Spread of excitation and channel interaction in single- and dual-electrode cochlear implant stimulation

Effects of pulse shape on pitch sensitivity of cochlear implant users

Contemporary cochlear implants (CIs) use cathodic-leading symmetric biphasic (C-BP) pulses for electrical stimulation. It remains unclear whether asymmetric pulses emphasizing the anodic or cathodic phase may improve spectral and temporal coding with CIs. This study tested place- and temporal-pitch sensitivity with C-BP, anodic-centered triphasic (A-TP), and cathodic-centered triphasic (C-TP) pulse trains on apical, middle, and basal electrodes in 10 implanted ears. Virtual channel ranking (VCR) thresholds (for place-pitch sensitivity) were measured at both a low and a high pulse rate of 99 (Experiment 1) and 1000 (Experiment 2) pulses per second (pps), and amplitude modulation frequency ranking (AMFR) thresholds (for temporal-pitch sensitivity) were measured at a 1000-pps pulse rate in Experiment 3. All stimuli were presented in monopolar mode. Results of all experiments showed that detection thresholds, most comfortable levels (MCLs), VCR thresholds, and AMFR thresholds were higher on more basal electrodes. C-BP pulses had longer active phase duration and thus lower detection thresholds and MCLs than A-TP and C-TP pulses. Compared to C-TP pulses, A-TP pulses had lower detection thresholds at the 99-pps but not the 1000-pps pulse rate, and had lower MCLs at both pulse rates. A-TP pulses led to lower VCR thresholds than C-BP pulses, and in turn than C-TP pulses, at the 1000-pps pulse rate. However, pulse shape did not affect VCR thresholds at the 99-pps pulse rate (possibly due to the fixed temporal pitch) or AMFR thresholds at the 1000-pps pulse rate (where the overall high performance may have reduced the changes with different pulse shapes). Notably, stronger polarity effect on VCR thresholds (or more improvement in VCR with A-TP than with C-TP pulses) at the 1000-pps pulse rate was associated with stronger polarity effect on detection thresholds at the 99-pps pulse rate (consistent with more degeneration of auditory nerve peripheral processes). The results suggest that A-TP pulses may improve place-pitch sensitivity or spectral coding for CI users, especially in situations with peripheral process degeneration.

Full-array channel discrimination in cochlear implants: validation and clinical application

OBJECTIVE

We sought to validate our proposed tool for estimating channel discrimination of cochlear implant (CI) users along the full electrode array and to assess associations between place-pitch discrimination and speech perception.

DESIGN

In two tests, participants identified one stimulus (probe) as the odd-one-out compared with two reference stimuli. Probe stimuli were evoked using dual electrode stimulation characterised by the current steering coefficient α. The first test measured psychometric functions (PFs) on pre-defined contacts, with just a noticeable difference (JNDα) as the outcome variable. The second test estimated channel discrimination on the full electrode array, yielding a discrimination score of Dα. We measured speech perception as free-field consonant-vowel-consonant phoneme recognition scores.

STUDY SAMPLE

We included 25 adults with at least 6 months of CI experience.

RESULTS

JNDα and Dα scores measured on the same contact correlated significantly (rs = 0.64, p < 0.001). Mean JNDα and speech perception scores showed significant relationships in quiet and in noise.

CONCLUSIONS

Dα correlated strongly with JNDα scores obtained with the PFs. For poor performers, the full-array test may underestimate JNDα. The full-array pitch discrimination test could be a helpful clinical tool, such as for fitting regions of lesser pitch discrimination ability.

Exponential fitting of spread of excitation response measurements in cochlear implants

BACKGROUND

Hearing performance in cochlear implant (CI) users is variable. An objective measure which can allow a prediction of this performance is desirable. Spread of neural excitation (SoE) curves are an objective measure that can be obtained using the fitting software of cochlear implants and might be able to be used as a predictor. A novel method to interpret SoE curves is presented. New Method Spread of excitation measurements for three recording sites were fitted using two exponential functions. An asymmetric width measure was developed, defined as the distance in mm to the point, where 25% or 50% of peak normalized amplitude was reached, for each half of the SoE separately. Also, a novel population of subjects with MED-EL CIs is used. Furthermore, speech perception (speech reception threshold, SRT) was evaluated using a matrix sentence test in a multi-source noise field.

RESULTS

SoE width was narrowest for the basal recording site and widest for the apical recording site. Fitted SoE exponential functions were most asymmetric for the apical recording site. A significant positive correlation between sentence test SRT and SoE width at the apical recording site was found. Comparison with Existing Methods The use of an asymmetric width measure correlated strongly and positively with speech perception for apical recording sites, unlike the symmetric width measure used in previous studies. Presumably, longer electrodes allow stimulation of a more apical part of the cochlear. At the apical part of the cochlea, dendrites from a large region of the basilar membrane map to a narrow portion on the spiral ganglion, which might explain the observed asymmetry.

