Rate-versus-intensity functions and related AP responses in normal and pathological guinea pig and human cochleas

Sensorineural Hearing Loss Diminishes Use of Temporal Envelope Cues: Evidence From Roving-Level Tone-in-Noise Detection

OBJECTIVES

The objective of our study is to understand how listeners with and without sensorineural hearing loss (SNHL) use energy and temporal envelope cues to detect tones in noise. Previous studies of low-frequency tone-in-noise detection have shown that when energy cues are made less reliable using a roving-level paradigm, thresholds of listeners with normal hearing (NH) are only slightly increased. This result is consistent with studies demonstrating the importance of temporal envelope cues for masked detection. In contrast, roving-level detection thresholds are more elevated in listeners with SNHL at the test frequency, suggesting stronger weighting of energy cues. The present study extended these tests to a wide range of frequencies and stimulus levels. The authors hypothesized that individual listeners with SNHL use energy and temporal envelope cues differently for masked detection at different frequencies and levels, depending on the degree of hearing loss.

DESIGN

Twelve listeners with mild to moderate SNHL and 12 NH listeners participated. Tone-in-noise detection thresholds at 0.5, 1, 2, and 4 kHz in 1/3 octave bands of simultaneously gated Gaussian noise were obtained using a novel, two-part tracking paradigm. A track refers to the sequence of trials in an adaptive test procedure; the signal to noise ratio was the tracked variable. Each part of the track consisted of a two-alternative, two-interval, forced-choice procedure. The initial portion of the track estimated detection threshold using a fixed masker level. When the track continued, stimulus levels were randomly varied over a 20-dB rove range (±10 dB with respect to mean masker level), and a second threshold was estimated. Rove effect (RE) was defined as the difference between thresholds for the fixed- and roving-level tests. The size of the RE indicated how strongly a listener weighted energy-based cues for masked detection. Participants were tested at one to three masker levels per frequency, depending on audibility.

RESULTS

Across all stimulus frequencies and levels, NH listeners had small REs (≈1 dB), whereas listeners with SNHL typically had larger REs. Some listeners with SNHL had larger REs at higher frequencies, where pure-tone audiometric thresholds were typically elevated. RE did not vary significantly with masker level for either group. Increased RE for the SNHL group was consistent with simulations in which energy cues were more heavily weighted than envelope cues.

CONCLUSIONS

Tone-in-noise detection thresholds in NH listeners were typically elevated only slightly by the roving-level paradigm at any frequency or level tested, consistent with the primary use of level-independent cues, such as temporal envelope cues that are conveyed by fluctuations in neural responses. In comparison, thresholds of listeners with SNHL were more affected by the roving-level paradigm, suggesting stronger weighting of energy cues. For listeners with SNHL, the largest RE was observed at 4000 Hz, for which pure-tone audiometric thresholds were most elevated. Specifically, RE size at 4000 Hz was significantly correlated with higher pure-tone audiometric thresholds at the same frequency, after controlling for the effect of age. Future studies will explore strategies for restoring or enhancing neural fluctuation cues that may lead to improved hearing in noise for listeners with SNHL.

Tinnitus development is associated with synaptopathy of inner hair cells in Mongolian gerbils

Human hearing loss (HL) is often accompanied by comorbidities like tinnitus, which is affecting up to 15% of the adult population. Rodent animal studies could show that tinnitus may not only be a result of apparent HL due to cochlear hair cell damage but can also be a consequence of synaptopathy at the inner hair cells (IHCs) already induced by moderate sound traumata. Here, we investigate synaptopathy previously shown in mice in our animal model, the Mongolian gerbil, and relate it to behavioral signs of tinnitus. Tinnitus was induced by a mild monaural acoustic trauma leading to monaural noise induced HL in the animals, quantified by auditory brainstem response (ABR) audiometry. Behavioral signs of tinnitus percepts were detected by measurement of prepulse inhibition of the acoustic startle response in a gap-noise paradigm. Fourteen days after trauma, the cochleae of both ears were isolated, and IHC synapses were counted within several spectral regions of the cochlea. Behavioral signs of tinnitus were only found in animals with IHC synaptopathy, independent of type of HL. On the other hand, animals with apparent HL but without behavioral signs of tinnitus showed a reduction in amplitudes of ABR waves I&II but no significant changes in the number of synapses at the IHC. We conclude-in line with the literature-that HL is caused by damage to the IHC or by other reasons but that the development of tinnitus, at least in our animal model, is closely linked to synaptopathy at the IHC.

Frequency tuning curves derived from auditory steady state evoked potentials: a proof-of-concept study

OBJECTIVES

Assess the feasibility of drawing tuning curves from the masking function of steady state potentials. Develop a noninvasive tool for research applications on cochlear frequency selectivity in sedated animals. Obtain pilot human data validating auditory steady state evoked potential-derived (ASSEP) tuning curves against psychophysical data.

DESIGN

ASSEP tuning curves were drawn in 10 Beagle puppies and six human adults using amplitude-modulated probes. Two probe frequencies (1 and 2 kHz) were used in dogs and only one (2 kHz) in humans. The modulation rates of the two probes were set to 81 and 88 Hz, respectively. Psychophysical tuning curves were obtained in 12 normal human subjects using the same maskers and either a pure-tone or an amplitude-modulated probe to verify if the latter had a specific effect on tuning curve parameters. Six of these 12 subjects participated in the electrophysiologic measurements. For each tuning curve, the intensity of the narrowband masker required just to mask the fixed probe was plotted for different masker center frequencies. Masker center frequencies extended to about half an octave above and an octave below the probe frequencies in 100-Hz steps. Tuning curve width (Q10 dB values), high- and low-frequency slopes (in dB/octave) and the masker frequency yielding the lowest masking threshold (maximal masker frequency) were computed. Canine Q10 dB values obtained were compared with those published for several species with other techniques. For humans, ASSEP and psychophysical tuning curves were directly compared in the same subjects and with published data.

