Acoustic-electric interactions in the guinea pig auditory nerve: simultaneous and forward masking of the electrically evoked compound action potential

The Relationship Between Cochlear Implant Speech Perception Outcomes and Electrophysiological Measures of the Electrically Evoked Compound Action Potential

OBJECTIVE

This study assessed the relationship between electrophysiological measures of the electrically evoked compound action potential (eCAP) and speech perception scores measured in quiet and in noise in postlingually deafened adult cochlear implant (CI) users. It tested the hypothesis that how well the auditory nerve (AN) responds to electrical stimulation is important for speech perception with a CI in challenging listening conditions.

DESIGN

Study participants included 24 postlingually deafened adult CI users. All participants used Cochlear Nucleus CIs in their test ears. In each participant, eCAPs were measured at multiple electrode locations in response to single-pulse, paired-pulse, and pulse-train stimuli. Independent variables included six metrics calculated from the eCAP recordings: the electrode-neuron interface (ENI) index, the neural adaptation (NA) ratio, NA speed, the adaptation recovery (AR) ratio, AR speed, and the amplitude modulation (AM) ratio. The ENI index quantified the effectiveness of the CI electrodes in stimulating the targeted AN fibers. The NA ratio indicated the amount of NA at the AN caused by a train of constant-amplitude pulses. NA speed was defined as the speed/rate of NA. The AR ratio estimated the amount of recovery from NA at a fixed time point after the cessation of pulse-train stimulation. AR speed referred to the speed of recovery from NA caused by previous pulse-train stimulation. The AM ratio provided a measure of AN sensitivity to AM cues. Participants' speech perception scores were measured using Consonant-Nucleus-Consonant (CNC) word lists and AzBio sentences presented in quiet, as well as in noise at signal-to-noise ratios (SNRs) of +10 and +5 dB. Predictive models were created for each speech measure to identify eCAP metrics with meaningful predictive power.

RESULTS

The ENI index and AR speed individually explained at least 10% of the variance in most of the speech perception scores measured in this study, while the NA ratio, NA speed, the AR ratio, and the AM ratio did not. The ENI index was identified as the only eCAP metric that had unique predictive power for each of the speech test results. The amount of variance in speech perception scores (both CNC words and AzBio sentences) explained by the eCAP metrics increased with increased difficulty under the listening condition. Over half of the variance in speech perception scores measured in +5 dB SNR noise (both CNC words and AzBio sentences) was explained by a model with only three eCAP metrics: the ENI index, NA speed, and AR speed.

CONCLUSIONS

Of the six electrophysiological measures assessed in this study, the ENI index is the most informative predictor for speech perception performance in CI users. In agreement with the tested hypothesis, the response characteristics of the AN to electrical stimulation are more important for speech perception with a CI in noise than they are in quiet.

A Computational Model of a Single Auditory Nerve Fiber for Electric-Acoustic Stimulation

Cochlear implant (CI) recipients with preserved acoustic low-frequency hearing in the implanted ear are a growing group among traditional CI users who benefit from hybrid electric-acoustic stimulation (EAS). However, combined ipsilateral electric and acoustic stimulation also introduces interactions between the two modalities that can affect the performance of EAS users. A computational model of a single auditory nerve fiber that is excited by EAS was developed to study the interaction between electric and acoustic stimulation. Two existing models of sole electric or acoustic stimulation were coupled to simulate responses to combined EAS. Different methods of combining both models were implemented. In the coupled model variant, the refractoriness of the simulated fiber leads to suppressive interaction between electrically evoked and acoustically evoked spikes as well as spontaneous activity. The second model variant is an uncoupled EAS model without electric-acoustic interaction. By comparing predictions between the coupled and the noninteracting EAS model, it was possible to infer electric-acoustic interaction at the level of the auditory nerve. The EAS model was used to simulate fiber populations with realistic inter-unit variability, where each unit was represented by the single-fiber model. Predicted thresholds and dynamic ranges, spike rates, latencies, jitter, and vector strengths were compared to empirical data. The presented EAS model provides a framework for future studies of peripheral electric-acoustic interaction.

