Mechanical Effects of Cochlear Implant on Acoustic Hearing

Residual hearing loss in cochlear implant users is investigated using the mechanical-human-cochlear model. Hearing loss due to stiffening of the round window increases significantly as input frequencies decrease from 3 kHz to 1 kHz but remains constant at lower frequencies, whereas loss due to the presence of an electrode insert becomes significantly higher at lower frequencies ([Formula: see text] kHz). The latter also shifts the characteristic frequency map toward the basal end of the cochlea. In the region away from the end of the electrode insert, cochlear function recovers, but the user still suffers from hearing loss caused by round window stiffening.

Investigating time-efficiency of forward masking paradigms for estimating basilar membrane input-output characteristics

It is well known that pure-tone audiometry does not sufficiently describe individual hearing loss (HL) and that additional measures beyond pure-tone sensitivity might improve the diagnostics of hearing deficits. Specifically, forward masking experiments to estimate basilar-membrane (BM) input-output (I/O) function have been proposed. However, such measures are very time consuming. The present study investigated possible modifications of the temporal masking curve (TMC) paradigm to improve time and measurement efficiency. In experiment 1, estimates of knee point (KP) and compression ratio (CR) of individual BM I/Os were derived without considering the corresponding individual “off-frequency” TMC. While accurate estimation of KPs was possible, it is difficult to ensure that the tested dynamic range is sufficient. Therefore, in experiment 2, a TMC-based paradigm, referred to as the “gap method”, was tested. In contrast to the standard TMC paradigm, the maker level was kept fixed and the “gap threshold” was obtained, such that the masker just masks a low-level (12 dB sensation level) signal. It is argued that this modification allows for better control of the tested stimulus level range, which appears to be the main drawback of the conventional TMC method. The results from the present study were consistent with the literature when estimating KP levels, but showed some limitations regarding the estimation of the CR values. Perspectives and limitations of both approaches are discussed.

Intracochlear Pressure Transients During Cochlear Implant Electrode Insertion

HYPOTHESIS

Cochlear implant (CI) electrode insertion into the round window induces pressure transients in the cochlear fluid comparable to high-intensity sound transients.

BACKGROUND

Many patients receiving a CI have some remaining functional hearing at low frequencies; thus, devices and surgical techniques have been developed to use this residual hearing. To maintain functional acoustic hearing, it is important to retain function of any hair cells and auditory nerve fibers innervating the basilar membrane; however, in a subset of patients, residual low-frequency hearing is lost after CI insertion. Here, we test the hypothesis that transient intracochlear pressure spikes are generated during CI electrode insertion, which could cause damage and compromise residual hearing.

METHODS

Human cadaveric temporal bones were prepared with an extended facial recess. Pressures in the scala vestibuli and tympani were measured with fiber-optic pressure sensors inserted into the cochlea near the oval and round windows, whereas CI electrodes (five styles from two manufacturers) were inserted into the cochlea via a round window approach.

RESULTS

Pressures in the scala tympani tended to be larger in magnitude than pressures in the scala vestibuli, consistent with electrode insertion into the scala tympani. CI electrode insertion produced a range of pressure transients in the cochlea that could occur alone or as part of a train of spikes with equivalent peak sound pressure levels in excess of 170 dB sound pressure level. Instances of pressure transients varied with electrode styles.

CONCLUSION

Results suggest electrode design, insertion mechanism, and surgical technique affect the magnitude and rate of intracochlear pressure transients during CI electrode insertion. Pressure transients showed intensities similar to those elicited by high-level sounds and thus could cause damage to the basilar membrane and/or hair cells.

Fatigue Modeling via Mammalian Auditory System for Prediction of Noise Induced Hearing Loss

Noise induced hearing loss (NIHL) remains as a severe health problem worldwide. Existing noise metrics and modeling for evaluation of NIHL are limited on prediction of gradually developing NIHL (GDHL) caused by high-level occupational noise. In this study, we proposed two auditory fatigue based models, including equal velocity level (EVL) and complex velocity level (CVL), which combine the high-cycle fatigue theory with the mammalian auditory model, to predict GDHL. The mammalian auditory model is introduced by combining the transfer function of the external-middle ear and the triple-path nonlinear (TRNL) filter to obtain velocities of basilar membrane (BM) in cochlea. The high-cycle fatigue theory is based on the assumption that GDHL can be considered as a process of long-cycle mechanical fatigue failure of organ of Corti. Furthermore, a series of chinchilla experimental data are used to validate the effectiveness of the proposed fatigue models. The regression analysis results show that both proposed fatigue models have high corrections with four hearing loss indices. It indicates that the proposed models can accurately predict hearing loss in chinchilla. Results suggest that the CVL model is more accurate compared to the EVL model on prediction of the auditory risk of exposure to hazardous occupational noise.

Comparing auditory filter bandwidths, spectral ripple modulation detection, spectral ripple discrimination, and speech recognition: Normal and impaired hearing

Some listeners with hearing loss show poor speech recognition scores in spite of using amplification that optimizes audibility. Beyond audibility, studies have suggested that suprathreshold abilities such as spectral and temporal processing may explain differences in amplified speech recognition scores. A variety of different methods has been used to measure spectral processing. However, the relationship between spectral processing and speech recognition is still inconclusive. This study evaluated the relationship between spectral processing and speech recognition in listeners with normal hearing and with hearing loss. Narrowband spectral resolution was assessed using auditory filter bandwidths estimated from simultaneous notched-noise masking. Broadband spectral processing was measured using the spectral ripple discrimination (SRD) task and the spectral ripple depth detection (SMD) task. Three different measures were used to assess unamplified and amplified speech recognition in quiet and noise. Stepwise multiple linear regression revealed that SMD at 2.0 cycles per octave (cpo) significantly predicted speech scores for amplified and unamplified speech in quiet and noise. Commonality analyses revealed that SMD at 2.0 cpo combined with SRD and equivalent rectangular bandwidth measures to explain most of the variance captured by the regression model. Results suggest that SMD and SRD may be promising clinical tools for diagnostic evaluation and predicting amplification outcomes.

Comparison of distortion-product otoacoustic emission growth rates and slopes of forward-masked psychometric functions

Slopes of forward-masked psychometric functions (FM PFs) were compared with distortion-product otoacoustic emission (DPOAE) input/output (I/O) parameters at 1 and 6 kHz to test the hypothesis that these measures provide similar estimates of cochlear compression. Implicit in this hypothesis is the assumption that both DPOAE I/O and FM PF slopes are functionally related to basilar-membrane (BM) response growth. FM PF-slope decreased with signal level, but this effect was reduced or reversed with increasing hearing loss; there was a trend of decreasing psychometric function (PF) slope with increasing frequency, consistent with greater compression at higher frequencies. DPOAE I/O functions at 6 kHz exhibited an increase in the breakpoint of a two-segment slope as a function of hearing loss with a concomitant decrease in the level of the distortion product (L(d)). Results of the comparison between FM PF and DPOAE I/O parameters revealed only a weak correlation, suggesting that one or both of these measures may provide unreliable information about BM compression.

