Impact of morphometry, myelinization and synaptic current strength on spike conduction in human and cat spiral ganglion neurons

Auditory nerve fiber excitability for alternative electrode placement in the obstructed human cochlea: electrode insertion in scala vestibuli versus scala tympani

Objective. The cochlear implant (CI) belongs to the most successful neuro-prostheses. Traditionally, the stimulating electrode arrays are inserted into the scala tympani (ST), the lower cochlear cavity, which enables simple surgical access. However, often deep insertion is blocked, e.g. by ossification, and the auditory nerve fibers (ANFs) of lower frequency regions cannot be stimulated causing severe restrictions in speech understanding. As an alternative, the CI can be inserted into the scala vestibuli (SV), the other upper cochlear cavity.Approach. In this computational study, the excitability of 25 ANFs are compared for stimulation with ST and SV implants. We employed a 3-dimensional realistic human cochlear model with lateral wall electrodes based on aμ-CT dataset and manually traced fibers. A finite element approach in combination with a compartment model of a spiral ganglion cell was used to simulate monophasic stimulation with anodic (ANO) and cathodic (CAT) pulses of 50μs.Main results. ANO thresholds are lower in ST (mean/std =μ/σ= 189/55μA) stimulation compared to SV (μ/σ= 323/119μA) stimulation. Contrary, CAT thresholds are higher for the ST array (μ/σ= 165/42μA) compared to the SV array (μ/σ= 122/46μA). The threshold amplitude depends on the specific fiber-electrode spatial relationship, such as lateral distance from the cochlear axis, the angle between electrode and target ANF, and the curvature of the peripheral process. For CAT stimulation the SV electrodes show a higher selectivity leading to less cross-stimulation of additional fibers from different cochlear areas.Significance. We present a first simulation study with a human cochlear model that investigates an additional CI placement into the SV and its impact on the excitation behavior. Results predict comparable outcomes to ST electrodes which confirms that SV implantation might be an alternative for patients with a highly obstructed ST.

Asymmetric pulses delivered by a cochlear implant allow a reduction in evoked firing rate and in spatial activation in the guinea pig auditory cortex

Despite that fact that the cochlear implant (CI) is one of the most successful neuro-prosthetic devices which allows hearing restoration, several aspects still need to be improved. Interactions between stimulating electrodes through current spread occurring within the cochlea drastically limit the number of discriminable frequency channels and thus can ultimately result in poor speech perception. One potential solution relies on the use of new pulse shapes, such as asymmetric pulses, which can potentially reduce the current spread within the cochlea. The present study characterized the impact of changing electrical pulse shapes from the standard biphasic symmetric to the asymmetrical shape by quantifying the evoked firing rate and the spatial activation in the guinea pig primary auditory cortex (A1). At a fixed charge, the firing rate and the spatial activation in A1 decreased by 15 to 25 % when asymmetric pulses were used to activate the auditory nerve fibers, suggesting a potential reduction of the spread of excitation inside the cochlea. A strong "polarity-order" effect was found as the reduction was more pronounced when the first phase of the pulse was cathodic with high amplitude. These results suggest that the use of asymmetrical pulse shapes in clinical settings can potentially reduce the channel interactions in CI users.

The human OPA1delTTAG mutation induces adult onset and progressive auditory neuropathy in mice

Dominant optic atrophy (DOA) is one of the most prevalent forms of hereditary optic neuropathies and is mainly caused by heterozygous variants in OPA1, encoding a mitochondrial dynamin-related large GTPase. The clinical spectrum of DOA has been extended to a wide variety of syndromic presentations, called DOAplus, including deafness as the main secondary symptom associated to vision impairment. To date, the pathophysiological mechanisms underlying the deafness in DOA remain unknown. To gain insights into the process leading to hearing impairment, we have analyzed the Opa1delTTAG mouse model that recapitulates the DOAplus syndrome through complementary approaches combining morpho-physiology, biochemistry, and cellular and molecular biology. We found that Opa1delTTAG mutation leads an adult-onset progressive auditory neuropathy in mice, as attested by the auditory brainstem response threshold shift over time. However, the mutant mice harbored larger otoacoustic emissions in comparison to wild-type littermates, whereas the endocochlear potential, which is a proxy for the functional state of the stria vascularis, was comparable between both genotypes. Ultrastructural examination of the mutant mice revealed a selective loss of sensory inner hair cells, together with a progressive degeneration of the axons and myelin sheaths of the afferent terminals of the spiral ganglion neurons, supporting an auditory neuropathy spectrum disorder (ANSD). Molecular assessment of cochlea demonstrated a reduction of Opa1 mRNA level by greater than 40%, supporting haploinsufficiency as the disease mechanism. In addition, we evidenced an early increase in Sirtuin 3 level and in Beclin1 activity, and subsequently an age-related mtDNA depletion, increased oxidative stress, mitophagy as well as an impaired autophagic flux. Together, these results support a novel role for OPA1 in the maintenance of inner hair cells and auditory neural structures, addressing new challenges for the exploration and treatment of OPA1-linked ANSD in patients.

Models of Cochlea Used in Cochlear Implant Research: A Review

As the first clinically translated machine-neural interface, cochlear implants (CI) have demonstrated much success in providing hearing to those with severe to profound hearing loss. Despite their clinical effectiveness, key drawbacks such as hearing damage, partly from insertion forces that arise during implantation, and current spread, which limits focussing ability, prevent wider CI eligibility. In this review, we provide an overview of the anatomical and physical properties of the cochlea as a resource to aid the development of accurate models to improve future CI treatments. We highlight the advancements in the development of various physical, animal, tissue engineering, and computational models of the cochlea and the need for such models, challenges in their use, and a perspective on their future directions.

A computational model to simulate spectral modulation and speech perception experiments of cochlear implant users

Speech understanding in cochlear implant (CI) users presents large intersubject variability that may be related to different aspects of the peripheral auditory system, such as the electrode–nerve interface and neural health conditions. This variability makes it more challenging to proof differences in performance between different CI sound coding strategies in regular clinical studies, nevertheless, computational models can be helpful to assess the speech performance of CI users in an environment where all these physiological aspects can be controlled. In this study, differences in performance between three variants of the HiRes Fidelity 120 (F120) sound coding strategy are studied with a computational model. The computational model consists of (i) a processing stage with the sound coding strategy, (ii) a three-dimensional electrode-nerve interface that accounts for auditory nerve fiber (ANF) degeneration, (iii) a population of phenomenological ANF models, and (iv) a feature extractor algorithm to obtain the internal representation (IR) of the neural activity. As the back-end, the simulation framework for auditory discrimination experiments (FADE) was chosen. Two experiments relevant to speech understanding were performed: one related to spectral modulation threshold (SMT), and the other one related to speech reception threshold (SRT). These experiments included three different neural health conditions (healthy ANFs, and moderate and severe ANF degeneration). The F120 was configured to use sequential stimulation (F120-S), and simultaneous stimulation with two (F120-P) and three (F120-T) simultaneously active channels. Simultaneous stimulation causes electric interaction that smears the spectrotemporal information transmitted to the ANFs, and it has been hypothesized to lead to even worse information transmission in poor neural health conditions. In general, worse neural health conditions led to worse predicted performance; nevertheless, the detriment was small compared to clinical data. Results in SRT experiments indicated that performance with simultaneous stimulation, especially F120-T, were more affected by neural degeneration than with sequential stimulation. Results in SMT experiments showed no significant difference in performance. Although the proposed model in its current state is able to perform SMT and SRT experiments, it is not reliable to predict real CI users' performance yet. Nevertheless, improvements related to the ANF model, feature extraction, and predictor algorithm are discussed.