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.

Postlingually Deafened Adult Cochlear Implant Users With Prolonged Recovery From Neural Adaptation at the Level of the Auditory Nerve Tend to Have Poorer Speech Perception Performance

OBJECTIVE

This study investigated the effects of two temporal response properties of the auditory nerve (i.e., neural adaptation and recovery from neural adaptation) on speech perception performance in postlingually deafened adult cochlear implant (CI) users.

DESIGN

Study participants included 18 postlingually deafened adults who were Cochlear Nucleus device users with a full electrode array insertion in the test ear(s). Neural adaptation and adaptation recovery of the auditory nerve (AN) were evaluated using electrophysiological measures of the electrically evoked compound action potential (eCAP). The amount of neural adaptation was quantified by the adaptation index within three time windows: 0 to 8.89 (window 1), 44.44 to 50.00 (window 2), and 94.44 to 100.00 ms (window 3). The speed of neural adaptation was estimated using a two-parameter power law function. To evaluate adaptation recovery of the AN, eCAPs to the last pulse of the 100-ms pulse train were recorded at masker-probe-intervals ranging from 1.054 to 256 ms in logarithmic steps. The amount of adaptation recovery was quantified by the adaptation recovery ratio. The time-constant of adaptation recovery was estimated using an exponential function with up to three components. Speech perception performance was evaluated by measuring consonant-nucleus-consonant (CNC) word scores presented in quiet and in speech-shaped noise at a signal-to-noise ratio (SNR) of +10 dB. One-tailed Pearson Product Moment correlation tests were used (1) to assess the associations among parameters of neural adaptation and adaptation recovery and (2) to evaluate the strength of association between these parameters and CNC word scores measured in quiet and in noise. The contributions of different parameters quantifying neural adaptation and adaptation recovery on speech perception scores were evaluated using multivariable linear regression analyses.

RESULTS

The Pearson Product Moment correlation coefficient demonstrated a moderate, negative correlation between the speed of adaptation recovery and CNC word scores measured in quiet and in noise. The speed of adaptation recovery accounted for 14.1% of variability in CNC word scores measured in quiet and 16.7% of variability in CNC word scores measured in noise. The correlation strengths between CNC word scores and the adaptation index, the adaptation recovery ratio and the speed of neural adaptation ranged from negligible to weak.

CONCLUSIONS

The speed of adaptation recovery plays a more important role than other features of neural adaptation and adaptation recovery of the AN in speech perception in postlingually deafened adult CI users. Patients with prolonged adaptation recovery tend to show poorer speech perception performance.

Characteristics of the Adaptation Recovery Function of the Auditory Nerve and Its Association With Advanced Age in Postlingually Deafened Adult Cochlear Implant Users

OBJECTIVE

This study aimed to (1) characterize the amount and the speed of recovery from neural adaptation at the auditory nerve (AN) and (2) assess their associations with advanced age in postlingually deafened adult cochlear implant users.

DESIGN

Study participants included 25 postlingually deafened adult, Cochlear Nucleus device users, ranging in age between 24.83 and 83.21 years at the time of testing. The stimulus was a 100-ms pulse train presented at four pulse rates: 500, 900, 1800, and 2400 pulses per second (pps). The pulse trains were presented at the maximum comfortable level measured for the 2400-pps pulse train. The electrically evoked compound action potential (eCAP) evoked by the last pulse of the pulse train (i.e., the probe pulse) was recorded. The remaining pulses of the pulse train served as the pulse-train masker. The time interval between the probe pulse and the last pulse of the pulse-train masker [i.e., masker-probe-interval (MPI)] systematically increased from 0.359 ms up to 256 ms. The adaptation recovery function (ARF) was obtained by plotting normalized eCAP amplitudes (re: the eCAP amplitude measured at the MPI of 256 ms) as a function of MPIs. The adaptation recovery ratio (ARR) was defined as the ratio between the eCAP amplitude measured at the MPI of 256 ms and that measured for the single-pulse stimulus presented at the same stimulation level. The time constants of the ARF were estimated using a mathematical model with an exponential function with up to three components. Generalized Linear Mixed effects Models were used to compare ARRs and time constants measured at different electrode locations and pulse rates, as well as to assess the effect of advanced age on these dependent variables.