CONCLUSIONS

For subjects implanted with long electrode arrays, an asymmetric width measure correlates positively with apical SoE distance. However, due to lack of a sufficient amount of data, the results are currently less conclusive and need to be consolidated in a larger cohort of subjects.

Dynamic Current Focusing Compared to Monopolar Stimulation in a Take-Home Trial of Cochlear Implant Users

OBJECTIVES

This study compared the performance of a dynamic partial tripolar cochlear implant speech encoding strategy termed dynamic current focusing (DCF) to monopolar stimulation (MP) using spectro-temporal, temporal, and speech-in-noise recognition testing.

DESIGN

DCF is a strategy that utilizes tripolar or high partial tripolar stimulation at threshold level and increases loudness by slowly widening current spread towards most comfortable level. Thirteen cochlear implant users were fitted with DCF and a non-steered MP matched on pulse rate, pulse width, and active electrodes. Nine participants completed the single-blinded within-subject crossover trial. Repeated testing consisted of four sessions. Strategies were allocated in a DCF-MP-DCF-MP or MP-DCF-MP-DCF design. Three-week adaptation periods ended with a test session in which speech-in-noise recognition (matrix speech-in-noise sentence test), spectro-temporal ripple tests (SMRT and STRIPES) and a temporal amplitude modulation detection test were conducted. All participants recorded their subjective experiences with both strategies using the Speech, Spatial and Qualities of Hearing Scale questionnaire.

RESULTS

Participants' SMRT thresholds improved 0.40 ripples per octave ( p = 0.02, Bonferroni-corrected: p = 0.1) with DCF over MP at 65 dB SPL. No significant differences between the strategies were found on speech-in-noise recognition at conversational (65 dB SPL) and soft (45 dB SPL) loudness levels, temporal testing, STRIPES, or the SMRT at 45 dB SPL. After Bonferroni correction, a learning effect remained on the matrix speech-in-noise sentence test at both loudness levels (65 dB SPL: p = 0.01; 45 dB SPL: p = 0.02). There was no difference in learning effects over time between DCF and MP. Similarly, no significant differences were found in subjective experience on the Speech, Spatial and Qualities of Hearing Scale questionnaire. DCF reduced average battery life by 48% (5.1 hours) ( p < 0.001) compared to MP.

CONCLUSIONS

DCF may improve spectral resolution over MP at comfortable loudness (65 dB SPL) in cochlear implant users. However, the evidence collected in this study was weak and the significant result disappeared after Bonferroni correction. Also, not all spectral tests revealed this improvement. As expected, battery life was reduced for DCF. Although the current study is limited by its small sample size, considering previous studies, DCF does not consistently improve speech recognition in noise over MP strategies.

Effect of stimulation parameters on sequential current-steered stimuli in cochlear implants

Manipulation of cochlear implant (CI) place pitch was carried out with current steering by stimulating two CI electrodes sequentially. The objective was to investigate whether shifts in activated neural populations could be achieved to produce salient pitch differences and to determine which stimulation parameters would be more effective in steering of current. These were the pulse rate and pulse width of electrical stimuli and the distance between the two current-steering electrodes. Nine CI users participated, and ten ears were tested. The pattern of pitch changes was not consistent across listeners, but the data suggest that individualized selection of stimulation parameters may be used to effect place pitch changes with sequential current steering. Individual analyses showed that pulse width generally had little influence on the effectiveness of current steering with sequential stimuli, while more salient place pitch shifts were often achieved at wider electrode spacing or when the stimulation pulse rate was the same as that indicated on the clinical MAP (the set of stimulation parameters) of the listener. Results imply that current steering may be used in CIs that allow only sequential stimulation to achieve place pitch manipulation.

Impact of Cochlear Implant Array Placement on Speech Perception

PURPOSE

To assess the role of flat panel computed tomography (FPCT) in the evaluation of cochlear implant (CI) electrode position and its relation to speech perception.

METHODS

From March 2015 to March 2019, we retrospectively enrolled deaf subjects ≥ 18 years who underwent unilateral CI by one surgeon, imaged with FPCT and assessed with disyllabic words score before CI and at 6 months of follow-up. We calculated the disyllabic score difference before CI and after CI (ΔSDS) and divided the subjects in favorable and unfavorable outcome groups using the median ΔSDS as a cutoff. We compared the demographic, clinical, electrode characteristics, and the CI positioning variables scalar position, surgical insertion depth (SID), linear insertion depth (LID), angular insertion depth (AID) and wrapping factor (WF).