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.

Effects of Degrees of Degeneration on the Electrical Excitation of Human Spiral Ganglion Neurons Based on a High-Resolution Computer Model

After hearing loss retrograde degeneration of spiral ganglion neurons (SGNs) has been described. Studies modeling the effects of degeneration mostly omitted peripheral processes (dendrites). Recent experimental observations indicated that degenerating SGNs manifested also a reduced diameter of their dendrites. We simulated populations of 400 SGNs inside a high resolution cochlear model with a cochlear implant, based on μCT scans of a human temporal bone. Cochlear implant stimuli were delivered as biphasic pulses in a monopolar configuration. Three SGN situations were simulated, based on our previous measurements of human SGN dendrites: (A) SGNs with intact dendrites (before degeneration), (B) degenerating SGNs, dendrites with a smaller diameter but original length, (C) degenerating SGNs, dendrites omitted. SGN fibers were mapped to characteristic frequency, and place pitch was estimated from excitation profiles. Results from degenerating SGNs (B, C) were similar. Most action potentials were initiated in the somatic area for all cases (A, B, C), except for areas near stimulating electrodes in the apex with intact SGNs (A), where action potentials were initiated in the distal dendrite. In most cases, degenerating SGNs had lower thresholds than intact SGNs (A) (down to -2 dB). Excitation profiles showed increased ectopic activation, i.e., activation of unintended neuronal regions, as well as similar neuronal regions excited by different apical electrodes, for degenerating SGNs (B, C). The estimated pitch showed cases of pitch reversals in apical electrodes for intact SGNs (A), as well as mostly identical pitches evoked by the four most apical electrodes for degenerating SGNs (B, C). In conclusion, neuronal excitation profiles to electrical stimulation exhibited similar traits in both ways of modeling SGN degeneration. Models showed degeneration of dendrites caused increased ectopic activation, as well as similar excitation profiles and pitch evoked by different apical electrodes. Therefore, insertion of electrodes beyond approximately 450° may not provide any benefit if SGN dendrites are degenerated.

A phenomenological computational model of the evoked action potential fitted to human cochlear implant responses

There is a growing interest in biomedical engineering in developing procedures that provide accurate simulations of the neural response to electrical stimulus produced by implants. Moreover, recent research focuses on models that take into account individual patient characteristics.

We present a phenomenological computational model that is customized with the patient’s data provided by the electrically evoked compound action potential (ECAP) for simulating the neural response to electrical stimulus produced by the electrodes of cochlear implants (CIs). The model links the input currents of the electrodes to the simulated ECAP.

Potentials and currents are calculated by solving the quasi-static approximation of the Maxwell equations with the finite element method (FEM). In ECAPs recording, an active electrode generates a current that elicits action potentials in the surrounding auditory nerve fibers (ANFs). The sum of these action potentials is registered by other nearby electrode. Our computational model emulates this phenomenon introducing a set of line current sources replacing the ANFs by a set of virtual neurons (VNs). To fit the ECAP amplitudes we assign a suitable weight to each VN related with the probability of an ANF to be excited. This probability is expressed by a cumulative beta distribution parameterized by two shape parameters that are calculated by means of a differential evolution algorithm (DE). Being the weights function of the current density, any change in the design of the CI affecting the current density produces changes in the weights and, therefore, in the simulated ECAP, which confers to our model a predictive capacity.

The results of the validation with ECAP data from two patients are presented, achieving a satisfactory fit of the experimental data with those provided by the proposed computational model.

The cochlea, found in the inner ear, is the organ where the sound is transformed into an electrical pulse to be transmitted by the neurons to the auditory cortex. Hearing loss can be caused by damage to the hair cells, in which case neuronal excitation is impaired. CIs are devices that replace the normal function of the impaired/damaged Organ of Corti. Computational models allow a better understanding of the mechanisms involved in the electrical stimulation of the auditory nerve. These models can help biomedical engineers to develop new CIs with improved auditory performance. One important aspect of our model is its customization with the patient’s data provided by the recording of the evoked compound action potential (the synchronous firing of a population of electrically stimulated auditory nerve fibers). This phenomenological model allows us to predict the registers of neural stimulation produced when the auditory nerve is stimulated with the CIs. We have validated the proposed model with real data obtained from two patients with CIs.

Signatures of cochlear processing in neuronal coding of auditory information

The vertebrate ear is endowed with remarkable perceptual capabilities. The faintest sounds produce vibrations of magnitudes comparable to those generated by thermal noise and can nonetheless be detected through efficient amplification of small acoustic stimuli. Two mechanisms have been proposed to underlie such sound amplification in the mammalian cochlea: somatic electromotility and active hair-bundle motility. These biomechanical mechanisms may work in concert to tune auditory sensitivity. In addition to amplitude sensitivity, the hearing system shows exceptional frequency discrimination allowing mammals to distinguish complex sounds with great accuracy. For instance, although the wide hearing range of humans encompasses frequencies from 20 Hz to 20 kHz, our frequency resolution extends to one-thirtieth of the interval between successive keys on a piano. In this article, we review the different cochlear mechanisms underlying sound encoding in the auditory system, with a particular focus on the frequency decomposition of sounds. The relation between peak frequency of activation and location along the cochlea - known as tonotopy - arises from multiple gradients in biophysical properties of the sensory epithelium. Tonotopic mapping represents a major organizational principle both in the peripheral hearing system and in higher processing levels and permits the spectral decomposition of complex tones. The ribbon synapses connecting sensory hair cells to auditory afferents and the downstream spiral ganglion neurons are also tuned to process periodic stimuli according to their preferred frequency. Though sensory hair cells and neurons necessarily filter signals beyond a few kHz, many animals can hear well beyond this range. We finally describe how the cochlear structure shapes the neural code for further processing in order to send meaningful information to the brain. Both the phase-locked response of auditory nerve fibers and tonotopy are key to decode sound frequency information and place specific constraints on the downstream neuronal network.

A simple model considering spiking probability during extracellular axon stimulation

The spiking probability of an electrically stimulated axon as a function of stimulus amplitude increases in a sigmoidal dependency from 0 to 1. However, most computer simulation studies for neuroprosthetic applications calculate thresholds for neural targets with a deterministic model and by reducing the sigmoid curve to a step function, they miss an important information about the control signal, namely how the spiking efficiency increases with stimulus intensity. Here, this spiking efficiency is taken into account in a compartment model of the Hodgkin Huxley type where a noise current is added in every compartment with an active membrane. A key parameter of the model is a common factor knoise which defines the ion current fluctuations across the cell membrane for every compartment by its maximum sodium ion conductance. In the standard model Gaussian signals are changed every 2.5 μs as a compromise of accuracy and computational costs. Additionally, a formula for other noise transmission times is presented and numerically tested. Spiking probability as a function of stimulus intensity can be approximated by the cumulative distribution function of the normal distribution with RS = σ/μ. Relative spread RS, introduced by Verveen, is a measure for the spread (normalized by the threshold intensity μ), that decreases inversely with axon diameter. Dynamic range, a related measure used in neuroprosthetic studies, defines the intensity range between 10% and 90% spiking probability. We show that (i) the dynamic range normalized by threshold is 2.56 times RS, (ii) RS increases with electrode—axon distance and (iii) we present knoise values for myelinated and unmyelinated axon models in agreement with recoded RS data. The presented method is applicable for other membrane models and can be extended to whole neurons that are described by multi-compartment models.