RESULTS

There were three ARF types observed in this study. The ARF type observed in the same study participant could be different at different electrode locations and/or pulse rates. Substantial variations in both the amount and the speed of neural adaptation recovery among study participants were observed. The ARR was significantly affected by pulse rate but was not affected by electrode location. The effect of electrode location on the time constants of the ARF was not statistically significant. Pulse rate had a statistically significant effect on τ 1, but not on τ 2 or τ 3 . There was no statistically significant effect of age on the ARR or the time constants of the ARF.

CONCLUSIONS

Neural adaptation recovery processes at the AN demonstrate substantial variations among human cochlear implant users. The recovery pattern can be nonmonotonic with up to three phases. While the amount of neural adaptation recovery decreases as pulse rate increases, only the speed of the first phase of neural adaptation recovery is affected by pulse rate. Electrode location or advanced age has no robust effect on neural adaptation recovery processes at the level of the AN for a 100-ms pulse-train masker with pulse rates of 500 to 2400 pps. The lack of sufficient participants in this study who were 40 years of age or younger at the time of testing might have precluded a thorough assessment of the effect of advanced age.

The role of electroneural versus electrophonic stimulation on psychoacoustic electric-acoustic masking in cochlear implant users with residual hearing

Cochlear implant (CI) candidates with residual low-frequency hearing are nowadays often implanted with CI electrode arrays that allow preserving their acoustic hearing in the implanted ear. These subjects receiving combined electric-acoustic stimulation (EAS) show enhanced speech perception scores when compared to traditional CI users without acoustic component. However, these benefits are limited by interaction effects such as masking between electric and acoustic stimulation. This study evaluates ipsilateral electric-acoustic masking in a psychophysical experiment conducted in 5 EAS subjects. The elevation of acoustic pure tone thresholds through simultaneous presentation of electric pulse trains and vice versa is measured for different acoustic frequencies and different settings of the electric stimuli. Electric-acoustic interaction could originate either from electroneural stimulation of auditory nerve fibers or from electrophonic stimulation of hair cells. The two fundamental goals of this study are to investigate the effects of stimulation rate and phase duration of the electric stimulus on electric-acoustic masking and to investigate the origin of electric-acoustic masking by assessing the contributions of electroneural versus electrophonic stimulation. The amount of electric-acoustic masking in the present study was independent of pulse rate and phase duration of the electric stimuli. Moreover, the results demonstrate that electric-acoustic masking depends on the spatial distance between the locations of electric or acoustic excitation in the cochlea, but not on the spectral content of the electric stimulus. We thereby conclude that psychoacoustic electric-acoustic masking in EAS users is dominated by electroneural-acoustic interaction, whereas the contribution of electrophonic stimulation is negligible.