RESULTS

We studied 50 subjects (F/M = 27/23; median age = 60.5 years, IQR: 50-70 years). The median ΔSDS was 80% (interquartile range [IQR]: 60-100%) in quiet and 80% (IQR: 47.5-100%) in noise. Of the subjects 23 demonstrated a favorable outcome and had earlier age at CI (median 52 years; IQR 45-67 years versus median 62 years; IQR: 56-71 years p = 0.032) and a significantly higher SID (median: 4.02 mm IQR: 3.00-5.35 mm versus median: 2.94 mm IQR: 2.06-3.90 mm; p = 0.029). No difference was found for LID (p = 0.977), AID (p = 0.302), and WF (p = 0.224). A logistic regression model built with the age at CI, number of CI electrodes, and the SID was significant χ2 ((df = 3, N = 50) = 14.517, p = 0.002). The model explained 33.7% (Nagelkerke R2) of ΔSDS variance and correctly classified 76% of the cases.

CONCLUSION

The SID measured by FPCT predicts the ΔSDS at 6 months follow-up, alongside with age at implantation and number of CI electrodes.

Word Recognition and Frequency Selectivity in Cochlear Implant Simulation: Effect of Channel Interaction

In cochlear implants (CI), spread of neural excitation may produce channel interaction. Channel interaction disturbs the spectral resolution and, among other factors, seems to impair speech recognition, especially in noise. In this study, two tests were performed with 20 adult normal-hearing (NH) subjects under different vocoded simulations. First, there was a measurement of word recognition in noise while varying the number of selected channels (4, 8, 12 or 16 maxima out of 20) and the degree of simulated channel interaction (“Low”, “Medium” and “High”). Then, there was an evaluation of spectral resolution function of the degree of simulated channel interaction, reflected by the sharpness (Q10dB) of psychophysical tuning curves (PTCs). The results showed a significant effect of the simulated channel interaction on word recognition but did not find an effect of the number of selected channels. The intelligibility decreased significantly for the highest degree of channel interaction. Similarly, the highest simulated channel interaction impaired significantly the Q10dB. Additionally, a strong intra-individual correlation between frequency selectivity and word recognition in noise was observed. Lastly, the individual changes in frequency selectivity were positively correlated with the changes in word recognition when the degree of interaction went from “Low” to “High”. To conclude, the degradation seen for the highest degree of channel interaction suggests a threshold effect on frequency selectivity and word recognition. The correlation between frequency selectivity and intelligibility in noise supports the hypothesis that PTCs Q10dB can account for word recognition in certain conditions. Moreover, the individual variations of performances observed among subjects suggest that channel interaction does not have the same effect on each individual. Finally, these results highlight the importance of taking into account subjects’ individuality and to evaluate channel interaction through the speech processor.

Music perception and training for pediatric cochlear implant users

INTRODUCTION

Cochlear implants (CIs) are biomedical devices that restore sound perception for people with severe-to-profound sensorineural hearing loss. Most postlingually deafened CI users are able to achieve excellent speech recognition in quiet environments. However, current CI sound processors remain limited in their ability to deliver fine spectrotemporal information, making it difficult for CI users to perceive complex sounds. Limited access to complex acoustic cues such as music, environmental sounds, lexical tones, and voice emotion may have significant ramifications on quality of life, social development, and community interactions.

AREAS COVERED

The purpose of this review article is to summarize the literature on CIs and music perception, with an emphasis on music training in pediatric CI recipients. The findings have implications on our understanding of noninvasive, accessible methods for improving auditory processing and may help advance our ability to improve sound quality and performance for implantees.

EXPERT OPINION

Music training, particularly in the pediatric population, may be able to continue to enhance auditory processing even after performance plateaus. The effects of these training programs appear generalizable to non-trained musical tasks, speech prosody and, emotion perception. Future studies should employ rigorous control groups involving a non-musical acoustic intervention, standardized auditory stimuli, and the provision of feedback.

Perception and prediction of loudness in sound coding strategies using simultaneous electric stimulation

Cochlear Implant (CI) sound coding strategies based on simultaneous stimulation lead to an increased loudness percept when compared to sequential stimulation using the same current levels. This is due to loudness summation as a result of channel interactions. Studying the loudness perception evoked by dual-channels compared to single-channels can be useful to optimize sound coding strategies that use simultaneous current pulses. Fourteen users of HiRes90k implants and one user of a CII implant loudness balanced single-channel to dual-channel stimuli with varying distance between simultaneous channels. In this study each component of a dual channel was a virtual channel, which shared current across two adjacent electrodes. Balancing was performed at threshold and comfortable level, for two spatial references (apical and basal) and for dual-channels with different relative current ratios. Increasing distance between dual-channels decreased the amount of current compensation in the dual-channel required to reach equal loudness to a single channel component by an average of 0.24 dB / mm without a significant difference between threshold and most comfortable level. If the components of the dual-channels were not at equal loudness, the loudness summation was reduced with respect to the equal loudness case. The results were incorporated into an existing loudness model by McKay et al. (2003). The predictions from the adapted model were evaluated by comparing the loudness evoked by simultaneous and sequential sound coding strategies. The application of the adapted model resulted in a deviation between predicted and actual behavioral loudness balancing adjustments in electrical level between simultaneous and sequential processing strategies of 0.24 dB on average.