Differences in loudness of positive and negative Schroeder-phase tone complexes as a function of the fundamental frequency

Tone complexes with positive (m+) and negative (m-) Schroeder phase show large differences in masking efficiency. This study investigated whether the different phase characteristics also affect loudness. Loudness matches between m+ and m- complexes were measured as a function of (1) the fundamental frequency (f0) for different frequency bands in normal-hearing and hearing-impaired subjects, and (2) intensity level in normal-hearing subjects. In normal-hearing subjects, the level of the m+ stimulus was up to 10 dB higher than that of the corresponding m- stimulus at the point of equal loudness. The largest differences in loudness were found for levels between 20 and 60 dB SL. In hearing-impaired listeners, the difference was reduced, indicating the relevance of active cochlear mechanisms. Loudness matches of m+ and m- stimuli to a common noise reference (experiment 3) showed differences as a function of f0 that were in line with direct comparisons from experiment 1 and indicated additionally that the effect is mainly due to the specific internal processing of m+. The findings are roughly consistent with studies pertaining to masking efficiency and can probably not be explained by current loudness models, supporting the need for incorporating more realistic cochlea simulations in future loudness models.

Finite element model of the stapes-inner ear interface

Since 1958, stapedotomy has been the method of choice for middle ear surgeons who operate on patients suffering from otosclerosis, especially stiffening of the interface between the stapes footplate of the middle ear and the oval window, which is a part of the cochlea of the inner ear. Later, many surgeons started to use the Schuknecht prosthesis, which consists of cartilage and is inserted into the complete opened oval window during stapedectomy. Our study shows that basilar membrane (BM) displacement is increased with an increasing stapes footplate area by a numerical simulation including the different geometries. An increase in the footplate area leads to an increase in BM displacement equivalent to 13 dB. Therefore, we recommend prostheses with areas as big as the normal stapes footplate area.

3D finite element analyses of insertion of the Nucleus standard straight and the Contour electrode arrays into the human cochlea

Previous experimental studies of insertion of the Nucleus standard straight and the Contour arrays into the scala tympani have reported that the electrode arrays cause damage to various cochlear structures. However, the level of insertion-induced damage by these electrode arrays to cochlear structures (the spiral ligament, the basilar membrane and the osseous spiral lamina) has not been quantified. Although it has been suggested that rotation can overcome this resistance and prevent the basilar membrane from being pierced by the tip of the Nucleus standard straight array, there has not been any attempt to study the relationship between the rotation and the reduction of damage to the basilar membrane. In this study, 3D finite element analyses of insertions of the Nucleus standard straight array and the Contour array into the scala tympani have been undertaken. The perforation of the basilar membrane by the tip of the Nucleus standard straight array at the region of 11-14 mm from the round window appears to be compounded by the geometry of the spiral passage of the scala tympani. Anti-clockwise rotations between 25 degrees and 90 degrees applied at the basal end of the electrode array (for the right cochlea) were shown to significantly reduce the contact stresses exerted by the tip on the basilar membrane which support the practice of applying small rotation partway through insertion of electrode array to minimize damage to the basilar membrane. Although the Contour array (with its stylet intact) is stiffer than the Nucleus standard straight array, a slight withdrawal of the stylet from the Contour array before insertion was found to significantly reduce damage by the electrode array to the spiral ligament and the basilar membrane.

Eyes as fenestrations to the ears: a novel mechanism for high-frequency and ultrasonic hearing

Intense airborne ultrasound has been associated with hearing loss, tinnitus, and various nonauditory subjective effects, such as headaches, dizziness, and fullness in the ear. Yet, when people detect ultrasonic components in music, ultrasound adds to the pleasantness of the perception and evokes changes in the brain as measured in electroencephalograms, behavior, and imaging. How does the airborne ultrasound get into the ear to create such polar-opposite human effects? Surprisingly, ultrasound passes first through the eyes; thus, the eye becomes but another window into the inner ear.

Temporal masking curves for hearing-impaired listeners

The decay of forward masking was investigated for three subjects with moderate sensorineural hearing loss. For such subjects, compression on the basilar membrane (BM) is thought to be largely absent, enabling one to determine the decay of masking without the influence of compression. Temporal masking curves (TMCs), plots of the masker level at threshold against delay between masker offset and signal onset, were measured for delays of 0, 15, 30, 45, 60, and 75 ms, for signal frequencies, fs, of 500, 1000, 2000, 4000, and 6000 Hz. Masker frequencies were 0.5, 0.8, 1.0, 1.15, and 1.3 times fs. Most of the TMCs were well fitted with single-segment straight lines, which, except for high masker levels, were roughly parallel for each fs, supporting the belief that BM compression was largely absent in these subjects. However, the slopes of the TMCs were greater for fs = 500 and 1000 Hz than for higher frequencies, which may indicate that the decay of forward masking is not the same for all signal frequencies. The results suggest that it may not be valid to infer BM compression at low signal frequencies by using a reference TMC for a high fs.

Cochlear compression in listeners with moderate sensorineural hearing loss

Psychophysical estimates of basilar membrane (BM) responses suggest that normal-hearing (NH) listeners exhibit constant compression for tones at the characteristic frequency (CF) across the CF range from 250 to 8000 Hz. The frequency region over which compression occurs is broadest for low CFs. This study investigates the extent that these results differ for three hearing-impaired (HI) listeners with sensorineural hearing loss. Temporal masking curves (TMCs) were measured over a wide range of probe (500-8000 Hz) and masker frequencies (0.5-1.2 times the probe frequency). From these, estimated BM response functions were derived and compared with corresponding functions for NH listeners. Compressive responses for tones both at and below CF occur for the three HI ears across the CF range tested. The maximum amount of compression was uncorrelated with absolute threshold. It was close to normal for two of the three HI ears, but was either slightly (at CFs < or =1000 Hz) or considerably (at CFs > or =4000 Hz) reduced for the third ear. Results are interpreted in terms of the relative damage to inner and outer hair cells affecting each of the HI ears. Alternative interpretations for the results are also discussed, some of which cast doubts on the assumptions of the TMC-based method and other behavioral methods for estimating human BM compression.

[Diagnosis of endolymphatic hydrops using low frequency modulated distortion product otoacoustic emissions]

BACKGROUND

The low frequency modulation of distortion product otoacoustic emissions (DPOAEs) is an objective audiometric method that appears to be a useful tool for the diagnosis of endolymphatic hydrops (EH), e.g. in patients with Menière's disease, or in those who present only some of the symptoms of the disease.

METHOD

Low-frequency modulated DPOAEs were registered in 20 patients with unilateral Menière's disease (13 women and 7 men, aged 40-66 years) and were compared to a control group matched in age and gender. As a diagnostic parameter, the 'modulation index' MI=1/2 MS/DM was used (MS or modulation span, being the difference between the maximal and the minimal DPOAE-amplitude, and DM, being the mean of the suppressed stationary DPOAE-amplitude).