Amplitude growth of intracochlear electrocochleography in cochlear implant users with residual hearing

In cochlear implant (CI) users with residual hearing, the electrode-nerve interface can be investigated combining electric-acoustic stimulation (EAS) via electrocochleography (ECochG), a technique to record cochlear potentials evoked by acoustic stimulation. EAS interaction was shown in previous studies using psychoacoustic experiments. This work characterizes EAS interaction through psychophysical experiments and the amplitude growth of cochlear microphonics (CM) and auditory nerve neurophonics (ANN) derived from intracochlear ECochG recordings. Significant CM responses were recorded at psychoacoustic threshold levels. The mean difference between psychoacoustic and CM threshold was 17.5 dB. No significant ANN responses were recorded at the psychoacoustic threshold level. At the psychoacoustic most comfortable level, significant CM and ANN responses were recorded. In the presence of electrical stimulation, the psychoacoustic detection thresholds were elevated on average by 2.38 dB while the recorded CM amplitudes were attenuated on average by 1.15 dB. No significant differences in electrophysiological EAS interaction across acoustic stimulation levels were observed from CM recordings. The presence of psychophysical and electrophysiological EAS interaction demonstrates that some aspects of psychoacoustic EAS interaction can be measured via intracochlear ECochG.

Electric-acoustic interaction measurements in cochlear-implant users with ipsilateral residual hearing using electrocochleography

Cochlear implantation is increasingly being used as a hearing-loss treatment for patients with residual hearing in the low acoustic frequencies. These patients obtain combined electric-acoustic stimulation (EAS). Substantial residual hearing and relatively long electrode arrays can lead to interactions between the electric and acoustic stimulation. This work investigated EAS interaction through psychophysical and electrophysiological measures. Moreover, cone-beam computed-tomography data was used to characterize the interaction along spatial cochlear locations. Psychophysical EAS interaction was estimated based on the threshold of audibility of an acoustic probe stimulus in the presence of a simultaneously presented electric masker stimulus. Intracochlear electrocochleography was used to estimate electrophysiological EAS interaction via the telemetry capability of the cochlear implant. EAS interaction was observed using psychophysical and electrophysiological measurements. While psychoacoustic EAS interaction was most pronounced close to the electrical stimulation site, electrophysiological EAS interaction was observed over a wider range of spatial cochlear locations. Psychophysical EAS interaction was significantly larger than electrophysiological EAS interaction for acoustic probes close to the electrode position.

The effect of cross-over frequency on binaural hearing performance of adults using electric-acoustic stimulation

Objective: To investigate the effect of varying cross-over frequency (CF) settings for electric-acoustic (EA) stimulation in one ear combined with acoustic (A) hearing in the opposite ear on binaural speech perception, localization and functional performance in real life. Methods: Performance with three different CF settings set according to audiometric-based criterion were compared, following a four week familiarisation period with each, in ten adult cochlear implant recipients with residual hearing in both ears. On completion of all trials participants selected their preferred CF setting. Results: On average, CF settings did not have a significant effect on performance scores. However, higher ratings on device usage were associated with the preferred CF settings. Conclusion: Individuals who use EA + A stimulation may benefit from access to different CF settings to achieve maximal device usage.

Physiological Mechanisms in Combined Electric-Acoustic Stimulation

OBJECTIVE

Electrical stimulation is normally performed on ears that have no hearing function, i.e., lack functional hair cells. The properties of electrically-evoked responses in these cochleae were investigated in several previous studies. Recent clinical developments have introduced cochlear implantation (CI) in residually-hearing ears to improve speech understanding in noise. The present study documents the known physiological differences between electrical stimulation of hair cells and of spiral ganglion cells, respectively, and reviews the mechanisms of combined electric and acoustic stimulation in the hearing ears.

DATA SOURCES

Literature review from 1971 to 2016.

CONCLUSIONS

Compared with pure electrical stimulation the combined electroacoustic stimulation provides additional low-frequency information and expands the dynamic range of the input. Physiological studies document a weaker synchronization of the evoked activity in electrically stimulated hearing ears compared with deaf ears that reduces the hypersynchronization of electrically-evoked activity. The findings suggest the possibility of balancing the information provided by acoustic and electric input using stimulus intensity. Absence of distorting acoustic-electric interactions allows exploiting these clinical benefits of electroacoustic stimulation.