Impact of electrode position on the dynamic range of a human auditory nerve fiber

OBJECTIVE

Electrodes of a cochlear implant generate spikes in auditory nerve fibers (ANFs). While the insertion depth of each of the electrodes is linked to a frequency section of the acoustic signal, the amplitude of the stimulating pulses controls the loudness of the related frequency band. However, in comparison to acoustic stimulation the dynamic range of an electrically stimulated ANF is quite small.

APPROACH

The dynamic range of an electrically stimulated ANF is defined as the interval of stimulus amplitudes that causes firing probabilities between 10% and 90%. A compartment model that includes sodium ion current fluctuations as the stochastic key component for spiking was evaluated for different electrode placements and fiber diameters.

MAIN RESULTS

The dynamic range is reversely related to ANF diameter. An increased dynamic range is expected to improve the quality of auditory perception for cochlear implant users. Electrodes are often placed as close to the center axis of the cochlea as possible. The analysis of the simulated auditory nerve firing showed that this placement is disadvantageous for the dynamic range of a selected ANF.

SIGNIFICANCE

Five times larger dynamic ranges are expected for electrodes close to the terminal of the dendrite or at mid-dendritic placement as opposed to electrodes close to the modiolus.

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.

Cochlear Implant Research and Development in the Twenty-first Century: A Critical Update

Cochlear implants (CIs) are the world's most successful sensory prosthesis and have been the subject of intense research and development in recent decades. We critically review the progress in CI research, and its success in improving patient outcomes, from the turn of the century to the present day. The review focuses on the processing, stimulation, and audiological methods that have been used to try to improve speech perception by human CI listeners, and on fundamental new insights in the response of the auditory system to electrical stimulation. The introduction of directional microphones and of new noise reduction and pre-processing algorithms has produced robust and sometimes substantial improvements. Novel speech-processing algorithms, the use of current-focusing methods, and individualised (patient-by-patient) deactivation of subsets of electrodes have produced more modest improvements. We argue that incremental advances have and will continue to be made, that collectively these may substantially improve patient outcomes, but that the modest size of each individual advance will require greater attention to experimental design and power. We also briefly discuss the potential and limitations of promising technologies that are currently being developed in animal models, and suggest strategies for researchers to collectively maximise the potential of CIs to improve hearing in a wide range of listening situations.

Spike-rate adaptation in a computational model of human-shaped spiral ganglion neurons

OBJECTIVES

The purpose of this study is to develop a biophysical model of human spiral ganglion neurons (SGNs) that includes voltage-gated hyperpolarization-activated cation (HCN) channels and low-threshold potassium voltage-gated, delayed-rectifier low-threshold potassium (KLT) channels, providing for a more complete simulation of spike-rate adaptation, a feature of most spiking neurons in which spiking activity is reduced in response to sustained stimulation.

METHODS

Our model incorporates features of spike-rate adaptation reported from in vivo studies, whilst also displaying similar behaviour to existing models of human SGNs, including the dependence of electrically evoked thresholds on the polarity of electrical pulses.

RESULTS

Hypothesizing that the mode of stimulation intracellular or extracellular predicts features of spike-rate adaptation similar to in vivo studies, including the influence of stimulus intensity and pulse-rate, we find that the mode of stimulation alters features of spike-rate adaptation. In particular, the reduction in spiking over time with sustained input was generally greater for extracellular, compared to intracellular, stimulation, when simulating a multi-compartment SGN with human morphological features. In contrast, time-constants of spike-rate adaption reported for in vivo data did not fit our predicted responses, highlighting the need for a more complete physiological understanding of the factors contributing to spike-rate adaptation in electrically stimulated human SGNs.

CONCLUSION

Our model extends previous computational models of SGNs with human morphology with ionic channels accounting for features of spike-rate adaptation.

SIGNIFICANCE

The significance of this work resides in the ability to improve the modeling of cochlear implant (CI) stimulation and its effects on neural responses. This will help develop novel, and perhaps personalised, stimulation strategies to reduce variability in CI user outcomes.

Translational and interdisciplinary insights into presbyacusis: A multidimensional disease

There are multiple etiologies and phenotypes of age-related hearing loss or presbyacusis. In this review we summarize findings from animal and human studies of presbyacusis, including those that provide the theoretical framework for distinct metabolic, sensory, and neural presbyacusis phenotypes. A key finding in quiet-aged animals is a decline in the endocochlear potential (EP) that results in elevated pure-tone thresholds across frequencies with greater losses at higher frequencies. In contrast, sensory presbyacusis appears to derive, in part, from acute and cumulative effects on hair cells of a lifetime of environmental exposures (e.g., noise), which often result in pronounced high frequency hearing loss. These patterns of hearing loss in animals are recognizable in the human audiogram and can be classified into metabolic and sensory presbyacusis phenotypes, as well as a mixed metabolic+sensory phenotype. However, the audiogram does not fully characterize age-related changes in auditory function. Along with the effects of peripheral auditory system declines on the auditory nerve, primary degeneration in the spiral ganglion also appears to contribute to central auditory system aging. These inner ear alterations often correlate with structural and functional changes throughout the central nervous system and may explain suprathreshold speech communication difficulties in older adults with hearing loss. Throughout this review we highlight potential methods and research directions, with the goal of advancing our understanding, prevention, diagnosis, and treatment of presbyacusis.

Axon-glia interactions in the ascending auditory system

The auditory system detects and encodes sound information with high precision to provide a high-fidelity representation of the environment and communication. In mammals, detection occurs in the peripheral sensory organ (the cochlea) containing specialized mechanosensory cells (hair cells) that initiate the conversion of sound-generated vibrations into action potentials in the auditory nerve. Neural activity in the auditory nerve encodes information regarding the intensity and frequency of sound stimuli, which is transmitted to the auditory cortex through the ascending neural pathways. Glial cells are critical for precise control of neural conduction and synaptic transmission throughout the pathway, allowing for the precise detection of the timing, frequency, and intensity of sound signals, including the sub-millisecond temporal fidelity is necessary for tasks such as sound localization, and in humans, for processing complex sounds including speech and music. In this review, we focus on glia and glia-like cells that interact with hair cells and neurons in the ascending auditory pathway and contribute to the development, maintenance, and modulation of neural circuits and transmission in the auditory system. We also discuss the molecular mechanisms of these interactions, their impact on hearing and on auditory dysfunction associated with pathologies of each cell type.

