A new method for dealing with the stimulus artefact in electrically evoked compound action potential measurements

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.

Comparison of response properties of the electrically stimulated auditory nerve reported in human listeners and in animal models

Cochlear implants (CIs) provide acoustic information to implanted patients by electrically stimulating nearby auditory nerve fibers (ANFs) which then transmit the information to higher-level neural structures for further processing and interpretation. Computational models that simulate ANF responses to CI stimuli enable the exploration of the mechanisms underlying CI performance beyond the capacity of in vivo experimentation alone. However, all ANF models developed to date utilize to some extent anatomical/morphometric data, biophysical properties and/or physiological data measured in non-human animal models. This review compares response properties of the electrically stimulated auditory nerve (AN) in human listeners and different mammalian models. Properties of AN responses to single pulse stimulation, paired-pulse stimulation, and pulse-train stimulation are presented. While some AN response properties are similar between human listeners and animal models (e.g., increased AN sensitivity to single pulse stimuli with long interphase gaps), there are some significant differences. For example, the AN of most animal models is typically more sensitive to cathodic stimulation while the AN of human listeners is generally more sensitive to anodic stimulation. Additionally, there are substantial differences in the speed of recovery from neural adaptation between animal models and human listeners. Therefore, results from animal models cannot be simply translated to human listeners. Recognizing the differences in responses of the AN to electrical stimulation between humans and other mammals is an important step for creating ANF models that are more applicable to various human CI patient populations.

Recording EEG in Cochlear Implant Users: Guidelines for Experimental Design and Data Analysis for Optimizing Signal Quality and Minimizing Artifacts

Cochlear implants (CI) are neural prostheses that can restore hearing in individuals with severe to profound hearing loss. Although CIs significantly improve quality of life, clinical outcomes are still highly variable. An important part of this variability is explained by the brain reorganization following cochlear implantation. Therefore, clinicians and researchers are seeking objective measurements to investigate post-implantation brain plasticity. Electroencephalography (EEG) is a promising technique because it is objective, non-invasive, and implant-compatible, but is nonetheless susceptible to massive artifacts generated by the prosthesis's electrical activity. CI artifacts can blur and distort brain responses; thus, it is crucial to develop reliable techniques to remove them from EEG recordings. Despite numerous artifact removal techniques used in previous studies, there is a paucity of documentation and consensus on the optimal EEG procedures to reduce these artifacts. Herein, and through a comprehensive review process, we provide a guideline for designing an EEG-CI experiment minimizing the effect of the artifact. We provide some technical guidance for recording an accurate neural response from CI users and discuss the current challenges in detecting and removing CI-induced artifacts from a recorded signal. The aim of this paper is also to provide recommendations to better appraise and report EEG-CI findings.

Effect of Increasing Pulse Phase Duration on Neural Responsiveness of the Electrically Stimulated Cochlear Nerve

OBJECTIVES

The aim of this study is to (1) investigate the effects of increasing the pulse phase duration (PPD) on the neural response of the electrically stimulated cochlear nerve (CN) in children with CN deficiency (CND) and (2) compare the results from the CND population to those measured in children with normal-sized CNs.

DESIGN

Study participants included 30 children with CND and 30 children with normal-sized CNs. All participants used a Cochlear Nucleus device in the test ear. For each subject, electrically evoked compound action potential (eCAP) input/output (I/O) functions evoked by single biphasic pulses with different PPDs were recorded at three electrode locations across the electrode array. PPD durations tested in this study included 50, 62, 75, and 88 μsec/phase. For each electrode tested for each study participant, the amount of electrical charge corresponding to the maximum comfortable level measured for the 88 μsec PPD was used as the upper limit of stimulation. The eCAP amplitude measured at the highest electrical charge level, the eCAP threshold (i.e., the lowest level that evoked an eCAP), and the slope of the eCAP I/O function were measured. Generalized linear mixed effect models with study group, electrode location, and PPD as the fixed effects and subject as the random effect were used to compare these dependent variables measured at different electrode locations and PPDs between children with CND and children with normal-sized CNs.

RESULTS

Children with CND had smaller eCAP amplitudes, higher eCAP thresholds, and smaller slopes of the eCAP I/O function than children with normal-sized CNs. Children with CND who had fewer electrodes with a measurable eCAP showed smaller eCAP amplitudes and flatter eCAP I/O functions than children with CND who had more electrodes with eCAPs. Increasing the PPD did not show a statistically significant effect on any of these three eCAP parameters in the two subject groups tested in this study.

