matching the centerline of the electrode array geometry with the trajectory line of the insertion [32,35]. An electrode array design with 12 electrodes (19 contacts) was used based on Med-EL Flex 28 design. The ANF around the cochlea were generated automatically according to the patient's cochlear shape, considering different anatomical landmarks, such as the organ of Corti and the beginning of the basilar membrane [32].…”

“…On the other hand, some authors observed that the distance between the electrode and the modiolus did not affect, or affected minimally, this neural response threshold, or that its relation could not be clearly determined [27,30]. Generally, a better speech perception of CI patients is obtained when the electrode is completely placed on the scala tympani [28,29]. Lateral wall electrode arrays provide a higher probability to maintain this intra-cochlear position, whereas perimodiolar electrodes are likely to be misplaced, affecting directly the residual hearing preservation [28].…”

“…Dynamic range, place pitch or spread of excitation on the auditory nerves have been a focus of interest due to their direct relation to the CI performance [6,13,17,20,23]. ANF response according to the intra-cochlear electrode position has also been investigated in several studies, in which different and often contradictory conclusions were reported [24][25][26][27][28][29]. To reduce the spread of excitation and obtain a better pitch discrimination, an electrode position closer to the modiolus has been reported to decrease the threshold in which the first response on the ANF is obtained [24,26].…”

“…Generally, a better speech perception of CI patients is obtained when the electrode is completely placed on the scala tympani [28,29]. Lateral wall electrode arrays provide a higher probability to maintain this intra-cochlear position, whereas perimodiolar electrodes are likely to be misplaced, affecting directly the residual hearing preservation [28]. This issue is still an open discussion, yet it plays an important role when the electrode design needs to be chosen; thus, further studies are required.…”