Simulated auditory fiber myelination heterogeneity desynchronizes population responses to electrical stimulation limiting inter-aural timing difference representation

Auditory nerve responses to electrical stimulation exhibit aberrantly synchronous response latencies to low-rate pulse trains, nevertheless, cochlear implant users generally have elevated inter-aural timing difference detection thresholds. These findings present an apparent paradox in which single units are unusually precise but downstream within the auditory pathway access to this precision is lost. Auditory nerves innervating a region of cochlea exhibit natural heterogeneity in their diameter, myelination, and other structural properties; a key question is whether this diversity may contribute to the loss of temporal fidelity. In this work, responses of simulated auditory neuron populations with realistic intrinsic diameter and myelination heterogeneity to low-rate pulse trains were produced. By performing a receiver operating characteristic analysis on response latency distributions, ideal-observer interaural timing difference (ITD) detection limits were produced for each population. Fiber heterogeneity produced dispersion of inter-fiber latencies that produced ITD thresholds like that observed in the best performing cochlear implant users. Incorporation of myelin loss into these populations further increased inter-fiber latency variance and elevated ITD detection limits. These findings suggest that the interaction of applied currents with fibers' specific intrinsic properties may introduce fundamental limits on presentation of fine temporal structure in electrical stimulation.

Dendritic Degeneration of Human Auditory Nerve Fibers and Its Impact on the Spiking Pattern Under Regular Conditions and During Cochlear Implant Stimulation

Due to limitations of human in vivo studies, detailed computational models enable understanding the neural signaling in the degenerated auditory system and cochlear implants (CIs). Four human cochleae were used to quantify hearing levels depending on dendritic changes in diameter and myelination thickness from type I of the auditory nerve fibers (ANFs). Type I neurons transmit the auditory information as spiking pattern from the inner hair cells (IHCs) to the cochlear nucleus. The impact of dendrite diameter and degree of myelination on neural signal transmission was simulated for (1) synaptic excitation via IHCs and (2) stimulation from CI electrodes. An accurate three-dimensional human cochlear geometry, along with 30 auditory pathways, mimicked the CI environment. The excitation properties of electrical potential distribution induced by two CI were analyzed. Main findings: (1) The unimodal distribution of control dendrite diameters becomes multimodal for hearing loss cases; a group of thin dendrites with diameters between 0.3 and 1 μm with a peak at 0.5 μm appeared. (2) Postsynaptic currents from IHCs excite such thin dendrites easier and earlier than under control conditions. However, this advantage is lost as their conduction velocity decreases proportionally with the diameter and causes increased spike latency and jitter in soma and axon. Firing probability reduces through the soma passage due to the low intracellular current flow in thin dendrites during spiking. (3) Compared with dendrite diameter, variations in myelin thickness have a small impact on spiking performance. (4) Contrary to synaptic excitation, CIs cause several spike initiation sites in dendrite, soma region, and axon; moreover, fiber excitability reduces with fiber diameter. In a few cases, where weak stimuli elicit spikes of a target neuron (TN) in the axon, dendrite diameter reduction has no effect. However, in many cases, a spike in a TN is first initiated in the dendrite, and consequently, dendrite degeneration demands an increase in threshold currents. (5) Threshold currents of a TN and co-stimulation of degenerated ANFs in other frequency regions depend on the electrode position, including its distance to the outer wall, the cochlear turn, and the three-dimensional pathway of the TN.

Neural Tissue Degeneration in Rosenthal's Canal and Its Impact on Electrical Stimulation of the Auditory Nerve by Cochlear Implants: An Image-Based Modeling Study

Sensorineural deafness is caused by the loss of peripheral neural input to the auditory nerve, which may result from peripheral neural degeneration and/or a loss of inner hair cells. Provided spiral ganglion cells and their central processes are patent, cochlear implants can be used to electrically stimulate the auditory nerve to facilitate hearing in the deaf or severely hard-of-hearing. Neural degeneration is a crucial impediment to the functional success of a cochlear implant. The present, first-of-its-kind two-dimensional finite-element model investigates how the depletion of neural tissues might alter the electrically induced transmembrane potential of spiral ganglion neurons. The study suggests that even as little as 10% of neural tissue degeneration could lead to a disproportionate change in the stimulation profile of the auditory nerve. This result implies that apart from encapsulation layer formation around the cochlear implant electrode, tissue degeneration could also be an essential reason for the apparent inconsistencies in the functionality of cochlear implants.

HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Neuronal diversity in the cochlea is largely determined by ion channels. Among voltage‐gated channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels open with hyperpolarization and depolarize the cell until the resting membrane potential. The functions for hearing are not well elucidated and knowledge about localization is controversial. We created a detailed map of subcellular location and co‐expression of all four HCN subunits across different mammalian species including CBA/J, C57Bl/6N, Ly5.1 mice, guinea pigs, cats, and human subjects. We correlated age‐related hearing deterioration in CBA/J and C57Bl/6N with expression levels of HCN1, −2, and −4 in individual auditory neurons from the same cohort. Spatiotemporal expression during murine postnatal development exposed HCN2 and HCN4 involvement in a critical phase of hair cell innervation. The huge diversity of subunit composition, but lack of relevant heteromeric pairing along the perisomatic membrane and axon initial segments, highlighted an active role for auditory neurons. Neuron clusters were found to be the hot spots of HCN1, −2, and −4 immunostaining. HCN channels were also located in afferent and efferent fibers of the sensory epithelium. Age‐related changes on HCN subtype expression were not uniform among mice and could not be directly correlated with audiometric data. The oldest mice groups revealed HCN channel up‐ or downregulation, depending on the mouse strain. The unexpected involvement of HCN channels in outer hair cell function where HCN3 overlaps prestin location emphasized the importance for auditory function. A better understanding may open up new possibilities to tune neuronal responses evoked through electrical stimulation by cochlear implants.

Electrical stimulation in the cochlea: Influence of modiolar microstructures on the activation of auditory nerve fibres

Existing computational studies of cochlear implants have demonstrated that the structural detail of threedimensional (3D) cochlear models exerts influence on the current spread within the cochlea. Nevertheless, the significance of including the microstructures inside the modiolar bone in a cochlear model is still unclear in the literature. We employed two different multi-compartment neuron models to simulate auditory nerve fibres, and compared response characteristics of the fibre population between a detailed and a simplified 3D cochlear model. Results showed that although the prediction of firing is dependent on the details of the neuron model, the responses of the fibre population to the electrical stimulus, especially the location of the initiation of action potential, varied between the detailed and the simplified models. Therefore, the inclusion of the modiolar microstructures in a cochlear model may be necessary for fully understanding the firing of auditory nerve fibres.

Mitochondrial Dysfunction and Therapeutic Targets in Auditory Neuropathy

Sensorineural hearing loss (SNHL) becomes an inevitable worldwide public health issue, and deafness treatment is urgently imperative; yet their current curative therapy is limited. Auditory neuropathies (AN) were proved to play a substantial role in SNHL recently, and spiral ganglion neuron (SGN) dysfunction is a dominant pathogenesis of AN. Auditory pathway is a high energy consumption system, and SGNs required sufficient mitochondria. Mitochondria are known treatment target of SNHL, but mitochondrion mechanism and pathology in SGNs are not valued. Mitochondrial dysfunction and pharmacological therapy were studied in neurodegeneration, providing new insights in mitochondrion-targeted treatment of AN. In this review, we summarized mitochondrial biological functions related to SGNs and discussed interaction between mitochondrial dysfunction and AN, as well as existing mitochondrion treatment for SNHL. Pharmaceutical exploration to protect mitochondrion dysfunction is a feasible and effective therapeutics for AN.

