Effects of hearing preservation on psychophysical responses to cochlear implant stimulation

Cochlear Health and Cochlear-implant Function

The cochlear implant (CI) is widely considered to be one of the most innovative and successful neuroprosthetic treatments developed to date. Although outcomes vary, CIs are able to effectively improve hearing in nearly all recipients and can substantially improve speech understanding and quality of life for patients with significant hearing loss. A wealth of research has focused on underlying factors that contribute to success with a CI, and recent evidence suggests that the overall health of the cochlea could potentially play a larger role than previously recognized. This article defines and reviews attributes of cochlear health and describes procedures to evaluate cochlear health in humans and animal models in order to examine the effects of cochlear health on performance with a CI. Lastly, we describe how future biologic approaches can be used to preserve and/or enhance cochlear health in order to maximize performance for individual CI recipients.

Parameter-Specific Morphing Reveals Contributions of Timbre to the Perception of Vocal Emotions in Cochlear Implant Users

Objectives:

Research on cochlear implants (CIs) has focused on speech comprehension, with little research on perception of vocal emotions. We compared emotion perception in CI users and normal-hearing (NH) individuals, using parameter-specific voice morphing.

Design:

Twenty-five CI users and 25 NH individuals (matched for age and gender) performed fearful-angry discriminations on bisyllabic pseudoword stimuli from morph continua across all acoustic parameters (Full), or across selected parameters (F0, Timbre, or Time information), with other parameters set to a noninformative intermediate level.

Results:

Unsurprisingly, CI users as a group showed lower performance in vocal emotion perception overall. Importantly, while NH individuals used timbre and fundamental frequency (F0) information to equivalent degrees, CI users were far more efficient in using timbre (compared to F0) information for this task. Thus, under the conditions of this task, CIs were inefficient in conveying emotion based on F0 alone. There was enormous variability between CI users, with low performers responding close to guessing level. Echoing previous research, we found that better vocal emotion perception was associated with better quality of life ratings.

Conclusions:

Some CI users can utilize timbre cues remarkably well when perceiving vocal emotions.

Estimating health of the implanted cochlea using psychophysical strength-duration functions and electrode configuration

It is generally believed that the efficacy of cochlear implants is partly dependent on the condition of the stimulated neural population. Cochlear pathology is likely to affect the manner in which neurons respond to electrical stimulation, potentially resulting in differences in perception of electrical stimuli across cochlear implant recipients and across the electrode array in individual cochlear implant users. Several psychophysical and electrophysiological measures have been shown to predict cochlear health in animals and were used to assess conditions near individual stimulation sites in humans. In this study, we examined the relationship between psychophysical strength-duration functions and spiral ganglion neuron density in two groups of guinea pigs with cochlear implants who had minimally-overlapping cochlear health profiles. One group was implanted in a hearing ear (N = 10) and the other group was deafened by cochlear perfusion of neomycin, inoculated with an adeno-associated viral vector with an Ntf3-gene insert (AAV.Ntf3) and implanted (N = 14). Psychophysically measured strength-duration functions for both monopolar and tripolar electrode configurations were then compared for the two treatment groups. Results were also compared to their histological outcomes. Overall, there were considerable differences between the two treatment groups in terms of their psychophysical performance as well as the relation between their functional performance and histological data. Animals in the neomycin-deafened, neurotrophin-treated, and implanted group (NNI) exhibited steeper strength-duration function slopes; slopes were positively correlated with SGN density (steeper slopes in animals that had higher SGN densities). In comparison, the implanted hearing (IH) group had shallower slopes and there was no relation between slopes and spiral ganglion density. Across all animals, slopes were negatively correlated with ensemble spontaneous activity levels (shallower slopes with higher ensemble spontaneous activity levels). We hypothesize that differences in strength-duration function slopes between the two treatment groups were related to the condition of the inner hair cells, which generate spontaneous activity that could affect the across-fiber synchrony and/or the size of the population of neural elements responding to electrical stimulation. In addition, it is likely that spiral ganglion neuron peripheral processes were present in the IH group, which could affect membrane properties of the stimulated neurons. Results suggest that the two treatment groups exhibited distinct patterns of variation in conditions near the stimulating electrodes that altered detection thresholds. Overall, the results of this study suggest a complex relationship between psychophysical detection thresholds for cochlear implant stimulation and nerve survival in the implanted cochlea. This relationship seems to depend on the characteristics of the electrical stimulus, the electrode configuration, and other biological features of the implanted cochlea such as the condition of the inner hair cells and the peripheral processes.

eABR THR Estimation Using High-Rate Multi-Pulse Stimulation in Cochlear Implant Users

We estimated the electrically-evoked auditory brainstem response thresholds (eABR THRs) in response to multi-pulses with high burst rate of 10,000 pulses-per-second (pps). Growth functions of wave eV amplitudes, root mean square (RMS) values, peak of phase-locking value (PLV), and the lowest valid data point (LVDP) were calculated in 1-, 2-, 4-, 8-, and 16-pulses conditions. The growth functions were then fitted and extrapolated with linear and exponential functions to find eABR THRs. The estimated THRs were compared to psychophysical THRs determined for multi-pulse conditions as well as to the clinical THRs measured behaviorally at the rate of 1,000 pps. The growth functions of features showed shallower growth slopes when the number of pulses increased. eABR THRs estimated in 4-, 8-, and 16-pulses conditions were closer to the clinical THRs, when compared to 1- and 2-pulses conditions. However, the smallest difference between estimated eABR THRs and clinical THRs was not always achieved from the same number of pulses. The smallest absolute difference of 30.3 μA was found for the linear fittings on growth functions of eABR RMS values in 4-pulses condition. Pearson's correlation coefficients (PCCs) between eABR THRs and psychophysical THRs were significant and relatively large in all but 16-pulses conditions. The PCCs between eABR THRs and clinical THRs, however, were smaller and in less cases significant. Results of this study showed that eABRs to multi-pulse stimulation could, to some extent, represent clinical stimulation paradigms, and thus in comparison to single pulses, could estimate clinical THRs with smaller errors.

The Neural Processing of Vocal Emotion After Hearing Reconstruction in Prelingual Deaf Children: A Functional Near-Infrared Spectroscopy Brain Imaging Study

As elucidated by prior research, children with hearing loss have impaired vocal emotion recognition compared with their normal-hearing peers. Cochlear implants (CIs) have achieved significant success in facilitating hearing and speech abilities for people with severe-to-profound sensorineural hearing loss. However, due to the current limitations in neuroimaging tools, existing research has been unable to detail the neural processing for perception and the recognition of vocal emotions during early stage CI use in infant and toddler CI users (ITCI). In the present study, functional near-infrared spectroscopy (fNIRS) imaging was employed during preoperative and postoperative tests to describe the early neural processing of perception in prelingual deaf ITCIs and their recognition of four vocal emotions (fear, anger, happiness, and neutral). The results revealed that the cortical response elicited by vocal emotional stimulation on the left pre-motor and supplementary motor area (pre-SMA), right middle temporal gyrus (MTG), and right superior temporal gyrus (STG) were significantly different between preoperative and postoperative tests. These findings indicate differences between the preoperative and postoperative neural processing associated with vocal emotional stimulation. Further results revealed that the recognition of vocal emotional stimuli appeared in the right supramarginal gyrus (SMG) after CI implantation, and the response elicited by fear was significantly greater than the response elicited by anger, indicating a negative bias. These findings indicate that the development of emotional bias and the development of emotional perception and recognition capabilities in ITCIs occur on a different timeline and involve different neural processing from those in normal-hearing peers. To assess the speech perception and production abilities, the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) and Speech Intelligibility Rating (SIR) were used. The results revealed no significant differences between preoperative and postoperative tests. Finally, the correlates of the neurobehavioral results were investigated, and the results demonstrated that the preoperative response of the right SMG to anger stimuli was significantly and positively correlated with the evaluation of postoperative behavioral outcomes. And the postoperative response of the right SMG to anger stimuli was significantly and negatively correlated with the evaluation of postoperative behavioral outcomes.