RESULTS

In the patients with unilateral Menière's disease, MI was lower than in the control group. This difference was highly significant. In 56% of the patients' contralateral ears MI was lower than the cut off-value and significantly lower than in the control group, but did not differ significantly from the patients' ipsilateral ears.

CONCLUSION

The registration of low-frequency modulated DPOAEs is comparable to the generally applied transtympanic electrocochleography in its diagnostic validity. The method is fast and non-invasive and could be applied to monitor the course of the disease.

Bone-Conducted Sound: Physiological and Clinical Aspects

OBJECTIVE

The fact that vibration of the skull causes a hearing sensation has been known since the 19th century. This mode of hearing was termed hearing by bone conduction. Although there has been more than a century of research on hearing by bone conduction, its physiology is not completely understood. Lately, new insights into the physiology of hearing by bone conduction have been reported. Knowledge of the physiology, clinical aspects, and limitations of bone conduction sound is important for clinicians dealing with hearing loss and is the purpose of this review.

DATA SOURCES

The data were compiled from the published literature in the areas of clinical bone conduction hearing, bone conduction hearing aids, basic research on bone conduction physiology, and recent research on bone conduction hearing from our laboratory.

CONCLUSION

Five factors contributing to bone conduction hearing have been identified: 1) sound radiated into the external ear canal, 2) middle ear ossicle inertia, 3) inertia of the cochlear fluids, 4) compression of the cochlear walls, and 5) pressure transmission from the cerebrospinal fluid. Of these five, inertia of the cochlear fluid seems most important. Bone conduction sound is believed to reflect the true cochlear function; however, certain conditions such as middle ear diseases can affect bone conduction sensitivity, but less than for air conduction. The bone conduction route can also be used for hearing aids; since the bone conduction route is less efficient than the air conduction route, bone conduction hearing aids are primarily used for hearing losses where air conduction hearing aids are contraindicated.

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.

The effects of neural synchronization and peripheral compression on the acoustic-reflex threshold

This study investigates the acoustic reflex threshold (ART) dependency on stimulus phase utilizing low-level reflex audiometry [Neumann et al., Audiol. Neuro-Otol. 1, 359-369 (1996)]. The goal is to obtain optimal broadband stimuli for elicitation of the acoustic reflex and to obtain objective determinations of cochlear hearing loss. Three types of tone complexes with different phase characteristics were investigated: A stimulus that compensates for basilar-membrane dispersion, thus causing a large overall neural synchrony (basilar-membrane tone complex-BMTC), the temporally inversed stimulus (iBMTC), and random-phase tone complexes (rTC). The ARTs were measured in eight normal-hearing and six hearing-impaired subjects. Five different conditions of peak amplitude and stimulus repetition rate were used for each stimulus type. The results of the present study suggest that the ART is influenced by at least two different factors: (a) the degree of synchrony of neural activity across frequency, and (b) the fast-acting compression mechanism in the cochlea that is reduced in the case of a sensorineural hearing loss. The results allow a clear distinction of the two subjects groups based on the different ART for the utilized types and conditions of the stimuli. These differences might be useful for objective recruitment detection in clinical diagnostics.

Comparing different estimates of cochlear compression in listeners with normal and impaired hearing

A loss of cochlear compression may underlie many of the difficulties experienced by hearing-impaired listeners. Two behavioral forward-masking paradigms that have been used to estimate the magnitude of cochlear compression are growth of masking (GOM) and temporal masking (TM). The aim of this study was to determine whether these two measures produce within-subjects results that are consistent across a range of signal frequencies and, if so, to compare them in terms of reliability or efficiency. GOM and TM functions were measured in a group of five normal-hearing and five hearing-impaired listeners at signal frequencies of 1000, 2000, and 4000 Hz. Compression values were derived from the masking data and confidence intervals were constructed around these estimates. Both measures produced comparable estimates of compression, but both measures have distinct advantages and disadvantages, so that the more appropriate measure depends on factors such as the frequency region of interest and the degree of hearing loss. Because of the long testing times needed, neither measure is suitable for clinical use in its current form.

Masker phase effects in normal-hearing and hearing-impaired listeners: evidence for peripheral compression at low signal frequencies

The presence of cochlear-based compression at low frequencies was investigated by measuring phase effects in harmonic maskers. In normal-hearing listeners, the amount of masking produced depends strongly on the phase relationships between the individual masker components. This effect is thought to be determined primarily by properties of the cochlea, including the phase dispersion and compressive input-output function of the basilar membrane. Thresholds for signals of 250 and 1000 Hz were measured in harmonic maskers with fundamental frequencies of 12.5 and 100 Hz as a function of the masker phase curvature. Results from 12 listeners with sensorineural hearing loss showed reduced masker phase effects, when compared with data from normal-hearing listeners, at both 250- and 1000-Hz signal frequencies. The effects of hearing impairment on phase-related masking differences were not well simulated in normal-hearing listeners by an additive white noise, suggesting that the effects of hearing impairment are not simply due to reduced sensation level. Maximum differences in masked threshold were correlated with auditory filter bandwidths at the respective frequencies, suggesting that both measures are affected by a common underlying mechanism, presumably related to cochlear outer hair cell function. The results also suggest that normal peripheral compression remains strong even at 250 Hz.

Evaluation of cochlear function in an acute endolymphatic hydrops model in the guinea pig by measuring low-level DPOAEs

During and after microinjection of artificial endolymph into scala media of the guinea pig, the 2f1- f2 -DPOAE at 4.5 kHz generated by low-level primaries was recorded. Reproducible changes were measured when 1.1 microl of artificial endolymph was injected at a rate of 1.65 nl/s (1.53-1.83). This volume corresponds with an acute endolymphatic hydrops of 23%. After the onset of injection the inner ear pressure immediately increased to a mean higher level of 22 Pa, whereas the 2f1- f2 -amplitude and -phase did not change for about 1 min. Thereafter, the amplitude decreased 2.6 dB (+/- 0.7) on average and slowly regained almost its initial value, with recovery frequently starting within the period of injection. In an attempt to explain the observed changes in 2f1- f2 -amplitude the basilar membrane displacement towards scala tympani at the 2f1- f2 generation site is estimated to be 19 nm for a 1.1 microl increase of endolymph volume. A small deflection of the outer hair cell stereocilia and as a consequence a change in cell conductance may explain the 2f1- f2 -amplitude changes. However, the precise mechanism of cochlear function change caused by endolymph volume increase (hydrops) remains to be elucidated.