Gradients in the biophysical properties of neonatal auditory neurons align with synaptic contact position and the intensity coding map of inner hair cells

Sound intensity is encoded by auditory neuron subgroups that differ in thresholds and spontaneous rates. Whether variations in neuronal biophysics contributes to this functional diversity is unknown. Because intensity thresholds correlate with synaptic position on sensory hair cells, we combined patch clamping with fiber labeling in semi-intact cochlear preparations in neonatal rats from both sexes. The biophysical properties of auditory neurons vary in a striking spatial gradient with synaptic position. Neurons with high thresholds to injected currents contact hair cells at synaptic positions where neurons with high thresholds to sound-intensity are found in vivo. Alignment between in vitro and in vivo thresholds suggests that biophysical variability contributes to intensity coding. Biophysical gradients were evident at all ages examined, indicating that cell diversity emerges in early post-natal development and persists even after continued maturation. This stability enabled a remarkably successful model for predicting synaptic position based solely on biophysical properties.

Finite element analysis and three-dimensional reconstruction of tonotopically aligned human auditory fiber pathways: A computational environment for modeling electrical stimulation by a cochlear implant based on micro-CT

The application of cochlear implants can be studied with computational models. The electrical potential distribution induced by an implanted device is evaluated with a volume conductor model, which is used as input for neuron models to simulate the reaction of cochlear neurons to micro-stimulation. In order to reliably predict the complex excitation profiles it is vital to consider an accurate representation of the human cochlea geometry including detailed three-dimensional pathways of auditory neurons reaching from the organ of Corti through the cochlea-volume. In this study, high-resolution micro-CT imaging (Δx = Δy = Δz = 3 μm) was used to reconstruct the pathways of 30 tonotopically organized nerve fiber bundles, distributed over eight octaves (11500-40 Hz). Results of the computational framework predict: (i) the peripheral process is most sensitive to cathodic stimulation (CAT), (ii) in many cases CAT elicits spikes in the peripheral terminal at threshold but with larger stimuli there is a second spike initiation site within the peripheral process, (iii) anodic stimuli (ANO) can excite the central process even at threshold, (iv) the recruitment of fibers by electrodes located in the narrowing middle- and apical turn is complex and impedes focal excitation of low frequency fibers, (v) degenerated cells which lost the peripheral process are more sensitive to CAT when their somata are totally covered with 2 membranes of a glial cell but they become ANO sensitive when the myelin covering is reduced.

Electrical Stimulation in the Human Cochlea: A Computational Study Based on High-Resolution Micro-CT Scans

Background: Many detailed features of the cochlear anatomy have not been included in existing 3D cochlear models, including the microstructures inside the modiolar bone, which in turn determines the path of auditory nerve fibers (ANFs).

Method: We captured the intricate modiolar microstructures in a 3D human cochlea model reconstructed from μCT scans. A new algorithm was developed to reconstruct ANFs running through the microstructures within the model. Using the finite element method, we calculated the electrical potential as well as its first and second spatial derivatives along each ANF elicited by the cochlear implant electrodes. Simulation results of electrical potential was validated against intracochlear potential measurements. Comparison was then made with a simplified model without the microstructures within the cochlea.

Results: When the stimulus was delivered from an electrode located deeper in the apex, the extent of the auditory nerve influenced by a higher electric potential grew larger; at the same time, the maximal potential value at the auditory nerve also became larger. The electric potential decayed at a faster rate toward the base of the cochlea than toward the apex. Compared to the cochlear model incorporating the modiolar microstructures, the simplified version resulted in relatively small differences in electric potential. However, in terms of the first and second derivatives of electric potential along the fibers, which are relevant for the initiation of action potentials, the two models exhibited large differences: maxima in both derivatives with the detailed model were larger by a factor of 1.5 (first derivative) and 2 (second derivative) in the exemplary fibers. More importantly, these maxima occurred at different locations, and opposite signs were found for the values of second derivatives between the two models at parts along the fibers. Hence, while one model predicts depolarization and spike initiation at a given location, the other may instead predict a hyperpolarization.

Conclusions: Although a cochlear model with fewer details seems sufficient for analysing the current spread in the cochlear ducts, a detailed-segmented cochlear model is required for the reconstruction of ANF trajectories through the modiolus, as well as the prediction of firing thresholds and spike initiation sites.

Comparison of Multi-Compartment Cable Models of Human Auditory Nerve Fibers

Background: Multi-compartment cable models of auditory nerve fibers have been developed to assist in the improvement of cochlear implants. With the advancement of computational technology and the results obtained from in vivo and in vitro experiments, these models have evolved to incorporate a considerable degree of morphological and physiological details. They have also been combined with three-dimensional volume conduction models of the cochlea to simulate neural responses to electrical stimulation. However, no specific rules have been provided on choosing the appropriate cable model, and most models adopted in recent studies were chosen without a specific reason or by inheritance. Methods: Three of the most cited biophysical multi-compartment cable models of the human auditory nerve, i.e., Rattay et al. (2001b), Briaire and Frijns (2005), and Smit et al. (2010), were implemented in this study. Several properties of single fibers were compared among the three models, including threshold, conduction velocity, action potential shape, latency, refractory properties, as well as stochastic and temporal behaviors. Experimental results regarding these properties were also included as a reference for comparison. Results: For monophasic single-pulse stimulation, the ratio of anodic vs. cathodic thresholds in all models was within the experimental range despite a much larger ratio in the model by Briaire and Frijns. For biphasic pulse-train stimulation, thresholds as a function of both pulse rate and pulse duration differed between the models, but none matched the experimental observations even coarsely. Similarly, for all other properties including the conduction velocity, action potential shape, and latency, the models presented different outcomes and not all of them fell within the range observed in experiments. Conclusions: While all three models presented similar values in certain single fiber properties to those obtained in experiments, none matched all experimental observations satisfactorily. In particular, the adaptation and temporal integration behaviors were completely missing in all models. Further extensions and analyses are required to explain and simulate realistic auditory nerve fiber responses to electrical stimulation.

The use of nonhuman primates in studies of noise injury and treatment

Exposure to prolonged or high intensity noise increases the risk for permanent hearing impairment. Over several decades, researchers characterized the nature of harmful noise exposures and worked to establish guidelines for effective protection. Recent laboratory studies, primarily conducted in rodent models, indicate that the auditory system may be more vulnerable to noise-induced hearing loss (NIHL) than previously thought, driving renewed inquiries into the harmful effects of noise in humans. To bridge the translational gaps between rodents and humans, nonhuman primates (NHPs) may serve as key animal models. The phylogenetic proximity of NHPs to humans underlies tremendous similarity in many features of the auditory system (genomic, anatomical, physiological, behavioral), all of which are important considerations in the assessment and treatment of NIHL. This review summarizes the literature pertaining to NHPs as models of hearing and noise-induced hearing loss, discusses factors relevant to the translation of diagnostics and therapeutics from animals to humans, and concludes with some of the practical considerations involved in conducting NHP research.