Recovery from forward masking in cochlear implant listeners: Effects of age and the electrode-neuron interface

Older adults exhibit deficits in auditory temporal processing relative to younger listeners. These age-related temporal processing difficulties may be further exacerbated in older adults with cochlear implant (CIs) when CI electrodes poorly interface with their target auditory neurons. The aim of this study was to evaluate the potential interaction between chronological age and the estimated quality of the electrode-neuron interface (ENI) on psychophysical forward masking recovery, a measure that reflects single-channel temporal processing abilities. Fourteen CI listeners (age 15 to 88 years) with Advanced Bionics devices participated. Forward masking recovery was assessed on two channels in each ear (i.e., the channels with the lowest and highest signal detection thresholds). Results indicated that the rate of forward masking recovery declined with advancing age, and that the effect of age was more pronounced on channels estimated to interface poorly with the auditory nerve. These findings indicate that the quality of the ENI can influence the time course of forward masking recovery for older CI listeners. Channel-to-channel variability in the ENI likely interacts with central temporal processing deficits secondary to auditory aging, warranting further study of programming and rehabilitative approaches tailored to older listeners.

Development of a chronically-implanted mouse model for studies of cochlear health and implant function

Mice with chronic cochlear implants can significantly contribute to our understanding of the relationship between cochlear health and implant function because of the availability of molecular tools for controlling conditions in the cochlea and transgenic lines modeling human disease. To date, research in implanted mice has mainly consisted of short-term studies, but since there are large changes in implant function following implant insertion trauma, and subsequent recovery in many cases, longer-term studies are needed to evaluate function and perception under stable conditions. Because frequent anesthetic administration can be especially problematic in mice, a chronic model that can be tested in the awake condition is desirable. Electrically-evoked compound action potentials (ECAPs) recorded with multichannel cochlear implants are useful functional measures because they can be obtained daily without anesthesia. In this study, we assessed changes and stability of ECAPs, electrically-evoked auditory brainstem responses (EABRs), ensemble spontaneous activity (ESA), and impedance data over time after implanting mice with multichannel implants. We then compared these data to histological findings in these implanted cochleae, and compared results from this chronic mouse model to data previously obtained in a well-established chronically-implanted guinea pig model. We determined that mice can be chronically implanted with cochlear implants, and ECAP recordings can be obtained frequently in an awake state for up to at least 42 days after implantation. These recordings can effectively monitor changes or stability in cochlear function over time. ECAP and EABR amplitude-growth functions (AGFs), AGF slopes, ESA levels and impedances in mice with multichannel implants appear similar to those found in guinea pigs with long-term multichannel implants. Animals with better survival of spiral ganglion neurons (SGNs) and inner hair cells (IHCs) have steeper AGF slopes, and larger ESA responses. The time course of post-surgical ear recovery may be quicker in mice and can show different patterns of recovery which seem to be dependent on the degree of insertion trauma and subsequent histological conditions. Histology showed varying degrees of cochlear damage with fibrosis present in all implanted mouse ears and small amounts of new bone in a few ears. Impedance changes over time varied within and across animals and may represent changes over time in multiple variables in the cochlear environment post-implantation. Due to the small size of the mouse, susceptibility to stress, and the higher potential for implant failure, chronic implantation in mice can be challenging, but overall is feasible and useful for cochlear implant research.

How electrically evoked compound action potentials in chronically implanted guinea pigs relate to auditory nerve health and electrode impedance

This study examined how multiple measures based on the electrically evoked compound action potential (ECAP) amplitude-growth functions (AGFs) were related to estimates of neural [spiral ganglion neuron (SGN) density and cell size] and electrode impedance measures in 34 specific pathogen free pigmented guinea pigs that were chronically implanted (4.9-15.4 months) with a cochlear implant electrode array. Two interphase gaps (IPGs) were used for the biphasic pulses and the effect of the IPG on each ECAP measure was measured ("IPG effect"). When using a stimulus with a constant IPG, SGN density was related to the across-subject variance in ECAP AGF linear slope, peak amplitude, and N1 latency. The SGN density values also help to explain a significant proportion of variance in the IPG effect for AGF linear slope and peak amplitude measures. Regression modeling revealed that SGN density was the primary dependent variable contributing to across-subject variance for ECAP measures; SGN cell size did not significantly improve the fitting of the model. Results showed that simple impedance measures were weakly related to most ECAP measures but did not typically improve the fit of the regression model.

Evaluating and Comparing Behavioural and Electrophysiological Estimates of Neural Health in Cochlear Implant Users

Variations in neural health along the cochlea can degrade the spectral and temporal representation of sounds conveyed by cochlear implants (CIs). We evaluated and compared one electrophysiological measure and two behavioural measures that have been proposed as estimates of neural health patterns, in order to explore the extent to which the different measures provide converging and consistent neural health estimates. All measures were obtained from the same 11 users of the Cochlear Corporation CI. The two behavioural measures were multipulse integration (MPI) and the polarity effect (PE), both measured on each of seven electrodes per subject. MPI was measured as the difference between thresholds at 80 pps and 1000 pps, and PE as the difference in thresholds between cathodic- and anodic-centred quadraphasic (QP) 80-pps pulse trains. It has been proposed that good neural health corresponds to a large MPI and to a large negative PE (lower thresholds for cathodic than anodic pulses). The electrophysiological measure was the effect of interphase gap (IPG) on the offset of the ECAP amplitude growth function (AGF), which has been correlated with spiral ganglion neuron density in guinea pigs. This 'IPG offset' was obtained on the same subset of electrodes used for the behavioural measures. Despite high test-retest reliability, there were no significant correlations between the neural health estimates for either within-subject comparisons across the electrode array, or between-subject comparisons of the means. A phenomenological model of a population of spiral ganglion neurons was then used to investigate physiological mechanisms that might underlie the different neural health estimates. The combined experimental and modelling results provide evidence that PE, MPI and IPG offset may reflect different characteristics of the electrode-neural interface.

The Influence of Cochlear Implant-Based Electric Stimulation on the Electrophysiological Characteristics of Cultured Spiral Ganglion Neurons

Background

Cochlear implant-based electrical stimulation may be an important reason to induce the residual hearing loss after cochlear implantation. In our previous study, we found that charge-balanced biphasic electrical stimulation inhibited the neurite growth of spiral ganglion neurons (SGNs) and decreased Schwann cell density in vitro. In this study, we want to know whether cochlear implant-based electrical stimulation can induce the change of electrical activity in cultured SGNs.

Methods

Spiral ganglion neuron electrical stimulation in vitro model is established using the devices delivering cochlear implant-based electrical stimulation. After 48 h treatment by 50 μA or 100 μA electrical stimulation, the action potential (AP) and voltage depended calcium current (I Ca) of SGNs are recorded using whole-cell electrophysiological method.

Results

The results show that the I Ca of SGNs is decreased significantly in 50 μA and 100 μA electrical stimulation groups. The reversal potential of I Ca is nearly +80 mV in control SGN, but the reversal potential decreases to +50 mV in 50 μA and 100 μA electrical stimulation groups. Interestingly, the AP amplitude, the AP latency, and the AP duration of SGNs have no statistically significant differences in all three groups.

Conclusion

Our study suggests cochlear implant-based electrical stimulation only significantly inhibit the I Ca of cultured SGNs but has no effect on the firing of AP, and the relation of I Ca inhibition and SGN damage induced by electrical stimulation and its mechanism needs to be further studied.

Cochlear implants and other inner ear prostheses: today and tomorrow

Cochlear implants (CIs) are implantable auditory prostheses designed to restore access to sound in deaf individuals via direct electrical stimulation of the auditory nerve. While CIs have been successful in restoring speech perception to many deaf patients, outcomes are variable and speech recognition in noise remains a problem. This chapter will review the factors underlying this variability, and discuss significant recent innovations to address these issues including neural health preservation, characterization, and regeneration, and other inner ear prostheses. The emerging role of central auditory plasticity will also be discussed. Together, these advances will point to the likely future directions for advancing the next generation of CIs and other inner ear prostheses.