Effectiveness of Phantom Stimulation in Shifting the Pitch Percept in Cochlear Implant Users

OBJECTIVES

Phantom electrode stimulation was developed for cochlear implant (CI) systems to provide a lower pitch percept by stimulating more apical regions of the cochlea, without inserting the electrode array deeper into the cochlea. Phantom stimulation involves simultaneously stimulating a primary and a compensating electrode with opposite polarity, thereby shifting the electrical field toward the apex and eliciting a lower pitch percept. The current study compared the effect sizes (in shifts of place of excitation) of multiple phantom configurations by matching the perceived pitch with phantom stimulation to that perceived with monopolar stimulation. Additionally, the effects of electrode location, type of electrode array, and stimulus level on the perceived pitch were investigated.

DESIGN

Fifteen adult advanced bionics CI users participated in this study, which included four experiments to eventually measure the shifts in place of excitation with five different phantom configurations. The proportions of current delivered to the compensating electrode, expressed as σ, were 0.5, 0.6, 0.7, and 0.8 for the symmetrical biphasic pulses (SBC0.5, SBC0.6, SBC0.7, and SBC0.8) and 0.75 for the pseudomonophasic pulse shape (PSA0.75). A pitch discrimination experiment was first completed to determine which basal and apical electrode contacts should be used for the subsequent experiments. An extensive loudness balancing experiment followed where both the threshold level (T-level) and most comfortable level (M-level) were determined to enable testing at multiple levels of the dynamic range. A pitch matching experiment was then performed to estimate the shift in place of excitation at the chosen electrode contacts. These rough shifts were then used in the subsequent experiment, where the shifts in place of excitation were determined more accurately.

RESULTS

Reliable data were obtained from 20 electrode contacts. The average shifts were 0.39, 0.53, 0.64, 0.76, and 0.53 electrode contacts toward the apex for SBC0.5, SBC0.6, SBC0.7, SBC0.8, and PSA0.75, respectively. When only the best configurations per electrode contact were included, the average shift in place of excitation was 0.92 electrode contacts (range: 0.25 to 2.0). While PSA0.75 leads to equal results as the SBC configurations in the apex, it did not result in a significant shift at the base. The shift in place of excitation was significantly larger at the apex and with lateral wall electrode contacts. The stimulus level did not affect the shift.

CONCLUSIONS

Phantom stimulation results in significant shifts in place of excitation, especially at the apical part of the electrode array. The phantom configuration that leads to the largest shift in place of excitation differs between subjects. Therefore, the settings of the phantom electrode should be individualized so that the phantom stimulation is optimized for each CI user. The real added value to the sound quality needs to be established in a take-home trial.

Channel discrimination along all contacts of the cochlear implant electrode array and its relation to speech perception

OBJECTIVE

To test the channel discrimination of cochlear implant (CI) users along all contacts of the electrode array and assess whether this is related to speech perception.

DESIGN

CI recipients were tested with a custom-made channel discrimination test. They were asked to distinguish a target stimulus from two reference stimuli in a three-alternative forced choice (3AFC) task. The target stimulus was evoked using current steering, with current steering coefficients (α) of 1, 0.5 and 0.25. The test provided a discrimination score (Dα) for each electrode contact along the array.

STUDY SAMPLE

Thirty adults implanted with a CI from Advanced Bionics.

RESULTS

Large variations in Dα scores were observed, both across the electrode array and between subjects. Statistical analysis revealed a significant channel-to-channel variability in Dα score (p < 0.01). Further, there was a significant relationship between subjects' Dα scores and their speech perception in quiet (p < 0.001).

CONCLUSIONS

The large variations in Dα score emphasise the importance of testing pitch discrimination across the complete electrode array. The relationship between Dα score and speech perception indicates that pitch discrimination might be a contributing factor to the performance of individual implant users.