CONCLUSIONS

For the same amount of electrical charge, increasing the PPD from 50 to 88 μsec for a biphasic pulse with a 7 μsec interphase gap did not significantly affect CN responsiveness to electrical stimulation in human cochlear implant users. Further studies with different electrical pulse configurations are warranted to determine whether evaluating the eCAP sensitivity to changes in the PPD can be used as a testing paradigm to estimate neural survival of the CN for individual cochlear implant users.

Electrophysiology and genetic testing in the precision medicine of congenital deafness: A review

BACKGROUND

Congenital hearing loss is remarkably heterogeneous, with over 130 deafness genes and thousands of variants, making for innumerable genotype/phenotype combinations. Understanding both the pathophysiology of hearing loss and molecular site of lesion along the auditory pathway permits for significantly individualized counseling. Electrophysiologic techniques such as electrocochleography (ECochG) and electrically-evoked compound action potentials (eCAP) are being studied to localize pathology and estimate residual cochlear vs. neural health. This review describes the expanding roles of genetic and electrophysiologic evaluation in the precision medicine of congenital hearing loss.The basics of genetic mutations in hearing loss and electrophysiologic testing (ECochG and eCAP) are reviewed, and how they complement each other in the diagnostics and prognostication of hearing outcomes. Used together, these measures improve the understanding of insults to the auditory system, allowing for individualized counseling for CI candidacy/outcomes or other habilitation strategies.

CONCLUSION

Despite tremendous discovery in deafness genes, the effects of individual genes on neural function remain poorly understood. Bridging the understanding between molecular genotype and neural and functional phenotype is paramount to interpreting genetic results in clinical practice. The future hearing healthcare provider must consolidate an ever-increasing amount of genetic and phenotypic information in the precision medicine of hearing loss.

Level coding by phase duration and asymmetric pulse shape reduce channel interactions in cochlear implants

Conventional loudness coding with CIs by pulse current amplitude has a disadvantage: Increasing the stimulation current increases the spread of excitation in the auditory nerve, resulting in stronger channel interactions at high stimulation levels. These limit the number of effective information channels that a CI user can perceive. Stimulus intensity information (loudness) can alternatively be transmitted via pulse phase duration. We hypothesized that loudness coding by phase duration avoids the increase in the spread of the electric field and thus leads to less channel interactions at high stimulation levels. To avoid polarity effects, we combined this coding with pseudomonophasic stimuli. To test whether this affects the spread of excitation, 16 acutely deafened guinea pigs were implanted with CIs and neural activity from the inferior colliculus was recorded while stimulating with either biphasic, amplitude-coded pulses, or pseudomonophasic, duration- or amplitude-coded pulses. Pseudomonophasic stimuli combined with phase duration loudness coding reduced the lowest response thresholds and the spread of excitation. We investigated the channel interactions at suprathreshold levels by computing the phase-locking to a pulse train in the presence of an interacting pulse train on a different electrode on the CI. Pseudomonophasic pulses coupled with phase duration loudness coding reduced the interference by 4-5% compared to biphasic pulses, depending on the place of stimulation. This effect of pseudomonophasic stimuli was achieved with amplitude coding only in the basal cochlea, indicating a distance- or volume dependent effect. Our results show that pseudomonophasic, phase-duration-coded stimuli slightly reduce channel interactions, suggesting a potential benefit for speech understanding in humans.

Late electrically-evoked compound action potentials as markers for acute micro-lesions of spiral ganglion neurons