Relationships between Intrascalar Tissue, Neuron Survival, and Cochlear Implant Function

Fibrous tissue and/or new bone are often found surrounding a cochlear implant in the cochlear scalae. This new intrascalar tissue could potentially limit cochlear implant function by increasing impedance and altering signaling pathways between the implant and the auditory nerve. In this study, we investigated the relationship between intrascalar tissue and 5 measures of implant function in guinea pigs. Variation in both spiral ganglion neuron (SGN) survival and intrascalar tissue was produced by implanting hearing ears, ears deafened with neomycin, and neomycin-deafened ears treated with a neurotrophin. We found significant effects of SGN density on 4 functional measures but adding intrascalar tissue level to the analysis did not explain more variation in any measure than was explained by SGN density alone. These results suggest that effects of intrascalar tissue on electrical hearing are relatively unimportant in comparison to degeneration of the auditory nerve, although additional studies in human implant recipients are still needed to assess the effects of this tissue on complex hearing tasks like speech perception. The results also suggest that efforts to minimize the trauma that aggravates both tissue development and SGN loss could be beneficial.

Investigation of Electrically Evoked Auditory Brainstem Responses to Multi-Pulse Stimulation of High Frequency in Cochlear Implant Users

We investigated the effects of electric multi-pulse stimulation on electrically evoked auditory brainstem responses (eABRs). Multi-pulses with a high burst rate of 10,000 pps were assembled from pulses of 45-μs phase duration. Conditions of 1, 2, 4, 8, and 16 pulses were investigated. Psychophysical thresholds (THRs) and most comfortable levels (MCLs) in multi-pulse conditions were measured. Psychophysical temporal integration functions (slopes of THRs/MCLs as a function of number of pulses) were -1.30 and -0.93 dB/doubling of the number of pulses, which correspond to the doubling of pulse duration. A total of 15 eABR conditions with different numbers of pulses and amplitudes were measured. The morphology of eABRs to multi-pulse stimuli did not differ from those to conventional single pulses. eABR wave eV amplitudes and latencies were analyzed extensively. At a fixed stimulation amplitude, an increasing number of pulses caused increasing wave eV amplitudes up to a certain, subject-dependent number of pulses. Then, amplitudes either saturated or even decreased. This contradicted the conventional amplitude growth functions and also contradicted psychophysical results. We showed that destructive interference could be a possible reason for such a finding, where peaks and troughs of responses to the first pulses were suppressed by those of successive pulses in the train. This study provides data on psychophysical THRs and MCLs and corresponding eABR responses for stimulation with single-pulse and multi-pulse stimuli with increasing duration. Therefore, it provides insights how pulse trains integrate at the level of the brainstem.

Anatomical Correlates and Surgical Considerations for Localized Therapeutic Hypothermia Application in Cochlear Implantation Surgery

Application of localized, mild therapeutic hypothermia during cochlear implantation (CI) surgery is feasible for residual hearing preservation.

Effects of Electrode Location on Estimates of Neural Health in Humans with Cochlear Implants

There are a number of psychophysical and electrophysiological measures that are correlated with SGN density in animal models, and these same measures can be performed in humans with cochlear implants (CIs). Thus, these measures are potentially applicable in humans for estimating the condition of the neural population (so called "neural health" or "cochlear health") at individual sites along the electrode array and possibly adjusting the stimulation strategy in the CI sound processor accordingly. Some measures used to estimate neural health in animals have included the electrically evoked compound potential (ECAP), psychophysical detection thresholds, and multipulse integration (MPI). With regard to ECAP measures, it has been shown that the change in the ECAP response as a function of increasing the stimulus interphase gap ("IPG Effect") also reflects neural density in implanted animals. These animal studies have typically been conducted using preparations in which the electrode was in a fixed position with respect to the neural population, whereas in human cochlear implant users, the position of individual electrodes varies widely within an electrode array and also across subjects. The current study evaluated the effects of electrode location in the implanted cochlea (specifically medial-lateral location) on various electrophysiological and psychophysical measures in eleven human subjects. The results demonstrated that some measures of interest, specifically ECAP thresholds, psychophysical detection thresholds, and ECAP amplitude-growth function (AGF) linear slope, were significantly related to the distances between the electrode and mid-modiolar axis (MMA). These same measures were less strongly related or not significantly related to the electrode to medial wall (MW) distance. In contrast, neither the IPG Effect for the ECAP AGF slope or threshold, nor the MPI slopes were significantly related to MMA or MW distance from the electrodes. These results suggest that "within-channel" estimates of neural health such as the IPG Effect and MPI slope might be more suitable for estimating nerve condition in humans for clinical application since they appear to be relatively independent of electrode position.

Residual Cochlear Function in Adults and Children Receiving Cochlear Implants: Correlations With Speech Perception Outcomes

OBJECTIVES

Variability in speech perception outcomes with cochlear implants remains largely unexplained. Recently, electrocochleography, or measurements of cochlear potentials in response to sound, has been used to assess residual cochlear function at the time of implantation. Our objective was to characterize the potentials recorded preimplantation in subjects of all ages, and evaluate the relationship between the responses, including a subjective estimate of neural activity, and speech perception outcomes.

DESIGN

Electrocochleography was recorded in a prospective cohort of 284 candidates for cochlear implant at University of North Carolina (10 months to 88 years of ages). Measurement of residual cochlear function called the "total response" (TR), which is the sum of magnitudes of spectral components in response to tones of different stimulus frequencies, was obtained for each subject. The TR was then related to results on age-appropriate monosyllabic word score tests presented in quiet. In addition to the TR, the electrocochleography results were also assessed for neural activity in the forms of the compound action potential and auditory nerve neurophonic.

RESULTS

The TR magnitude ranged from a barely detectable response of about 0.02 µV to more than 100 µV. In adults (18 to 79 years old), the TR accounted for 46% of variability in speech perception outcome by linear regression (r = 0.46; p < 0.001). In children between 6 and 17 years old, the variability accounted for was 36% (p < 0.001). In younger children, the TR accounted for less of the variability, 15% (p = 0.012). Subjects over 80 years old tended to perform worse for a given TR than younger adults at the 6-month testing interval. The subjectively assessed neural activity did not increase the information compared with the TR alone, which is primarily composed of the cochlear microphonic produced by hair cells.

CONCLUSIONS

The status of the auditory periphery, particularly of hair cells rather than neural activity, accounts for a large fraction of variability in speech perception outcomes in adults and older children. In younger children, the relationship is weaker, and the elderly differ from other adults. This simple measurement can be applied with high throughput so that peripheral status can be assessed to help manage patient expectations, create individually-tailored treatment plans, and identify subjects performing below expectations based on residual cochlear function.

Use of the guinea pig in studies on the development and prevention of acquired sensorineural hearing loss, with an emphasis on noise

Guinea pigs have been used in diverse studies to better understand acquired hearing loss induced by noise and ototoxic drugs. The guinea pig has its best hearing at slightly higher frequencies relative to humans, but its hearing is more similar to humans than the rat or mouse. Like other rodents, it is more vulnerable to noise injury than the human or nonhuman primate models. There is a wealth of information on auditory function and vulnerability of the inner ear to diverse insults in the guinea pig. With respect to the assessment of potential otoprotective agents, guinea pigs are also docile animals that are relatively easy to dose via systemic injections or gavage. Of interest, the cochlea and the round window are easily accessible, notably for direct cochlear therapy, as in the chinchilla, making the guinea pig a most relevant and suitable model for hearing. This article reviews the use of the guinea pig in basic auditory research, provides detailed discussion of its use in studies on noise injury and other injuries leading to acquired sensorineural hearing loss, and lists some therapeutics assessed in these laboratory animal models to prevent acquired sensorineural hearing loss.