Reduced order models of myelinated axonal compartments

The paper presents a hierarchical series of computational models for myelinated axonal compartments. Three classes of models are considered, either with distributed parameters (2.5D EQS-ElectroQuasi Static, 1D TL-Transmission Lines) or with lumped parameters (0D). They are systematically analyzed with both analytical and numerical approaches, the main goal being to identify the best procedure for order reduction of each case. An appropriate error estimator is proposed in order to assess the accuracy of the models. This is the foundation of a procedure able to find the simplest reduced model having an imposed precision. The most computationally efficient model from the three geometries proved to be the analytical 1D one, which is able to have accuracy less than 0.1%. By order reduction with vector fitting, a finite model is generated with a relative difference of 10^- 4 for order 5. The dynamical models thus extracted allow an efficient simulation of neurons and, consequently, of neuronal circuits. In such situations, the linear models of the myelinated compartments coupled with the dynamical, non-linear models of the Ranvier nodes, neuronal body (soma) and dendritic tree give global reduced models. In order to ease the simulation of large-scale neuronal systems, the sub-models at each level, including those of myelinated compartments should have the lowest possible order. The presented procedure is a first step in achieving simulations of neural systems with accuracy control.

Subcellular Abnormalities of Vestibular Nerve Morphology in Patients With Intractable Meniere's Disease

Objective: Few studies so far have focused on the retrocochlear lesions in Meniere's disease (MD). This study aims to investigate pathological alterations in the central portion of the vestibular nerve (VN) in patients with intractable Meniere's disease (MD) and to explore retrocochlear lesions and their relationship with disease severity. Methods: Eight MD patients with refractory vertigo received vestibular neurectomy via a retrosigmoid or translabyrinthine approach. Segments of VN were carefully removed and immediately fixed for histopathological examination. Five VN specimens were examined by light microscopy after hematoxylin/eosin staining; three specimens were extensively analyzed using transmission electron microscopy, to identify VN ultrastructural lesions. Correlations between lesions and patient clinical characteristics were examined. Results: Histopathological examination revealed evidence of various types of chronic VN impairment, including the formation of corpora amylacea (CA), axon atrophy, and severe damage to the myelin sheath. Electron microscopy revealed membranous whorls within dilated Schmidt-Lanterman incisures, the formation of myeloid bodies, dysmyelination, and demyelination. Unexpectedly, we observed a positive correlation between the density of CA in VN tissue and the duration of disease, as well as the degree of hearing impairment, independent of age. Conclusion: Our findings indicate that deformation of subcellular organelles in the central portion of the VN is one of the key pathological indicators for the progressive severity and intractability of vertigo and support a vestibular nerve degeneration.

The Summating Potential Is a Reliable Marker of Electrode Position in Electrocochleography: Cochlear Implant as a Theragnostic Probe

OBJECTIVE

For the increasing number of cochlear implantations in subjects with residual hearing, hearing preservation, and thus the prevention of implantation trauma, is crucial. A method for monitoring the intracochlear position of a cochlear implant (CI) and early indication of imminent cochlear trauma would help to assist the surgeon to achieve this goal. The aim of this study was to evaluate the reliability of the different electric components recorded by an intracochlear electrocochleography (ECochG) as markers for the cochleotopic position of a CI. The measurements were made directly from the CI, combining intrasurgical diagnostics with the therapeutical use of the CI, thus, turning the CI into a "theragnostic probe."

DESIGN

Intracochlear ECochGs were measured in 10 Dunkin Hartley guinea pigs of either sex, with normal auditory brainstem response thresholds. All subjects were fully implanted (4 to 5 mm) with a custom six contact CI. The ECochG was recorded simultaneously from all six contacts with monopolar configuration (retroauricular reference electrode). The gross ECochG signal was filtered off-line to separate three of its main components: compound action potential, cochlear microphonic, and summating potential (SP). Additionally, five cochleae were harvested and histologically processed to access the spatial position of the CI contacts. Both ECochG data and histological reconstructions of the electrode position were fitted with the Greenwood function to verify the reliability of the deduced cochleotopic position of the CI.

RESULTS

SPs could be used as suitable markers for the frequency position of the recording electrode with an accuracy of ±1/4 octave in the functioning cochlea, verified by histology. Cochlear microphonics showed a dependency on electrode position but were less reliable as positional markers. Compound action potentials were not suitable for CI position information but were sensitive to "cochlear health" (e.g., insertion trauma).

CONCLUSIONS

SPs directly recorded from the contacts of a CI during surgery can be used to access the intracochlear frequency position of the CI. Using SP monitoring, implantation may be stopped before penetrating functioning cochlear regions. If the technique was similarly effective in humans, it could prevent implantation trauma and increase hearing preservation during CI surgery. Diagnostic hardware and software for recording biological signals with a CI without filter limitations might be a valuable add-on to the portfolios of CI manufacturers.

Cochlea and auditory nerve

The transduction process in the cochlea requires patent hair cells. Population responses that reflect this patency are the cochlear microphonic (CM) and summating potential (SP). They can be measured using electrocochleography (ECochG). The CM reflects the sound waveform in the form of outer hair cell (OHC) depolarization and hyperpolarization, and the SP reflects the average voltage difference of the OHC membrane potential for depolarization and hyperpolarization. The CM can be measured using ECochG or via the so-called otoacoustic emissions, using a sensitive microphone in the ear canal. Neural population responses are called the compound action potentials (CAPs), which by frequency selective masking can be decomposed into narrow-band action potentials (NAPs) reflecting CAPs evoked by activity from small cochlear regions. Presence of CM and absence of CAPs are the diagnostic hallmarks of auditory neuropathy. Increased and prolonged SPs are often found in Ménière's disease but are too often in the normal range to be diagnostic. When including NAP waveforms, Ménière's disease can be differentiated from vestibular schwannomas, which often feature overlapping symptoms such as dizziness, hearing loss, and tinnitus. The patency of the efferent system, particularly the olivocochlear bundle, can be tested using the suppressive effect of contralateral stimulation on the otoacoustic emission amplitude.

Effects of nerve bundle geometry on neurotrauma evaluation

OBJECTIVE

We confirm that alteration of a neuron structure can induce abnormalities in signal propagation for nervous systems, as observed in brain damage. Here, we investigate the effects of geometrical changes and damage of a neuron structure in 2 scaled nerve bundle models, made of myelinated nerve fibers or unmyelinated nerve fibers.

METHODS

We propose a 3D finite element model of nerve bundles, combining a real-time full electromechanical coupling, a modulated threshold for spiking activation, and independent alteration of the electrical properties for each fiber. With the inclusion of plasticity, we then simulate mechanical compression and tension to induce damage at the membrane of a nerve bundle made of 4 fibers. We examine the resulting changes in strain and neural activity by considering in turn the cases of intact and traumatized nerve membranes.

RESULTS

Our results show lower strain and lower electrophysiological impairments in unmyelinated fibers than in myelinated fibers, higher deformation levels in larger bundles, and higher electrophysiological impairments in smaller bundles.

CONCLUSION

We conclude that the insulation sheath of myelin constricts the membrane deformation and scatters plastic strains within the bundle, that larger bundles deform more than small bundles, and that small fibers tolerate a higher level of elongation before mechanical failure.

Anatomical basis of drug delivery to the inner ear

The isolated anatomical position and blood-labyrinth barrier hampers systemic drug delivery to the mammalian inner ear. Intratympanic placement of drugs and permeation via the round- and oval window are established methods for local pharmaceutical treatment. Mechanisms of drug uptake and pathways for distribution within the inner ear are hard to predict. The complex microanatomy with fluid-filled spaces separated by tight- and leaky barriers compose various compartments that connect via active and passive transport mechanisms. Here we provide a review on the inner ear architecture at light- and electron microscopy level, relevant for drug delivery. Focus is laid on the human inner ear architecture. Some new data add information on the human inner ear fluid spaces generated with high resolution microcomputed tomography at 15 μm resolution. Perilymphatic spaces are connected with the central modiolus by active transport mechanisms of mesothelial cells that provide access to spiral ganglion neurons. Reports on leaky barriers between scala tympani and the so-called cortilymph compartment likely open the best path for hair cell targeting. The complex barrier system of tight junction proteins such as occludins, claudins and tricellulin isolates the endolymphatic space for most drugs. Comparison of relevant differences of barriers, target cells and cell types involved in drug spread between main animal models and humans shall provide some translational aspects for inner ear drug applications.