Cochlear implants (CIs) are the treatment of choice for profoundly hearing impaired people. It has been proposed that speech perception in CI users is influenced by the neural health (deafferentation, demyelination and degeneration) of the cochlea, which may be heterogeneous along an individual cochlea. Several options have been put forward to account for these local differences in neural health when fitting the speech processor settings, however with mixed results. The interpretation of the results is hampered by the fact that reliable markers of locally restricted changes in spiral ganglion neuron (SGN) health are lacking. The aim of the study was (i) to establish mechanical micro-lesions in the guinea pig as a model of heterogeneous SGN deafferentation and degeneration and (ii) to assess potential electrophysiological markers that can also be used in human subjects. First, we defined the extent of micro-lesions in normal hearing animals using acoustically-evoked compound action potentials (aCAPs); second, we measured electrically-evoked CAPs (eCAPs) before and after focal lesioning in neomycin-deafened and implanted animals. Therefore, we inserted guinea pig adjusted 6-contact CIs through a cochleostomy in the scala tympani. The eCAP was recorded from a ball electrode at the round window niche in response to monopolar or bipolar, 50 µs/phase biphasic pulses of alternating anodic- and cathodic-leading polarity. To exclude the large electrical artifact from the analysis, we focused on the late eCAP component. We systematically isolated the eCAP parameter that showed local pre- versus post-lesion changes and lesion-target specificity. Histological evaluation of the cleared cochleae revealed focal damage of an average size of 0.0036 mm³ with an apical-basal span of maximal 440 µm. We found that the threshold of the late N2P2 eCAP component was significantly elevated after lesioning when stimulating at basal (near the lesion), but not apical (distant to the lesion) CI contacts. To circumvent the potentially conflicting influence of the apical-basal gradient in eCAP thresholds, we used the polarity effect (PE=cathodic-anodic) as a relative measure. During monopolar stimulation, but not bipolar stimulation, the PE was sensitive to the lesion target and showed significantly better cathodic than anodic thresholds after soma lesions. We conclude that the difference in N2P2 thresholds in response to cathodic versus anodic-leading monopolar stimulation corresponds to the presence of SGN soma damage, and may therefore be a marker for SGN loss. We consider this electrophysiological estimate of local neural health a potentially relevant tool for human applications because of the temporal separation from the stimulation artifact and possible implementation into common eCAP measurements.

Ramped pulse shapes are more efficient for cochlear implant stimulation in an animal model

In all commercial cochlear implant (CI) devices, the electric stimulation is performed with a rectangular pulse that generally has two phases of opposite polarity. To date, developing new stimulation strategies has relied on the efficacy of this shape. Here, we investigate the potential of a novel stimulation paradigm that uses biophysically-inspired electrical ramped pulses. Using electrically-evoked auditory brainstem response (eABR) recordings in mice, we found that less charge, but higher current level amplitude, is needed to evoke responses with ramped shapes that are similar in amplitude to responses obtained with rectangular shapes. The most charge-efficient pulse shape had a rising ramp over both phases, supporting findings from previous in vitro studies. This was also true for longer phase durations. Our study presents the first physiological data on CI-stimulation with ramped pulse shapes. By reducing charge consumption ramped pulses have the potential to produce more battery-efficient CIs and may open new perspectives for designing other efficient neural implants in the future.

Test/Retest Variability of the eCAP Threshold in Advanced Bionics Cochlear Implant Users

The reliability of the electrically evoked compound action potential (eCAP) threshold depends on its precision and accuracy. The precision of the eCAP threshold reflects its variability, while the accuracy of the threshold shows how close it is to the actual value. The objective of this study was to determine the test/retest variability of the eCAP threshold in Advanced Bionics cochlear implant users, which has never been reported before. We hypothesized that the test/retest variability is dependent on the presence of random noise in the recorded eCAP waveforms. If this holds true, the recorded error should be reduced by approximately the square-root of the number of averages. As secondary objectives, we assessed the effects of the slope of the amplitude growth function (AGF), cochlear location, and eCAP threshold on eCAP threshold precision. We hypothesized that steeper slopes should result in better precision of the linearly extrapolated eCAP threshold. As other studies have shown that apical regions have steeper slopes and larger eCAPs, we recorded eCAPs in three different cochlear locations. The difference of the precision between two commonly applied stimulus-artifact reduction paradigms on eCAP threshold precision was compared, namely averaging of alternating stimulus polarities (AP averaging) and forward masking (FM). FM requires the addition of more waveforms than AP averaging, and hence we expected FM to have lower precision than AP.

Effect of Stimulus Polarity on Physiological Spread of Excitation in Cochlear Implants

BACKGROUND

Contemporary cochlear implants (CIs) use cathodic-leading, symmetrical, biphasic current pulses, despite a growing body of evidence that suggests anodic-leading pulses may be more effective at stimulating the auditory system. However, since much of this research on humans has used pseudomonophasic pulses or biphasic pulses with unusually long interphase gaps, the effects of stimulus polarity are unclear for clinically relevant (i.e., symmetric biphasic) stimuli.

