Objective. Proposing a good electrode-electrolyte interface (EEI) model and properly identifying relevant parameters may help designing safer and more optimized auditory nerve fiber stimulation and recording in cochlear implants (CI). However, in literature, EEI model parameter values exhibit large variability. We aim to explain some root causes of this variability using the Cole model and its simpler form, the Basic RC model. Approach. We use temporal and spectral methods and fit the models to stimulation pulse voltage response (SPVR) and electrochemical impedance spectroscopy (EIS) data. Main Results. Temporal fittings show that there are multiple sets of model parameters that provide a good fit to the SPVR data. Therefore, small methodological differences in literature may result in different model fits. While these models share similar characteristics at high frequencies >500 Hz, the SPVR fitting is blind to low frequencies, thus it cannot correctly estimate the Faradaic resistor. Similarly, the polarization capacitor and its fractional order are not estimated robustly (capacitor variations in the nano- to micro-farad range) due to limited observation of mid-range frequencies. EIS provides a good model fit down to ∼3Hz, and thus robust estimation for the polarization capacitor. At lower frequencies charge mechanisms may modify the EEI, requiring multi-compartment Cole model fitting to EIS to improve the estimation of Faradaic characteristics. Our EIS data measurements down to 0.05Hz show that a two-compartment Cole model is sufficient to explain the data. Significance. Our study describes the scope and limitation of SPVR and EIS fitting methods, by which literature variability is explained among CI EEI models. The estimation of mid-to-low-frequency characteristics of the CI EEI is not in the scope of the SPVR method. EIS provides a better fit; however, its results should not be extrapolated to unobserved frequencies where new charge transfer mechanisms may emerge at the EEI.

This paper presents a novel cochlear implant (CI) sound coding strategy called Spectral Feature Extraction (SFE). The SFE is a novel Fast Fourier Transform (FFT)-based Continuous Interleaved Sampling (CIS) strategy that provides less-smeared spectral cues to CI patients compared to Crystalis, a predecessor strategy used in Oticon Medical devices. The study also explores how the SFE can be enhanced into a Temporal Fine Structure (TFS)-based strategy named Spectral Event Extraction (SEE), combining spectral sharpness with temporal cues. Background/Objectives: Many CI recipients understand speech in quiet settings but struggle with music and complex environments, increasing cognitive effort. De-smearing the power spectrum and extracting spectral peak features can reduce this load. The SFE targets feature extraction from spectral peaks, while the SEE enhances TFS-based coding by tracking these features across frames. Methods: The SFE strategy extracts spectral peaks and models them with synthetic pure tone spectra characterized by instantaneous frequency, phase, energy, and peak resemblance. This deblurs input peaks by estimating their center frequency. In SEE, synthetic peaks are tracked across frames to yield reliable temporal cues (e.g., zero-crossings) aligned with stimulation pulses. Strategy characteristics are analyzed using electrodograms. Results: A flexible Frequency Allocation Map (FAM) can be applied to both SFE and SEE strategies without being limited by FFT bandwidth constraints. Electrodograms of Crystalis and SFE strategies showed that SFE reduces spectral blurring and provides detailed temporal information of harmonics in speech and music. Conclusions: SFE and SEE are expected to enhance speech understanding, lower listening effort, and improve temporal feature coding. These strategies could benefit CI users, especially in challenging acoustic environments.

Neurophysiologic measures like the electrically evoked Stapedius Reflex Threshold (eSRT) offer valuable insights into personalized responses to Cochlear Implant (CI) stimuli. This study evaluates the effects of stimulation current amplitude and the number of stimulating electrodes on eSRT and its correlation with the Most Comfortable Loudness level (MCL or C-level) in all-polar, pseudo-monophasic, pulse-width loudness coded stimulations. The study was conducted on seventeen adult patients with Oticon Medical implants. In these implants, current amplitude remains constant while loudness is coded through pulse-width. Stimulation amplitudes were set to low (default clinical) and high values, using single or multi electrode groups of 1, 3, and 5 (G1, G3, and G5) across five cochlear regions, spanning from apical to basal. For each amplitude and group, the eSRT detection rate and correlation with MCL were analyzed regionally and overall. Higher eSRT detection rates were observed in the contra-lateral ear, with G3 and G5 significantly enhancing detection over G1 for both low and high amplitudes. The current amplitude facilitated eSRT invocation, particularly for G1 and patients with higher MCLs. The correlation between eSRT and MCL was relatively high (r = 0.64-0.87) across all conditions, indicating accurate MCL estimation from eSRT at both low and high amplitudes. Among all condition interactions (amplitude, side, electrodes), low-amplitude contra-lateral G5 and high-amplitude contra-lateral G3 showed optimal modes for estimating MCL from eSRT, with linear slopes near 1.0, and detection rates of 54% and 63%, respectively. High-amplitude contra-lateral G5 further increased detection to 68%, though with a slope of 0.8, requiring a correction factor. These findings provide insights into eSRT mechanisms and its application in pseudo-monophasic all-polar CI programming. The eSRT detection rate can be improved in pulse-width modulation cochlear implants by using multi-electrode stimulation and/or increasing the stimulation amplitude beyond the default clinical value, without significantly affecting the correlation coefficient between eSRT and MCL.

Objective: This study assesses the electrically evoked stapedius reflex threshold (eSRT) as an objective method for generating auditory maps in individuals with pulse-width-modulation cochlear implants (CIs). It investigates the impact of both single- and multiple-electrode stimulations on eSRT detection rates and their association with patients' comfort loudness levels (C-levels). Despite eSRT exploration by several CI brands incorporating pulse-amplitude-modulation stimulation technology, there is a lack of information on systems using pulse-width modulation. Approach: The eSRT was measured in 19 ears ipsilaterally and contralaterally in response to stimulations in groups of 1, 3, and 5 electrodes across five distinct cochlear regions, spanning from apical to basal. For each group, the eSRT detection rate and its correlation with the C-levels were analyzed both regionally and overall. Main results: In both contralateral and ipsilateral sides, higher electrically evoked stapedius reflex (eSR) detection rates were obtained for multielectrode (67%, 47%) than for single-electrode (41%, 23%) stimulation. Invoking eSR in the apical and basal regions was easier than in the middle region. The multiple stimulation just increased slightly the correlation coefficients between eSRT and C-levels from 0.73 to 0.77 and from 0.77 to 0.85 (p < 0.05) for the contralateral and ipsilateral ears, respectively. Significance: Our research indicates that using a multielectrode stimulation approach enhances the detection rate of eSRT and slightly improves the correlation coefficient between eSRT and C-levels. It is found preferable to obtain measurements from the contralateral side due to its higher detection rate, although the ipsilateral side yields a marginally higher correlation coefficient. The findings suggest that eSRTs obtained from pulse width modulation implants could aid audiologists in programming the device.