Changes over time in the electrically evoked compound action potential (ECAP) interphase gap (IPG) effect following cochlear implantation in Guinea pigs

The electrically-evoked compound action potential (ECAP) is correlated with spiral ganglion neuron (SGN) density in cochlear implanted animals. In a previous study, we showed that ECAP amplitude growth function (AGF) linear slopes for stimuli with a constant interphase gap (IPG) changed significantly over time following implantation. Related studies have also shown that 1) IPG sensitivity for ECAP measures ("IPG Effect") is related to SGN density in animals and 2) the ECAP IPG Effect is related to speech recognition performance in humans with cochlear implants. The current study examined how the ECAP IPG Effect changed following cochlear implantation in four non-deafened guinea pigs with residual inner hair cells (IHCs) and 5 deafened, neurotrophin-treated guinea pigs. Simple impedances were measured on the same days as the ECAP measures. Generally, non-deafened implanted animals with higher SGN survival demonstrated higher ECAP AGF linear slope and peak amplitude values than the deafened, implanted guinea pigs. The ECAP IPG Effect for the AGF slopes and peak amplitudes was also larger in the hearing animals. The N1 latencies for a constant IPG were not different between groups, but the N1 latency IPG Effect was smaller in the non-deafened, implanted animals. Similar to previously reported results, ECAP measures using a fixed or changing IPG required as many as three months after implantation before a stable point could be calculated, but this was dependent on the animal and condition. For all ECAP measures most animals showed greater variance in the first 30 days post-implantation. Post-implantation changes in ECAPs and impedances were not correlated with one another. Results from this study are helpful for estimating the mechanisms underlying ECAP characteristics and have implications for clinical application of the ECAP measures in long-term human cochlear implant recipients. Specifically, these measures could help to monitor neural health over a period of time, or during a time of stability these measures could be used to help select electrode sites for activation in clinical programming.

Rhythm processing in cochlear implant-mediated music perception

Cochlear implants (CIs) are biomedical devices that provide sound to people with severe-to-profound hearing loss by direct electrical stimulation of auditory neurons in the cochlea. Despite the remarkable achievements with respect to speech perception in quiet environments, music perception with CIs remains generally poor due to the degradation of auditory input. Prior studies have shown that both pitch perception and timbre discrimination are poor in CI users, whereas the performance on rhythmic tasks is nearly equivalent to normal hearing participants. There are several caveats, however, to this generalization regarding rhythm processing for CI users. The purpose of this article is to summarize the literature on rhythmic perception for CI users while highlighting important limitations within these studies. We will also identify areas for future research and development of CI-mediated music processing. It is likely that rhythm processing will continue to advance as our understanding of electrical current delivery to the auditory nerve improves.

Assessing the Relationship Between the Electrically Evoked Compound Action Potential and Speech Recognition Abilities in Bilateral Cochlear Implant Recipients

OBJECTIVES

The primary objective of the present study was to examine the relationship between suprathreshold electrically evoked compound action potential (ECAP) measures and speech recognition abilities in bilateral cochlear implant listeners. We tested the hypothesis that the magnitude of ear differences in ECAP measures within a subject (right-left) could predict the difference in speech recognition performance abilities between that subject's ears (right-left).

DESIGN

To better control for across-subject variables that contribute to speech understanding, the present study used a within-subject design. Subjects were 10 bilaterally implanted adult cochlear implant recipients. We measured ECAP amplitudes and slopes of the amplitude growth function in both ears for each subject. We examined how each of these measures changed when increasing the interphase gap of the biphasic pulses. Previous animal studies have shown correlations between these ECAP measures and auditory nerve survival. Speech recognition measures included speech reception thresholds for sentences in background noise, as well as phoneme discrimination in quiet and in noise.

RESULTS

Results showed that the between-ear difference (right-left) of one specific ECAP measure (increase in amplitude growth function slope as the interphase gap increased from 7 to 30 µs) was significantly related to the between-ear difference (right-left) in speech recognition. Frequency-specific response patterns for ECAP data and consonant transmission cues support the hypothesis that this particular ECAP measure may represent localized functional acuity.

CONCLUSIONS

The results add to a growing body of literature suggesting that when using a well-controlled research design, there is evidence that underlying neural function is related to postoperative performance with a cochlear implant.

Outlook and future of inner ear therapy

Drug delivery to the inner ear is an ideal method to treat a wide variety of otologic conditions. A broad range of potential applications is just beginning to be explored. New approaches combine principles of inner ear pharmacokinetics with emerging technologies of drug delivery including novel delivery systems, drug-device combinations, and new categories of drugs. Strategies include cell-specific targeting, manipulation of gene expression, local activation following systemic delivery, and use of stem cells, viral vectors, and gene editing systems. Translation of these therapies to the clinic remains challenging given the potential risks of intracochlear and intralabyrinthine trauma, our limited understanding of the etiologies of particular inner ear disorders, and paucity of accurate diagnostic tools at the cellular level. This review provides an overview of future methods, delivery systems, disease targets, and clinical considerations required for translation to clinical medicine.

Evaluating Multipulse Integration as a Neural-Health Correlate in Human Cochlear-Implant Users: Relationship to Psychometric Functions for Detection

In electrical hearing, multipulse integration (MPI) describes the rate at which detection threshold decreases with increasing stimulation rate in a fixed-duration pulse train. In human subjects, MPI has been shown to be dependent on the psychophysically estimated spread of neural excitation at a high stimulation rate, with broader spread predicting greater integration. The first aim of the present study was to replicate this finding using alternative methods for measuring MPI and spread of neural excitation. The second aim was to test the hypothesis that MPI is related to the slope of the psychometric function for detection. Specifically, a steep d' versus stimulus level function would predict shallow MPI since the amount of current reduction necessary to compensate for an increase in stimulation rate to maintain threshold would be small. The MPI function was measured by obtaining adaptive detection thresholds at 160 and 640 pulses per second. Spread of neural excitation was measured by forward-masked psychophysical tuning curves. All psychophysical testing was performed in a monopolar stimulation mode (MP 1 + 2). Results showed that MPI was correlated with the slopes of the tuning curves, with broader tuning predicting steeper MPI, confirming the earlier finding. However, there was no relationship between MPI and the slopes of the psychometric functions. These results suggest that a broad stimulation of the cochlea facilitates MPI. MPI however is not related to the estimated neural excitation growth with current level near the behavioral threshold, at least in monopolar stimulation.

Auditory Performance and Electrical Stimulation Measures in Cochlear Implant Recipients With Auditory Neuropathy Compared With Severe to Profound Sensorineural Hearing Loss

OBJECTIVES

The aim of the study was to compare auditory and speech outcomes and electrical parameters on average 8 years after cochlear implantation between children with isolated auditory neuropathy (AN) and children with sensorineural hearing loss (SNHL).

DESIGN

The study was conducted at a tertiary, university-affiliated pediatric medical center. The cohort included 16 patients with isolated AN with current age of 5 to 12.2 years who had been using a cochlear implant for at least 3.4 years and 16 control patients with SNHL matched for duration of deafness, age at implantation, type of implant, and unilateral/bilateral implant placement. All participants had had extensive auditory rehabilitation before and after implantation, including the use of conventional hearing aids. Most patients received Cochlear Nucleus devices, and the remainder either Med-El or Advanced Bionics devices. Unaided pure-tone audiograms were evaluated before and after implantation. Implantation outcomes were assessed by auditory and speech recognition tests in quiet and in noise. Data were also collected on the educational setting at 1 year after implantation and at school age. The electrical stimulation measures were evaluated only in the Cochlear Nucleus implant recipients in the two groups. Similar mapping and electrical measurement techniques were used in the two groups. Electrical thresholds, comfortable level, dynamic range, and objective neural response telemetry threshold were measured across the 22-electrode array in each patient. Main outcome measures were between-group differences in the following parameters: (1) Auditory and speech tests. (2) Residual hearing. (3) Electrical stimulation parameters. (4) Correlations of residual hearing at low frequencies with electrical thresholds at the basal, middle, and apical electrodes.

RESULTS

The children with isolated AN performed equally well to the children with SNHL on auditory and speech recognition tests in both quiet and noise. More children in the AN group than the SNHL group were attending mainstream educational settings at school age, but the difference was not statistically significant. Significant between-group differences were noted in electrical measurements: the AN group was characterized by a lower current charge to reach subjective electrical thresholds, lower comfortable level and dynamic range, and lower telemetric neural response threshold. Based on pure-tone audiograms, the children with AN also had more residual hearing before and after implantation. Highly positive coefficients were found on correlation analysis between T levels across the basal and midcochlear electrodes and low-frequency acoustic thresholds.

