Predicting Cochlear Implant Electrode Placement Using Monopolar, Three-Point and Four-Point Impedance Measurements.

OBJECTIVE: This study aimed to investigate the relationship between cochlear implant (CI) electrode distances to the cochlea's inner wall (the modiolus) and electrical impedance measurements made at the CI's electrode contacts. We introduced a protocol for "three-point impedances" in which we recorded bipolar impedances in response to monopolar stimulation at a neighboring electrode. We aimed to assess the usability of three-point impedances and two existing CI impedance measurement methods (monopolar and four-point impedances) for predicting electrode positioning during CI insertion. METHODS: Impedances were recorded during stepwise CI electrode array insertions in cadaveric human temporal bones. The positioning of the electrodes with respect to the modiolus was assessed at each step using cone beam computed tomography. Linear mixed regression analysis was performed to assess the relationship between the impedances and electrode-modiolar distances. The experimental results were compared to clinical impedance data and to an existing lumped-element model of an implanted CI. RESULTS: Three-point and four-point impedances strongly correlated with electrode-modiolar distance. In contrast, monopolar impedances were only minimally affected by changes in electrode positioning with respect to the modiolus. An overall model specificity of 62% was achieved when incorporating all impedance parameters. This specificity could be increased beyond 73% when prior expectations of electrode positioning were incorporated in the model. CONCLUSION: Three-point and four-point impedances are promising measures to predict electrode-modiolar distance in real-time during CI insertion. SIGNIFICANCE: This work shows how electrical impedance measurements can be used to predict the CI's electrode positioning in a biologically realistic model.

Phenomenological model of auditory nerve population responses to cochlear implant stimulation.

BACKGROUND: Models of auditory nerve fiber (ANF) responses to electrical stimulation are helpful to develop advanced coding for cochlear implants (CIs). A phenomenological model of ANF population responses to CI electrical stimulation with a lower computational complexity compared to a biophysical model would be beneficial to evaluate new CI coding strategies. NEW METHOD: This study presents a phenomenological model which combines four temporal characteristics of ANFs (refractoriness, facilitation, accommodation and spike rate adaptation) in addition to a spatial spread of the electric field. RESULTS: The model predicts the performances of CI subjects in the melodic contour identification (MCI) experiment. The simulations for the MCI experiment were consistent with CI recipients' experimental outcomes that were not predictable from the electrical stimulation patterns themselves. COMPARISON WITH EXISTING METHODS: Previously, no phenomenological population model of ANFs has combined all four aforementioned temporal phenomena. CONCLUSIONS: The proposed model would help the further investigations of ANFs responses to different electrical stimulation patterns and comparison of different sound coding strategies in CIs.

A bio-inspired coding (BIC) strategy for cochlear implants.

A bio-inspired coding (BIC) strategy was implemented in this study with the goal of better representation of spectral and temporal information. The auditory nerve fibers' (ANFs) characteristics such as refractory recovery, facilitation and spatial spread were obtained from ECAP data recorded in 11 CI recipients. These characteristics, together with a non-individualized model-derived adaptation effect, were integrated into the BIC strategy for a better selection of channels. Two variations of the BIC strategy were compared to the conventional advanced combination encoder (ACE) coding strategy: the BIC-I strategy based on the individual CI recipients' ECAP parameters, and the BIC-G strategy based on the median values of ECAP parameters from all CI recipients who participated in the study. The melodic contour identification (MCI) and Oldenburg sentence recognition in noise (OLSA) tests were used to assess and compare the three coding strategies. A significantly better performance in the transformed MCI test results with the rationalized arcsine transformation, was observed for both BIC strategy variations compared to the ACE strategy. There was no significant difference between the two variations of the BIC strategy and the ACE strategy in the OLSA test. No correlation was found between recovery time constants, absolute refractory periods, left and right width of SOE functions from three test electrodes and CI recipients' performances in the two experiments. However, significant correlations were found between facilitation time constant and amplitude and the results of the MCI and OLSA tests for the two variations of the BIC strategy.

Measuring temporal response properties of auditory nerve fibers in cochlear implant recipients.

Auditory nerve fibers' (ANFs) refractoriness and facilitation can be quantified in electrically evoked compound action potentials (ECAPs) recorded via neural response telemetry (NRT). Although facilitation has been observed in animals and human cochlear implant (CI) recipients, no study has modeled this in human CI users until now. In this study, recovery and facilitation effects at different masker and probe levels for three test electrodes (E6, E12 and E18) in 11 CI subjects were recorded. The ECAP recovery and facilitation were modeled by exponential functions and the same function used for +10 CL masker offset condition can be applied to all other masker offsets measurements. Goodness of fit was evaluated for the exponential functions. A significant effect of probe level was observed on a recovery time constant which highlights the importance of recording the recovery function at the maximum acceptable stimulus level. Facilitation time constant and amplitude showed no dependency on the probe level. However, facilitation was stronger for masker level at or around the threshold of the ECAP (T-ECAP). There was a positive correlation between facilitation magnitude and amplitude growth function (AGF) slope, which indicates that CI subjects with better peripheral neural survival have stronger facilitation.

Investigating the use of a Gammatone filterbank for a cochlear implant coding strategy.

BACKGROUND: Contemporary speech processing strategies in cochlear implants (CIs) such as the Advanced Combination Encoder (ACE) use a standard Fast Fourier Transform (FFT) filterbank to extract envelopes. The assignment of the FFT bins to approximate the frequency resolution of the basilar membrane is only partly based on physiology, especially since the bins are distributed linearly below 1000Hz and logarithmically above 1000Hz. NEW METHOD: A Gammatone filterbank which provides a closer approximation to the bandwidths of filters in the human auditory system could replace the standard FFT filterbank in the ACE strategy. An infinite impulse response (IIR) all-pole design of the Gammatone filterbank was compared to the FFT filterbank with 128, 256 and 512 points resolutions and the effect of the frequency boundaries of the filters was also investigated. RESULTS: Melodic contour identification (MCI) and just noticeable difference (JND) experiments, both involving synthetic clarinet notes in octaves 3 and 4, were conducted with 6 normal hearing (NH) participants using noise vocoded stimuli; and 10 CI recipients just performed the MCI experiment. The MCI results for both NH and CI subjects, showed a significant effect of the filterbank on the percentage correct responses of the participants. COMPARISON WITH EXISTING METHODS: The Gammatone filterbank can better resolve the harmonics of tested synthetic clarinet notes which led to better performances in the MCI experiment. CONCLUSIONS: The total delay of the Gammatone filterbank can be made smaller than the delay of the FFT filterbank with the same frequency resolution at low frequencies.