Relationship between tone burst discharge pattern and spontaneous firing rate of auditory nerve fibres in the guinea pig

A level adjusted cochlear frequency-to-place map for estimating tonotopic frequency mismatch with a cochlear implant

Objectives

To provide a level-adjusted correction to the current standard relating anatomical cochlear place to characteristic frequency in humans, and to re-evaluate anatomical frequency mismatch in cochlear implant (CI) recipients considering this correction. It is hypothesized that a level-adjusted place-frequency function may represent a more accurate tonotopic benchmark for CIs in comparison to the current standard.

Design

The present analytical study compiled data from fifteen previous animal studies that reported iso-intensity responses from cochlear structures at different stimulation levels. Extracted outcome measures were characteristic frequencies and centroid-based best frequencies at 70 dB SPL input from 47 specimens spanning a broad range of cochlear locations. A simple relationship was used to transform these measures to human estimates of characteristic and best frequencies, and non-linear regression was applied to these estimates to determine how the standard human place-frequency function should be adjusted to reflect best frequency rather than characteristic frequency. The proposed level-adjusted correction was then compared to average place-frequency positions of commonly used CI devices when programmed with clinical settings.

Results

The present study showed that the best frequency at 70 dB SPL (BF70) tends to shift away from characteristic frequency (CF). The amount of shift was statistically significant (signed-rank test z = 5.143, p < 0.001), but the amount and direction of shift depended on cochlear location. At cochlear locations up to 600° from the base, BF70 shifted downwards in frequency relative to CF by about 4 semitones on average. Beyond 600° from the base, BF70 shifted upwards in frequency relative to CF by about 6 semitones on average. In terms of spread (90% prediction interval), the amount of shift between CF and BF70 varied from relatively no shift to nearly an octave of shift. With the new level-adjusted frequency-place function, the amount of anatomical frequency mismatch for devices programmed with standard of care settings is less extreme than originally thought, and may be nonexistent for all but the most apical electrodes.

Conclusions

The present study validates the current standard for relating cochlear place to characteristic frequency, and introduces a level-adjusted correction for how best frequency shifts away from characteristic frequency at moderately loud stimulation levels. This correction may represent a more accurate tonotopic reference for CIs. To the extent that it does, its implementation may potentially enhance perceptual accommodation and speech understanding in CI users, thereby improving CI outcomes and contributing to advancements in the programming and clinical management of CIs.

Evidence of cochlear neural degeneration in normal-hearing subjects with tinnitus

Tinnitus, reduced sound-level tolerance, and difficulties hearing in noisy environments are the most common complaints associated with sensorineural hearing loss in adult populations. This study aims to clarify if cochlear neural degeneration estimated in a large pool of participants with normal audiograms is associated with self-report of tinnitus using a test battery probing the different stages of the auditory processing from hair cell responses to the auditory reflexes of the brainstem. Self-report of chronic tinnitus was significantly associated with (1) reduced cochlear nerve responses, (2) weaker middle-ear muscle reflexes, (3) stronger medial olivocochlear efferent reflexes and (4) hyperactivity in the central auditory pathways. These results support the model of tinnitus generation whereby decreased neural activity from a damaged cochlea can elicit hyperactivity from decreased inhibition in the central nervous system.

A Fundamental Inequality Governing the Rate Coding Response of Sensory Neurons

A fundamental inequality governing the spike activity of peripheral neurons is derived and tested against auditory data. This inequality states that the steady-state firing rate must lie between the arithmetic and geometric means of the spontaneous and peak activities during adaptation. Implications towards the development of auditory mechanistic models are explored.

Modeling temporal information encoding by the population of fibers in the healthy and synaptopathic auditory nerve