PURPOSE

The purpose of this study was to examine the effects of stimulus polarity on basic characteristics of physiological spread-of-excitation (SOE) measures obtained with the electrically evoked compound action potential (ECAP) in CI recipients using clinically relevant stimuli.

RESEARCH DESIGN

Using a within-subjects (repeated measures) design, we examined the differences in mean amplitude, peak electrode location, area under the curve, and spatial separation between SOE curves obtained with anodic- and cathodic-leading symmetrical, biphasic pulses.

STUDY SAMPLE

Fifteen CI recipients (ages 13-77) participated in this study. All were users of Cochlear Ltd. devices.

DATA COLLECTION AND ANALYSIS

SOE functions were obtained using the standard forward-masking artifact reduction method. Probe electrodes were 5-18, and they were stimulated at an 8 (of 10) loudness rating ("loud"). Outcome measures (mean amplitude, peak electrode location, curve area, and spatial separation) for each polarity were compared within subjects.

RESULTS

Anodic-leading current pulses produced ECAPs with larger average amplitudes, greater curve area, and less spatial separation between SOE patterns compared with that for cathodic-leading pulses. There was no effect of polarity on peak electrode location.

CONCLUSIONS

These results indicate that for equal current levels, the anodic-leading polarity produces broader excitation patterns compared with cathodic-leading pulses, which reduces the spatial separation between functions. This result is likely due to preferential stimulation of the central axon. Further research is needed to determine whether SOE patterns obtained with anodic-leading pulses better predict pitch discrimination.

Effects of Stimulus Polarity and Artifact Reduction Method on the Electrically Evoked Compound Action Potential

OBJECTIVE

Previous research from our laboratory comparing electrically evoked compound action potential (ECAP) artifact reduction methods has shown larger amplitudes and lower thresholds with cathodic-leading forward masking (CathFM) than with alternating polarity (AltPol). One interpretation of this result is that the anodic-leading phase used with AltPol elicits a less excitatory response (in contrast to results from recent studies with humans), which when averaged with responses to cathodic-leading stimuli, results in smaller amplitudes. Another interpretation is that the latencies of the responses to anodic- and cathodic-leading pulses differ, which when averaged together, result in smaller amplitudes than for either polarity alone due to temporal smearing. The purpose of this study was to separate the effects of stimulus polarity and artifact reduction method to determine the relative effects of each.

DESIGN

This study used a within-subjects design. ECAP growth functions were obtained using CathFM, anodic-leading forward masking (AnodFM), and AltPol for 23 CI recipients (N = 13 Cochlear and N = 10 Advanced Bionics). N1 latency, amplitude, slope of the amplitude growth function, and threshold were compared across methods. Data were analyzed separately for each manufacturer due to inherent differences between devices.

RESULTS

N1 latencies were significantly shorter for AnodFM than for CathFM and AltPol for both Cochlear and Advanced Bionics participants. Amplitudes were larger for AnodFM than for either CathFM or AltPol for Cochlear recipients; amplitude was not significantly different across methods for Advanced Bionics recipients. Slopes were shallowest for CathFM for Cochlear subjects, but were not significantly different among methods for Advanced Bionics subjects. Thresholds with AltPol were significantly higher than both FM methods for Cochlear recipients; there was no difference in threshold across methods for the Advanced Bionics recipients.

CONCLUSIONS

For Cochlear devices, the smaller amplitudes and higher thresholds observed for AltPol seem to be the result of latency differences between polarities. These results suggest that AltPol is not ideal for managing stimulus artifact for ECAP recordings. For the Advanced Bionics group, there were no significant differences among methods for amplitude, slope, or threshold, which suggests that polarity and artifact reduction method have little influence in these devices. We postulate that polarity effects are minimized for symmetrical biphasic pulses that lack an interphase gap, such as those used with Advanced Bionics devices; however, this requires further investigation.

A Comparison of Alternating Polarity and Forward Masking Artifact-Reduction Methods to Resolve the Electrically Evoked Compound Action Potential

OBJECTIVE

Cochlear implant manufacturers utilize different artifact-reduction methods to measure electrically evoked compound action potentials (ECAPs) in the clinical software. Two commercially available artifact-reduction techniques include forward masking (FwdMsk) and alternating polarity (AltPol). AltPol assumes that responses to the opposing polarities are equal, which is likely problematic. On the other hand, FwdMsk can yield inaccurate waveforms if the masker does not effectively render all neurons into a refractory state. The goal of this study was to compare ECAP thresholds, amplitudes, and slopes of the amplitude growth functions (AGFs) using FwdMsk and AltPol to determine whether the two methods yield similar results.