Physiological vulnerability of distortion product otoacoustic emissions from the amphibian ear

The physiological vulnerability of distortion product otoacoustic emissions (DPOAEs) was investigated in the leopard frog, Rana pipiens pipiens. For each frog, DPOAEs were recorded from the amphibian and the basilar papillae. Measurements were taken before and after either the arrest of oxygen supply due to cardioectomy, or the destruction of the central nervous system (CNS). DPOAEs in response to high-level stimuli (> 75 dB SPL) were rather robust to these insults during the first two hours post surgery. In contrast, DPOAE amplitudes in response to low-level stimuli (< 75 dB SPL) decreased significantly. On average, low-level emissions from the amphibian papilla disappeared within 6 min for cardioectomy, and after 13 min for CNS destruction. In the basilar papilla, low-level DPOAEs disappeared more slowly: on average after 34 min following cardioectomy, and after 58 min for CNS destruction. The difference in physiological vulnerability between low- and high-level emissions is similar to that in mammals and a lizard. The difference between the DPOAE decay rate of the frog's amphibian and basilar papillae suggests important differences between the hearing mechanisms of the papillae.

Cochlear compression estimates from measurements of distortion-product otoacoustic emissions

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.

Suppression tuning in noise-exposed rabbits

Psychophysical, basilar-membrane (BM), and single nerve-fiber tuning curves, as well as suppression of distortion-product otoacoustic emissions (DPOAEs), all give rise to frequency tuning patterns with stereotypical features. Similarities and differences between the behaviors of these tuning functions, both in normal conditions and following various cochlear insults, have been documented. While neural tuning curves (NTCs) and BM tuning curves behave similarly both before and after cochlear insults known to disrupt frequency selectivity, DPOAE suppression tuning curves (STCs) do not necessarily mirror these responses following either administration of ototoxins [Martin et al., J. Acoust. Soc. Am. 104, 972-983 (1998)] or exposure to temporarily damaging noise [Howard et al., J. Acoust. Soc. Am. 111, 285-296 (2002)]. However, changes in STC parameters may be predictive of other changes in cochlear function such as cochlear immaturity in neonatal humans [Abdala, Hear. Res. 121, 125-138 (1998)]. To determine the effects of noise-induced permanent auditory dysfunction on STC parameters, rabbits were exposed to high-level noise that led to permanent reductions in DPOAE level, and comparisons between pre- and postexposure DPOAE levels and STCs were made. Statistical comparisons of pre- and postexposure STC values at CF revealed consistent basal shifts in the frequency region of greatest cochlear damage, whereas thresholds, Q10dB, and tip-to-tail gain values were not reliably altered. Additionally, a large percentage of high-frequency lobes associated with third tone interference phenomena, that were exhibited in some data sets, were dramatically reduced following noise exposure. Thus, previously described areas of DPOAE interference above f2 may also be studied using this type of experimental manipulation [Martin et al., Hear. Res. 136, 105-123 (1999); Mills, J. Acoust. Soc. Am. 107, 2586-2602 (2002)].

Further efforts to predict pure-tone thresholds from distortion product otoacoustic emission input/output functions

Recently, Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] fit linear equations to distortion product otoacoustic emission (DPOAE) input/output (UO) functions after the DPOAE level (in dB SPL) was converted into pressure (in microPa). Significant correlations were observed between these DPOAE thresholds and audiometric thresholds. The present study extends their work by (1) evaluating the effect of frequency, (2) determining the behavioral thresholds in those conditions that did not meet inclusion criteria, and (3) including a wider range of stimulus levels. DPOAE I/O functions were measured in as many as 278 ears of subjects with normal and impaired hearing. Nine f2 frequencies (500 to 8000 Hz in 1/2-octave steps) were used, L2 ranged from 10 to 85 dB SPL (5-dB steps), and L1 was set according to the equation L1 = 0.4L2 + 39 dB [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] for L2 levels up to 65 dB SPL, beyond which L1 = L2. For the same conditions as those used by Boege and Janssen, we observed a frequency effect such that correlations were higher for mid-frequency threshold comparisons. In addition, a larger proportion of conditions not meeting inclusion criteria at mid and high frequencies had hearing losses exceeding 30 dB HL, compared to lower frequencies. These results suggest that DPOAE I/O functions can be used to predict audiometric thresholds with greater accuracy at mid and high frequencies, but only when certain inclusion criteria are met. When the SNR inclusion criterion is not met, the expected amount of hearing loss increases. Increasing the range of input levels from 20-65 dB SPL to 10-85 dB SPL increased the number of functions meeting inclusion criteria and increased the overall correlation between DPOAE and behavioral thresholds.

[Aural fullness and endolymphatic hydrops]

BACKGROUND

Low-frequency modulated DPOAEs were registrated to investigate whether the subjective symptom of aural fullness can indicate an endolymphatic hydrops.

METHOD

The cochlear partition is periodically moved towards scala vestibuli and scala tympani by a low-frequency suppressor tone. The level of simultaneously registrated DPOAEs is modulated depending on the phase of the suppressor. This modulation may be reduced when the displacement of the organ of Corti is inhibited by mechanical stiffening of the basilar membrane (e. g. in endolymphatic hydrops).

SUBJECTS

Low-frequency modulated DPOAEs were registrated in 15 patients with Menière's disease, in 8 patients with aural fullness and tinnitus, but without vertigo, and in 21 normal hearing adults.

RESULTS

In both patient groups the modulation depth is high significantly lower than in the control group. The results do not differ significantly in the patients with and without vertigo.

CONCLUSION

The reduction of the modulation depth can be interpreted as a sign of an endolymphatic hydrops. In the group of patients with aural fullness a cochlear hydrops can be assumed. The measurement of low-frequency modulated DPOAEs is a new tool to diagnose and monitor the course of endolymphatic hydrops and to evaluate the effectiveness of therapeutic methods.

Auditory processing efficiency deficits in children with developmental language impairments

The "temporal processing hypothesis" suggests that individuals with specific language impairments (SLIs) and dyslexia have severe deficits in processing rapidly presented or brief sensory information, both within the auditory and visual domains. This hypothesis has been supported through evidence that language-impaired individuals have excess auditory backward masking. This paper presents an analysis of masking results from several studies in terms of a model of temporal resolution. Results from this modeling suggest that the masking results can be better explained by an "auditory efficiency" hypothesis. If impaired or immature listeners have a normal temporal window, but require a higher signal-to-noise level (poor processing efficiency), this hypothesis predicts the observed small deficits in the simultaneous masking task, and the much larger deficits in backward and forward masking tasks amongst those listeners. The difference in performance on these masking tasks is predictable from the compressive nonlinearity of the basilar membrane. The model also correctly predicts that backward masking (i) is more prone to training effects, (ii) has greater inter- and intrasubject variability, and (iii) increases less with masker level than do other masking tasks. These findings provide a new perspective on the mechanisms underlying communication disorders and auditory masking.