RESULTS

In dogs, the ASSEP method yielded reproducible tuning curves with qualitative and quantitative parameters similar to other physiologic measures of tuning obtained in various animals. Q10 dB values were greater at 2 than at 1 kHz, reflecting the well-known correlation between sharpness of tuning and central frequency. In humans, ASSEP Q10 dB values were slightly smaller than the psychophysical ones, but were greater by a factor of 2 than those obtained with previously published electrophysiologic procedures. In both species, detuning-a shift of the tip of the curve away from the probe frequency-was frequently observed as upward shifts with a maximal value of 200 Hz. Human psychophysical tuning curves also showed a certain amount of upward detuning. The intraindividual comparison of the two types of probes performed on human subjects with the psychophysical method did not indicate a specific effect of the amplitude-modulated probe on the curve parameters. Neither did the intraindividual comparisons indicate that an amplitude-modulated probe per se promoted detuning. Detuning has been observed with several other techniques and is usually attributed to nonlinear interactions between masker and probe in simultaneous masking.

CONCLUSIONS

The results demonstrate the feasibility of measuring realistic ASSEP tuning curves in sedated dogs and in sleeping human adults. The ASSEP tuning curves exhibit a series of classical features similar to those obtained with time-honored methods. These results pave the way for the development of a noninvasive electrophysiologic method for tuning curve recording and its applications in noncooperative experimental animals or clinical subjects.

Loudness perception in the domestic cat: reaction time estimates of equal loudness contours and recruitment effects

The domestic cat is the primary physiological model of loudness coding and recruitment. At present, there are no published descriptions of loudness perception in this species. This study used a reaction time task to characterize loudness perception in six behaviorally trained cats. The psychophysical approach was based on the assumption that sounds of equal loudness elicit responses of equal latency. The resulting equal latency contours reproduced well-known features of human equal loudness contours. At the completion of normal baseline measures, the cats were exposed to intense sound to investigate the behavioral correlates of loudness recruitment, the abnormally rapid growth of loudness that is commonly associated with hearing loss. Observed recruitment effects were similar in magnitude to those that have been reported in hearing-impaired humans. Linear hearing aid amplification is known to improve speech intelligibility but also exacerbate recruitment in impaired listeners. The effects of speech spectra and amplification on recruitment were explored by measuring the growth of loudness for natural and amplified vowels before and after sound exposure. Vowels produced more recruitment than tones, and the effect was exacerbated by the selective amplification of formant structure. These findings support the adequacy of the domestic cat as a model system for future investigations of the auditory processes that underlie loudness perception, recruitment, and hearing aid design.

Encoding intensity in ventral cochlear nucleus following acoustic trauma: implications for loudness recruitment

Loudness recruitment, an abnormally rapid growth of perceived loudness with sound level, is a common symptom of sensorineural hearing loss. Following acoustic trauma, auditory-nerve rate responses are reduced, and rate grows more slowly with sound level, which seems inconsistent with recruitment (Heinz et al., J. Assoc. Res. Otolaryngol. 6:91-105, 2005). However, rate-level functions (RLFs) in the central nervous system may increase in either slope or saturation value following trauma (e.g., Salvi et al., Hear. Res. 147:261-274, 2000), suggesting that recruitment may arise from central changes. In this paper, we studied RLFs of neurons in ventral cochlear nucleus (VCN) of the cat after acoustic trauma. Trauma did not change the general properties of VCN neurons, and the usual VCN functional classifications remained valid (chopper, primary-like, onset, etc.). After trauma, non-primary-like neurons, most noticeably choppers, exhibited elevated maximum discharge rates and steeper RLFs for frequencies at and near best frequency (BF). Primary-like neurons showed the opposite changes. To relate the neurons' responses to recruitment, rate-balance functions were computed; these show the sound level required to give equal rates in a normal and a traumatized ear and are analogous to loudness balance functions that show the sound levels giving equal perceptual loudness in the two ears of a monaurally hearing-impaired person. The rate-balance functions showed recruitment-like steepening of their slopes in non-primary-like neurons in all conditions. However, primary-like neurons showed recruitment-like behavior only when rates were summated across neurons of all BFs. These results suggest that the non-primary-like, especially chopper, neurons may be the most peripheral site of the physiological changes in the brain that underlie recruitment.

Illusory percepts from auditory adaptation

Phenomena resembling tinnitus and Zwicker phantom tone are seen to result from an auditory gain adaptation mechanism that attempts to make full use of a fixed-capacity channel. In the case of tinnitus, the gain adaptation enhances internal noise of a frequency band otherwise silent due to damage. This generates a percept of a phantom sound as a consequence of hearing loss. In the case of Zwicker tone, a frequency band is temporarily silent during the presentation of a notched broadband sound, resulting in a percept of a tone at the notched frequency. The model suggests a link between tinnitus and the Zwicker tone percept, in that it predicts different results for normal and tinnitus subjects due to a loss of instantaneous nonlinear compression. Listening experiments on 44 subjects show that tinnitus subjects (11 of 44) are significantly more likely to hear the Zwicker tone. This psychoacoustic experiment establishes the first empirical link between the Zwicker tone percept and tinnitus. Together with the modeling results, this supports the hypothesis that the phantom percept is a consequence of a central adaptation mechanism confronted with a degraded sensory apparatus.