DESIGN

ECAP AGFs were obtained from three electrode regions (basal, middle, and apical) across 24 ears in 20 Cochlear Ltd. recipients using both FwdMsk and AltPol methods. AltPol waveforms could not be resolved for recipients of devices with the older-generation chip (CI24R(CS); N = 6).

RESULTS

Results comparing FwdMsk and AltPol in the CI24RE- and CI512-generation devices showed significant differences in threshold, AGF slope, and amplitude between methods. FwdMsk resulted in lower visual-detection thresholds (p < 0.001), shallower slopes (p = 0.004), and larger amplitudes (p = 0.03) compared with AltPol.

CONCLUSIONS

Results from this study are consistent with recent findings showing differences in ECAP amplitude and latency between polarities for human CI recipients. When averaged, these differences likely result in a reduced ECAP response with AltPol. The next step will be to separate the effects of artifact-reduction method and stimulus polarity to determine the relative effects of each.

Effects of Pulse Shape and Polarity on Sensitivity to Cochlear Implant Stimulation: A Chronic Study in Guinea Pigs

Most cochlear implants (CIs) stimulate the auditory nerve with trains of symmetric biphasic pulses consisting of two phases of opposite polarity. Animal and human studies have shown that both polarities can elicit neural responses. In human CI listeners, studies have shown that at suprathreshold levels, the anodic phase is more effective than the cathodic phase. In contrast, animal studies usually show the opposite trend. Although the reason for this discrepancy remains unclear, computational modelling results have proposed that the degeneration of the peripheral processes of the neurons could lead to a higher efficiency of anodic stimulation. We tested this hypothesis in ten guinea pigs who were deafened with an injection of sysomycin and implanted with a single ball electrode inserted in the first turn of the cochlea. Animals were tested at regular intervals between 1 week after deafening and up to 1 year for some of them. Our hypothesis was that if the effect of polarity is determined by the presence or absence of peripheral processes, the difference in polarity efficiency should change over time because of a progressive neural degeneration. Stimuli consisted of charge-balanced symmetric and asymmetric pulses allowing us to observe the response to each polarity individually. For all stimuli, the inferior colliculus evoked potential was measured. Results show that the cathodic phase was more effective than the anodic phase and that this remained so even several months after deafening. This suggests that neural degeneration cannot entirely account for the higher efficiency of anodic stimulation observed in human CI listeners.

Electrically evoked compound action potential artifact rejection by independent component analysis: technique validation

The electrically-evoked compound action potential (ECAP) is the synchronous whole auditory nerve activity in response to an electrical stimulus, and can be recorded in situ on cochlear implant (CI) electrodes. A novel procedure (ECAP-ICA) to isolate the ECAP from the stimulation artifact, based on independent component analysis (ICA), is described here. ECAPs with artifact (raw-ECAPs) were sequentially recorded for the same stimulus on 9 different intracochlear recording electrodes. The raw-ECAPs were fed to ICA, which separated them into independent sources. Restricting the ICA projection to 4 independent components did not induce under-fitting and was found to explain most of the raw-data variance. The sources were identified and only the source corresponding to the neural response was retained for artifact-free ECAP reconstruction. The validity of the ECAP-ICA procedure was supported as follows: N₁ and P₁ peaks occurred at usual latencies; and ECAP-ICA and artifact amplitude-growth functions (AGFs) had different slopes. Concatenation of raw-ECAPs from multiple stimulus currents, including some below the ECAP-ICA threshold, improved the source separation process. The main advantage of ECAP-ICA is that use of maskers or alternating polarity stimulation are not needed.

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Novel technique to cancel artifact in electrically-evoked compound action potentials.

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No maskers or alternate polarity are required.

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ECAPs output from the procedure are compared to forward-masking technique.