A Novel Algorithm to Derive Spread of Excitation Based on Deconvolution

OBJECTIVE

The width of the spread of excitation (SOE) curve has been widely thought to represent an estimate of SOE. Therefore, correlates between psychophysical parameters, such as pitch discrimination and speech perception, and the width of SOE curves, have long been investigated. However, to date, no relationships between these objective and subjective measurements have yet been determined. In a departure from the current thinking, the authors now propose that the SOE curve, recorded with forward masking, is the equivalent of a convolution operation. As such, deconvolution would be expected to retrieve the excitation areas attributable to either masker or probe, potentially more closely revealing the actual neural SOE. This study aimed to develop a new analytical tool with which to derive SOE using this principle.

DESIGN

Intraoperative SOE curve measurements of 16 subjects, implanted with an Advanced Bionics implant, were analyzed. Evoked compound action potential (ECAP)-based SOE curves were recorded on electrodes 3 to 16, using the forward masker paradigm, with variable masker. The measured SOE curves were then compared with predicted SOE curves, built by the convolution of basic excitation density profiles (EDPs). Predicted SOE curves were fitted to the measured SOEs by iterative adjustment of the EDPs for the masker and the probe.

RESULTS

It was possible to generate a good fit between the predicted and measured SOE curves, inclusive of their asymmetry. The rectangular EDP was of least value in terms of its ability to generate a good fit; smoother SOE curves were modeled using the exponential or Gaussian EDPs. In most subjects, the EDP width (i.e., the size of the excitation area) gradually changed from wide at the apex of the electrode array, to narrow at the base. A comparison of EDP widths to SOE curve widths, as calculated in the literature, revealed that the EDPs now provide a measure of the SOE that is qualitatively distinct from that provided using conventional methods.

CONCLUSIONS

This study shows that an eCAP-based SOE curve, measured with forward masking, can be treated as a convolution of EDPs for masker and probe. The poor fit achieved for the measured and modeled data using the rectangular EDP, emphasizes the requirement for a sloping excitation area to mimic actual SOE recordings. Our deconvolution method provides an explanation for the frequently observed asymmetry of SOE curves measured along the electrode array, as this is a consequence of a wider excitation area in the apical part of the cochlea, in the absence of any asymmetry in the actual EDP. In addition, broader apical EDPs underlie the higher eCAP amplitudes found for apical stimulation.

Analysis of a purely conductance-based stochastic nerve fibre model as applied to compound models of populations of human auditory nerve fibres used in cochlear implant simulations

The study presents the application of a purely conductance-based stochastic nerve fibre model to human auditory nerve fibres within finite element volume conduction models of a semi-generic head and user-specific cochleae. The stochastic, threshold and temporal characteristics of the human model are compared and successfully validated against physiological feline results with the application of a mono-polar, bi-phasic, cathodic first stimulus. Stochastic characteristics validated include: (i) the log(Relative Spread) versus log(fibre diameter) distribution for the discharge probability versus stimulus intensity plots and (ii) the required exponential membrane noise versus transmembrane voltage distribution. Intra-user, and to a lesser degree inter-user, comparisons are made with respect to threshold and dynamic range at short and long pulse widths for full versus degenerate single fibres as well as for populations of degenerate fibres of a single user having distributed and aligned somas with varying and equal diameters. Temporal characteristics validated through application of different stimulus pulse rates and different stimulus intensities include: (i) discharge rate, latency and latency standard deviation versus stimulus intensity, (ii) period histograms and (iii) interspike interval histograms. Although the stochastic population model does not reduce the modelled single deterministic fibre threshold, the simulated stochastic and temporal characteristics show that it could be used in future studies to model user-specific temporally encoded information, which influences the speech perception of CI users.

Take-Home Trial Comparing Fast Fourier Transformation-Based and Filter Bank-Based Cochlear Implant Speech Coding Strategies

Previous studies have demonstrated no improved or deteriorated speech intelligibility with the HiResolution Fidelity 120™ speech coding strategy (HiResF120) over the original HiRes strategy. Improved spectral and deteriorated temporal sensitivities have been shown, making it plausible that the beneficial effect in the spectral domain was offset by the worsened temporal sensitivity. We hypothesize that the implementation of fast Fourier transform (FFT) processing, instead of the traditionally used bandpass filters, explains the reduction of temporal sensitivity. In this study, spectral ripple discrimination, temporal modulation detection, and speech intelligibility in noise were assessed in a two-week take-home trial with 3 speech coding strategies: one with conventional bandpass filters (HiRes), one with FFT-based filters (HiRes FFT), and one with FFT-based filters and current steering (HiRes Optima). One participant dropped out due to discomfort with both research programs. The 10 remaining participants performed equally well on all tasks with all three speech coding strategies, implying that FFT processing does not change the ability of CI recipients to discriminate spectral or temporal information or speech understanding.