Computational Evaluation of Cochlear Implant Surgery Outcomes Accounting for Uncertainty and Parameter Variability

Cochlear implantation (CI) is a complex surgical procedure that restores hearing in patients with severe deafness. The successful outcome of the implanted device relies on a group of factors, some of them unpredictable or difficult to control. Uncertainties on the electrode array position and the electrical properties of the bone make it difficult to accurately compute the current propagation delivered by the implant and the resulting neural activation. In this context, we use uncertainty quantification methods to explore how these uncertainties propagate through all the stages of CI computational simulations. To this end, we employ an automatic framework, encompassing from the finite element generation of CI models to the assessment of the neural response induced by the implant stimulation. To estimate the confidence intervals of the simulated neural response, we propose two approaches. First, we encode the variability of the cochlear morphology among the population through a statistical shape model. This allows us to generate a population of virtual patients using Monte Carlo sampling and to assign to each of them a set of parameter values according to a statistical distribution. The framework is implemented and parallelized in a High Throughput Computing environment that enables to maximize the available computing resources. Secondly, we perform a patient-specific study to evaluate the computed neural response to seek the optimal post-implantation stimulus levels. Considering a single cochlear morphology, the uncertainty in tissue electrical resistivity and surgical insertion parameters is propagated using the Probabilistic Collocation method, which reduces the number of samples to evaluate. Results show that bone resistivity has the highest influence on CI outcomes. In conjunction with the variability of the cochlear length, worst outcomes are obtained for small cochleae with high resistivity values. However, the effect of the surgical insertion length on the CI outcomes could not be clearly observed, since its impact may be concealed by the other considered parameters. Whereas the Monte Carlo approach implies a high computational cost, Probabilistic Collocation presents a suitable trade-off between precision and computational time. Results suggest that the proposed framework has a great potential to help in both surgical planning decisions and in the audiological setting process.

Challenging aspects of contemporary cochlear implant electrode array design

Objective

A design comparison of current perimodiolar and lateral wall electrode arrays of the cochlear implant (CI) is provided. The focus is on functional features such as acoustic frequency coverage and tonotopic mapping, battery consumption and dynamic range. A traumacity of their insertion is also evaluated.

Methods

Review of up-to-date literature.

Results

Perimodiolar electrode arrays are positioned in the basal turn of the cochlea near the modiolus. They are designed to initiate the action potential in the proximity to the neural soma located in spiral ganglion. On the other hand, lateral wall electrode arrays can be inserted deeper inside the cochlea, as they are located along the lateral wall and such insertion trajectory is less traumatic. This class of arrays targets primarily surviving neural peripheral processes. Due to their larger insertion depth, lateral wall arrays can deliver lower acoustic frequencies in manner better corresponding to cochlear tonotopicity. In fact, spiral ganglion sections containing auditory nerve fibres tuned to low acoustic frequencies are located deeper than 1 and half turn inside the cochlea. For this reason, a significant frequency mismatch might be occurring for apical electrodes in perimodiolar arrays, detrimental to speech perception. Tonal languages such as Mandarin might be therefore better treated with lateral wall arrays. On the other hand, closer proximity to target tissue results in lower psychophysical threshold levels for perimodiolar arrays. However, the maximal comfort level is also lower, paradoxically resulting in narrower dynamic range than that of lateral wall arrays. Battery consumption is comparable for both types of arrays.

Conclusions

Lateral wall arrays are less likely to cause trauma to cochlear structures. As the current trend in cochlear implantation is the maximal protection of residual acoustic hearing, the lateral wall arrays seem more suitable for hearing preservation CI surgeries. Future development could focus on combining the advantages of both types: perimodiolar location in the basal turn extended to lateral wall location for higher turn locations.

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.

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

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

Early impairment of intracranial conduction time predicts mortality in deeply sedated critically ill patients: a prospective observational pilot study

Background

Somatosensory (SSEP) and brainstem auditory (BAEP) evoked potentials are neurophysiological tools which, respectively, explore the intracranial conduction time (ICCT) and the intrapontine conduction time (IPCT). The prognostic values of prolonged cerebral conduction times in deeply sedated patients have never been assessed. Sedated patients are at risk of developing new neurological complications, undetected. In this prospective observational bi-center pilot study, we investigated whether early impairment of SSEP’s ICCT and/or BAEP’s IPCT could predict in-ICU mortality or altered mental status (AMS), in deeply sedated critically ill patients.

Methods

SSEP by stimulation of the median nerve and BAEP were assessed in critically ill patients receiving deep sedation on day 3 following ICU admission. Deep sedation was defined by a Richmond Assessment sedation Scale (RASS) <−3. Mean left- and right-side ICCT and IPCT were measured for each patient. Primary and secondary outcomes were, respectively, in-ICU mortality and AMS defined as the occurrence of delirium and/or delayed awakening after discontinuation of sedation.

Results

Eighty-six patients were studied of which 49 (57%) were non-brain-injured and 37 (43%) were brain-injured. Impaired ICCT was a predictor of in-ICU mortality after adjustment on the global Sequential Organ Failure Assessment score (SOFA) [OR (95% CI) = 2.69 (1.05–6.85); p = 0.039] and on the non-neurological SOFA components [2.67 (1.05–6.81); p = 0.040]. IPCT was more frequently delayed in the subgroup of patients who developed post-sedation AMS (24%) compared those without AMS (0%). However, this difference did not reach statistical significance (p = 0.053). Impairment rates of ICCT and IPCT were not found to be significantly different between non-brain- and brain-injured subgroups of patients.

Conclusion

In critically ill patients receiving deep sedation, early ICCT impairment was associated with mortality. Somatosensory and brainstem auditory evoked potentials may be useful early warning indicators of brain dysfunction as well as prognostic markers in deeply sedated critically ill patients.

Neurophysiological assessment of brain dysfunction in critically ill patients: an update

The aim of this review was to provide up-to-date information about the usefulness of clinical neurophysiology testing in the management of critically ill patients. Evoked potentials (EPs) and electroencephalogram (EEG) are non-invasive clinical neurophysiology tools that allow an objective assessment of the central nervous system's function at the bedside in intensive care unit (ICU). These tests are quite useful in diagnosing cerebral complications, and establishing the vital and functional prognosis in ICU. EEG keeps a particularly privileged importance in detecting seizures phenomena such as subclinical seizures and non-convulsive status epilepticus. Quantitative EEG (QEEG) analysis techniques commonly called EEG Brain mapping can provide obvious topographic displays of digital EEG signals characteristics, showing the potential distribution over the entire scalp including filtering, frequency, and amplitude analysis and color mapping. Evidences of usefulness of QEEG for seizures detection in ICU are provided by several recent studies. Furthermore, beyond detection of epileptic phenomena, changes of some QEEG panels are early warning indicators of sedation level as well as brain damage or dysfunction in ICU. EPs offer the opportunity for assessing brainstem's functional integrity, as well as subcortical and cortical brain areas. A multimodal use, combining EEG and various modalities of EPs is recommended since this allows a more accurate functional exploration of the brain and helps caregivers to tailor therapeutic measures according to neurological worsening trends and to anticipate the prognosis in ICU.