The polarity sensitivity of the electrically stimulated human auditory nerve measured at the level of the brainstem

Recent behavioral studies have suggested that the human auditory nerve of cochlear implant (CI) users is mainly excited by the positive (anodic) polarity. Those findings were only obtained using asymmetric pseudomonophasic (PS) pulses where the effect of one phase was measured in the presence of a counteracting phase of opposite polarity, longer duration, and lower amplitude than the former phase. It was assumed that only the short high-amplitude phase was responsible for the excitation. Similarly, it has been shown that electrically evoked compound action potentials could only be obtained in response to the anodic phases of asymmetric pulses. Here, experiment 1 measured electrically evoked auditory brainstem responses to standard symmetric, PS, reversed pseudomonophasic, and reversed pseudomonophasic with inter-phase gap (6 ms) pulses presented for both polarities. Responses were time locked to the short high-amplitude phase of asymmetric pulses and were smaller, but still measurable, when that phase was cathodic than when it was anodic. This provides the first evidence that cathodic stimulation can excite the auditory system of human CI listeners and confirms that this stimulation is nevertheless less effective than for the anodic polarity. A second experiment studied the polarity sensitivity at different intensities by means of a loudness balancing task between pseudomonophasic anodic (PSA) and pseudomonophasic cathodic (PSC) stimuli. Previous studies had demonstrated greater sensitivity to anodic stimulation only for stimuli producing loud percepts. The results showed that PSC stimuli required higher amplitudes than PSA stimuli to reach the same loudness and that this held for current levels ranging from 10 to 100% of the dynamic range.

Evaluating the noise in electrically evoked compound action potential measurements in cochlear implants

Electrically evoked compound action potentials (ECAPs) are widely used to study the excitability of the auditory nerve and stimulation properties in cochlear implant (CI) users. However, ECAP detection can be difficult and very subjective at near-threshold stimulation levels or in spread of excitation measurements. In this study, we evaluated the statistical properties of the background noise (BN) and the postaverage residual noise (RN) in ECAP measurements in order to determine an objective detection criterion. For the estimation of the BN and the RN, a method currently used in auditory brainstem response measurements was applied. The potential benefit of using weighted (Bayesian) averages was also examined. All estimations were performed with a set of approximately 360 ECAP measurements recorded from five human CI users of the CII or HiRes90K device (advanced bionics). Results demonstrated that the BN was normally distributed and the RN decreased according to the square root of the number of averages. No additional benefit was observed by using weighted averaging. The noise was not significantly different either at different stimulation intensities or across recording electrodes along the cochlea. The analysis of the statistical properties of the noise indicated that a signal-to-noise ratio of 1.7 dB as a detection criterion corresponds to a false positive detection rate of 1% with the used measurement setup.

Spatial channel interactions in cochlear implants

The modern multi-channel cochlear implant is widely considered to be the most successful neural prosthesis owing to its ability to restore partial hearing to post-lingually deafened adults and to allow essentially normal language development in pre-lingually deafened children. However, the implant performance varies greatly in individuals and is still limited in background noise, tonal language understanding, and music perception. One main cause for the individual variability and the limited performance in cochlear implants is spatial channel interaction from the stimulating electrodes to the auditory nerve and brain. Here we systematically examined spatial channel interactions at the physical, physiological, and perceptual levels in the same five modern cochlear implant subjects. The physical interaction was examined using an electric field imaging technique, which measured the voltage distribution as a function of the electrode position in the cochlea in response to the stimulation of a single electrode. The physiological interaction was examined by recording electrically evoked compound action potentials as a function of the electrode position in response to the stimulation of the same single electrode position. The perceptual interactions were characterized by changes in detection threshold as well as loudness summation in response to in-phase or out-of-phase dual-electrode stimulation. To minimize potentially confounding effects of temporal factors on spatial channel interactions, stimulus rates were limited to 100 Hz or less in all measurements. Several quantitative channel interaction indexes were developed to define and compare the width, slope and symmetry of the spatial excitation patterns derived from these physical, physiological and perceptual measures. The electric field imaging data revealed a broad but uniformly asymmetrical intracochlear electric field pattern, with the apical side producing a wider half-width and shallower slope than the basal side. In contrast, the evoked compound action potential and perceptual channel interaction data showed much greater individual variability. It is likely that actual reduction in neural and higher level interactions, instead of simple sharpening of the electric current field, would be the key to predicting and hopefully improving the variable cochlear implant performance. The present results are obtained with auditory prostheses but can be applied to other neural prostheses, in which independent spatial channels, rather than a high stimulation rate, are critical to their performance.