The Electrically Evoked Compound Action Potential: From Laboratory to Clinic

The electrically evoked compound action potential (eCAP) represents the synchronous firing of a population of electrically stimulated auditory nerve fibers. It can be directly recorded on a surgically exposed nerve trunk in animals or from an intra-cochlear electrode of a cochlear implant. In the past two decades, the eCAP has been widely recorded in both animals and clinical patient populations using different testing paradigms. This paper provides an overview of recording methodologies and response characteristics of the eCAP, as well as its potential applications in research and clinical situations. Relevant studies are reviewed and implications for clinicians are discussed.

Systematic review of compound action potentials as predictors for cochlear implant performance

OBJECTIVES/HYPOTHESIS

The variability in speech perception between cochlear implant users is thought to result from the degeneration of the auditory nerve. Degeneration of the auditory nerve, histologically assessed, correlates with electrophysiologically acquired measures, such as electrically evoked compound action potentials (eCAPs) in experimental animals. To predict degeneration of the auditory nerve in humans, where histology is impossible, this paper reviews the correlation between speech perception and eCAP recordings in cochlear implant patients.

DATA SOURCES

PubMed and Embase.

REVIEW METHODS

We performed a systematic search for articles containing the following major themes: cochlear implants, evoked potentials, and speech perception. Two investigators independently conducted title-abstract screening, full-text screening, and critical appraisal. Data were extracted from the remaining articles.

RESULTS

Twenty-five of 1,429 identified articles described a correlation between speech perception and eCAP attributes. Due to study heterogeneity, a meta-analysis was not feasible, and studies were descriptively analyzed. Several studies investigating presence of the eCAP, recovery time constant, slope of the amplitude growth function, and spatial selectivity showed significant correlations with speech perception. In contrast, neural adaptation, eCAP threshold, and change with varying interphase gap did not significantly correlate with speech perception in any of the identified studies.

CONCLUSIONS

Significant correlations between speech perception and parameters obtained through eCAP recordings have been documented in literature; however, reporting was ambiguous. There is insufficient evidence for eCAPs as a predictive factor for speech perception. More research is needed to further investigate this relation. Laryngoscope, 2016 127:476-487, 2017.

Assessing the Electrode-Neuron Interface with the Electrically Evoked Compound Action Potential, Electrode Position, and Behavioral Thresholds

Variability in speech perception scores among cochlear implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual cochlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.

Excitation Patterns of Standard and Steered Partial Tripolar Stimuli in Cochlear Implants

Current steering in partial tripolar (pTP) mode has been shown to improve pitch perception and spectral resolution with cochlear implants (CIs). In this mode, a fraction (σ) of the main electrode current is returned within the cochlea and steered between the basal and apical flanking electrodes (with a proportion of α and 1 - α, respectively). Pitch generally decreases when α increases from 0 to 1, although the salience of pitch change varies across CI users. This study aimed to identify the mechanism of pitch changes with pTP-mode current steering and the factors contributing to the intersubject variability in pitch-ranking sensitivity. The electrical fields were measured for steered pTP stimuli on the same main electrode with α = 0, 0.5, and 1 in five implanted ears using electrical field imaging (EFI). The related excitation patterns were also measured physiologically using evoked compound action potential (ECAP) and psychophysically using psychophysical forward masking (PFM). Consistent with the pitch-ranking results in this study, the EFI, ECAP, and PFM centroids shifted apically with increasing α. An apical shift was also observed for the PFM peak but not for the EFI or ECAP peak. The pattern width was similar with different α values within a given measure (e.g., EFI, ECAP, or PFM), but the ECAP patterns were broader than the EFI and PFM patterns, possibly because ECAP was measured with smaller σ values than EFI and PFM. The amount of pattern shift with α depended on σ (i.e., the total amount of current used for steering) but was not correlated with the pitch-ranking sensitivity across subjects. The results revealed that the pitch changes elicited by pTP-mode current steering were not only driven by the shifts of excitation centroid.

Pitch ranking, electrode discrimination, and physiological spread of excitation using current steering in cochlear implants

The first objective of this study was to determine whether adaptive pitch-ranking and electrode-discrimination tasks with cochlear-implant (CI) recipients produce similar results for perceiving intermediate "virtual-channel" pitch percepts using current steering. Previous studies have not examined both behavioral tasks in the same subjects with current steering. A second objective was to determine whether a physiological metric of spatial separation using the electrically evoked compound action potential spread-of-excitation (ECAP SOE) function could predict performance in the behavioral tasks. The metric was the separation index (Σ), defined as the difference in normalized amplitudes between two adjacent ECAP SOE functions, summed across all masker electrodes. Eleven CII or 90 K Advanced Bionics (Valencia, CA) recipients were tested using pairs of electrodes from the basal, middle, and apical portions of the electrode array. The behavioral results, expressed as d', showed no significant differences across tasks. There was also no significant effect of electrode region for either task. ECAP Σ was not significantly correlated with pitch ranking or electrode discrimination for any of the electrode regions. Therefore, the ECAP separation index is not sensitive enough to predict perceptual resolution of virtual channels.