We report a theoretical study aimed at investigating the impact of cochlear synapse loss (synaptopathy) on the encoding of the envelope (ENV) and temporal fine structure (TFS) of sounds by the population of auditory nerve fibers. A computational model was used to simulate auditory-nerve spike trains evoked by sinusoidally amplitude-modulated (AM) tones at 10 Hz with various carrier frequencies and levels. The model included 16 cochlear channels with characteristic frequencies (CFs) from 250 Hz to 8 kHz. Each channel was innervated by 3, 4 and 10 fibers with low (LSR), medium (MSR), and high spontaneous rates (HSR), respectively. For each channel, spike trains were collapsed into three separate 'population' post-stimulus time histograms (PSTHs), one per fiber type. Information theory was applied to reconstruct the stimulus waveform, ENV, and TFS from one or more PSTHs in a mathematically optimal way. The quality of the reconstruction was regarded as an estimate of the information present in the used PSTHs. Various synaptopathy scenarios were simulated by removing fibers of specific types and/or cochlear regions before stimulus reconstruction. We found that the TFS was predominantly encoded by HSR fibers at all stimulus carrier frequencies and levels. The encoding of the ENV was more complex. At lower levels, the ENV was predominantly encoded by HSR fibers with CFs near the stimulus carrier frequency. At higher levels, the ENV was equally well or better encoded by HSR fibers with CFs different from the AM carrier frequency as by LSR fibers with CFs at the carrier frequency. Altogether, findings suggest that a healthy population of HSR fibers (i.e., including fibers with CFs around and remote from the AM carrier frequency) might be sufficient to encode the ENV and TFS over a wide range of stimulus levels. Findings are discussed regarding their relevance for diagnosing synaptopathy using non-invasive ENV- and TFS-based measures.

Effects of Calcitonin-Gene-Related-Peptide on Auditory Nerve Activity

Calcitonin-gene-related peptide (CGRP) is a lateral olivocochlear (LOC) efferent neurotransmitter. Depression of sound-driven auditory brainstem response amplitude in CGRP-null mice suggests the potential for endogenous CGRP release to upregulate spontaneous and/or sound-driven auditory nerve (AN) activity. We chronically infused CGRP into the guinea pig cochlea and evaluated changes in AN activity as well as outer hair cell (OHC) function. The amplitude of both round window noise (a measure of ensemble spontaneous activity) and the synchronous whole-nerve response to sound (compound action potential, CAP) were enhanced. Lack of change in both onset adaptation and steady state amplitude of sound-evoked distortion product otoacoustic emission (DPOAE) responses indicated CGRP had no effect on OHCs, suggesting the origin of the observed changes was neural. Combined with results from the CGRP-null mice, these results appear to confirm that endogenous CGRP enhances auditory nerve activity when released by the LOC neurons. However, infusion of the CGRP receptor antagonist CGRP (8–37) did not reliably influence spontaneous or sound-driven AN activity, or OHC function, results that contrast with the decreased ABR amplitude measured in CGRP-null mice.

A simplified physiological model of rate-level functions of auditory-nerve fibers

Several approaches have been used to describe the rate-level functions of auditory-nerve fibers (ANFs). One approach uses descriptive models that can be fitted easily to data. Another derives rate-level functions from comprehensive physiological models of auditory peripheral processing. Here, we seek to identify the minimal set of components needed to provide a physiologically plausible account of rate-level functions. Our model consists of a first-order Boltzmann mechanoelectrical transducer function relating the instantaneous stimulus pressure to an instantaneous output, followed by a lowpass filter that eliminates the AC component, followed by an exponential synaptic transfer function relating the DC component to the mean spike rate. This is perhaps the simplest physiologically plausible model capable of accounting for rate-level functions under the assumption that the model parameters for a given ANF and stimulus frequency are level-independent. We find that the model typically accounts well for rate-level functions from cat ANFs for all stimulus frequencies. More complicated model variants having saturating synaptic transfer functions do not perform significantly better, implying the system operates far away from synaptic saturation. Rate saturation in the model is caused by saturation of the DC component of the filter output (e.g., the receptor potential), which in turn is due to the saturation of the transducer function. The maximum mean spike rate is approximately constant across ANFs, such that the slope parameter of the exponential synaptic transfer function decreases with increasing spontaneous rate. If the synaptic parameters for a given ANF are assumed to be constant across stimulus frequencies, then frequency- and level-dependent input nonlinearities are derived that are qualitatively similar to those reported in the literature. Contrary to assumptions in the literature, such nonlinearities are obtained even for ANFs having high spontaneous rates. Finally, spike-rate adaptation is examined and found to be accounted for by a decrease in the slope parameter of the synaptic transfer function over time following stimulus onset.