Electric-acoustic forward masking in cochlear implant users with ipsilateral residual hearing

In order to investigate the temporal mechanisms of the auditory system, psychophysical forward masking experiments were conducted in cochlear implant users who had preserved acoustic hearing in the ipsilateral ear. This unique electric-acoustic stimulation (EAS) population allowed the measurement of threshold recovery functions for acoustic or electric probes in the presence of electric or acoustic maskers, respectively. In the electric masking experiment, the forward masked threshold elevation of acoustic probes was measured as a function of the time interval after the offset of the electric masker, i.e. the masker-to-probe interval (MPI). In the acoustic masking experiment, the forward masked threshold elevation of electric probe stimuli was investigated under the influence of a preceding acoustic masker. Since electric pulse trains directly stimulate the auditory nerve, this novel experimental setup allowed the acoustic adaptation properties (attributed to the physiology of the hair cells) to be differentiated from the subsequent processing by more central mechanisms along the auditory pathway. For instance, forward electric masking patterns should result more from the auditory-nerve response to electrical stimulation, while forward acoustic masking patterns should primarily be the result of the recovery from adaptation at the hair-cell neuron interface. Electric masking showed prolonged threshold elevation of acoustic probes, which depended significantly on the masker-to-probe interval. Additionally, threshold elevation was significantly dependent on the similarity between acoustic stimulus frequency and electric place frequency, the electric-acoustic frequency difference (EAFD). Acoustic masking showed a reduced, but statistically significant effect of electric threshold elevation, which did not significantly depend on MPI. Lastly, acoustic masking showed longer decay times than electric masking and a reduced dependency on EAFD. In conclusion, the forward masking patterns observed for combined electric-acoustic stimulation provide further insights into the temporal mechanisms of the auditory system. For instance, the asymmetry in the amount of threshold elevation, the dependency on EAFD and the time constants for the recovery functions of acoustic and electric masking all indicate that there must be several processes with different latencies (e.g. neural adaptation, depression of spontaneous activity, efferent systems) that are involved in forward masking recovery functions.

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.

Loss of Afferent Vestibular Input Produces Central Adaptation and Increased Gain of Vestibular Prosthetic Stimulation

Implanted vestibular neurostimulators are effective in driving slow phase eye movements in monkeys and humans. Furthermore, increases in slow phase velocity and electrically evoked compound action potential (vECAP) amplitudes occur with increasing current amplitude of electrical stimulation. In intact monkeys, protracted intermittent stimulation continues to produce robust behavioral responses and preserved vECAPs. In lesioned monkeys, shorter duration studies show preserved but with somewhat lower or higher velocity behavioral responses. It has been proposed that such changes are due to central adaptive changes in the electrically elicited vestibulo-ocular reflex (VOR). It is equally possible that these differences are due to changes in the vestibular periphery in response to activation of the vestibular efferent system. In order to investigate the site of adaptive change in response to electrical stimulation, we performed transtympanic gentamicin perfusions to induce rapid changes in vestibular input in monkeys with long-standing stably functioning vestibular neurostimulators, disambiguating the effects of implantation from the effects of ototoxic lesion. Gentamicin injection was effective in producing a large reduction in natural VOR only when it was performed in the non-implanted ear, suggesting that the implanted ear contributed little to the natural rotational response before injection. Injection of the implanted ear produced a reduction in the vECAP responses in that ear, suggesting that the intact hair cells in the non-functional ipsilateral ear were successfully lesioned by gentamicin, reducing the efficacy of stimulation in that ear. Despite this, injection of both ears produced central plastic changes that resulted in a dramatically increased slow phase velocity nystagmus elicited by electrical stimulation. These results suggest that loss of vestibular afferent activity, and a concurrent loss of electrically elicited vestibular input, produces an increase in the efficacy of a vestibular neurostimulator by eliciting centrally adapted behavioral responses without concurrent adaptive increase of galvanic afferent activation in the periphery.