CONCLUSIONS

Prelingual children with isolated AN who fail to show expected oral and auditory progress after extensive rehabilitation with conventional hearing aids should be considered for cochlear implantation. Children with isolated AN had similar pattern as children with SNHL on auditory performance tests after cochlear implantation. The lower current charge required to evoke subjective and objective electrical thresholds in children with AN compared with children with SNHL may be attributed to the contribution to electrophonic hearing from the remaining neurons and hair cells. In addition, it is also possible that mechanical stimulation of the basilar membrane, as in acoustic stimulation, is added to the electrical stimulation of the cochlear implant.

Evaluating Multipulse Integration as a Neural-Health Correlate in Human Cochlear Implant Users: Effects of Stimulation Mode

Previous psychophysical studies have shown that a steep detection-threshold-versus-stimulation-rate function (multipulse integration; MPI) is associated with laterally positioned electrodes producing a broad neural excitation pattern. These findings are consistent with steep MPI depending on either a certain width of neural excitation allowing a large population of neurons operating at a low point on their dynamic range to respond to an increase in stimulation rate or a certain slope of excitation pattern that allows recruitment of neurons at the excitation periphery. Results of the current study provide additional support for these mechanisms by demonstrating significantly flattened MPI functions in narrow bipolar than monopolar stimulation. The study further examined the relationship between the steepness of the psychometric functions for detection (d' versus log current level) and MPI. In contrast to findings in monopolar stimulation, current data measured in bipolar stimulation suggest that steepness of the psychometric functions explained a moderate amount of the across-site variance in MPI. Steepness of the psychometric functions, however, cannot explain why MPI flattened in bipolar stimulation, since slopes of the psychometric functions were comparable in the two stimulation modes. Lastly, our results show that across-site mean MPI measured in monopolar and bipolar stimulation correlated with speech recognition in opposite signs, with steeper monopolar MPI being associated with poorer performance but steeper bipolar MPI being associated with better performance. If steeper MPI requires broad stimulation of the cochlea, the correlation between monopolar MPI and speech recognition can be interpreted as the detrimental effect of poor spectral resolution on speech recognition. Assuming bipolar stimulation produces narrow excitation, and MPI measured in bipolar stimulation reflects primarily responses of the on-site neurons, the correlation between bipolar MPI and speech recognition can be understood in light of the importance of neural survival for speech recognition.

Neurotrophin Gene Therapy in Deafened Ears with Cochlear Implants: Long-term Effects on Nerve Survival and Functional Measures

Because cochlear implants function by stimulating the auditory nerve, it is assumed that the condition of the nerve plays an important role in the efficacy of the prosthesis. Thus, considerable research has been devoted to methods of preserving the nerve following deafness. Neurotrophins have been identified as a potential contributor to neural health, but most of the research to date has been done in young animals and for short periods (less than 3 to 6 months) after the onset of treatment. The first objective of the current experiment was to examine the effects of a neurotrophin gene therapy delivery method on spiral ganglion neuron (SGN) preservation and function in the long term (5 to 14 months) in mature guinea pigs with cochlear implants. The second objective was to examine several potential non-invasive monitors of auditory nerve health following the neurotrophin gene therapy procedure. Eighteen mature adult male guinea pigs were deafened by cochlear perfusion of neomycin and then one ear was inoculated with an adeno-associated viral vector with an Nft3-gene insert (AAV.Ntf3) and implanted with a cochlear implant electrode array. Five control animals were deafened and inoculated with an empty AAV and implanted. Data from 43 other guinea pig ears from this and previous experiments were used for comparison: 24 animals implanted in a hearing ear, nine animals deafened and implanted with no inoculation, and ten normal-hearing non-implanted ears. After 4 to 21 months of psychophysical and electrophysiological testing, the animals were prepared for histological examination of SGN densities and inner hair cell (IHC) survival. Seventy-eight percent of the ears deafened and inoculated with AAV.Ntf3 showed better SGN survival than the 14 deafened-control ears. The degree of SGN preservation following the gene therapy procedure was variable across animals and across cochlear turns. Slopes of psychophysical multipulse integration (MPI) functions were predictive of SGN density, but only in animals with preserved IHCs. MPI was not affected by the AAV.Ntf3 treatment, but there was a minor improvement in temporal integration (TI). AAV.Ntf3 treatment had significant effects on ECAP and EABR amplitude growth func-tion (AGF) slopes; the reduction in slope in deafened ears was ameliorated by the AAV.Ntf3 treatment. Slopes of the ECAP and EABR AGFs were predictive of SGN density in a broad area near and just apical to the implant. The highest ensemble spontaneous activity (ESA) values were seen in animals with surviving IHCs, but AAV.Ntf3 treatment in deafened ears resulted in slightly higher ESA values compared to deafened untreated ears. Overall, a combination of the psychophysical and electrophysiological measures can be useful for monitoring the health of the implanted cochlea in guinea pigs. These measures should be applicable for assessing cochlear health in human subjects.

Recovery from forward masking in cochlear implant listeners depends on stimulation mode, level, and electrode location

Psychophysical recovery from forward masking was measured in adult cochlear implant users of Cochlear^TM and Advanced Bionics^TM devices, in monopolar and in focused (bipolar and tripolar) stimulation modes, at four electrode sites across the arrays, and at two levels (loudness balanced across modes and electrodes). Results indicated a steeper psychophysical recovery from forward masking in monopolar over bipolar and tripolar modes, modified by differential effects of electrode and level. The interactions between factors varied somewhat across devices. It is speculated that psychophysical recovery from forward masking may be driven by different populations of neurons in the different modes, with a broader stimulation pattern resulting in a greater likelihood of response by healthier and/or faster-recovering neurons within the stimulated population. If a more rapid recovery from prior stimulation reflects responses of neurons not necessarily close to the activating site, the spectral pattern of the incoming acoustic signal may be distorted. These results have implications for speech processor implementations using different degrees of focusing of the electric field. The primary differences in the shape of the recovery function were observed in the earlier portion (between 2 and 45 ms) of recovery, which is significant in terms of the speech envelope.

Voice emotion perception and production in cochlear implant users

Voice emotion is a fundamental component of human social interaction and social development. Unfortunately, cochlear implant users are often forced to interface with highly degraded prosodic cues as a result of device constraints in extraction, processing, and transmission. As such, individuals with cochlear implants frequently demonstrate significant difficulty in recognizing voice emotions in comparison to their normal hearing counterparts. Cochlear implant-mediated perception and production of voice emotion is an important but relatively understudied area of research. However, a rich understanding of the voice emotion auditory processing offers opportunities to improve upon CI biomedical design and to develop training programs benefiting CI performance. In this review, we will address the issues, current literature, and future directions for improved voice emotion processing in cochlear implant users.

Evaluating multipulse integration as a neural-health correlate in human cochlear-implant users: Relationship to spatial selectivity

The decrease of psychophysical detection thresholds as a function of pulse rate for a fixed-duration electrical pulse train is referred to as multipulse integration (MPI). The MPI slopes correlate with anatomical and physiological indices of cochlear health in guinea pigs with cochlear implants. The aim of the current study was to assess whether the MPI slopes were related to the spatial spread of activation by electrical stimulation. The hypothesis was that MPI is dependent on the total number of excitable neurons at the stimulation site, with broader neural excitation producing a steeper threshold decrease as a function of stimulation rate. MPI functions were measured at all stimulation sites in 22-site electrode arrays in human subjects. Some sites with steep MPI functions and other sites with shallow functions were assessed for spatial spread of excitation at 900 pps using a forward-masking paradigm. The results showed a correlation between the slopes of the forward-masking functions and the steepness of MPI, with broader stimulation predicting greater integration. The results are consistent with the idea that integration of multiple pulses in a pulse train relies on the number of excitable neurons at the stimulation site.

