A computational model for rate-level functions from cat auditory-nerve fibers

A computationally tractable form of the rate-level model proposed by Sachs and Abbas (1974) is presented. The first stage of the model is a compressive nonlinearity whose input-output function is chosen to reflect current data on basilar-membrane displacement. The output of this nonlinearity is converted to driven discharge rate by the saturating nonlinearity originally used by Sachs and Abbas (1974). In fitting the model to data four model parameters are chosen to minimize the mean squared error between rate functions generated by the model and the data. With parameters chosen in this way, the model provides good fits to the range of rate-level shapes from flat saturations to sloping saturations. One important parameter in the model is the 'threshold for compression'. For low- and medium-spontaneous rate fibers with similar best frequencies (BFs), the minimum mean squared error compression threshold is roughly constant at about 30 dB above the thresholds of the most sensitive (high-spontaneous rate) fibers at that BF.     

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

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

Effects of electrical stimulation on the acoustically evoked auditory-nerve response in guinea pigs with a high-frequency hearing loss

Criteria for cochlear implantation keep expanding and people with substantial residual low-frequency hearing are considered candidates for implantation nowadays. Therefore, electro-acoustical stimulation in the same ear (EAS) is receiving increasing interest. We have investigated the effects of intracochlear electrical stimulation on acoustically evoked auditory-nerve activity, using a forward masking paradigm. The stimulation electrode was placed in the basal turn of the cochlea. Compound action potential (CAP) recordings were performed in guinea pigs with severe high-frequency hearing loss and in normal-hearing control animals. In normal-hearing animals, electrical stimulation generally suppressed CAPs, especially at high acoustic frequencies (8 and 16?kHz) and low sound levels. At low frequencies (0.5 and 1?kHz), suppression was observed only at high sound levels. In animals with a high-frequency hearing loss, suppression of CAPs at low frequencies was substantially less compared to control animals, even at high current levels and temporal overlap of acoustic and electric stimuli. Hence, effects of electrical stimulation substantially differed between normal-hearing animals and animals with a high-frequency hearing loss. These findings stress the need for a proper animal model when investigating EAS. We conclude that in case of high-frequency loss, the basal part of the cochlea can be stimulated electrically with little effect on responses to low-frequency acoustic stimuli.     

Effects of electrical stimulation of efferent olivocochlear neurons on cat auditory-nerve fibers. II. Spontaneous rate

In order to increase our understanding of cochlear mechanisms, we measured changes in the rate of spontaneous firing (SR) of single auditory-nerve fibers in response to the stimulation of medial olivocochlear efferents in cats. During the first second of efferent stimulation, SR was depressed by up to 35%, except in one very sensitive animal in which depressions up to 80% were found. With data from this aberrant cat excluded, the SR depression, on the average, increased as auditory-nerve fiber sensitivity increased, increased as the original SR decreased (data were not obtained for SRs less than two spikes/sec), and had a broad maximum at CFs of about 10 kHz. After the efferent stimulation was turned off, there was an "overshoot" in which the SR increased past the original rate in some fibers. The "overshoot" was larger for fibers with lower SRs and for fibers which showed larger "adaptation" in the efferent-induced depression of SR. The data on SR depression during efferent stimulation are consistent with two hypotheses: (1) that the stronger than usual efferent suppression of "spontaneous" rate found in some very sensitive fibers occurs because the "spontaneous" firing was, in part, a response to sound, and (2) that "true spontaneous" firing is reduced by the efferent-induced hyperpolarization of outer hair cells (OHCs) being electrically coupled through the endocochlear potential to inner hair cells (IHCs). It is suggested that (1) the efferent-induced suppression of "true spontaneous" activity is largest at CFs near 10 kHz because this CF region receives the greatest OHC innervation from medial efferents and the efferent-induced change in OHCs is electrically coupled to IHCs, whereas (2) the efferent suppression of responses to sound is largest at lower CFs because the efferent endings on OHCs act to decrease the motion of the basilar membrane and this change is propagated apically from the active efferent synapses on OHCs.     