Alterations of basilar membrane response phase and velocity after acoustic overstimulation

To investigate the physiology of noise-induced hearing loss, the sound-induced vibrations of the basilar membrane (BM) of the inner ear were measured in living anesthetized guinea pigs before and after intense sound exposure. The vibrations were measured using a laser Doppler velocimeter after placing reflective glass beads on the BM. Pseudo-random noise waveforms containing frequencies between 4 and 24 kHz were used to generate velocity tuning curves. Before overstimulation, sharp response peaks were seen at stimulus frequencies between 15 and 17 kHz, consistent with the expected best frequency of the recording location. The response to low level stimuli lagged the high level ones by up to 90 degrees at the characteristic frequency. Following exposure to loud sound, the BM vibrations showed a pronounced reduction in amplitude, primarily at low stimulus levels, and the best frequency moved to approximately 12 kHz. At higher levels, the reduction was either absent or much smaller. In addition to the amplitude changes, increased phase lags were seen at frequencies near the characteristic frequency. In animals with more severe exposures, response phases were altered also at frequencies showing no change of the amplitude. The phase was independent of stimulus level after severe exposures.

Sources of DPOAEs revealed by suppression experiments, inverse fast Fourier transforms, and SFOAEs in impaired ears

DPOAE sources are modeled by intermodulation distortion generated near the f2 place and a reflection of this distortion near the DP place. In a previous paper, inverse fast Fourier transforms (IFFTs) of DPOAE filter functions in normal ears were consistent with this model [Konrad-Martin et al., J. Acoust. Soc. Am. 109, 2862-2879 (2001)]. In the present article, similar measurements were made in ears with specific hearing-loss configurations. It was hypothesized that hearing loss at f2 or DP frequencies would influence the relative contributions to the DPOAE from the corresponding basilar membrane places, and would affect the relative magnitudes of SFOAEs at frequencies equal to f2 and fDP. DPOAEs were measured with f2 = 4 kHz, f1 varied, and a suppressor near fDP. L2 was 25-55 dB SPL (L1 = L2 + 10 dB). SFOAEs were measured at f2 and at 2.7 kHz (the average fDP produced by the f1 sweep) for stimulus levels of 20-60 dB SPL. SFOAE results supported predictions of the pattern of amplitude differences between SFOAEs at 4 and 2.7 kHz for sloping losses, but did not support predictions for the rising- and flat-loss categories. Unsuppressed IFFTs for rising losses typically had one peak. IFFTs for flat or sloping losses typically have two or more peaks; later peaks were more prominent in ears with sloping losses compared to normal ears. Specific predictions were unambiguously supported by the results for only four of ten cases, and were generally supported in two additional cases. Therefore, the relative contributions of the two DPOAE sources often were abnormal in impaired ears, but not always in the predicted manner.

Auditory filter nonlinearity in mild/moderate hearing impairment

Sensorineural hearing loss has frequently been shown to result in a loss of frequency selectivity. Less is known about its effects on the level dependence of selectivity that is so prominent a feature of normal hearing. The aim of the present study is to characterize such changes in nonlinearity as manifested in the auditory filter shapes of listeners with mild/moderate hearing impairment. Notched-noise masked thresholds at 2 kHz were measured over a range of stimulus levels in hearing-impaired listeners with losses of 20-50 dB. Growth-of-masking functions for different notch widths are more parallel for hearing-impaired than for normal-hearing listeners, indicating a more linear filter. Level-dependent filter shapes estimated from the data show relatively little change in shape across level. The loss of nonlinearity is also evident in the input/output functions derived from the fitted filter shapes. Reductions in nonlinearity are clearly evident even in a listener with only 20-dB hearing loss.

Distortion product otoacoustic emission input/output functions in normal-hearing and hearing-impaired human ears

DPOAE input/output (I/O) functions were measured at 7f2 frequencies (1 to 8 kHz; f2/f1 = 1.22) over a range of levels (-5 to 95 dB SPL) in normal-hearing and hearing-impaired human ears. L1-L2 was level dependent in order to produce the largest 2f1-f2 responses in normal ears. System distortion was determined by collecting DP data in six different acoustic cavities. These data were used to derive a multiple linear regression model to predict system distortion levels. The model was tested on cochlear-implant users and used to estimate system distortion in all other ears. At most but not all f2's, measurements in cochlear implant ears were consistent with model predictions. At all f2 frequencies, the ears with normal auditory thresholds produced I/O functions characterized by compressive nonlinear regions at moderate levels, with more rapid growth at low and high stimulus levels. As auditory threshold increased, DPOAE threshold increased, accompanied by DPOAE amplitude reductions, notably over the range of levels where normal ears showed compression. The slope of the I/O function was steeper in impaired ears. The data from normal-hearing ears resembled direct measurements of basilar membrane displacement in lower animals. Data from ears with hearing loss showed that the compressive region was affected by cochlear damage; however, responses at high levels of stimulation resembled those observed in normal ears.

Hair cell death in a hearing-deficient canary

Cell death has been documented in bird auditory inner ear epithelia after induced damage. This cell death is quickly followed by an increase in supporting cell division and regeneration of the epithelium, thereby suggesting a possible relationship between these two processes. However, aspects of this relationship still need to be better understood. The Belgian Waterslager (BWS) canary is an ideal system in which to study cell death and subsequent cell division. In contrast to mixed breed (MB) canaries, cell division normally occurs in the auditory end organ of the BWS without any external manipulation. In addition, some of the cells in the auditory epithelium may be dying through an apoptotic-like process. In the present study two methods were used to quantify dying cells in the BWS and MB canary auditory epithelia: morphological criteria and TUNEL. Results confirm that some of the abnormal hair cells in the BWS auditory epithelium are apoptotic-like. The presence of both cell death and cell division indicates that these processes act concurrently in the adult end organ. Future studies are needed to determine if cell death is a stimulus for the observed cell division.

Differential display and gene arrays to examine auditory plasticity

Differential gene expression forms the basis for development, differentiation, regeneration, and plasticity of tissues and organs. We describe two methods to identify differentially expressed genes. Differential display, a PCR-based approach, compares the expression of subsets of genes under two or more conditions. Gene arrays, or DNA microarrays, contain cDNAs from both known genes and novel genes spotted on a solid support (nylon membranes or glass slides). Hybridization of the arrays with RNA isolated from two different experimental conditions allows the simultaneous analysis of large numbers of genes, from hundreds to thousands to whole genomes. Using differential display to examine differential gene expression after noise trauma in the chick basilar papilla, we identified the UBE3B gene that encodes a new member of the E3 ubiquitin ligase family (UBE3B). UBE3B is highly expressed immediately after noise in the lesion, but not in the undamaged ends, of the chick basilar papilla. UBE3B is most similar to a ubiquitin ligase gene from Caenorhabditis elegans, suggesting that this gene has been conserved throughout evolution. We also describe preliminary experiments to profile gene expression in the cochlea and brain with commercially available low density gene arrays on nylon membranes and discuss potential applications of this and DNA microarray technology to the auditory system.