Auditory steady-state responses in cochlear implant users: Effect of modulation frequency and stimulation artifacts

Previous studies have shown that objective measures based on stimulation with low-rate pulse trains fail to predict the threshold levels of cochlear implant (CI) users for high-rate pulse trains, as used in clinical devices. Electrically evoked auditory steady-state responses (EASSRs) can be elicited by modulated high-rate pulse trains, and can potentially be used to objectively determine threshold levels of CI users. The responsiveness of the auditory pathway of profoundly hearing-impaired CI users to modulation frequencies is, however, not known. In the present study we investigated the responsiveness of the auditory pathway of CI users to a monopolar 500 pulses per second (pps) pulse train modulated between 1 and 100 Hz. EASSRs to forty-three modulation frequencies, elicited at the subject's maximum comfort level, were recorded by means of electroencephalography. Stimulation artifacts were removed by a linear interpolation between a pre- and post-stimulus sample (i.e., blanking). The phase delay across modulation frequencies was used to differentiate between the neural response and a possible residual stimulation artifact after blanking. Stimulation artifacts were longer than the inter-pulse interval of the 500pps pulse train for recording electrodes ipsilateral to the CI. As a result the stimulation artifacts could not be removed by artifact removal on the bases of linear interpolation for recording electrodes ipsilateral to the CI. However, artifact-free responses could be obtained in all subjects from recording electrodes contralateral to the CI, when subject specific reference electrodes (Cz or Fpz) were used. EASSRs to modulation frequencies within the 30-50 Hz range resulted in significant responses in all subjects. Only a small number of significant responses could be obtained, during a measurement period of 5 min, that originate from the brain stem (i.e., modulation frequencies in the 80-100 Hz range). This reduced synchronized activity of brain stem responses in long-term severely-hearing impaired CI users could be an attribute of processes associated with long-term hearing impairment and/or electrical stimulation.

Preoperative steroids for hearing preservation cochlear implantation: A review

Preoperative steroids have been shown to be beneficial in reducing the hearing loss associated with cochlear implantation. This review article discusses the mechanism of action, effects of differing routes of administration, and side effects of steroids administered to the inner ear. Studies on the role of preoperative steroids in animal and human studies are also examined and future directions for research in this area are discussed.

Evaluating multipulse integration as a neural-health correlate in human cochlear-implant users: Relationship to forward-masking recovery

The present study evaluated the slopes of threshold-versus-pulse-rate functions (multipulse integration, MPI) in humans with cochlear implants in relation to recovery from 300-ms forward maskers. MPI has been correlated with spiral ganglion cell density in animals. The present study showed that steeper MPI functions were correlated with faster recovery from forward masking. The findings suggested that the variations in the MPI slopes are explained not only by the quantity of neurons contributing to the integration process but also by the neurons' temporal response characteristics and possibly central inhibition.

Recognition of Modified Conditioning Sounds by Competitively Trained Guinea Pigs

The guinea pig (GP) is an often-used species in hearing research. However, behavioral studies are rare, especially in the context of sound recognition, because of difficulties in training these animals. We examined sound recognition in a social competitive setting in order to examine whether this setting could be used as an easy model. Two starved GPs were placed in the same training arena and compelled to compete for food after hearing a conditioning sound (CS), which was a repeat of almost identical sound segments. Through a 2-week intensive training, animals were trained to demonstrate a set of distinct behaviors solely to the CS. Then, each of them was subjected to generalization tests for recognition of sounds that had been modified from the CS in spectral, fine temporal and tempo (i.e., intersegment interval, ISI) dimensions. Results showed that they discriminated between the CS and band-rejected test sounds but had no preference for a particular frequency range for the recognition. In contrast, sounds modified in the fine temporal domain were largely perceived to be in the same category as the CS, except for the test sound generated by fully reversing the CS in time. Animals also discriminated sounds played at different tempos. Test sounds with ISIs shorter than that of the multi-segment CS were discriminated from the CS, while test sounds with ISIs longer than that of the CS segments were not. For the shorter ISIs, most animals initiated apparently positive food-access behavior as they did in response to the CS, but discontinued it during the sound-on period probably because of later recognition of tempo. Interestingly, the population range and mean of the delay time before animals initiated the food-access behavior were very similar among different ISI test sounds. This study, for the first time, demonstrates a wide aspect of sound discrimination abilities of the GP and will provide a way to examine tempo perception mechanisms using this animal species.

Continuous topical drug delivery using osmotic pump in animal cochlear implant model: Continuous steroid delivery is effective for hearing preservation

CONCLUSIONS

Continuous topical drug delivery using an osmotic pump is an effective supplementary technique for hearing preservation after cochlear implantation, as demonstrated in a guinea pig model.

OBJECTIVE

To evaluate the effect of continuous topical steroid delivery via an osmotic pump in an animal cochlear implant model.

METHODS

Twenty-three guinea pigs were used for the study. The animals were divided into three groups: control group (n = 8), simple topical dexamethasone delivery group (sDEXA group, n = 7) and continuous topical dexamethasone delivery group (cDEXA, n = 8). The hearing thresholds of all animals were measured by pre-operative auditory brain stem responses (ABRs) at 2, 8, 16, 24, and 32 kHz. ABRs were re-evaluated after cochlear implantation, and the animals were sacrificed for hematoxylin and eosin staining.

RESULTS

The ABR threshold at 1 week post-operatively was significantly lower in the cDEXA group than in the control and sDEXA groups at most frequencies. Threshold shifts from baseline were statistically smaller in the cDEXA group than in the control and sDEXA groups at all frequencies. Histological analysis revealed decreased numbers of multi-nucleated giant cells and thinner histiocyte layers.

Insertion trauma and recovery of function after cochlear implantation: Evidence from objective functional measures

Partial loss and subsequent recovery of cochlear implant function in the first few weeks following cochlear implant surgery has been observed in previous studies using psychophysical detection thresholds. In the current study, we explored this putative manifestation of insertion trauma using objective functional measures: electrically-evoked compound action potential (ECAP) amplitude-growth functions (ECAP amplitude as a function of stimulus level). In guinea pigs implanted in a hearing ear with good post-implant hearing and good spiral ganglion neuron (SGN) survival, consistent patterns of ECAP functions were observed. The slopes of ECAP growth functions were moderately steep on the day of implant insertion, decreased to low levels over the first few days after implantation and then increased slowly over several weeks to reach a relatively stable level. In parallel, ECAP thresholds increased over time after implantation and then recovered, although more quickly, to a relatively stable low level as did thresholds for eliciting a facial twitch. Similar results were obtained in animals deafened but treated with an adenovirus with a neurotrophin gene insert that resulted in good SGN preservation. In contrast, in animals implanted in deaf ears that had relatively poor SGN survival, ECAP slopes reached low levels within a few days after implantation and remained low. These results are consistent with the idea that steep ECAP growth functions require a healthy population of auditory nerve fibers and that cochlear implant insertion trauma can temporarily impair the function of a healthy SGN population.

Morphological correlates of hearing loss after cochlear implantation and electro-acoustic stimulation in a hearing-impaired Guinea pig model

Hybrid or electro-acoustic stimulation (EAS) cochlear implants (CIs) are designed to provide high-frequency electric hearing together with residual low-frequency acoustic hearing. However, 30-50% of EAS CI recipients lose residual hearing after implantation. The objective of this study was to determine the mechanisms of EAS-induced hearing loss in an animal model with high-frequency hearing loss. Guinea pigs were exposed to 24 h of noise (12-24 kHz at 116 dB) to induce a high-frequency hearing loss. After recovery, two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem responses were recorded biweekly to monitor changes in hearing. The organ of Corti was immunolabeled with phalloidin, anti-CtBP2, and anti-GluR2 to quantify hair cells, ribbons and post-synaptic receptors. The lateral wall was immunolabeled with phalloidin and lectin to quantify stria vascularis capillary diameters. Bimodal or trimodal diameter distributions were observed; the number and location of peaks were objectively determined using the Aikake Information Criterion and Expectation Maximization algorithm. Noise exposure led to immediate hearing loss at 16-32 kHz for all groups. Cochlear implantation led to additional hearing loss at 4-8 kHz; this hearing loss was negatively and positively correlated with minimum and maximum peaks of the bimodal or trimodal distributions of stria vascularis capillary diameters, respectively. After chronic stimulation, no significant group changes in thresholds were seen; however, elevated thresholds at 1 kHz in implanted, stimulated animals were significantly correlated with decreased presynaptic ribbon and postsynaptic receptor counts. Inner and outer hair cell counts did not differ between groups and were not correlated with threshold shifts at any frequency. As in the previous study in a normal-hearing model, stria vascularis capillary changes were associated with immediate hearing loss after implantation, while little to no hair cell loss was observed even in cochlear regions with threshold shifts as large as 40-50 dB. These findings again support a role of lateral wall blood flow changes, rather than hair cell loss, in hearing loss after surgical trauma, and implicate the endocochlear potential as a factor in implantation-induced hearing loss. Further, the analysis of the hair cell ribbons and post-synaptic receptors suggest that delayed hearing loss may be linked to synapse or peripheral nerve loss due to stimulation excitotoxicity or inflammation. Further research is needed to separate these potential mechanisms of delayed hearing loss.