Effect of electrode impedance on spread of excitation and pitch perception using electrically coupled "dual-electrode" stimulation

OBJECTIVES

In newer-generation Cochlear Ltd. cochlear implants, two adjacent electrodes can be electrically coupled to produce a single contact or "dual electrode" (DE). The goal of the present study was to evaluate whether relatively large impedance differences (>3.0 kOhms) between coupled electrodes affect the excitation pattern and pitch percepts produced by the DE.

DESIGN

Fifteen electrode pairs in six recipients were tested. Neural spread-of-excitation patterns and pitch perception were measured for adjacent physical electrodes (PEs) and the resulting DE to determine if the lower-impedance PE in the pair dominates the DE response pattern. The results were compared with a "normative sample" (impedance differences <3.0 kOhms) from two earlier studies.

RESULTS

In general, spread-of-excitation patterns for DEs more closely approximated those of the lower-impedance PE in each pair. The DE was more easily distinguished in pitch from the higher-impedance PE than the lower-impedance PE. The electrically evoked compound action potential and perceptual results generally differed from those of the normative group.

CONCLUSIONS

Impedance differences between adjacent PEs should be considered if DE stimulation is implemented in future research studies or clinical coding strategies.

Pitch ranking, electrode discrimination, and physiological spread-of-excitation using Cochlear's dual-electrode mode

This study compared pitch ranking, electrode discrimination, and electrically evoked compound action potential (ECAP) spatial excitation patterns for adjacent physical electrodes (PEs) and the corresponding dual electrodes (DEs) for newer-generation Cochlear devices (Cochlear Ltd., Macquarie, New South Wales, Australia). The first goal was to determine whether pitch ranking and electrode discrimination yield similar outcomes for PEs and DEs. The second goal was to determine if the amount of spatial separation among ECAP excitation patterns (separation index, Σ) between adjacent PEs and the PE-DE pairs can predict performance on the psychophysical tasks. Using non-adaptive procedures, 13 subjects completed pitch ranking and electrode discrimination for adjacent PEs and the corresponding PE-DE pairs (DE versus each flanking PE) from the basal, middle, and apical electrode regions. Analysis of d' scores indicated that pitch-ranking and electrode-discrimination scores were not significantly different, but rather produced similar levels of performance. As expected, accuracy was significantly better for the PE-PE comparison than either PE-DE comparison. Correlations of the psychophysical versus ECAP Σ measures were positive; however, not all test/region correlations were significant across the array. Thus, the ECAP separation index is not sensitive enough to predict performance on behavioral tasks of pitch ranking or electrode discrimination for adjacent PEs or corresponding DEs.

ECAP spread of excitation with virtual channels and physical electrodes

The primary goal of this study was to evaluate physiological spatial excitation patterns for stimulation of adjacent physical electrodes and intermediate virtual channels. Two experiments were conducted that utilized electrically evoked compound action potential (ECAP) spread-of-excitation (SOE) functions obtained with the traditional forward-masking subtraction method. These two experiments examined spatial excitation patterns for virtual-channel maskers and probes, respectively. In Experiment 1, ECAP SOE patterns were obtained for maskers applied to physical electrodes and virtual channels to determine whether virtual-channel maskers yield SOE patterns similar to those predicted from physical electrodes. In Experiment 2, spatial separation of SOE functions was compared for two adjacent physical probe electrodes and the intermediate virtual channel to determine the extent to which ECAP SOE patterns for virtual-channel probes are spatially separate from those obtained with physical electrodes. Data were obtained for three electrode regions (basal, middle, apical) for 35 ears implanted with Cochlear (N = 16) or Advanced Bionics (N = 19) devices. Results from Experiment 1 showed no significant difference between predicted and measured ECAP amplitudes for Advanced Bionics subjects. Measured ECAP amplitudes for virtual-channel maskers were significantly larger than the predicted amplitudes for Cochlear subjects; however, the difference was <2 μV and thus is likely not clinically significant. Results from Experiment 2 showed that the probe set in the apical region demonstrated the least amount of spatial separation amongst SOE functions, which may be attributed to more uniform nerve survival patterns, closer electrode spacing, and/or the tapered geometry of the cochlea. As expected, adjacent physical probes demonstrated greater spatial separation than for comparisons between each physical probe and the intermediate virtual channel. Finally, the virtual-channel SOE functions were generally weighted toward the basal electrode in the pair.