The glucocorticoid antagonist mifepristone attenuates sound-induced long-term deficits in auditory nerve response and central auditory processing in female rats

Systemic corticosteroids have been the mainstay of treatment for various hearing disorders for more than 30 yr. Accordingly, numerous studies have described glucocorticoids (GCs) and stressors to be protective in the auditory organ against damage associated with a variety of health conditions, including noise exposure. Conversely, stressors are also predictive risk factors for hearing disorders. How both of these contrasting stress actions are linked has remained elusive. Here, we demonstrate that higher corticosterone levels during acoustic trauma in female rats is highly correlated with a decline of auditory fiber responses in high-frequency cochlear regions, and that hearing thresholds and the outer hair cell functions (distortion products of otoacoustic emissions) are left unaffected. Moreover, when GC receptor (GR) or mineralocorticoid receptor (MR) activation was antagonized by mifepristone or spironolactone, respectively, GR, but not MR, inhibition significantly and permanently attenuated trauma-induced effects on auditory fiber responses, including inner hair cell ribbon loss and related reductions of early and late auditory brainstem responses. These findings strongly imply that higher corticosterone stress levels profoundly impair auditory nerve processing, which may influence central auditory acuity. These changes are likely GR mediated as they are prevented by mifepristone.-Singer, W., Kasini, K., Manthey, M., Eckert, P., Armbruster, P., Vogt, M. A., Jaumann, M., Dotta, M., Yamahara, K., Harasztosi, C., Zimmermann, U., Knipper, M., Rüttiger, L. The glucocorticoid antagonist mifepristone attenuates sound-induced long-term deficits in auditory nerve response and central auditory processing in female rats.

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.

The function of BDNF in the adult auditory system

The inner ear of vertebrates is specialized to perceive sound, gravity and movements. Each of the specialized sensory organs within the cochlea (sound) and vestibular system (gravity, head movements) transmits information to specific areas of the brain. During development, brain-derived neurotrophic factor (BDNF) orchestrates the survival and outgrowth of afferent fibers connecting the vestibular organ and those regions in the cochlea that map information for low frequency sound to central auditory nuclei and higher-auditory centers. The role of BDNF in the mature inner ear is less understood. This is mainly due to the fact that constitutive BDNF mutant mice are postnatally lethal. Only in the last few years has the improved technology of performing conditional cell specific deletion of BDNF in vivo allowed the study of the function of BDNF in the mature developed organ. This review provides an overview of the current knowledge of the expression pattern and function of BDNF in the peripheral and central auditory system from just prior to the first auditory experience onwards. A special focus will be put on the differential mechanisms in which BDNF drives refinement of auditory circuitries during the onset of sensory experience and in the adult brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Auditory nerve fibre responses in the ferret

The ferret (Mustela putorius) is a medium-sized, carnivorous mammal with good low-frequency hearing; it is relatively easy to train, and there is therefore a good body of behavioural data detailing its detection thresholds and localization abilities. However, despite extensive studies of the physiology of the central nervous system of the ferret, even extending to the prefrontal cortex, little is known of the functioning of the auditory periphery. Here, we provide an insight into this peripheral function by detailing responses of single auditory nerve fibres. Our expectation was that the ferret auditory nerve responsiveness would be similar that of its near relative, the cat. However, by comparing a range of variables (the frequency tuning, the variation of rate-level functions with spontaneous rate, and the high-frequency cut-off of phase locking) across several species, we show that the auditory nerve (and hence cochlea) in the ferret is more similar to that of the guinea-pig and chinchilla than to that of the cat. Animal models of hearing are often chosen on the basis of the similarity of their audiogram to that of the human, particularly in the low-frequency region. We show here that whereas the ferret hears well at low frequencies, this is likely to occur via fibres with higher characteristic frequencies. These qualitative differences in response characteristics in auditory nerve fibres are important in interpreting data across all of auditory science, as it has been argued recently that tuning in animals is broader than in humans.

Temporal response properties of the auditory nerve: data from human cochlear-implant recipients

The primary goal of this study was to characterize the variability in auditory-nerve temporal response patterns obtained with the electrically evoked compound action potential (ECAP) within and across a relatively large group of cochlear-implant recipients. ECAPs were recorded in response to each of 21 pulses in a pulse train for five rates (900, 1200, 1800, 2400, and 3500 pps) and three cochlear regions (basal, middle, and apical). An alternating amplitude pattern was typically observed across the pulse train for slower rates, reflecting refractory properties of individual nerve fibers. For faster rates, the alternation ceased and overall amplitudes were substantially lower relative to the first pulse in the train, reflecting cross-fiber desynchronization. The following specific parameters were examined: (1) the rate at which the alternating pattern ceased (termed stochastic rate), (2) the alternation depth and the rate at which the maximum alternation occurred, and (3) the average normalized ECAP amplitude across the pulse train (measure of overall adaptation/desynchronization). Data from 29 ears showed that stochastic rates for the group spanned the entire range of rates tested. The majority of subjects (79%) had different stochastic rates across the three cochlear regions. The stochastic rate occurred most frequently at 2400 pps for basal and middle electrodes, and at 3500 pps for apical electrodes. Stimulus level was significantly correlated with stochastic rate, where higher levels yielded faster stochastic rates. The maximum alternation depth averaged 19% of the amplitude for the first pulse. Maximum alternation occurred most often at 1800 pps for basal and apical electrodes, and at 1200 pps for middle electrodes. These differences suggest some independence between alternation depth and stochastic rate. Finally, the overall amount of adaptation or desynchronization ranged from 63% (for 900 pps) to 23% (for 3500 pps) of the amplitude for the first pulse. Differences in temporal response properties across the cochlea within subjects may have implications for developing new speech-processing strategies that employ varied rates across the array.