Development of tinnitus-related neuronal hyperactivity through homeostatic plasticity after hearing loss: a computational model

Tinnitus, the perception of a sound in the absence of acoustic stimulation, is often associated with hearing loss. Animal studies indicate that hearing loss through cochlear damage can lead to behavioral signs of tinnitus that are correlated with pathologically increased spontaneous firing rates, or hyperactivity, of neurons in the auditory pathway. Mechanisms that lead to the development of this hyperactivity, however, have remained unclear. We address this question by using a computational model of auditory nerve fibers and downstream auditory neurons. The key idea is that mean firing rates of these neurons are stabilized through a homeostatic plasticity mechanism. This homeostatic compensation can give rise to hyperactivity in the model neurons if the healthy ratio between mean and spontaneous firing rate of the auditory nerve is decreased, for example through a loss of outer hair cells or damage to hair cell stereocilia. Homeostasis can also amplify non-auditory inputs, which then contribute to hyperactivity. Our computational model predicts how appropriate additional acoustic stimulation can reverse the development of such hyperactivity, which could provide a new basis for treatment strategies.

Dorsal cochlear nucleus response properties following acoustic trauma: response maps and spontaneous activity

Recordings from single neurons in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats were done to characterize the effects of acoustic trauma. Trauma was produced by a 250 Hz band of noise centered at 10 kHz, presented at 105-120 dB SPL for 4h. After a one-month recovery period, neurons were recorded in the DCN. The threshold shift, determined from compound action-potential audiograms, showed a sharp threshold elevation of about 60 dB at BFs above an edge frequency of 5-10 kHz. The response maps of neurons with best frequencies (BFs) above the edge did not show the typical organization of excitatory and inhibitory areas seen in the DCN of unexposed animals. Instead, neurons showed no response to sound, weak responses that were hard to tune and characterize, or "tail" responses, consisting of broadly-tuned, predominantly excitatory responses, with a roughly low-pass shape similar to the tuning curves of auditory nerve fibers with similar threshold shifts. In some tail responses whose BFs were near the edge of the threshold elevation, a second weak high-frequency response was seen that suggests convergence of auditory nerve inputs with widely separated BFs on these cells. Spontaneous rates among neurons with elevated thresholds were not increased over those in populations of principal neurons in unexposed animals.

Auditory-nerve rate responses are inconsistent with common hypotheses for the neural correlates of loudness recruitment

A number of perceptual phenomena related to normal and impaired level coding can be accounted for by the degree of compression in the basilar-membrane (BM) magnitude response. However, the narrow dynamic ranges of auditory-nerve (AN) fibers complicate these arguments. Because the AN serves as an information bottleneck, an improved understanding of the neural coding of level may clarify some of the limitations of current hearing aids. Here three hypotheses for the neural correlate of loudness recruitment were evaluated based on AN responses from normal-hearing cats and from cats with a noise-induced hearing loss (NIHL). Auditory-nerve fiber rate-level functions for tones were analyzed to test the following hypotheses: Loudness recruitment results from steeper AN rate functions after impairment. This hypothesis was not supported; AN rate functions were not steeper than normal following NIHL, despite steeper estimated BM responses based on the AN data. Loudness is based on the total AN discharge count, and recruitment results from an abnormally rapid spread of excitation after impairment. Whereas abnormal spread of excitation can be observed, steeper growth of total AN rate is not seen over the range of sound levels where recruitment is observed in human listeners. Loudness of a narrowband stimulus is based on AN responses in a narrow BF region, and recruitment results from compression of the AN-fiber threshold distribution after impairment. This hypothesis was not supported because there was no evidence that impaired AN threshold distributions were compressed and the growth of AN activity summed across BFs near the stimulus frequency was shallower than normal.Overall, these results suggest that loudness recruitment cannot be accounted for based on summed AN rate responses and may depend on neural mechanisms involved in the central representation of intensity.

Response growth with sound level in auditory-nerve fibers after noise-induced hearing loss

People with sensorineural hearing loss are often constrained by a reduced acoustic dynamic range associated with loudness recruitment; however, the neural correlates of loudness and recruitment are still not well understood. The growth of auditory-nerve (AN) activity with sound level was compared in normal-hearing cats and in cats with a noise-induced hearing loss to test the hypothesis that AN-fiber rate-level functions are steeper in impaired ears. Stimuli included best-frequency and fixed-frequency tones, broadband noise, and a brief speech token. Three types of impaired responses were observed. 1) Fibers with rate-level functions that were similar across all stimuli typically had broad tuning, consistent with outer-hair-cell (OHC) damage. 2) Fibers with a wide dynamic range and shallow slope above threshold often retained sharp tuning, consistent with primarily inner-hair-cell (IHC) damage. 3) Fibers with very steep rate-level functions for all stimuli had thresholds above approximately 80 dB SPL and very broad tuning, consistent with severe IHC and OHC damage. Impaired rate-level slopes were on average shallower than normal for tones, and were steeper in only limited conditions. There was less variation in rate-level slopes across stimuli in impaired fibers, presumably attributable to the lack of suppression-induced reductions in slopes for complex stimuli relative to BF-tone slopes. Sloping saturation was observed less often in impaired fibers. These results illustrate that AN fibers do not provide a simple representation of the basilar-membrane I/O function and suggest that both OHC and IHC damage can affect AN response growth.