On a novel type of neuron with proposed mechanoreceptor function in the human round window membrane--an immunohistochemical study

An immunohistochemical study was performed on surgically obtained human fresh cochlear tissue, using synaptophysin antibodies. After immediate aldehyde fixation and decalcification in Na-EDTA serial cryosections were made of the cochlea including the round window membrane (RWM). Apart from highly specific immunostaining of spiral ganglion cells and unmyelinated nerve fibers an immunoreactive neuroreceptor could be demonstrated at the postero-medial insertion of the RWM. The perikaryon showed intense synaptophysin immunoreativity with a distal process projecting into the fibrous stroma of the RWM displaying structural specializations suggestive of a mechanoreceptor function. It is speculated whether the neuroreceptor may be involved in the proprioception and/or mechanoreception of tensile forces generated within the lamina propria during displacement of the yielding RWM in the bony labyrinth. Such a function could be important for the regulation of perilymph pressure.

Phase-dependent suppression of transient evoked and distortion product otoacoustic emissions by a low-frequency tone

The subjective recording of the masked threshold of short acoustical stimuli with a loud tone of 30 Hz (phase audiogram) has been used for the clinical diagnosis of endolymphatic hydrops (EH). In normally-hearing subjects, a marked modulation of the threshold was found, depending on the phase of the low-frequency tone. A very small dependence was found in patients with Menière's disease, due to the micromechanical changes in the basilar membrane (BM). The same phase relationship becomes apparent in low-frequency suppression of otoacoustic emissions. The amplitudes of TEOAEs are controlled by the phase-dependent displacement of the BM. The suppressed TEOAEs have to be measured separately in each phase relationship. During recording of suppressed DPOAEs, the low-frequency suppressor is permanently superimposed on the pair of primary tones. After time averaging and a moving short-time FFT, the spectral values of the DPOAEs are obtained depending on the phase of the low-frequency tone. Modulation depends also on the masker level, the levels of the primary tones, and on their frequency range. The method of low-frequency suppressed DPOAEs is an objective method to diagnose EH and could be a useful tool in human inner ear research.

Hair cell loss and regeneration after severe acoustic overstimulation in the adult pigeon

The extent of hair cell regeneration following acoustic overstimulation severe enough to destroy tall hair cells, was determined in adult pigeons. BrdU (5-bromo-2'-deoxyuridine) was used as a proliferation marker. Recovery of hearing thresholds in each individual animal was measured over a period of up to 16 weeks after trauma. In ears with loss of both short and tall hair cells, little or no functional recovery occurred. In ears with less damage, where significant functional recovery did occur, there were always a few rows of surviving hair cells left at the neural edge of the basilar papilla. In the region of hair cell loss, numerous BrdU labeled cells were found. However, only a small minority of these cells were regenerated hair cells, the majority being monolayer cells. Irrespective of the extent of the region of hair cell loss, regenerated hair cells were observed predominantly in a narrow strip at the transition from the abneural area of total hair cell loss and the neural area of hair cell survival. With increasing damage this strip moved progressively towards the neural edge of the papilla. No regeneration of hair cells was observed in the abneural region of total hair cell loss, even up to 16 weeks after trauma. The results indicate that there is a gradient in the destructive effect of loud sound across the width of the basilar papilla, from most detrimental at the abneural edge to least detrimental at the neural edge. Both tall and short hair cells can regenerate after sound trauma. Whether they do regenerate or not depends on the degree of damage to the area of the papilla where they normally reside. Regeneration of new hair cells occurs only in a narrow longitudinal band, which moves from abneural into the neural direction with increasing damage. In the area neural to this band, hair cells survive the overstimulation. In the area abneural to this band, sound damage is so severe, that no regeneration of hair cells occurs. As a consequence morphological and functional deficits persist.

Masking of tones and speech by Schroeder-phase harmonic complexes in normally hearing and hearing-impaired listeners

Tone detection and sentence recognition were measured for normally hearing and hearing-impaired listeners using maskers consisting of harmonic series with components summed in positive or negative Schroeder phase. Each task was carried out with the signal set at 60, 70, or 80 dB SPL. For listeners with normal hearing, positive Schroeder-phase complexes masked tones and sentences less than negative Schroeder-phase maskers. In the two experimental tasks, to achieve the same amount of masking, positive Schroeder-phase complexes had to be set as much as 12-15 dB higher than negative Schroeder-phase complexes. Large phase effects were observed on both tasks at all three test levels. The two maskers were more nearly equal in effectiveness in the presence of cochlear damage. The findings support an interpretation that involves differences in the shape of the basilar-membrane waveform generated by each masker and active cochlear processing which enhances the internal signal-to-masker ratio for signals presented in the positive Schroeder masker. This spectral enhancement appears to require nonlinear active gain that is characteristic of normal auditory processing at moderate presentation levels. The results of the sentence recognition task suggest that group differences observed in tone detection transfer fairly directly to speech perception under masking.

Short-term temporal integration: evidence for the influence of peripheral compression

Thresholds for a 6.5-kHz sinusoidal signal, temporally centered in a 400-ms broadband-noise masker, were measured as a function of signal duration for normally hearing listeners and listeners with cochlear hearing loss over a range of masker levels. For the normally hearing listeners, the slope of the function relating signal threshold to signal duration (integration function) was steeper at medium masker levels than at low or high levels by a factor of nearly 2, for signal durations between 2 and 10 ms, while no significant effect of level was found for signal durations of 20 ms and more. No effect of stimulus level was found for the hearing-impaired listeners at any signal duration. For signal durations greater than 10 ms, consistent with many previous studies, the slope of the integration function was shallower for the hearing-impaired listeners than for the normally hearing listeners. However, for shorter durations, there was no significant difference in slope between the results from the hearing-impaired listeners and those from the normally hearing listeners in the high- and low-level masker conditions. A model incorporating a compressive nonlinearity, representing the effect of basilar-membrane (BM) compression, and a short-term temporal integrator, postulated to be a more central process, can account well for changes in the short-term integration function with level, if it is assumed that the compression is greater at medium levels than at low or high levels by a factor of about 4. This is in reasonable agreement with physiological measurements of BM compression, and with previous psychophysical estimates.

A behavioral measure of basilar-membrane nonlinearity in listeners with normal and impaired hearing

This paper examines the possibility of estimating basilar-membrane (BM) nonlinearity using a psychophysical technique. The level of a forward masker required to mask a brief signal was measured for conditions where the masker was either at, or one octave below, the signal frequency. The level of the forward masker at masked threshold provided an indirect measure of the BM response to the signal, as follows. Consistent with physiological studies, it was assumed that the BM responds linearly to frequencies well below the characteristic frequency (CF). Thus the ratio of the slopes of the masking functions between a masker at the signal frequency and a masker well below the signal frequency should provide an estimate of BM compression at CF. Results obtained from normally hearing listeners were in quantitative agreement with physiological estimates of BM compression. Furthermore, differences between normally hearing listeners and listeners with cochlear hearing impairment were consistent with the physiological effects of damage to the cochlea. The results support the hypothesis that BM nonlinearity governs the nonlinear growth of the upward spread of masking, and suggest that this technique provides a straightforward method for estimating BM nonlinearity in humans.