Integration of Pulse Trains in Humans and Guinea Pigs with Cochlear Implants

Temporal integration (TI; threshold versus stimulus duration) functions and multipulse integration (MPI; threshold versus pulse rate) functions were measured behaviorally in guinea pigs and humans with cochlear implants. Thresholds decreased with stimulus duration at a fixed pulse rate and with pulse rate at a fixed stimulus duration. The rates of threshold decrease (slopes) of the TI and MPI functions were not statistically different between the guinea pig and human subject groups. A characteristic of the integration functions that the two groups shared was that the slopes of the TI functions were similar in magnitude to slopes of the MPI function only at low pulse rates (< approximately 300 pulses per second). This is consistent with the notion that the TI functions and the MPI functions at the low rates are mediated by a mechanism of long-term integration described in the statistical "multiple looks" model. Histological analysis of the guinea pig cochleae suggested that the slopes of both the MPI and the TI functions were dependent on sensory and neural health near the stimulated regions. The strongest predictor for spiral ganglion cell densities measured near the stimulation sites was the slope of the MPI functions below 1,000 pps. Several mechanisms may be considered to account for the association of shallow integration functions with poor sensory and neural status. These mechanisms are related to abnormal across-fiber synchronization, increased refractoriness and adaptation with impaired neural function, and steep growth of neural excitation with current level associated with neural pathology. The slope of the integration functions can potentially be used as a non-invasive measure for identifying stimulation sites with poor neural health and selecting those sites for removal or rehabilitation, but these applications remain to be tested.

Establishment of an experimental system to study the influence of electrical field on cochlear structures

Treatment of partial hearing loss with the combined electrical and acoustical stimulation (EAS) aims at restoring the hearing while preserving the residual hearing. The aim of present study was to establish an in vitro system to study the effects of an electrical field on the auditory hair cells and spiral ganglion cells. Cochlear tissues containing the organ of Corti, spiral limbus and spiral ganglion neurons were dissected from post-natal Wistar rats (p3-p5) and cultured in the micro-channels. Electric current was homogenously applied on the apical, medial and basal parts of explants. Biphasic rectangular pulses were applied continuously over a period of 30 h or 42 h and the explants were fixed and stained to visualize the hair cells and neurites. Application of electrical field for 30 h has not induced significant changes in the number of inner or outer hair cells when compared to the control. However, after 42 h of electric stimulation, the number of hair cells decreased significantly by about 30%. The medial and basal fragments were particularly affected. The number of neurites has not been influenced but significant neuritic beading, consistent with neurodegeneration, was observed after 42 h of electric stimulation. Although performed with immature auditory tissues, our findings hint at the possibility of particular electric current inducing damage or loss of auditory hair cells, which should be considered when designing EAS electrodes.

Importance of cochlear health for implant function

Amazing progress has been made in providing useful hearing to hearing-impaired individuals using cochlear implants, but challenges remain. One such challenge is understanding the effects of partial degeneration of the auditory nerve, the target of cochlear implant stimulation. Here we review studies from our human and animal laboratories aimed at characterizing the health of the implanted cochlea and the auditory nerve. We use the data on cochlear and neural health to guide rehabilitation strategies. The data also motivate the development of tissue-engineering procedures to preserve or build a healthy cochlea and improve performance obtained by cochlear implant recipients or eventually replace the need for a cochlear implant. This article is part of a Special Issue entitled .

Relationship between multipulse integration and speech recognition with cochlear implants

Comparisons of performance with cochlear implants and postmortem conditions in the cochlea in humans have shown mixed results. The limitations in those studies favor the use of within-subject designs and non-invasive measures to estimate cochlear conditions. One non-invasive correlate of cochlear health is multipulse integration, established in an animal model. The present study used this measure to relate neural health in human cochlear implant users to their speech recognition performance. The multipulse-integration slopes were derived based on psychophysical detection thresholds measured for two pulse rates (80 and 640 pulses per second). A within-subject design was used in eight subjects with bilateral implants where the direction and magnitude of ear differences in the multipulse-integration slopes were compared with those of the speech-recognition results. The speech measures included speech reception threshold for sentences and phoneme recognition in noise. The magnitude of ear difference in the integration slopes was significantly correlated with the magnitude of ear difference in speech reception thresholds, consonant recognition in noise, and transmission of place of articulation of consonants. These results suggest that multipulse integration predicts speech recognition in noise and perception of features that use dynamic spectral cues.

Effects of site-specific level adjustments on speech recognition with cochlear implants

OBJECTIVES

Modulation detection thresholds (MDTs) vary across stimulation sites in a cochlear implant (CI) electrode array in a manner that is subject and ear specific. Previous studies have demonstrated that speech recognition with a CI can be improved by site-selection strategies, where selected stimulation sites with poor modulation sensitivity are removed from a subject's processor MAP. Limitations of site-selection strategies are that they can compromise spectral resolution and distort frequency-place mapping because the frequencies assigned to the removed sites are usually reallocated to other sites, and site bandwidths are broadened. The objective of the present study was to test an alternative approach for rehabilitation that aimed at improving the across-site mean MDTs by adjusting stimulation parameters at the poorly performing sites. On the basis of previous findings that modulation detection contributes to speech recognition and improves significantly with stimulus level, the authors hypothesized that modulation sensitivity at the poor sites could be improved by artificially increasing stimulation levels at those sites in the speech processor, which then would lead to improved speech recognition.

DESIGN

Nine postlingually deafened ears implanted with Nucleus CIs were evaluated for MDTs, absolute-detection threshold levels (T levels), and the maximum loudness levels (C levels) on each of the available stimulation sites. For each ear, the minimum stimulation level settings in the speech-processor MAP were raised by 5%, and alternatively by 10%, of the dynamic range (DR) from true thresholds on five stimulation sites with the poorest MDTs. For comparison, a 5% level raise was applied globally to all stimulation sites. The C levels were fixed during these level manipulations. MDTs at the five poorest stimulation sites were compared at 20% DR before and after the level adjustments. Speech-reception thresholds (SRTs), that is, signal to noise ratios required for 50% correct speech recognition, were evaluated for these MAPs using CUNY sentences. The site-specific level-adjusted MAPs were compared with the global-level-adjusted MAP and the MAP without level adjustment. The effects on speech recognition of adjusting the minimal stimulation level settings on the five poorest stimulation sites were also compared with effects of removing these sites from the speech-processor MAP.

RESULTS

The 5% level increase on the five electrodes with the worst MDTs resulted in an improvement in the group mean SRT of 2.36 dB SNR relative to the MAP without level adjustment. The magnitude of level increase that resulted in the greatest SRT improvement for individuals varied across ears. MDTs measured at 20% DR significantly improved on the poor sites after the level adjustment that resulted in the best SRT for that ear was applied. Increasing the minimal stimulation levels on all stimulation sites or removing sites selected for rehabilitation, the parsimonious approaches, did not improve SRTs.

CONCLUSIONS

The site-specific adjustments of the T level settings improved modulation sensitivity at low levels and significantly improved subjects' SRTs. Thus, this site-rehabilitation strategy was an effective alternative to site-selection strategies for improving speech recognition in CI users.