Cochlear Compression: Perceptual Measures and Implications for Normal and Impaired Hearing

This article provides a review of recent developments in our understanding of how cochlear nonlinearity affects sound perception and how a loss of the nonlinearity associated with cochlear hearing impairment changes the way sounds are perceived. The response of the healthy mammalian basilar membrane (BM) to sound is sharply tuned, highly nonlinear, and compressive. Damage to the outer hair cells (OHCs) results in changes to all three attributes: in the case of total OHC loss, the response of the BM becomes broadly tuned and linear. Many of the differences in auditory perception and performance between normal-hearing and hearing-impaired listeners can be explained in terms of these changes in BM response. Effects that can be accounted for in this way include poorer audiometric thresholds, loudness recruitment, reduced frequency selectivity, and changes in apparent temporal processing. All these effects can influence the ability of hearing-impaired listeners to perceive speech, especially in complex acoustic backgrounds. A number of behavioral methods have been proposed to estimate cochlear nonlinearity in individual listeners. By separating the effects of cochlear nonlinearity from other aspects of hearing impairment, such methods may contribute towards identifying the different physiological mechanisms responsible for hearing loss in individual patients. This in turn may lead to more accurate diagnoses and more effective hearing-aid fitting for individual patients. A remaining challenge is to devise a behavioral measure that is sufficiently accurate and efficient to be used in a clinical setting.

An auditory-periphery model of the effects of acoustic trauma on auditory nerve responses

Acoustic trauma degrades the auditory nerve's tonotopic representation of acoustic stimuli. Recent physiological studies have quantified the degradation in responses to the vowel /E/ and have investigated amplification schemes designed to restore a more correct tonotopic representation than is achieved with conventional hearing aids. However, it is difficult from the data to quantify how much different aspects of the cochlear pathology contribute to the impaired responses. Furthermore, extensive experimental testing of potential hearing aids is infeasible. Here, both of these concerns are addressed by developing models of the normal and impaired auditory peripheries that are tested against a wide range of physiological data. The effects of both outer and inner hair cell status on model predictions of the vowel data were investigated. The modeling results indicate that impairment of both outer and inner hair cells contribute to degradation in the tonotopic representation of the formant frequencies in the auditory nerve. Additionally, the model is able to predict the effects of frequency-shaping amplification on auditory nerve responses, indicating the model's potential suitability for more rapid development and testing of hearing aid schemes.

Electrophysiological characteristics of inferior colliculus neurons in mutant mice with hereditary absence of cochlear outer hair cells

One strain of homozygous Kit(W-v) mice (formerly known as W(v)/W(v)) lack 98% of the cochlear outer hair cells (OHCs) from birth. Inner hair cells (IHCs) and supporting cells develop normally. Thus, this strain is an attractive model to study the effect of complete OHC absence on central frequency representation. Frequency threshold curves were recorded along the tonotopic axis of inferior colliculus (IC) in mutant and control mice of the genetic background strain (C57BL/6J) and a different outbred strain (NMRI/wild mouse hybrids) known to be free of any cochlear pathology. The average threshold level of neurons in the mutants was 100 dB sound pressure level, 60 dB higher than in C57BL/6J and NMRI mice. Their tuning curves lacked the sharply tuned tip. In the C57BL/6J mice, although younger than four months, abnormal tuning curves were found for about 30% of the neurons, especially in the high frequency range. No abnormal tuning curves were found in the NMRI mice. The bandwidth of the tuning curves, measured at 10 dB above threshold, was on average 1.27 octaves in mutants, 0.62 octaves in C57BL/6J mice, and 0.34 octaves in NMRI mice. The range for the high cut-off frequency of the tuning curves at 10 dB above threshold was 6.4-61.1 kHz in the NMRI and 7-59.5 kHz in C57BL/6J. In the mutants, the range was limited to 11.1-41.7 kHz. The tonotopic gradient based on the cut-off frequency was less steep in the IC of the mutants than in both control groups.

Suppression of distortion product otoacoustic emissions and hearing threshold

A distortion product otoacoustic emission (DPOAE) suppression tuning curve (STC) shows the minimum level of suppressor tone that is required to reduce DPOAE level by a fixed amount, as a function of suppressor frequency. Several years ago, Mills [J. Acoust. Soc. Am. 103, 507-523 (1998)] derived, theoretically, an approximately linear relationship between the tip-to-tail suppressor level difference on a DPOAE STC, and the gain of the cochlear amplifier, defined as the maximum increase in the active over the passive basilar membrane (BM) response. In this paper, preliminary data from adult human subjects are presented that establish a correlation between this tip-to-tail DPOAE STC difference and the threshold of hearing, the latter measured at the frequency of the f2 primary tone. Assuming that both suppression and the DPOAE are by-products of active, nonlinear BM dynamics, the above result suggests that threshold elevation in mild levels of hearing loss may be attributed, in part, to a reduction of cochlear amplifier gain, which is detectable with the suppression paradigm.

Properties of derived cochlear action potentials in forward tonal masking in guinea pigs

Chronic experiments on guinea pigs were used to study the characteristics of cochlear nerve action potentials and the derived potential in conditions of forward masking. The auditory nerve action potential was recorded from the round window of the cochlea. The derived potential was obtained by subtracting the action potential recorded with masking from the response obtained in silence. These studies showed that the derived potential showed higher sensitivity to masking than the traditional measure of masking, i.e., the decrement in the amplitude of the auditory nerve action potential. The derived potential reflects not only changes in the action potential during masking, but also changes in its shape. Differences between the derived potential and the action potential decrement provide evidence that the amplitude and time changes in the derived potential provide more complete information on the nature of activity in the auditory fibers.