The dependence of auditory nerve rate adaptation on electric stimulus parameters, electrode position, and fiber diameter: a computer model study

This paper describes results from a stochastic computational neuron model that simulates the effects of rate adaptation on the responses to electrical stimulation in the form of pulse trains. We recently reported results from a single-node computational model that included a novel element that tracks external potassium ion concentration so as to modify membrane voltage and cause adaptation-like responses. Here, we report on an improved version of the model that incorporates the anatomical components of a complete feline auditory nerve fiber (ANF) so that conduction velocity and effects of manipulating the site of excitation can be evaluated. Model results demonstrate rate adaptation and changes in spike amplitude similar to those reported for feline ANFs. Changing the site of excitation from a central to a peripheral axonal site resulted in plausible changes in latency and relative spread (i.e., dynamic range). Also, increasing the distance between a modeled ANF and a stimulus electrode tended to decrease the degree of rate adaptation observed in pulse-train responses. This effect was clearly observed for high-rate (5,000 pulse/s) trains but not low-rate (250 pulse/s) trains. Finally, for relatively short electrode-to-ANF distances, increases in modeled ANF diameter increased the degree of rate adaptation. These results are compared against available feline ANF data, and possible effects of individual parameters are discussed.

A phenomenological model of the synapse between the inner hair cell and auditory nerve: long-term adaptation with power-law dynamics

There is growing evidence that the dynamics of biological systems that appear to be exponential over short time courses are in some cases better described over the long-term by power-law dynamics. A model of rate adaptation at the synapse between inner hair cells and auditory-nerve (AN) fibers that includes both exponential and power-law dynamics is presented here. Exponentially adapting components with rapid and short-term time constants, which are mainly responsible for shaping onset responses, are followed by two parallel paths with power-law adaptation that provide slowly and rapidly adapting responses. The slowly adapting power-law component significantly improves predictions of the recovery of the AN response after stimulus offset. The faster power-law adaptation is necessary to account for the "additivity" of rate in response to stimuli with amplitude increments. The proposed model is capable of accurately predicting several sets of AN data, including amplitude-modulation transfer functions, long-term adaptation, forward masking, and adaptation to increments and decrements in the amplitude of an ongoing stimulus.

Spontaneous activity of auditory-nerve fibers: insights into stochastic processes at ribbon synapses

In several sensory systems, the conversion of the representation of stimuli from graded membrane potentials into stochastic spike trains is performed by ribbon synapses. In the mammalian auditory system, the spiking characteristics of the vast majority of primary afferent auditory-nerve (AN) fibers are determined primarily by a single ribbon synapse in a single inner hair cell (IHC), and thus provide a unique window into the operation of the synapse. Here, we examine the distributions of interspike intervals (ISIs) of cat AN fibers under conditions when the IHC membrane potential can be considered constant and the processes generating AN fiber activity can be considered stationary, namely in the absence of auditory stimulation. Such spontaneous activity is commonly thought to result from an excitatory Poisson point process modified by the refractory properties of the fiber, but here we show that this cannot be the case. Rather, the ISI distributions are one to two orders of magnitude better and very accurately described as a result of a homogeneous stochastic process of excitation (transmitter release events) in which the distribution of interevent times is a mixture of an exponential and a gamma distribution with shape factor 2, both with the same scale parameter. Whereas the scale parameter varies across fibers, the proportions of exponentially and gamma distributed intervals in the mixture, and the refractory properties, can be considered constant. This suggests that all of the ribbon synapses operate in a similar manner, possibly just at different rates. Our findings also constitute an essential step toward a better understanding of the spike-train representation of time-varying stimuli initiated at this synapse, and thus of the fundamentals of temporal coding in the auditory pathway.