Effects of electrical stimulation of efferent olivocochlear neurons on cat auditory-nerve fibers. I. Rate-level functions

In previous studies describing the effects of electrically stimulating the olivocochlear bundle, it seems possible that both medial and lateral (MOC and LOC) efferents may have been stimulated. To selectively stimulate MOC efferents, we used an electrode placed at the origin of the MOC efferents in the brainstem (MOC stimulation). For comparison, a stimulating electrode was placed in the fourth ventricle at the decussation of the crossed olivocochlear bundle where both MOC and LOC efferents are present (midline-OCB stimulation). Rate versus sound level functions from auditory-nerve fibers were obtained with and without efferent stimulation. Stimulation at either location shifted rate vs. level functions to higher sound levels and depressed the rate in the plateau. For fibers with high spontaneous rates, the level shifts and plateau depressions had slightly different distributions as a function of characteristic frequency. The average amplitudes of these effects were largest for midline-OCB stimulation, next largest for crossed MOC stimulation and smallest for uncrossed MOC stimulation. The qualitative pattern of the effects, however, did not depend on the location of the stimulus electrode. The amplitudes of the efferent-induced effects were different for auditory-nerve fibers with different spontaneous rates (by as much as a factor of three for the plateau depression). The results support several hypotheses: (1) the effects of midline-OCB stimulation are due only to the action of MOC efferents, (2) individual crossed and uncrossed MOC fibers produce similar effects, and (3) efferents differentially change the information carrying properties of auditory-nerve fibers in different spontaneous-rate categories. These results, taken together with anatomical data in the literature, are consistent with the hypothesis that, in the cat, MOC and midline-OCB stimulation have their effect solely through synapses on outer hair cells. The data are consistent with the hypothesis that the level shifts are produced by MOC efferents acting on outer hair cells to reduce the mechanical stimulus to inner hair cells. It seems likely that some other mechanism is required to produce the plateau depressions, at least for auditory-nerve fibers with high spontaneous rates.     

Curious oddments of auditory-nerve studies

Three interesting theoretical issues are presented to illustrate how certain isolated observations on auditory-nerve activity can be puzzling until other, seemingly unrelated phenomena are documented. The issues are (1) disinhibition; (2) 'peak-splitting'; and (3) independence of spike generation in primary neurons innervating the same inner-hair cell. (1) The issue of disinhibition is important for theories of lateral inhibition. For auditory-nerve fibers, the question can he phrased, 'If the rate of discharge to a tone at the characteristic frequency (CF) of a unit can he reduced by adding a second tone off the CF, is it possible to suppress this reduction by adding a third tone, even further off the CF?' The data are insufficient to conclude that disinhibition is found for auditory-nerve fibers and other explanations are available to account for the results of three-tone experiments. (2) Normally, only a single peak in the histogram of responses to low tones is phase-locked, but at high stimulus levels, the histograms will show two, or even three, peaks per stimulus cycle ('peak-splitting'). At still higher levels, the histograms again show only a single peak, but it is phase-shifted from the original peak for low stimulus levels. This complex sequence of events can be accounted for by simple models. (3) Although simultaneous recordings from pairs of auditory-nerve fibers have failed to show non-stimulus related correlations between spike trains, it has not been directly demonstrated that any two recorded fibers innervate the same hair cell. However, an indirect argument is offered to support the idea that fibers innervating a single inner-hair cell must have independent spike generators.     

对灰鼠延迟性神经元死亡模型中螺旋神经节细胞缺损的评估

目的探索观察耳蜗螺旋神经节细胞的简便定量研究方法 ,并在灰鼠延迟性螺旋神经节细胞死亡动物模型中验证本方法的实用性和可靠性。方法 15只成年灰鼠平均分为3组,第1组用于正常对照;第2组一次性同时肌肉注射庆大霉素（125mg/kg）和静脉注射利尿酸钠（40mg/kg）,并在用药后2个月处死;第3组接受与第2组同样的药物注射,但在用药后4个月处死。耳蜗样品被常规应用环氧树脂包埋并制作成耳蜗中轴半薄切片,计数耳蜗各回蜗轴螺旋管腔切片截面内的螺旋神经节数量并进行统计分析。结果灰鼠耳蜗底回起始端的蜗轴螺旋管腔比耳蜗底回中部和耳蜗中回大,因此耳蜗底回起始端蜗轴螺旋管内的螺旋神经节细胞数量多于耳蜗底回中部和耳蜗中回。耳蜗毛细胞被彻底破坏后2个月,与正常灰鼠耳蜗各回蜗轴螺旋管腔切片截面内的螺旋神经节细胞数量相比,耳蜗底回蜗轴螺旋管切片截面内的螺旋神经节细胞减少数量比耳蜗中回严重,提示延迟性螺旋神经节细胞死亡可能遵循着从耳蜗底回向顶回扩展的规律。耳蜗毛细胞被彻底破坏后4个月,全耳蜗蜗轴螺旋管内的螺旋神经节细胞基本上丧失殆尽。结论计数耳蜗中轴切片各回蜗轴螺旋管腔切片截面内的螺旋神经节细胞数量是一种简便可靠的定量分析方法 。     