Linearized response growth inferred from growth-of-masking slopes in ears with cochlear hearing loss

Growth of masking for OFF-frequency conditions (probe frequency above the masker spectrum) and ON-frequency conditions (probe within the masker spectrum) was investigated using simultaneous masking in three subjects with normal hearing and nine subjects with high-frequency sensorineural hearing loss. Growth-of-masking functions (probe thresholds as a function of masker intensity) for OFF-frequency conditions were obtained for probe tones placed at six frequencies above a 200-Hz-wide masker with an upper edge at 520 Hz. Growth-of-masking functions for ON-frequency conditions were obtained for probe tones placed within the 200-Hz-wide masker and for probe tones placed within 400-Hz-wide maskers with upper edges at 1040, 1300, 1627, and 2040 Hz (probe tones placed 20 Hz below the upper edge frequency). Growth-of-masking functions were fit with a power function of masker intensity added to an internal noise with intensity equal to the absolute threshold for the probe, and were well described by two free parameters and a threshold constant: the growth-of-masking slope (beta), a masking sensitivity constant (kappa) that indicated the minimum effective masker level at which masking began, and the intensity of the probe at absolute threshold (IT). For OFF-frequency masking conditions, growth-of-masking slopes (beta) decreased by a factor of 0.8 for every 10 dB of hearing loss. Comparisons with data from previous studies of upward spread of masking, and assumptions about underlying physiological mechanisms, led to the conclusion that more gradual than normal growth-of-masking slopes reflect larger (steeper) growth-of-response slopes at the probe frequency in regions of hearing loss. Derived response-growth exponents increased by a factor of 1.2 for every 10 dB of hearing loss (HL), from an exponent around 0.25 at 0 dB HL to an exponent around 1.0 at 75 dB HL (linear response growth). Masking sensitivity constants (kappa), the minimum effective masker levels, indicated that masking began at slightly higher masker levels in subjects with sensorineural hearing loss than in subjects with normal hearing. It was concluded that higher masked thresholds in regions of hearing loss were due primarily to a loss of active gain at the probe frequency and were not due to an excessive response at the probe frequency to the lower-frequency masker. For ON-frequency masking conditions, growth-of-masking slopes were not different from normal in hearing-impaired subjects. ON-frequency masking began when the effective power within an auditory filter at the probe frequency reached elevated absolute threshold at the probe frequency. Critical ratios were normal except for one subject with the most hearing loss.

Auditory-nerve fiber responses to clicks in guinea pigs with a damaged cochlea

This paper describes auditory-nerve single-fiber responses to clicks in noise-damaged cochleas. Poststimulus time histograms (PSTHs) were recorded for various click intensities and for the two click polarities. The PSTHs found in fibers with elevated thresholds are discussed in relation to the frequency threshold curves (FTCs) measured in these fibers. Five types of abnormal FTCs are distinguished. Type I is elevated as a whole, type II has an elevated (and often broadened) tip and a tail at normal level, type III has low thresholds in the tail (often hypersensitive), type IV represents a flat tuning, and type V has no tip but shows a clear appearance of the tail (often hypersensitive). The click PSTHs of abnormal fibers were compared to normal PSTHs at equal sound-pressure levels, and various abnormal trends were found corresponding to the type of FTC. PSTHs for type I have longer dominant-peak latencies and smaller amplitudes; PSTHs for type II were normal well above the fiber's threshold; PSTHs for type III revealed remarkable patterns with multiple peaks, part of them with a latency strongly varying with polarity; PSTHs for type IV showed narrow peaks and steep amplitude/intensity curves; PSTHs for type V showed a multiple peaked pattern and large amplitudes and steep amplitude/intensity curves to rarefaction polarity. The various features in the click responses were in most cases consistent with the type of FTC. The results can be used to explain deviations in whole-nerve recordings in abnormal cochleas.

Nonlinear input-output functions derived from the responses of guinea-pig cochlear nerve fibres: variations with characteristic frequency

Rate-versus-level functions (RLFs) were recorded from individual cochlear nerve fibres in anaesthetised guinea-pigs. Variations in the shapes of these functions with frequency were used to derive input-output (IO) relationships for the mechanical preprocessing mechanisms in the cochlea. It was assumed that these preprocessing mechanisms operated linearly at frequencies well below each fibre's characteristic frequency (CF). The IO functions derived at each fibre's CF provided strong evidence of compressively nonlinear preprocessing in most regions of the cochlea. However, the apparent degree of compression depended on the fibre's CF, and hence on the presumed site of cochlear innervation. For fibres with CFs of between 1.5 and 3.6 kHz, the CF derived IO functions grew at rates of around 0.5 dB/dB. For fibres with CFs above 4 kHz, the IO functions were more compressive, with high-intensity asymptotic slopes of around 0.13 dB/dB. In the highest (> or = 10 kHz) CF fibres, the degree of compression depended on the physiological condition of the cochlea; the derived IO functions becoming more linear as the cochlea became less sensitive. The derived IO technique was not well suited to analyse responses evoked by very low frequency (e.g., < 500 Hz) tones. Nonetheless, the CF RLFs from fibres with CFs lower than approximately 1 kHz provided little evidence of mechanical nonlinearity near the apex of the cochlea. These findings imply a longitudinal variation in the mechanisms of cochlear preprocessing, and provide important new tests for functional models of the cochlea.

Neonatal cochlear hearing loss results in developmental abnormalities of the central auditory pathways

We have used animal models of long term neonatal cochlear hearing loss to study developmental plasticity of the central auditory pathways. Newborn chinchilla pups and feline kittens were treated with the ototoxic drug amikacin, so as to induce basal lesions in the cochlea. At maturity these animals were used in single unit electrophysiological mapping studies, in which the cochleotopic organization of primary auditory cortex (of the cat) and the inferior colliculus of the midbrain (in the chinchilla) were mapped. We have observed, both in the midbrain and auditory cortex, massive reorganization of frequency representation. Most striking were the presence of large monotonic regions (i.e. large areas in which all neurons have similar tuning properties). Cochlear lesions which involve inner hair cells clearly modify the normal development of cochleotopic representation in the midbrain and cortical regions. We suggest that similar abnormal patterns of frequency representation will exist in human subjects with long term neonatal hearing loss.