Reducing blanking artifact in electrically evoked compound action potentials

The main source of distortion in the recording of the electrically evoked compound action potentials is the stimulus artifact. The popular hardware blanking technique tends to reduce this artifact, but generates a blanking artifact as a consequence of the transient state in the amplifier. In this paper we propose two techniques to deal with the blanking artifact. The proposed techniques are combined with conventional and generalized alternating stimulation in order to reduce both stimulus and blanking artifacts in the recording of the evoked potentials. A comparison over 126 evoked potential recordings reveals that the proposed blanking artifact reduction methods improve the quality of electrically evoked compound action potential recordings.

Complete optical neurophysiology: toward optical stimulation and recording of neural tissue

Direct optical methods to stimulate and record neural activity provide artifact-free, noninvasive, and noncontact neurophysiological procedures. For stimulation, focused mid-infrared light alters membrane potential and activates individual neural processes. Simultaneous intrinsic scattered light parameters, including birefringence changes, can record neural activity with signals similar to potentiometric dyes. The simultaneous combination of optical stimulation and optical recording techniques provide the potential for powerful tools that may someday remove the need for invasive wires during electrophysiological recordings.

Higher sensitivity of human auditory nerve fibers to positive electrical currents

Most contemporary cochlear implants (CIs) stimulate the auditory nerve with trains of amplitude-modulated, symmetric biphasic pulses. Although both polarities of a pulse can depolarize the nerve fibers and generate action potentials, it remains unknown which of the two (positive or negative) phases has the stronger effect. Understanding the effects of pulse polarity will help to optimize the stimulation protocols and to deliver the most relevant information to the implant listeners. Animal experiments have shown that cathodic (negative) current flows are more effective than anodic (positive) ones in eliciting neural responses, and this finding has motivated the development of novel speech-processing algorithms. In this study, we show electrophysiologically and psychophysically that the human auditory system exhibits the opposite pattern, being more sensitive to anodic stimulation. We measured electrically evoked compound action potentials in CI listeners for phase-separated pulses, allowing us to tease out the responses to each of the two opposite-polarity phases. At an equal stimulus level, the anodic phase yielded the larger response. Furthermore, a measure of psychophysical masking patterns revealed that this polarity difference was still present at higher levels of the auditory system and was therefore not solely due to antidromic propagation of the neural response. This finding may relate to a particular orientation of the nerve fibers relative to the electrode or to a substantial degeneration and demyelination of the peripheral processes. Potential applications to improve CI speech-processing strategies are discussed.

Advances in cochlear implant telemetry: evoked neural responses, electrical field imaging, and technical integrity

During the last decade, cochlear implantation has evolved into a well-established treatment of deafness, predominantly because of many improvements in speech processing and the controlled excitation of the auditory nerve. Cochlear implants now also feature telemetry, which is highly useful to monitor the proper functioning of the implanted electronics and electrode contacts. Telemetry can also support the clinical management in young children and difficult cases where neural unresponsiveness is suspected. This article will review recent advances in the telemetry of the electrically evoked compound action potential that have made these measurements simple and routine procedures in most cases. The distribution of the electrical stimulus itself sampled by "electrical field imaging" reveals general patterns of current flow in the normal cochlea and gross abnormalities in individual patients; models have been developed to derive more subtle insights from an individual electrical field imaging. Finally, some thoughts are given to the extended application of telemetry, for example, in monitoring the neural responses or in combination with other treatments of the deaf ear.

Unraveling the electrically evoked compound action potential

With the advent of eCAP recording tools such as NRT and NRI for cochlear implants, neural monitoring has become widely used to ascertain the integrity of the neural/electrode interface as well as for assisting in the setting of program levels. The basic concepts of eCAP recordings are deduced from the acoustical equivalent of the electrocochleogram. There are, however, indications that under electrical stimulation some of these do not hold, like the unitary response concept (i.e., the principle that every fiber produces the same contribution to the eCAP). Computer modeling has proven to be a valuable tool for gaining insight into the functioning of electrical stimulation. In this study the extension of a three-dimensional human cochlea, incorporating back-measuring capabilities, is described. Using this new model, the contribution of single fiber action potentials (SFAPs) to the measured eCAP is investigated. The model predicts that contrary to common belief--the compound action potential as measured by the cochlear implant system does not necessarily reflect the propagated action potential along the auditory nerve.