Effects of electrical stimulation of efferent olivocochlear neurons on cat auditory-nerve fibers. III. Tuning curves and thresholds at CF

In order to study the effects of efferent activity, olivocochlear efferents were stimulated with an electrode in the fourth ventricle at the decussation of the crossed olivocochlear bundle (midline-OCB stimulation) or with an electrode at the brainstem origin of medial efferents (MOC stimulation). Tuning curves, or similar measures of threshold, were obtained from auditory-nerve fibers in the presence or absence of efferent stimulation. Efferent stimulation raised the thresholds of fibers for tones at the characteristic frequency (CF) by an amount which varied with the spontaneous rate (SR) of the auditory-nerve fiber. On the average, high-SR fibers had the smallest threshold shifts, and low-SR fibers had the largest threshold shifts. The distribution of threshold shifts as a function of CF peaked at CFs of 3-8 kHz for high-SR and medium-SR fibers but appeared to peak at higher CFs for low-SR fibers. Within the high-SR or medium-SR groups, the fibers with the lowest thresholds had the largest threshold shifts. Efferent stimulation decreased the Q20 of the tuning curves from most fibers (i.e. it made the tuning curves wider), but increased the Q20 from some fibers with CFs below 2 kHz. For fibers with CFs above 4 kHz, efferent stimulation shifted the tuning-curve tails to higher sound levels by about 1 dB on the average. The qualitative patterns of the effects due to midline-OCB stimulation or to MOC stimulation were similar. The distribution of high-SR threshold shifts vs. CF appears to be displaced apically in the cochlea compared to the distribution of MOC endings on outer hair cells. This can be understood in terms of efferent activity depressing basilar membrane motion and affecting regions at, and apical to, the activated efferent synapses. To explain the low-SR threshold shifts, an additional way in which efferent activity inhibits responses appears to be required. The data are consistent with one function of the medial efferents being to raise the thresholds of auditory-nerve fibers and thereby adjust the effective range of the auditory system.     

Representation of voice pitch in discharge patterns of auditory-nerve fibers

Responses of populations of auditory-nerve fibers were measured for synthesized consonant-vowel stimuli. This paper explores the encoding of fundamental frequency (pitch) in these responses. Post-stimulus time (PST) histograms were computed from 25 ms segments of the spike trains. Discrete Fourier transforms with a 40 Hz resolution were computed from the histograms. Two representations of pitch are considered. The first representation is based on the pitch-related temporal properties of the speech signal. Histograms for individual units can show envelope modulations directly related to the pitch period. These modulations reflect the responses of these fibers to a number of stimulus harmonics near fiber CF. Responses of fibers near formant frequencies are dominated by a single large harmonic component, and thus show small or no pitch-related enveloped modulations. Envelope modulations are reduced in the presence of background noise. The second representation uses both temporal properties of auditory-nerve responses and cochlear place to encode the pitch-related harmonic structure of speech. As a measure of the response of the population of fibers to each harmonic of 40 Hz the magnitude of the component of the Fourier transform at that frequency was averaged across all fibers whose characteristic frequencies were within one-fourth octave of that harmonic. We call this measure the average localized synchronized rate (ALSR). The ALSR provides a good representation of stimulus spectrum, even in the presence of background noise. From the harmonic structure of the ALSR, we are able to extract the stimulus pitch frequency. The relationship of these two representations to pitch perception in both acoustic and electrical stimulation (via cochlear implants) is discussed.     