Cellular localization of voltage-gated calcium channels and synaptic vesicle-associated proteins in the guinea pig cochlea

The cellular localization of voltage-gated calcium channels (VGCCs) and synaptic vesicle-associated proteins, SV2, synapsin I, and vesicle-associated membrane protein (VAMP) (synaptobrevin), was investigated in the guinea pig cochlea using immunocytochemistry and confocal laser scanning microscopy. Reactivity, in guinea pig, of antibodies to the alpha1 subunits of L-type, alpha1C [Cav1.2] and alpha 1D [Cav1.3]; P/Q-type, alpha1A [Cav2.1]; and R-type, a1E [Cav2.3] high voltage-activated calcium channels, was determined by Western blotting and immunolabeling of cerebellum. In the cochlea the sensory inner hair cells of the organ of Corti displayed strong intracellular staining, predominantly localized to their basolateral poles, with an antibody directed against the alpha1C subunit. Some alpha1C labeling was also observed in the inner pillar cells, in cell bodies of afferent neurons in the spiral ganglion, and in the inferior region of the spiral ligament. The supporting pillar cells were strongly immunoreactive throughout for alpha1D, but no alpha1D labeling of the inner hair cells was seen. The alpha1A subunit showed a cytoplasmic distribution in all three rows of outer hair cells. alpha1E labeling localized to the outer hair cells, predominantly in the subcuticular plate region, and also to nerve fiber bundles beneath these hair cells. Strong immunoreactivity was consistently seen with antibodies directed against SV2 and synapsin I in neuronal structures surrounding the basolateral surfaces of both the inner and outer hair cells but was absent from the sensory cells themselves. VAMP labeling was found throughout the cytoplasm of the inner hair cells and in neuronal structures beneath the hair cells. These results reveal a differential distribution of VGCC-types in the sensory and nonsensory elements of the guinea pig cochlea, with the inner hair cells expressing alpha1C L-type channels and VAMP but not synapsin I or SV2.

Response properties of single auditory nerve fibers in the mouse

The availability of transgenic and mutant lines makes the mouse a valuable model for study of the inner ear, and a powerful window into cochlear function can be obtained by recordings from single auditory nerve (AN) fibers. This study provides the first systematic description of spontaneous and sound-evoked discharge properties of AN fibers in mouse, specifically in CBA/CaJ and C57BL/6 strains, both commonly used in auditory research. Response properties of 196 AN fibers from CBA/CaJ and 58 from C57BL/6 were analyzed, including spontaneous rates (SR), tuning curves, rate versus level functions, dynamic range, response adaptation, phase-locking, and the relation between SR and these response properties. The only significant interstrain difference was the elevation of high-frequency thresholds in C57BL/6. In general, mouse AN fibers showed similar responses to other mammals: sharpness of tuning increased with characteristic frequency, which ranged from 2.5 to 70 kHz; SRs ranged from 0 to 120 sp/s, and fibers with low SR (<1 sp/s) had higher thresholds, and wider dynamic ranges than fibers with high SR. Dynamic ranges for mouse high-SR fibers were smaller (<20 dB) than those seen in other mammals. Phase-locking was seen for tone frequencies <4 kHz. Maximum synchronization indices were lower than those in cat but similar to those found in guinea pig.

Purinergic receptors in auditory neurotransmission

The effects of ATP (adenosine 5' triphosphate) analogs on gross cochlear potentials and single primary afferent discharge properties were studied by intracochlear perfusion in anesthetized guinea pigs. ATP-gamma-S was most potent, with betagammamethylene-ATP and Bz-ATP being significantly less effective. These data are consistent with the notion that purinergic receptors activated by scala tympani perfusion contain subunits of the P2X(2) variant. The relative ineffectiveness of Bz-ATP (a P2X(7) agonist) suggests that while this variant has been reported to be expressed in the cochlea, it may not play a major functional role under normal conditions. Changes in the threshold of the gross DC receptor potential (summating potential, SP) and the compound action potential (CAP) were consistent with a combination of effects on both early and final stages of the transduction process, as reported by previous workers. Effects of ATP-gamma-S on single-neuron spontaneous firing rates varied according to the initial spontaneous rate of each primary afferent. Effects on single-neuron tuning curves were consistent with an action mainly on the outer hair cell transduction with betagammamethylene-ATP (elevation of tuning curve tips), but with ATP-gamma-S changes in sensitivity across the full extent of the tuning curve indicated an additional action on inner hair cell-afferent neurotransmission. In agreement with previous reports on ATP-gamma-S, it was found that all ATP analogs produced significant increases in the DC potential in scala media (endocochlear potential, EP). However, the relationship between changes in EP (a major component of the driving force on ions through hair cells) and the alterations in gross and single unit measures of cochlear activity was not clear.