Threshold levels of dual electrode stimulation in cochlear implants

Simultaneous stimulation on two contacts (current steering) creates intermediate pitches between the physical contacts in cochlear implants. All recent studies on current steering have focused on Most Comfortable Loudness levels and not at low stimulation levels. This study investigates the efficacy of dual electrode stimulation at lower levels, thereby focusing on the requirements to correct for threshold variations. With a current steered signal, threshold levels were determined on 4 different electrode pairs for 7 different current steering coefficients (α). This was done psychophysically in twelve postlingually deafened cochlear implant (HiRes90K, HiFocus1J) users and, in a computer model, which made use of three different neural morphologies. The analysis on the psychophysical data taking all subjects into account showed that in all conditions there was no significant difference between the threshold level of the physical contacts and the intermediate created percepts, eliminating the need for current corrections at these very low levels. The model data showed unexpected drops in threshold in the middle of the two physical contacts (both contacts equal current). Results consistent with this prediction were obtained for a subset of 5 subjects for the apical pair with wider spacing (2.2 mm). Further analysis showed that this decrease was only observed in subjects with a long duration of deafness. For current steering on adjacent contacts, the results from the psychophysical experiments were in line with the results from computational modelling. However, the dip in the threshold profile could only be replicated in the computational model with surviving peripheral processes without an unmyelinated terminal. On the basis of this result, we put forward that the majority of the surviving spiral ganglion cells in the cochlea in humans with a long duration of deafness still retain peripheral processes, but have lost their unmyelinated terminals.

Restoring speech perception with cochlear implants by spanning defective electrode contacts

CONCLUSION

Even with six defective contacts, spanning can largely restore speech perception with the HiRes 120 speech processing strategy to the level supported by an intact electrode array. Moreover, the sound quality is not degraded.

OBJECTIVES

Previous studies have demonstrated reduced speech perception scores (SPS) with defective contacts in HiRes 120. This study investigated whether replacing defective contacts by spanning, i.e. current steering on non-adjacent contacts, is able to restore speech recognition to the level supported by an intact electrode array.

METHODS

Ten adult cochlear implant recipients (HiRes90K, HiFocus1J) with experience with HiRes 120 participated in this study. Three different defective electrode arrays were simulated (six separate defective contacts, three pairs or two triplets). The participants received three take-home strategies and were asked to evaluate the sound quality in five predefined listening conditions. After 3 weeks, SPS were evaluated with monosyllabic words in quiet and in speech-shaped background noise.

RESULTS

The participants rated the sound quality equal for all take-home strategies. SPS with background noise were equal for all conditions tested. However, SPS in quiet (85% phonemes correct on average with the full array) decreased significantly with increasing spanning distance, with a 3% decrease for each spanned contact.

Consonant recognition as a function of the number of stimulation channels in the Hybrid short-electrode cochlear implant

Consonant recognition was measured as a function of the number of stimulation channels for Hybrid short-electrode cochlear implant (CI) users, long-electrode CI users, and normal-hearing (NH) listeners in quiet and background noise. Short-electrode CI subjects were tested with 1-6 channels allocated to a frequency range of 1063-7938 Hz. Long-electrode CI subjects were tested with 1-6, 8, or 22 channels allocated to 188-7938 Hz, or 1-6 or 15 channels from the basal 15 electrodes allocated to 1063-7938 Hz. NH listeners were tested with simulations of each CI group/condition. Despite differences in intracochlear electrode spacing for equivalent channel conditions, all CI subject groups performed similarly at each channel condition and improved up to at least four channels in quiet and noise. All CI subject groups underperformed relative to NH subjects. These preliminary findings suggest that the limited channel benefit seen for CI users may not be due solely to increases in channel interactions as a function of electrode density. Other factors such as pre-operative patient history, location of stimulation in the base versus apex, or a limit on the number of electric channels that can be processed cognitively, may also interact with the effects of electrode contact spacing along the cochlea.

Assessing the placement of a cochlear electrode array by multidimensional scaling

Correct placement of the electrode is crucial for cochlear implantation (CI) surgery. It determines the access to the auditory nerve and subsequent hearing performance. Here, we propose an objective measures tool that can partially verify the electrode position. The intracochlear spread of the electrical fields is measured and analyzed by means of multidimensional scaling resulting in an intuitive visual representation. The user can then detect major issues, such as electrode foldover or ossification. Other implantation issues, such as electrode migration into the scala vestibuli, may not significantly alter the electrical conduction pattern and remain undetected. Still, as the measurement is quick and readily available, it may be a valuable intraoperative verification tool.