A model of threshold for pulsatile electrical stimulation of cochlear implants

Threshold measures have been made as a function of the repetition rate and pulse duration of biphasic electrical pulses applied to the cochlea through a cochlear implant (Shannon, 1985). Nonmonotonicities in those data suggest that at least two separate processes are involved in the translation of an electrical stimulus into a threshold perception. This paper presents a phenomenological model which accounts for the key features of the threshold data. The model consists of two parallel processes which are each power-law functions of the instantaneous current amplitude. The output of each process is then integrated with a short time constant (approximately 1–2 ms). The maximum of these two outputs represents the sensory magnitude of that electrical stimulus. Threshold data from 14 patients implanted with three different devices are compared to model predictions over a wide range of pulse durations and pulse rates. Since the model accurately predicts thresholds over such a wide range of stimuli, it is possible that it can predict the threshold of an arbitrary electrical stimulus. This model could be used to construct a speech processor that would convert any acoustic waveform into an equivalent electrical waveform that would preserve threshold relationships.     

Phase-locking of auditory-nerve discharges to sinusoidal electric stimulation of the cochlea.

The activity of auditory-nerve fibers was recorded in anesthetized cats in response to sinusoidal electric stimuli applied through a bipolar electrode pair inserted about 5 mm into the cochlea through the round window. The synchronization index was calculated from period histograms for frequencies ranging from 0.2 to over 10 kHz. The stimulus artifact was largely eliminated through the use of differential micropipettes and an adaptive digital filter. Measured synchronization indices were many times larger than the indices that could be attributed to the residual stimulus artifact. Synchronization indices at each stimulus frequency varied considerably from fiber to fiber, even in the same animal. The dependence of synchrony on stimulus frequency was also variable, decreasing monotonically in some fibers and nonmonotonically in others. The average electric synchronization index for all fibers did not fall as steeply with frequency as does the average synchrony for acoustic stimuli. The finding of significant phase locking to electric stimuli well above 1 kHz suggests that the poor frequency discrimination of cochlear-implant recipients for single-channel stimulation above this frequency may be due to the inability of the central processor to make effective use of the available phase-locking information for monaural stimulation.     

Representation of whispered vowels in discharge patterns of auditory-nerve fibers

We have previously shown that the spectra of speech sounds can be represented in the temporal patterns of discharge of populations of auditory-nerve fibers. These results were obtained using perfectly periodic stimuli, for which a temporal representation is straightforward. In order to see if our results could be generalized to nonperiodic stimuli, we have studied responses to a whispered vowel with formant frequencies typical of /ε/. The whispered vowel was generated by exciting a vocal tract model with noise; this signal was therefore aperiodic. Temporal patterns of responses to the vowel in populations of auditory-nerve fibers were analyzed using interval histograms. Fourier transforms of these histograms show large components at frequencies near the formant frequencies of the vowel. With these Fourier transform components as a measure of the temporal response, a temporal-place representation of the response of populations of fibers preserves the spectral features of the aperiodic vowel stimulus. Profiles of average rate versus characteristic frequency for fibers with spontaneous rates greater than $\text{1}\text{s}$ show little if any formant-related structure. On the other hand, such profiles for fibers with spontaneous rates less than $\text{1}\text{s}$ may show peaks in the region of the formants.     

Spectral characteristics of the responses of primary auditory-nerve fibers to amplitude-modulated signals

The spectral responses of cat single primary auditory nerve fibers to sinusoidal amplitude-modulated (AM) and double-sideband (DSB) acoustic signals applied to the ear were examined. DSB is an amplitude-modulated signal with a suppressed carrier. Period histograms were compiled from the neural spike-train data, and the frequency spectrum was determined by Fourier transforming these histograms. For DSB signals, spectral components were found to be present at the frequencies of the stimulus as well as at certain combination frequencies. For AM signals, several clusters of spectral components were present. The lowest-frequency cluster consisted of components at DC, at the modulation frequency, and at its harmonics. A higher frequency cluster occurs around a component with the frequency of the carrier. The components of cluster are separated from the carrier by the modulation frequency and its harmonics. Yet higher-frequency clusters appear around multiples of the carrier frequency with components at frequencies separated from these multiples by the modulation frequency and its harmonics. The magnitudes of these spectral components were determined for carrier frequencies located below, at, and above the characteristic frequency of the units, and for different stimulus levels, modulation frequencies, and modulation depths. The low-frequency components present in the neural spike train appear to be the result of demodulation taking place in the inner ear. The demodulated components are strong and are present over a wide range of sound levels, carrier frequencies, modulation frequencies, and nerve-fiber characteristics. This demodulation may be significant for speech recognition.     