Growth of evoked potential amplitude in neonatal chicks exposed to intense sound

The recovery of auditory function at selected intervals following exposure to a 0.9 kHz tone for 48 h at 120 dB SPL is described in neonatal chicks. Evoked potentials recorded from the nucleus magnocellularis were used to measure threshold sensitivity and peak-to-peak response amplitude as a function of stimulus intensity. The relation between evoked-response amplitude and stimulus intensity was nearly linear in control animals. However, at 10 days post exposure, the evoked response in mid-range frequencies showed a severe threshold shift and an abnormally rapid growth in amplitude. At 3 days post exposure, the rate of growth was nearly identical to that measured in control animals and threshold sensitivity showed considerable recovery. Current theories of amplitude-intensity growth and studies of basilar papilla damage and repair following intense sound exposure were applied in the analysis of these results.

Single-fibre and whole-nerve responses to clicks as a function of sound intensity in the guinea pig

This paper describes a study of the intensity dependence of click-evoked responses of auditory-nerve fibres in relation to the simultaneously recorded compound action potential (CAP). Condensation and rarefaction clicks were presented to normal hearing guinea pigs over an intensity range of 60 dB. The recorded poststimulus time histograms (PSTHs) were characterized by the latency (tp), amplitude (Ap) and synchronization (Sp) of their dominant peak, parameters that are particularly important for the understanding of the CAP. For all fibres tp decreased monotonically with increasing intensity, in a continuous way for fibres with high characteristic frequency (CF greater than 3 kHz), and in discrete steps of one CF-cycle for low-CF (CF less than or equal to 3 kHz) fibres. An additional analysis of PSTH envelopes revealed that average latency shifts with intensity are similar for all CFs above 2 kHz. For all fibres Ap increased monotonically with intensity; the increase was stronger and maximum values were larger for low-CF than for high-CF fibres. A schematic model PSTH was then formulated on the basis of the experimental data. A sum of these model PSTHs from a hypothesized fibre population was convolved with an elemental unit response (Versnel et al., 1992) in order to simulate the compound action potential. Synthesized CAPs agreed with experimental CAPs in their main aspects.

Effects of adaptation on electrocochleography and auditory brain-stem responses in the elderly

Changes in the amplitude and latency of the evoked potentials in electrocochleography (ECochG) and auditory brainstem responses (ABR) produced by increased stimulus rates (adaptation) were investigated using an extratympanic ECochG technique with simultaneous recording of the ABR in 12 elderly patients, and compared with those of eight normally hearing young adult subjects. Although the absolute latencies of the action potential (AP) and ABR waves were delayed in the elderly, the shift in latency of these components with increased stimulus rate was similar in both groups of subjects. Amplitudes of the AP and wave III component were reduced with increased stimulus rate to a degree which again was similar in both the elderly and young adults. On this basis we suggest that synaptic connections to nerve fibres from surviving hair cells in the elderly are functioning so that disturbance of this part of the acoustic nerve is not a feature of presbycusis.

Reorganization of auditory cortex after neonatal high frequency cochlear hearing loss

Cochleotopic representation in cortex (AI) is extensively reorganized in cats having neonatal, bilateral high frequency cochlear hearing loss. Anterior areas of AI, normally devoted to high frequencies, contain neurons which are almost all tuned to one lower frequency. This frequency corresponds, at the level of the cochlea, to the border between normal and damaged haircell regions.

Age-related changes in cochleas of mongolian gerbils

The effects of aging on the gerbil cochlea were studied in 16 animals raised in a quiet environment. Animals were tested at ages ranging from 33 to 36 months, the approximate average lifespan of gerbils in our colony. Hearing sensitivity was assessed by measures of whole-nerve compound action potential (CAP) thresholds and surface preparations of the organ of Corti were subsequently examined by light microscopy for losses of sensory hair cells. These quiet-aged animals showed a wide range of hair-cell losses and threshold shifts. Outer hair cells often showed significant losses while inner hair cells were rarely absent. All animals had some threshold shift, especially at frequencies above 4 kHz. These shifts ranged from 1 to 68 dB. At high frequencies, threshold shifts often occurred without hair-cell losses at corresponding cochlear locations. At low frequencies, threshold shifts seldom reflected the losses of hair cells commonly found in the cochlear apex. Thus, the correlation of specific hair-cell losses and CAP threshold shifts at corresponding frequencies was poor. On the other hand, the total number of missing hair cells, irrespective of location, was a good, general indicator of the hearing capacity in a given ear. It appears that the factor or factors that makes cochleas susceptible to hair-cell loss with increasing age also affects other cochlear mechanisms that are necessary for normal functioning of the ear.

Recoveries of whole-nerve AP thresholds, amplitudes and tuning curves in gerbils following noise exposure

The recoveries of whole-nerve action potential (AP) thresholds, AP amplitudes and AP tuning curves in gerbils were monitored following two weeks of exposure to band-pass noise at 85 dBA. Recordings were made by means of electrodes chronically implanted in the subjects' bullas. The noise exposure caused threshold elevations in all of our subjects, with the greatest shifts occurring an octave or more above the 2 kHz upper cutoff frequency of the noise. The magnitudes of the shifts varied greatly (up to 26 dB) across subjects. Thresholds of animals with the smallest initial loss of sensitivity returned to pre-exposure values within 16 days, while those of animals with greater initial losses remained elevated beyond 16 days. AP amplitudes and AP tuning curves (APTCs) were most affected at frequencies where the initial threshold shifts were greatest. At these frequencies AP amplitudes were reduced, and APTCs showed broadened tips, reduced tip-to-tail ratios, and in some cases a shift in the frequency of the tip. These effects did not necessarily reverse with threshold recovery, suggesting that AP amplitudes and AP tuning curves are more sensitive indices of acoustic injury than are AP thresholds.