Matching the neural adaptation in the rat ventral cochlear nucleus produced by artificial (electric) and acoustic stimulation of the cochlea

To investigate neural adaptive properties, near-field evoked potentials were recorded from a chronically implanted electrode in the ventral cochlear nucleus in awake Long-Evans rats exposed to acoustic stimuli or receiving intracochlear electric stimulation. Stimuli were 250-ms trains of repetitive acoustic clicks (10, 30 and 50 dB SPL) or biphasic electric pulses (30, 50 and 70 microA) with intratrain pulse rates ranging from 100 to 1000 pulses per second (pps). The amplitude of the first negative (N(1)) to positive (P(1)) component of the average evoked potentials was measured for each consecutive individual pulse in the train. While a progressive exponential decrease in N(1)-P(1) amplitude was observed as a function of the position of the pulse within the train for both types of stimulation, the decrement of electric responses (adaptive pattern) was substantially less prominent than that observed for acoustic stimuli. Based on this difference, the present work was extended by modifying electric stimuli in order to try to restore normal adaptation phenomena. The results suggest the feasibility of mimicking acoustic adaptation by stimulation with exponentially decreasing electric pulse trains, which may be clinically applicable in the auditory implant field.

Effects of intensity of repetitive acoustic stimuli on neural adaptation in the ventral cochlear nucleus of the rat

To study neural adaptation as a function of stimulus intensity, auditory near-field evoked potentials were recorded from the ventral cochlear nucleus in awake Long Evans rats. Responses to 250-ms trains of repetitive clicks (pulse rates ranging from 100 to 1000 pulses per second) were collected at stimulus intensities of 5, 10, 30, 50 and 70 dB SPL. The amplitude of the first negative (N1) component of the average evoked potentials to individual pulses in the train was measured by using a subtraction method. The N1 responses were normalized with respect to the highest cochlear nucleus potential observed in the train, and then plotted as a function of click position in the train. As expected, the general trend of the curves was an exponential decay reaching a plateau more or less rapidly as a function of both intensity and rate of stimulation. Fitting these curves with exponential decay equations revealed that the rapid time constant decreased for increasing stimulus intensities whereas the short-term time constant is relatively independent of intensity. The amount of adaptation (expressed as the ratio of the plateau to the first peak amplitude) was substantially less prominent at low intensities (5-10 dB SPL) and low rates (100-200 pulses per second) than at higher intensities and high rates. These results indicate that adaptation patterns obtained in the ventral cochlear nucleus by using near-field evoked potentials exhibit properties comparable to those already present at the level of the auditory nerve.

Effects of trigeminal ganglion stimulation on unit activity of ventral cochlear nucleus neurons

The trigeminal ganglion sends a projection to the granule and magnocellular regions of the ventral cochlear nucleus (VCN; [J Comp Neurol 419 (2000) 271]), as well as to the cochlea ([Neuroscience 79 (1997) 605; Neuroscience 84 (1998a) 559]). We investigated the effects of electrically stimulating the trigeminal ganglion on unit responses in the guinea-pig VCN. Responses consisted of one, two or more phases of excitation, sometimes followed by a longer inhibitory phase. The latencies to the first excitation peak ranged between 5 and 17 ms from the onset of stimulation. These responses were preceded by a slow wave potential evoked by the stimulation. Applying kainic acid, which eliminates VIIIth nerve responses, diminished the firing rates of VCN units to trigeminal stimulation, and increased their first spike latencies. Cochlear destruction had a similar effect. The responses in VCN evoked by trigeminal ganglion stimulation therefore appear to result from direct stimulation of the trigeminal ganglion-cochlear nucleus pathway, as well as modulation by the trigeminal ganglion-cochlear pathway. Alternatively, a reduction in spontaneous rate of VCN neurons by removal of VIIIth nerve input could explain the decreased response to trigeminal stimulation after cochlear manipulations. The modulation of firing rate in second order auditory neurons by first order somatosensory neurons could influence central auditory targets and may be involved in generating or modulating perceptions of phantom sounds which can be modified by manipulations of somatic regions of the head and neck ("somatic tinnitus").