Effects of chronic cochlear de-efferentation on auditory-nerve response.

The olivocochlear bundle was sectioned at the floor of the fourth ventricle in a series of cats. From three to thirty weeks post-operatively, recordings were made from single auditory-nerve fibers. Tuning curves, spontaneous discharge rates, and rate-level functions for tones at the characteristic frequency were measured and compared to normal data. Light- and electron-microscopic analysis of the cochleas suggested the lesions were complete, for both classes of cochlear efferents, in three cases. Electrophysiological data from these cases showed normal thresholds, tuning curves and rate-level functions; however, the distributions of spontaneous activity suggested significant decreases in average rates in the de-efferented cases.     

A model of the electrically excited human cochlear neuron. II. Influence of the three-dimensional cochlear structure on neural excitability

A simplified spiraled model of the human cochlea is developed from a cross sectional photograph. The potential distribution within this model cochlea is calculated with the finite element technique for an active scala tympani implant. The method in the companion article [Rattay et al., 2001] allows for simulation of the excitation process of selected elements of the cochlear nerve. The bony boundary has an insulating influence along every nerve fiber which shifts the stimulation condition from that of a homogeneous extracellular medium towards constant field stimulation: for a target neuron which is stimulated by a ring electrode positioned just below the peripheral end of the fiber the extracellular voltage profile is rather linear. About half of the cochlear neurons of a completely innervated cochlea are excited with monopolar stimulation at three-fold threshold intensity, whereas bipolar and especially quadrupolar stimulation focuses the excited region even for stronger stimuli. In contrast to single fiber experiments with cats, the long peripheral processes in human cochlear neurons cause first excitation in the periphery and, consequently, neurons with lost dendrite need higher stimuli.     

耳蜗内高速率电刺激对大鼠下丘神经元兴奋性的影响

目的 探讨耳蜗内不同强度下高速率电刺激对大鼠下丘神经元兴奋性的影响。方法在刺激强度分别为电刺激听性脑干反应（electrically evoked auditory brainstem responses,EABR）阈上6 dB和阈上12dB,用500pps和1000pps两种不同的电刺激速率急性刺激大鼠耳蜗,记录刺激2小时内及刺激停止后2小时内下丘神经元近场电位幅值。结果 刺激强度为EABR阈上6dB时,应用500 ppsg刺激速率,下丘神经元近场电位幅值在刺激过程中及刺激后2小时内都呈现上升趋势;应用1000pps刺激速率刺激2小时内,下丘神经元兴奋性有所下降,至刺激停止时为87％,但刺激停止后2小时内幅值不仅得到恢复,而且高于刺激前水平;予EABR阈上12dB强度电流刺激,在两种电刺激速率下,刺激后均可见下丘神经元反应幅值明显下降,速率越高下降越明显,恢复趋势越微弱。急性刺激停止时,500pps和1000pps刺激速率下丘神经元反应幅值分别下降52％、69％（P〈0．05）,刺激停止后120分钟,分别下降10％、59％。结论在较低电流强度（EABR阈上6dB）刺激下,高速率电刺激（1000pps）会导致下丘神经元兴奋性下降,但刺激停止后可恢复甚至高于刺激前水平;高强度、高速率电刺激会导致下丘神经元兴奋性下降,且刺激停止后恢复较慢：与听觉神经纤维相比,下丘神经元兴奋性受耳蜗内高速率电刺激的影响更大。     

Towards a unifying basis of auditory thresholds: distributions of the first-spike latencies of auditory-nerve fibers