A Model of Electrically Stimulated Auditory Nerve Fiber Responses with Peripheral and Central Sites of Spike Generation

A computational model of cat auditory nerve fiber (ANF) responses to electrical stimulation is presented. The model assumes that (1) there exist at least two sites of spike generation along the ANF and (2) both an anodic (positive) and a cathodic (negative) charge in isolation can evoke a spike. A single ANF is modeled as a network of two exponential integrate-and-fire point-neuron models, referred to as peripheral and central axons of the ANF. The peripheral axon is excited by the cathodic charge, inhibited by the anodic charge, and exhibits longer spike latencies than the central axon; the central axon is excited by the anodic charge, inhibited by the cathodic charge, and exhibits shorter spike latencies than the peripheral axon. The model also includes subthreshold and suprathreshold adaptive feedback loops which continuously modify the membrane potential and can account for effects of facilitation, accommodation, refractoriness, and spike-rate adaptation in ANF. Although the model is parameterized using data for either single or paired pulse stimulation with monophasic rectangular pulses, it correctly predicts effects of various stimulus pulse shapes, stimulation pulse rates, and level on the neural response statistics. The model may serve as a framework to explore the effects of different stimulus parameters on psychophysical performance measured in cochlear implant listeners.

Development of a voltage-dependent current noise algorithm for conductance-based stochastic modelling of auditory nerve fibres

This study presents the development of an alternative noise current term and novel voltage-dependent current noise algorithm for conductance-based stochastic auditory nerve fibre (ANF) models. ANFs are known to have significant variance in threshold stimulus which affects temporal characteristics such as latency. This variance is primarily caused by the stochastic behaviour or microscopic fluctuations of the node of Ranvier's voltage-dependent sodium channels of which the intensity is a function of membrane voltage. Though easy to implement and low in computational cost, existing current noise models have two deficiencies: it is independent of membrane voltage, and it is unable to inherently determine the noise intensity required to produce in vivo measured discharge probability functions. The proposed algorithm overcomes these deficiencies while maintaining its low computational cost and ease of implementation compared to other conductance and Markovian-based stochastic models. The algorithm is applied to a Hodgkin-Huxley-based compartmental cat ANF model and validated via comparison of the threshold probability and latency distributions to measured cat ANF data. Simulation results show the algorithm's adherence to in vivo stochastic fibre characteristics such as an exponential relationship between the membrane noise and transmembrane voltage, a negative linear relationship between the log of the relative spread of the discharge probability and the log of the fibre diameter and a decrease in latency with an increase in stimulus intensity.

Temporal Considerations for Stimulating Spiral Ganglion Neurons with Cochlear Implants

A wealth of knowledge about different types of neural responses to electrical stimulation has been developed over the past 100 years. However, the exact forms of neural response properties can vary across different types of neurons. In this review, we survey four stimulus-response phenomena that in recent years are thought to be relevant for cochlear implant stimulation of spiral ganglion neurons (SGNs): refractoriness, facilitation, accommodation, and spike rate adaptation. Of these four, refractoriness is the most widely known, and many perceptual and physiological studies interpret their data in terms of refractoriness without incorporating facilitation, accommodation, or spike rate adaptation. In reality, several or all of these behaviors are likely involved in shaping neural responses, particularly at higher stimulation rates. A better understanding of the individual and combined effects of these phenomena could assist in developing improved cochlear implant stimulation strategies. We review the published physiological data for electrical stimulation of SGNs that explores these four different phenomena, as well as some of the recent studies that might reveal the biophysical bases of these stimulus-response phenomena.

A phenomenological model of myelinated nerve with a dynamic threshold

To evaluate coding strategies for cochlear implants a model of the human cochlear nerve is required. Nerve models based on voltage-clamp experiments, such as the Frankenhaeuser-Huxley model of myelinated nerve, can have over forty parameters and are not amenable for fitting to physiological data from a different animal or type of nerve. Phenomenological nerve models, such as leaky integrate-and-fire (LIF) models, have fewer parameters but have not been validated with a wide range of stimuli. In the absence of substantial cochlear nerve data, we have used data from a toad sciatic nerve for validation (50 Hz to 2 kHz with levels up to 20 dB above threshold). We show that the standard LIF model with fixed refractory properties and a single set of parameters cannot adequately predict the toad rate-level functions. Given the deficiency of this standard model, we have abstracted the dynamics of the sodium inactivation variable in the Frankenhaeuser-Huxley model to develop a phenomenological LIF model with a dynamic threshold. This nine-parameter model predicts the physiological rate-level functions much more accurately than the standard LIF model. Because of the low number of parameters, we expect to be able to optimize the model parameters so that the model is more appropriate for cochlear implant simulations.

Immediate and delayed cochlear neuropathy after noise exposure in pubescent mice

Moderate acoustic overexposure in adult rodents is known to cause acute loss of synapses on sensory inner hair cells (IHCs) and delayed degeneration of the auditory nerve, despite the completely reversible temporary threshold shift (TTS) and morphologically intact hair cells. Our objective was to determine whether a cochlear synaptopathy followed by neuropathy occurs after noise exposure in pubescence, and to define neuropathic versus non-neuropathic noise levels for pubescent mice. While exposing 6 week old CBA/CaJ mice to 8-16 kHz bandpass noise for 2 hrs, we defined 97 dB sound pressure level (SPL) as the threshold for this particular type of neuropathic exposure associated with TTS, and 94 dB SPL as the highest non-neuropathic noise level associated with TTS. Exposure to 100 dB SPL caused permanent threshold shift although exposure of 16 week old mice to the same noise is reported to cause only TTS. Amplitude of wave I of the auditory brainstem response, which reflects the summed activity of the cochlear nerve, was complemented by synaptic ribbon counts in IHCs using confocal microscopy, and by stereological counts of peripheral axons and cell bodies of the cochlear nerve from 24 hours to 16 months post exposure. Mice exposed to neuropathic noise demonstrated immediate cochlear synaptopathy by 24 hours post exposure, and delayed neurodegeneration characterized by axonal retraction at 8 months, and spiral ganglion cell loss at 8-16 months post exposure. Although the damage was initially limited to the cochlear base, it progressed to also involve the cochlear apex by 8 months post exposure. Our data demonstrate a fine line between neuropathic and non-neuropathic noise levels associated with TTS in the pubescent cochlea.

Basic response properties of auditory nerve fibers: a review

All acoustic information from the periphery is encoded in the timing and rates of spikes in the population of spiral ganglion neurons projecting to the central auditory system. Considerable progress has been made in characterizing the physiological properties of type-I and type-II primary auditory afferents and understanding the basic properties of type-I afferents in response to sounds. Here, we review some of these properties, with emphasis placed on issues such as the stochastic nature of spike timing during spontaneous and driven activity, frequency tuning curves, spike-rate-versus-level functions, dynamic-range and spike-rate adaptation, and phase locking to stimulus fine structure and temporal envelope. We also review effects of acoustic trauma on some of these response properties.