Evoked-response forward-masking functions in chinchillas with noise-induced permanent hearing loss

Evoked-response forward-masking functions were measured by chronic electrodes in the inferior colliculus of the chinchilla before and after exposure to an intense tone that produced a permanent hearing loss. Before exposure, the forward-masking time constants ranged from 50 to 90 ms. After exposure, the forward-masking time constants increased significantly in the region of hearing loss, but not in regions where hearing was normal. The effect of the hearing loss on the time course of forward masking was most pronounced once the hearing loss exceeded 20-25 dB. These physiological changes in the evoked-response forward-masking functions appear to parallel those observed psychophysically in human listeners.

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.

Electrophysiology of the auditory system

This review has attempted to summarise the properties of electro physiological responses in the auditory system. The treatment was broad and consequently somewhat sketchy. For a more detailed recent treatment of the physiology of the auditory system the reader is referred to Pickles (1982), Møller (1983), or Altschuller et al (1986). The data on acoustic injury have been reviewed recently by Schmiedt (1984). Discussions of a number of topics such as development, hair cell function and speech encoding are found in Berlin (1984).

Evoked response 'forward masking' patterns in chinchillas with temporary hearing loss

Evoked response "forward masking" data were measured from the inferior colliculus of the chinchilla before and during a temporary threshold shift. The hearing loss was induced by a 2 kHz pure tone of 85 dB SPL presented from 5-8 days. The exposure elevated thresholds by approximately 35 dB at the mid frequencies, but had no effect on low frequency hearing. The exposure also altered the time course of the evoked response forward masking data. Time constants fitting the forward masking data increased by up to a factor of three at the frequency with the greatest loss, but remained within normal limits at the low frequencies where hearing was normal. The increase in the forward masking time constants became most noticeable once the hearing loss exceeded 25 dB. These physiological results are consistent with psychophysical forward masking data from hearing impaired listeners.

Adaptation in hearing-impaired ears: effects of intermediate duration stimuli

Recovery from adaptation was measured in acoustically-traumatized ears of cats for pure-tone adapters 1-60 s in duration at levels of 60-100 dB SPL. Adaptation was assessed by measuring the whole nerve action potentials in response to pure-tone stimuli. Effects of changing probe frequency and of changing adapter duration were measured. Impaired ears showed less adaptation than normal ears for adapters of similar SPL, but they showed relatively normal spread of adaptation across probe frequency. The effect was larger for long duration stimuli than for short duration stimuli.

Functional properties of auditory-nerve fibers during postnatal development in the kitten

The discharges of the auditory-nerve fibers were studied in kittens between 2-40 days of age. Up to the 10th postnatal day, fibers could be divided into two main categories: fibers with spontaneous activity (SA) that respond to sound and fibers without SA but with evoked responses. A third, smaller, category, fibers having neither SA nor evoked activity, was also present. The development of SA comprises two phases. The first, lasting from birth up to the third postnatal week, shows a relatively fast increase and the second, lasting up to adulthood, a slower increase. Typical tone burst responses can be recorded at the end of the first postnatal week. Thereafter reactivity steadily increases especially after the 10th postnatal day. In young animals, rate level function is characterized by a steep segment with a low dynamic range followed by a decrease in activity that lasts until the end of the second week. At this point adult-like functions may be observed, although maximal firing still increases for some weeks. Tuning curves and threshold sensitivity tend to develop inversely at corresponding frequencies. Fibers with low characteristic frequencies reach adult threshold before that of high frequency fibers and high frequency fibers reach adult tuning before low frequency fibers. A comparison of auditory-nerve fiber activity in kittens show that maturation of most functional characteristics lasts several weeks after birth and in some cases continues after the first postnatal month.

Single cochlear fibre responses in guinea pigs with long-term endolymphatic hydrops

Some cochlear fibre response properties have been measured in two GPs approximately one year after induction of endolymphatic hydrops (by surgical obliteration of the endolymphatic sac and duct). These animals are considered as models of the effects of hydrops in Menière's disease, and the purpose of the study was to examine any modifications of fibre response properties which may underly auditory symptoms of the disease in man. Neurones towards more apical cochlear regions (with low characteristic frequencies) showed the greatest deterioration in tuning properties; on average, in the 1-6 kHz range, Q10dB values were reduced by a factor of two compared with normal animals. Discharge rate versus intensity functions of such units were abnormally steep, with dynamic ranges reduced by 10-20 dB. Towards higher frequency regions neurone response properties showed less deterioration (contrasting with many other types of cochlear pathology where, in general, the high frequency basal region exhibits greatest vulnerability). We have also observed in a few units an abnormal bursting in both spontaneous and driven discharge. Interspike intervals during burst are less than 1 ms (within relative refractory period). These findings are related to the auditory symptoms of Menière's disease, in particular, poor frequency selectivity, loudness recruitment and tinnitus.