Cochlear microphonics in Ménière's disease

The pathophysiology of hearing deterioration in Ménière's disease (MD) is unclear. Hair cell loss has been proposed to be the cause of severe hearing loss in Ménière's disease. The cochlear microphonic (CM) is known to be the receptor potential of the outer hair cells in the cochlea. This study measured the CM in Ménière's disease and investigated its relationship with the degree of hearing impairment and endolymphatic hydrops. Transtympanic electrocochleography (ECoG) using rarefaction (RAR) and condensation (CON) tonebursts at 1 kHz was performed on 130 ears of 119 patients. Ninety six ears were diagnosed to have MD and 34 were diagnosed non-Ménière's disease (NMD). The mean amplitude of the CM was 33.10 +/- 46.04 microV in the MD group and 13.15 +/- 12.77 microV in the NMD group (p < 0.001). Enlarged negative summating potential and action potential ratios (SP:AP > 40%) were found in 81.3% of the MD group and 17.6% of the NMD group. In the MD group, the CM in the group with an enlarged negative SP was 36.98 +/- 49.78 microV, and 16.31 +/- 15.88 microV in the group without (p < 0.01). The CM was 34.33 +/- 49.28 microV in the pure-tone average (PTA) < or = 25 dB group, 46.97 +/- 58.31 microV in the 26-40 dB group, 29.12 +/- 42.62 microV in the 41-70 dB group, and 26.20 +/- 22.41 microV in the > 70 dB group (p > 0.05). The CMs in 11 pairs of MD ears and sensorineural hearing loss (SNHL) ears with matching hearing (MD 44 dB, SNHL 45 dB) were measured. They were 71.42 +/- 75.94 microV and 7.90 +/- 5.89 microV, respectively (p < 0.01). Our study shows that the CM is higher in ears with endolymphatic hydrops, evidenced by an enlarged SP:AP ratio, than ears without and the CM shows no statistical difference in groups with different levels of hearing loss. These findings suggest that hearing loss with a large CM in Ménière's disease patients may be the result of an alteration of cochlear mechanics and only severe hearing loss with a small CM is caused by hair cell loss. The CM measurement, to evaluate the hair cell status, may be helpful in identifying patients whose hearing may be recoverable if the underlying hydrops can be corrected. Our data do not permit the conclusion that an enlarged CM can be used in the diagnosis of MD.

Measurement of pressure and displacement of the membranous labyrinth in endolymphatic hydrops by the tensile test

We measured the mechanical characteristics, particularly the strength of various regions of the membranous labyrinth by the penetration test with a specially designed machine. A load-displacement curve was drawn by the tensile test for the Reissner's and basement membranes. Additionally, a stress-strain curve was drawn. The modulus of elasticity was measured showing a straight line in the stress-strain curve: 1.5 x 10(2) mN/mm2 for Reissner's membrane and 9.3 x 10(2)-1.3 x 10(3) mN/mm2 for the basement membrane. Furthermore, the endolymphatic pressure was calculated at the point in time when the maximum strain as the limit of elasticity was 0.2. It was 81 Pa. The displacement of Reissner's membrane was 0.2 mm when the initial tension was taken as 0 in the formula, and that of the basement membrane was 3 to 37 microns when the initial tension was 0, 0.1, 0.2 and 0.3.

[Amplitude changes in distortion products of otoacoustic emissions after acute noise exposure]

BACKGROUND

The inner ear responds to acute impact of high-intensity noise with a temporary threshold shift (TTS), which represents a reaction of the outer hair cells. That is why TTS should be detectable as changes of distortion-product otoacoustic emissions (DPOAE).

METHODS

DPOAE were measured in 102 ears in 59 test subjects with normal hearing before and after definitive noise exposure (20 min of "white noise" at 90 dB HL). We also registrated the TTS (4kHz) immediately after noise and measured DP changes during a 30-min recovery period.

RESULTS

Typically we found a reduction of DP amplitude. It averaged between 2.0 and 2.5 dB HL in the frequency range from 2 to 5 kHz. There was no correlation between TTS and DP amplitude reduction at 4kHz. During the recovery period, individual ears showed an erratic pattern of DP amplitudes. However, a tendency toward incremental DP amplitude regeneration appeared after averaging. As many as 10% of the measured ears showed an atypical reaction with an increase of DP amplitudes. Whether the latter is due to errors in the test procedure or is evidence of a particularly stable inner ear remains to be discussed.

CONCLUSIONS

The results are further proof for the reaction of the outer hair cells on acute noise impact, which apparently leads to an increase of the stiffness of basilary membrane due to contraction processes. It may be possible to differentiate vulnerable inner ears from stable ones.

[Pathologic mechanoelectric transduction of outer hair cells as the cause of recruitment]

It is believed that the sound-induced travelling wave in the mammalian cochlea is enhanced and sharpened by a positive feedback mechanism. This causes the passive linear basilar membrane growth function to become non-linear. The present paper shows that nonlinear basilar membrane vibration is due to the nonlinear growth function of the receptor potential of outer hair cells, which can be described by a 2nd-order Boltzmann function. Since intensity coding in the inner ear depends on an interaction of nonlinear basilar membrane motion and nerve fibers with three different types of synaptic threshold and growth function, the process is directly dependent on an intact mechanoelectrical transduction of outer hair cells. According to the proposed model, a loss in efficiency of outer hair cell mechanoelectrical transduction must lead to both a reduction in gain (i.e., hearing loss) and a linearizing of the response. As a result, once above threshold, the changes of stereociliary displacement, basilar membrane displacement and neural firing rate per unit change of sound intensity must be larger than for the healthy cochlea with its compressive nonlinearity.

Incomplete recovery of chicken distortion product otoacoustic emissions following acoustic overstimulation

Distortion product otoacoustic emissions (DPOAEs) were measured in chickens before and after exposure to a 525-Hz pure tone (120 dB SPL, 48 h). The exposure caused extensive hair cell loss and destroyed the tectorial membrane along the abneural edge of the basilar papilla in the low-to-mid-frequency region of the cochlea. Although the lesion was restricted, DPOAEs were greatly depressed at all frequencies immediately after the exposure. The high-frequency DPOAEs gradually recovered to preexposure values after the exposure; however, there was little or no improvement in DPOAEs at test frequencies equal to or slightly above the exposure frequency even after 16 weeks of recovery. By 28 days of recovery, the previously damaged region of the basilar papilla had been repopulated by hair cells and the lower honeycomb layer of the tectorial membrane had regenerated, but not the upper fibrous layer. The upper fibrous layer of the tectorial membrane was still missing after 16 weeks of recovery and the region of damage corresponded closely to the frequency regions where the DPOAEs were depressed.