A nonlinear filter-bank model of the guinea-pig cochlear nerve: rate responses

The aim of this study is to produce a functional model of the auditory nerve (AN) response of the guinea-pig that reproduces a wide range of important responses to auditory stimulation. The model is intended for use as an input to larger scale models of auditory processing in the brain-stem. A dual-resonance nonlinear filter architecture is used to reproduce the mechanical tuning of the cochlea. Transduction to the activity on the AN is accomplished with a recently proposed model of the inner-hair-cell. Together, these models have been shown to be able to reproduce the response of high-, medium-, and low-spontaneous rate fibers from the guinea-pig AN at high best frequencies (BFs). In this study we generate parameters that allow us to fit the AN model to data from a wide range of BFs. By varying the characteristics of the mechanical filtering as a function of the BF it was possible to reproduce the BF dependence of frequency-threshold tuning curves, AN rate-intensity functions at and away from BF, compression of the basilar membrane at BF as inferred from AN responses, and AN iso-intensity functions. The model is a convenient computational tool for the simulation of the range of nonlinear tuning and rate-responses found across the length of the guinea-pig cochlear nerve.

Adaptation in a revised inner-hair cell model

A revised computational model of the inner-hair cell (IHC) and auditory-nerve (AN) complex was recently presented [Sumner et al., J. Acoust. Soc. Am. 111, 2178-2188 (2002)]. One key improvement is that the model reproduces the rate-intensity functions of low- (LSR), medium- (MSR), and high-spontaneous rate (HSR) fibers in the guinea-pig. Here we describe the adaptation characteristics of the model, and how they vary with model fiber type. Adaptation of the revised model for a HSR fiber is in line with an earlier version of the model [Meddis and Hewitt, J. Acoust. Soc. Am. 90, 904-917 (1991)]. In guinea-pig, poststimulus time histograms (PSTH) have been found to show less adaptation in LSR fibers. Evidence from chinchilla suggests that this is due to chronic adaptation resulting from short interstimulus intervals, and that fully recovered LSR fibers actually show more adaptation. However, the model is able to account for both variations of PSTH shape when fully recovered from adaptation. Interstimulus interval can also affect recovery in the model. The model is further tested against data previously used to evaluate models of AN adaptation. The tests are (i) recovery from adaptation of spontaneous rate and (ii) the recovery of response to acoustic stimuli ("forward masking"), (iii) the response to stimulus increments and (iv) decrements, and (v) the conservation of transient components. A HSR model fiber performs similarly to the earlier version of the model. However, there is considerable variation in response to increments and decrements between different model fibers.

Response adaptation of medial olivocochlear neurons is minimal

Response adaptation is a general characteristic of neurons. A number of studies have investigated the adaptation characteristics of auditory-nerve fibers, which send information to the brain about sound stimuli. However, there have been no previous adaptation studies of olivocochlear neurons, which provide efferent fibers to hair cells and auditory nerve dendrites in the auditory periphery. To study adaptation in efferent fibers, responses of single olivocochlear neurons were recorded to characteristic-frequency tones and noise, using anesthetized guinea pigs. To measure short-term adaptation, stimuli of 500 ms duration were presented, and the responses were displayed as peristimulus time histograms. These histograms showed regular peaks, indicating a "chopping" pattern of response. The rate during each chopping period as well as the general trend of the histogram could be well fit by an equation that expresses the firing rate as a sum of 1) a short-term adaptive rate that decays exponentially with time and 2) a constant steady-state rate. For the adaptation in medial olivocochlear (MOC) neurons, the average exponential time constant was 47 ms, which is roughly similar to that for short-term adaptation in auditory-nerve fibers. The amount of adaptation (expressed as a percentage decrease of onset firing rate), however, was substantially less in MOC neurons (average 31%) than in auditory-nerve fibers (average 63%). To test for adaptation over longer periods, we used noise and tones of 10 s duration. After the short-term adaptation, the responses of MOC neurons were almost completely sustained (average long-term adaptation 3%). However, in the same preparations, significant long-term adaptation was present in auditory-nerve fibers. These results indicate that the MOC response adaptation is minimal compared with that of auditory-nerve fibers. Such sustained responses may enable the MOC system to produce sustained effects in the periphery, supporting a role for this efferent system during ongoing stimuli of long duration.