Electroacoustic stimulation: now and into the future

Cochlear implants have provided hearing to hundreds of thousands of profoundly deaf people around the world. Recently, the eligibility criteria for cochlear implantation have been relaxed to include individuals who have some useful residual hearing. These recipients receive inputs from both electric and acoustic stimulation (EAS). Implant recipients who can combine these hearing modalities demonstrate pronounced benefit in speech perception, listening in background noise, and music appreciation over implant recipients that rely on electrical stimulation alone. The mechanisms bestowing this benefit are unknown, but it is likely that interaction of the electric and acoustic signals in the auditory pathway plays a role. Protection of residual hearing both during and following cochlear implantation is critical for EAS. A number of surgical refinements have been implemented to protect residual hearing, and the development of hearing-protective drug and gene therapies is promising for EAS recipients. This review outlines the current field of EAS, with a focus on interactions that are observed between these modalities in animal models. It also outlines current trends in EAS surgery and gives an overview of the drug and gene therapies that are clinically translatable and may one day provide protection of residual hearing for cochlear implant recipients.

Suppression of the acoustically evoked auditory-nerve response by electrical stimulation in the cochlea of the guinea pig

There is increasing interest in the use of electro-acoustical stimulation in people with a cochlear implant that have residual low-frequency hearing in the implanted ear. This raises the issue of how electrical and acoustical stimulation interact in the cochlea. We have investigated the effect of electrical stimulation on the acoustically evoked compound action potential (CAP) in normal-hearing guinea pigs. CAPs were evoked by tone bursts, and electric stimuli were delivered at the base of the cochlea using extracochlear electrodes. CAPs could be suppressed by electrical stimulation under various conditions. The dependence of CAP suppression on several parameters was investigated, including frequency and level of the acoustic stimulus, current level of the electric stimulus and the interval between electric and acoustic stimulus (EAI). Most pronounced suppression was observed when CAPs were evoked with high-frequency tones of low level. Suppression increased with current level and at high currents low-frequency evoked CAPs could also be suppressed. Suppression was typically absent several milliseconds after the electric stimulus. Suppression mediated by direct neural responses and hair cell mediated (electrophonic) responses is discussed. We conclude that the high-frequency part of the cochlea can be stimulated electrically with little detrimental effects on CAPs evoked by low-frequency tones.

Auditory nerve fiber responses to combined acoustic and electric stimulation

Persons with a prosthesis implanted in a cochlea with residual acoustic sensitivity can, in some cases, achieve better speech perception with "hybrid" stimulation than with either acoustic or electric stimulation presented alone. Such improvements may involve "across auditory-nerve fiber" processes within central nuclei of the auditory system and within-fiber interactions at the level of the auditory nerve. Our study explored acoustic-electric interactions within feline auditory nerve fibers (ANFs) so as to address two goals. First, we sought to better understand recent results that showed non-monotonic recovery of the electrically evoked compound action potential (ECAP) following acoustic masking (Nourski et al. 2007, Hear. Res. 232:87-103). We hypothesized that post-masking changes in ANF temporal properties and responsiveness (spike rate) accounted for the ECAP results. We also sought to describe, more broadly, the changes in ANF responses that result from prior acoustic stimulation. Five response properties-spike rate, latency, jitter, spike amplitude, and spontaneous activity-were examined. Post-masking reductions in spike rate, within-fiber jitter and across-fiber variance in latency were found, with the changes in temporal response properties limited to ANFs with high spontaneous rates. Thus, our results suggest how non-monotonic ECAP recovery occurs for ears with spontaneous activity, but cannot account for that pattern of recovery when there is no spontaneous activity, including the results from the presumably deafened ears used in the Nourski et al. (2007) study. Finally, during simultaneous (electric+acoustic) stimulation, the degree of electrically driven spike activity had a strong influence on spike rate, but did not affect spike jitter, which apparently was determined by the acoustic noise stimulus or spontaneous activity.