Factors associated with hearing loss in a normal-hearing guinea pig model of Hybrid cochlear implants

The Hybrid cochlear implant (CI), also known as Electro-Acoustic Stimulation (EAS), is a new type of CI that preserves residual acoustic hearing and enables combined cochlear implant and hearing aid use in the same ear. However, 30-55% of patients experience acoustic hearing loss within days to months after activation, suggesting that both surgical trauma and electrical stimulation may cause hearing loss. The goals of this study were to: 1) determine the contributions of both implantation surgery and EAS to hearing loss in a normal-hearing guinea pig model; 2) determine which cochlear structural changes are associated with hearing loss after surgery and EAS. Two groups of animals were implanted (n = 6 per group), with one group receiving chronic acoustic and electric stimulation for 10 weeks, and the other group receiving no direct acoustic or electric stimulation during this time frame. A third group (n = 6) was not implanted, but received chronic acoustic stimulation. Auditory brainstem response thresholds were followed over time at 1, 2, 6, and 16 kHz. At the end of the study, the following cochlear measures were quantified: hair cells, spiral ganglion neuron density, fibrous tissue density, and stria vascularis blood vessel density; the presence or absence of ossification around the electrode entry was also noted. After surgery, implanted animals experienced a range of 0-55 dB of threshold shifts in the vicinity of the electrode at 6 and 16 kHz. The degree of hearing loss was significantly correlated with reduced stria vascularis vessel density and with the presence of ossification, but not with hair cell counts, spiral ganglion neuron density, or fibrosis area. After 10 weeks of stimulation, 67% of implanted, stimulated animals had more than 10 dB of additional threshold shift at 1 kHz, compared to 17% of implanted, non-stimulated animals and 0% of non-implanted animals. This 1-kHz hearing loss was not associated with changes in any of the cochlear measures quantified in this study. The variation in hearing loss after surgery and electrical stimulation in this animal model is consistent with the variation in human patients. Further, these findings illustrate an advantage of a normal-hearing animal model for quantification of hearing loss and damage to cochlear structures without the confounding effects of chemical- or noise-induced hearing loss. Finally, this study is the first to suggest a role of the stria vascularis and damage to the lateral wall in implantation-induced hearing loss. Further work is needed to determine the mechanisms of implantation- and electrical-stimulation-induced hearing loss.

Can ECAP measures be used for totally objective programming of cochlear implants?

An experiment was conducted with eight cochlear implant subjects to investigate the feasibility of using electrically evoked compound action potential (ECAP) measures other than ECAP thresholds to predict the way that behavioral thresholds change with rate of stimulation, and hence, whether they can be used without combination with behavioral measures to determine program stimulus levels for cochlear implants. Loudness models indicate that two peripheral neural response characteristics contribute to the slope of the threshold versus rate function: the way that neural activity to each stimulus pulse decreases as rate increases and the slope of the neural response versus stimulus current function. ECAP measures related to these two characteristics were measured: the way that ECAP amplitude decreases with stimulus rate and the ECAP amplitude growth function, respectively. A loudness model (incorporating temporal integration and the two neural response characteristics) and regression analyses were used to evaluate whether the ECAP measures could predict the average slope of the behavioral threshold versus current function and whether individual variation in the measures could predict individual variation in the slope of the threshold function. The average change of behavioral threshold with increasing rate was well predicted by the model when using the average ECAP data. However, the individual variations in the slope of the thresholds versus rate functions were not well predicted by individual variations in ECAP data. It was concluded that these ECAP measures are not useful for fully objective programming, possibly because they do not accurately reflect the neural response characteristics assumed by the model, or are measured at current levels much higher than threshold currents.

Temporal processing in the auditory system: insights from cochlear and auditory midbrain implantees

Central auditory processing in humans was investigated by comparing the perceptual effects of temporal parameters of electrical stimulation in auditory midbrain implant (AMI) and cochlear implant (CI) users. Four experiments were conducted to measure the following: effect of interpulse intervals on detection thresholds and loudness; temporal modulation transfer functions (TMTFs); effect of duration on detection thresholds; and forward masking decay. The CI data were consistent with a phenomenological model that based detection or loudness decisions on the output of a sliding temporal integration window, the input to which was the hypothetical auditory nerve response to each stimulus pulse. To predict the AMI data, the model required changes to both the neural response input (i.e., midbrain activity to AMI stimuli, compared to auditory nerve activity to CI stimuli) and the shape of the integration window. AMI data were consistent with a neural response that decreased more steeply compared to CI stimulation as the pulse rate increased or interpulse interval decreased. For one AMI subject, the data were consistent with a significant adaptation of the neural response for rates above 200 Hz. The AMI model required an integration window that was significantly wider (i.e., decreased temporal resolution) than that for CI data, the latter being well fit using the same integration window shape as derived from normal-hearing data. These models provide a useful way to conceptualize how stimulation of central auditory structures differs from stimulation of the auditory nerve and to better understand why AMI users have difficulty processing temporal cues important for speech understanding.

The use of neurotrophin therapy in the inner ear to augment cochlear implantation outcomes

Severe to profound deafness is most often secondary to a loss of or injury to cochlear mechanosensory cells, and there is often an associated loss of the peripheral auditory neural structures, specifically the spiral ganglion neurons and peripheral auditory fibers. Cochlear implantation is currently our best hearing rehabilitation strategy for severe to profound deafness. These implants work by directly electrically stimulating the remnant auditory neural structures within the deafened cochlea. When administered to the deafened cochlea in animal models, neurotrophins, specifically brain derived neurotrophic factor and neurotrophin-3, have been shown to dramatically improve spiral ganglion neuron survival and stimulate peripheral auditory fiber regrowth. In animal models, neurotrophins administered in combination with cochlear implantation has resulted in significant improvements in the electrophysiological and psychophysical measures of outcome. While further research must be done before these therapies can be applied clinically, neurotrophin therapies for the inner ear show great promise in enhancing CI outcomes and the treatment of hearing loss.

Psychophysically based site selection coupled with dichotic stimulation improves speech recognition in noise with bilateral cochlear implants

The ability to perceive important features of electrical stimulation varies across stimulation sites within a multichannel implant. The aim of this study was to optimize speech processor MAPs for bilateral implant users by identifying and removing sites with poor psychophysical performance. The psychophysical assessment involved amplitude-modulation detection with and without a masker, and a channel interaction measure quantified as the elevation in modulation detection thresholds in the presence of the masker. Three experimental MAPs were created on an individual-subject basis using data from one of the three psychophysical measures. These experimental MAPs improved the mean psychophysical acuity across the electrode array and provided additional advantages such as increasing spatial separations between electrodes and/or preserving frequency resolution. All 8 subjects showed improved speech recognition in noise with one or more experimental MAPs over their everyday-use clinical MAP. For most subjects, phoneme and sentence recognition in noise were significantly improved by a dichotic experimental MAP that provided better mean psychophysical acuity, a balanced distribution of selected stimulation sites, and preserved frequency resolution. The site-selection strategies serve as useful tools for evaluating the importance of psychophysical acuities needed for good speech recognition in implant users.

Sheep as a large animal model for middle and inner ear implantable hearing devices: a feasibility study in cadavers

OBJECTIVE

Currently, no large animal model exists for surgical-experimental exploratory analysis of implantable hearing devices. In a histomorphometric study, we sought to investigate whether sheep or pig cochleae are suitable for this purpose and whether device implantation is feasible.

METHODS

Skulls of pig and sheep cadavers were examined using high-resolution 128-slice computed tomography (CT) to study anatomic relationships. A cochlear implant and an active middle ear implant could be successfully implanted into the sheep's inner and middle ear, respectively. Correct device placement was verified by CT and histology. The cochlear anatomy of the sheep was further studied by micro-CT and histology.

RESULTS

Our investigations indicate that the sheep is a suitable animal model for implantation of implantable hearing devices. The implantation of the devices was successfully performed by access through a mastoidectomy. The histologic, morphologic, and micro-CT study of the sheep cochlea showed that it is highly similar to the human cochlea. The temporal bone of the pig was not suitable for these microsurgical procedures because the middle and inner ear were not accessible owing to distinct soft and fatty tissue coverage of the mastoid.

CONCLUSION

The sheep is an appropriate large animal model for experimental studies with implantable hearing devices, whereas the pig is not.