Temporal aspects of the effects of cooling on responses of single auditory nerve fibers

During an investigation of the effects of cochlear cooling on frequency tuning and input/output relations of single auditory nerve fibers in gerbil (Ohlemiller and Siegel (1994) Hear. Res. 80, 174-190), cooling-related changes in post-stimulus time histogram (PSTH) shape and phase-locking to tonebursts were characterized in a small sample of neurons. Local cochlear cooling by 5-10 degrees C below normal core temperature did not alter overall PSTH shape, although some evidence was found for a reduction in the time constants of rapid and short term rate adaptation. The relative contributions of rapid and short term response components appeared unaltered. Effects of cooling on phase-locking were assessed by calculating the synchronization index for responses to intense ( > 70 dB SPL) tonebursts at 0.5, 1.0, and 2.0 kHz. Synchronization filter functions exhibited modest reductions in both magnitude and the upper frequency limit of phase-locking. The effects of cooling on the temporal character of responses appear distinct from those of a simple reduction in stimulus intensity. Results are interpreted in terms of cooling-related changes in responses of cochlear hair cells and afferent neurons, and suggest that temperature artifacts are unlikely to underlie reported species differences in PSTH shape and phase-locking.

Transmitter release in inner hair cell synapses: a model analysis of spontaneous and driven rate properties of cochlear nerve fibres

The inner hair cell (IHC) synapse is one of the stages of cochlear processing that determine the relation between sound pressure level and spike rate in auditory nerve fibres. Transmitter released in the non-stimulated condition is held responsible for the wide range of spontaneous spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC transmembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously developed computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of rate thresholds, shapes of rate-intensity functions and maximal driven rate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC potential, is elaborated. Its results lead to the hypothesis that the true release function is non-linear and variable across synapses generating different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. This approach leads to adequate reproduction of the variation in rate thresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function from given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear function of IHC potential which, by the systematic variation of its parameters across SR, effectively leads to the physiological variation in dynamic range across fibres of different SR. Possible relations of these results with ultrastructural morphology and basic physiology of IHC synapses are discussed.

Central projections of auditory nerve fibers of differing spontaneous rate, II: Posteroventral and dorsal cochlear nuclei

Response properties of auditory nerve fibers (ANFs), including threshold sensitivity, vary systematically with spontaneous discharge rate (SR) (Liberman, M.C.: J. Acoust. Soc Amer. 63:442-455, 1978). Thus, an understanding of the mechanisms underlying signal transformation in the cochlear nucleus (CN) must include a description of any SR-based difference in ANF projections. This study is the second of a pair describing the CN projections of intracellularly labeled ANFs of known SR, the first of which summarized projection to the anteroventral CN (Liberman, M.C.: J. Comp. Neurol. 313:240-258, 1991). For each swelling from each labeled fiber, the position (within CN subdivisions), the size, and the type of cell contacted (if determinable) was noted: roughly one in four labeled swellings appeared in intimate contact with the soma or proximal dendrites of a CN cell. In all such cases, cell size and swelling size were measured. As reported for auteroventral cochlear nucleus, the ANF innervation of the small-cell regions of posteroventral CN (PVCN) was almost exclusively by low- and medium-SR fibers. Other significant SR-based trends in ANF projections included 1) a tendency for high-SR fibers to contact larger cells in PVCN, 2) a meager projection of low- and medium-SR fibers to octopus cells, and 3) a tendency in the dorsal CN (DCN) for low-SR terminals to end closer to the fusiform cell layer than high-SR terminals. There were no significant SR-based difference in ANF swelling sizes in any subdivision. A consideration of the average cell sizes, ANF swelling sizes and estimated numbers of ANFs of different CF and SR converging on each CN cell help explain some of the differences in response transformation associated with different cell types in the CN.

The efferent modulation of mammalian inner hair cell afferents

The results of immunocytochemical, enzymatic and electrophysiological studies have indicated that acetylcholine and GABA may act as neurotransmitters in lateral olivocochlear efferent endings on inner hair cell afferent dendrites. Since spike activity can be recorded in the dendritic region of inner hair cells, microiontophoretic techniques were used testing the possible neurotransmitter candidates, acetylcholine and GABA, on spontaneous and induced firing of the afferent dendrites. The experiments were carried out in anaesthetised guinea-pigs, the third and fourth turns of the cochlea being exposed for electrode penetration. Ejection of acetylcholine resulted in a pronounced dose-dependent increase in subsynaptic spiking activity. Furthermore, acetylcholine enhanced glutamate-induced activity. In contrast, even at high doses, GABA had very little effect on the spontaneous cochlear firing rate. When the firing rate had first been enhanced by glutamate or N-methyl-D-aspartate, however, this activation could be reduced by the ejection of GABA. A similar reduction was observed when the firing rate had been enhanced with acetylcholine. The results of our studies support the hypothesis that these substances are involved in efferent neurotransmission on inner hair cell afferent fibres. It should be pointed out, however, that besides acetylcholine and GABA, several opioids such as enkephalins and dynorphins seem to be involved in efferent cochlear innervation.