Receptor potentials of lizard cochlear hair cells with free-standing stereocilia in response to tones

Intracellular potentials were recorded with micropipettes from hair cells with free-standing stereocilia in the cochleae of anaesthetized alligator lizards. Wave forms of intracellular responses to click stimuli were classified into three types: hair cells, supporting cells, and untuned cells. We studied primarily the responses of hair cells to tonal stimuli. For most frequencies, f, and levels, P, of tone-burst stimuli, the response envelope of the receptor potential increases monotonically at the tone-burst onset, and decreases monotonically at tone-burst offset. Overshoot in the envelope of the response at the onset and offset of tone bursts is observed only for tone bursts of low f, high P, and short (approximately equal to 1 msec) rise/fall time. The steady-state response to tones consists of a positive (depolarizing) d.c. component, V0, plus a.c. components (e.g. a fundamental component, V1, second harmonic, V2, and third harmonic, V3). The magnitudes of a.c. and d.c. components are functions of f and P, and show three characteristics: frequency selectivity, non-linearity, and low-pass filtering. The receptor potential is frequency selective. The frequency selectivity of V0 and V1 components was measured by means of iso-voltage (iso-V0 and iso-V1) contours. Iso-V0 and iso-V1 contours are V-shaped: the maximum sensitivity occurs at a characteristic frequency (c.f.). The shapes of these contours near the c.f. depend on the values of V0 and V1 at which the contours were measured and are sharper for lower values of V0 and V1. The mean slopes of the low- and high-frequency sides of these contours are: -45.0 and +85.1 dB/decade for iso-V0 contours (n = 26), and -33.6 and +103.8 dB/decade for iso-V1 contours (n = 28). The receptor potential has non-linear properties. The magnitudes and phase angles of V0, V1, V2, and V3 receptor-potential components were measured as a function of P for different f. The slopes of level functions (the dependence of log V0 and log magnitude of V1 on log P) were measured at low levels for different f. For values of f differing from c.f. by more than a half-octave, the slope for V0 is between 1 and 2 with a mean of 1.3; the slope for V1 is about 1, i.e. magnitude of V1 increases approximately linearly with P. For frequencies near c.f., the slopes for V0 and V1 are approximately 0.8 and 0.5, respectively, indicating the presence of a compressive non-linearity.(ABSTRACT TRUNCATED AT 400 WORDS)

Compound action-potential tuning curves in normal and acoustically traumatized cats

Compound action-potential tuning curves, using a forward-masking paradigm, were developed on both a control group and a group of acoustically traumatized cats. Differences observed between the two populations included a decrease in the sharpness of the tip, in the sensitivity of the tip, and/or in the sensitivity of the tail region. Phase-contrast light microscopy was performed on all exposed ears using a celloidin-embedding technique with horizontal sectioning. Whenever an abnormality in an action-potential tuning curve was seen, histological evidence of damage to the organ of Corti in an appropriate region corresponding to the signal frequency was observed. However, several cases of damage to the cochlea were observed with normal tuning curves. Whenever the tip region of the tuning curve was elevated, evidence of damage to all three rows of outer hair cells and to the inner hair cells was seen.

Neural mechanisms in sound detection and temporal summation

The psychophysics and neurophysiology of sound detection in quiet and under noise masking were studied in goldfish. Psychophysical masking is a linear function of masker level. For long duration signals, signal-to-noise ratios (S/N) at threshold are 15.5, 19, and 22.5 dB for 200, 400 and 800 Hz signals, respectively, and is -5 dB for a noise signal. Threshold declines with signal duration to about 700 ms. The slopes of the masked temporal summation functions are about unity, indicating that energy is constant at threshold. In quiet however, the slopes are generally less than 0.5, indicating that shorter signals are detected at lower energy. Neural correlates of the masked S/Ns and the slopes of temporal summation functions were sought in the response patterns of single saccular neurons. Rate- and synchronization-intensity functions were obtained for tone and noise signals in quiet and in noise. S/Ns at behavioral threshold correspond closely to those required to raise spike rate just above that evoked by the masker alone, but are well above those required to cause clear synchronization. Therefore, sound detection is probably based on spike rate and not synchronization criteria. The equivalence of behavioral and neural thresholds indicates that the filters used in behavioral sound detection are simply the bandwidths of saccular fibers. A model outlined by Zwislocki which predicts the rate of temporal summation from the rate of growth of neural activity with intensity accounts quite well for the observed slopes of temporal summation functions both in quiet and in noise.

Loudness recruitment: contributing mechanisms as revealed by cochlear AP measures in man

A measure of the (average) rate of discharge versus intensity function of cochlear fibers can be obtained from cochlear-evoked compound action potentials using a tone-on-tone forward masking technique. The rationale for the method is presented. This technique was used to investigate, indirectly, cochlear fiber responses in human subjects, both with normal hearing and with deafness of cochlear origin and showing signs of loudness recruitment. In animals with pathologic cochleas, and change in rate of fiber discharge with intensity is more rapid than in normal animals. The present study confirms that this also is the case in human cochlear pathology and suggests that this abnormal steepening of rate versus intensity functions contributes to the phenomenon of loudness recruitment.

Postnatal development of frequency and intensity sensitivity of neurons in the anteroventral cochlear nucleus of kittens

Tuning curves and spike count-vs.-intensity functions were derived form tone-burst responses of single neurons of the anteroventral cochlear nucleus of kittens 4-45 days of age. During the first postnatal week tuning curves are relatively shallow and thresholds are high. With advancing age there is a progressive reduction in threshold and sharpness of tuning. Sharpening of tuning during the first several weeks postpartum seems to be due to a differential reduction in threshold between CF and frequencies below CF. Spike count-vs.-intensity functions are steep in young kittens as compared to adults. During the first few postnatal weeks the dynamic range and shapes of the functions take on the characteristics of adult AVCN neurons.