Detecting sounds in quiet is the simplest task performed by the auditory system, but the neural mechanisms underlying perceptual detection thresholds for sounds in quiet are still not understood. Heil and Neubauer [Heil, P., Neubauer, H., 2003. A unifying basis of auditory thresholds based on temporal summation. Proc. Natl. Acad. Sci. USA 100, 6151-6156] have provided evidence for a simple probabilistic model according to which the stimulus, at any point in time, has a certain probability of exceeding threshold and being detected. Consequently, as stimulus duration increases, the cumulative probability of detection events increases, performance improves, and threshold amplitude decreases. The origin of these processes was traced to the first synapse in the auditory system, between the inner hair cell and the afferent auditory-nerve fiber (ANF). Here we provide further support for this probabilistic "continuous-look" model. It is derived from analyses of the distributions of the latencies of the first spikes of cat ANFs with very low spontaneous discharge rates to tones of different amplitudes. The first spikes in these fibers can be considered detection events. We show that, as predicted, the distributions can be explained by the joint probability of the occurrence of three independent sub-events, where the probability of each of those occurring is proportional to the low-pass filtered stimulus amplitude. The "temporal integration" functions of individual ANFs, derived from their first-spike latencies, are remarkably similar in shape to "temporal integration" functions, which relate threshold sound pressure level at the perceptual level to stimulus duration. This further supports a close link between the mechanisms determining the timing of the first (and other) evoked spikes at the level of the auditory nerve and detection thresholds at the perceptual level. The possible origin and some functional consequences of the expansive power-law non-linearity are discussed.     

Spread of excitation varies for different electrical pulse shapes and stimulation modes in cochlear implants

In Cochlear Implants (CI) Bipolar (BP) electrical stimulation has been suggested as a method to reduce the spread of current along the cochlea. However, behavioral measurements in BP mode have shown either similar or worse performance than in Monopolar (MP) mode. This could be explained by a bimodal excitation pattern, with two main excitation peaks at the sites of the stimulating electrodes. We measured the Spread of Excitation (SOE) by means of the Electrically Evoked Compound Action Potential (ECAP), obtained using the forward-masked paradigm. The aim was to measure the bimodality of the excitation and to determine whether it could be reduced by using asymmetric pulses. Three types of maskers shapes were used: Symmetric (SYM), Pseudomonophasic (PS), and Symmetric with a long Inter-Phase Gap (SYM-IPG) pulses. Maskers were presented in BP?+?9 (wide), BP?+?3 (narrow) and MP (only SYM) mode on fixed electrodes. The SOE obtained with the MP masker showed a main excitation peak close to the masker electrode. Wide SYM maskers produced bimodal excitation patterns showing two peaks close to the electrodes of the masker channel, whereas SYM-IPG maskers showed a single main peak near the electrode for which the masker's second phase (responsible for most of the masking) was anodic. Narrow SYM maskers showed complex and wider excitation patterns than asymmetric stimuli consistent with the overlap of the patterns produced by each channel's electrodes. The masking produced by narrow SYM-IPG and PS stimuli was more pronounced close to the masker electrode for which the effective phase was anodic. These results showed that the anodic polarity is the most effective one in BP mode and that the bimodal patterns produced by SYM maskers could be partially reduced by using asymmetric pulses.     

A new model for calculating auditory excitation patterns and loudness for cases of cochlear hearing loss

A model for calculating auditory excitation patterns and loudness for steady sounds for normal hearing is extended to deal with cochlear hearing loss. The filters used in the model have a double ROEX-shape, the gain of the narrow active filter being controlled by the output of the broad passive filter. It is assumed that the hearing loss at each audiometric frequency can be partitioned into a loss due to dysfunction of outer hair cells (OHCs) and a loss due to dysfunction of inner hair cells (IHCs). OHC loss is modeled by decreasing the maximum gain of the active filter, which results in increased absolute threshold, reduced compressive nonlinearity and reduced frequency selectivity. IHC loss is modeled by a level-dependent attenuation of excitation level, which results in elevated absolute threshold. The magnitude of OHC loss and IHC loss can be derived from measures of loudness recruitment and the measured absolute threshold, using an iterative procedure. The model accurately fits loudness recruitment data obtained using subjects with unilateral or highly asymmetric cochlear hearing loss who were required to make loudness matches between tones presented alternately to the two ears. With the same parameters, the model predicted loudness matches between narrowband and broadband sound reasonably well, reflecting loudness summation. The model can also predict when a dead region is present.