A human temporal bone study of changes in the basilar membrane of the apical turn in endolymphatic hydrops

We observed that some temporal bones with endolymphatic hydrops (EH) showed varying degrees of basalward displacement (towards the scala tympani) of the basilar membrane (BM) in the apical turn of the cochlea. In some, the BM was adherent to the bony wall of the scala tympani (i.e., the interscalar septum). Such mechanical distortion of the BM could conceivably alter cochlear mechanics and lead to sensorineural hearing loss. The results of a systematic evaluation of 234 temporal bones to characterize, quantify and determine the functional significance of this observation are presented. Four groups of bones were evaluated: normal (N = 78), presbycusis (N = 96), Ménière's disease (N = 23), and EH secondary to labyrinthitis (N = 37). The incidence of extreme displacement of the BM in the apical turn such that it adhered to the interscalar septum was 52% in Ménière's disease, 57% in EH secondary to labyrinthitis, 10% in presbycusis, and 1% in normals. These differences were significant and could not be explained on the basis of age, sex, postmortem time, or artifact of technique or processing. Displacement of the BM was not observed in other turns of the cochlea. Its pathogenesis is not known, but may be related to atrophy of the spiral ligament. It is likely that such BM displacement results in sensorineural hearing loss. However, our data and theoretical analyses both indicate that such a loss will be restricted to frequencies below 100 Hz and that this pathologic change alone is not likely to cause appreciable hearing loss at clinically tested frequencies of 250 HZ and higher. Hence, even though this pathologic finding is common in endolymphatic hydrops, it cannot explain the low-frequency hearing lost observed in Ménière's disease.

[Diagnosis of endolymphatic hydrops with low tone masked otoacoustic emissions]

BACKGROUND

A new method of diagnosis of endolymphatic hydrops by recording low-tone masked evoked otoacoustic emissions (TEOAE) is presented.

METHODS

A short acoustic stimulus and a masker tone of 30 Hz are applied in an adjustable phase relation simultaneously to the same ear. In the normal hearing ear the masker shows little influence on the TEOAE at 0 degrees, whereas the suppression at phase 270 degrees (maximal rarefaction at the eardrum) is nearly complete. However, in cases of endolymphatic hydrops this masking effect is reduced or absent, indicating impaired mobility of the basilar membrane.

RESULTS

The masked TEOAE were recorded of patients with normal hearing, Menière's disease, and sudden hearing loss without vertigo. In Menière cases with supposed endolymphatic hydrops, the amplitude modulation of the emissions was found to be much less than in the other groups.

CONCLUSIONS

Where TEOAE can be recorded, low-tone masking is a quick, objective, and noninvasive method for the diagnosis of endolymphatic hydrops.

Fibronectin-like immunoreactivity of the basilar membrane of celloidin-embedded human temporal bone sections

Dysfunction of the mechanical properties of the basilar membrane is a potential cause of presbycusis. In cases of minimal sensorineural or strial degeneration it is believed to play a major role. The membrane has been shown to be partly composed of fibronectin. Fibronectin immunoreactivity is diminished in aged rats. Mesothelial cell line the perilymphatic surface of the membrane and are reduced in number in the aged rat cochlea. Fibronectin immunoreactivity was examined in human temporal bone sections (6 months to 92 years old). Hematoxylin and eosin stained section (17 to 97 years) were immunoreactivity was demonstrable in the human cochlea, but was not reduced, even in the eldest cases examined The number of mesothelial cells was reduced, however, and was related to the age of the individual, but not to the clinical diagnosis or audiogram shape. These two factors do not, therefore, appear to give rise to hearing losses associated with presbycusis.

Modeling the fine structure of the 2f1-f2 acoustic distortion product. II. Model evaluation

In a previous article, a vector sum model was developed that successfully reproduces the ADP rippling pattern when a nonuniformity is introduced in the active damping factor, a parameter that is inversely related to the energy gain contribution of the outer hair cells [OHCs, Sun et al., J. Acoust. Soc. Am. 96, 2166-2174 (1994)]. Here, the mean of the damping factor is increased nonlinearly with input level, mimicking the saturation of the active feedback of the OHCs. The passive damping factor in the transmission line model was also nonlinearly increased with input level to reproduce the frequency shift of the peak of the traveling wave observed in experimental data. The resulting model simulates an ADP that is compatible with data from human subjects wherein the ADP fine structure does not saturate with level. Moreover, the model suggests that the shifting of the ADP pattern with level is a direct result of the peak shift of the traveling wave, thus implicating the nonlinear damping factors as the underlying basis of this phenomenon. The input/output (I/O) functions of the simulated ADP emissions at specific frequencies were also examined. The resulting functions show a variety of shapes, depending on the pattern of ADP fine structure around the I/O frequency, and the way the fine structure shifts as primary levels increase. These I/O functions are also similar to those observed in human subjects, even with regard to overall slopes which approximate one. Thus the model illustrates how cubic distortion generators coupled with damping on linearities can yield I/O function slopes on the order of one, rather than the expected three.(ABSTRACT TRUNCATED AT 250 WORDS)

The application of frequency and time domain kurtosis to the assessment of hazardous noise exposures

Five computer-synthesized broadband noises, each having the same average spectrum and the same unweighted Leq of 100 dB SPL but very different temporal structures, were used to produce hearing loss in chinchillas. Despite the same exposure energies and spectra, each noise exposure produced a different magnitude and frequency distribution of hearing loss and sensory cell loss. The results indicate that the statistical properties of a signal are important in the determination of hearing loss. When the audiometric and histological results are compared to a metric based upon kurtosis measured in the time and the frequency domain for each exposure, there is a clear indication that these statistical metrics are good predictors of the relative magnitude and frequency distribution of the acoustic trauma.

Frequency selectivity in patients with acoustic neuroma

Frequency selectivity was compared in subjects with hearing loss due to acoustic neuroma and cochlear pathology, and normal listeners. A particular interest was the role of probe tone parameters on the shape of the tuning curve. Psychophysical tuning curves (PTCs) were measured for each of two equal energy 2000-Hz probe tones (10 dB SL/300 msec and 17 dB SL/60 msec), using simultaneous 1/3-octave narrow-band noise maskers centered at 1, 1.25, 1.6, 2.5, 3.15, and 4 kHz. The results showed that the critical masker levels obtained for impaired listeners were significantly greater than those from normal subjects. The slope of the low-frequency limb of the PTC was steeper for normal compared to hearing-impaired listeners but there was no difference due to site of lesion. In all three groups, the critical masker levels obtained with the short probe were significantly greater than those for the long probe, negating the hypothesis that equal energy probes would yield the same outcomes. Tuning in listeners with hearing loss was highly correlated with audiometric threshold but not with tumor size, width of the internal auditory canal, or tumor location within the cerebellopontine angle. The main conclusion was that cochlear and retrocochlear hearing loss are similar with respect to their effect on frequency selectivity.

The application of frequency and time domain kurtosis to the assessment of hazardous noise exposures

Five computer-synthesized broadband noises, each having the same average spectrum and the same unweighted Leq of 100 dB SPL but very different temporal structures, were used to produce hearing loss in chinchillas. Despite the same exposure energies and spectra, each noise exposure produced a different magnitude and frequency distribution of hearing loss and sensory cell loss. The results indicate that the statistical properties of a signal are important in the determination of hearing loss. When the audiometric and histological results are compared to a metric based upon kurtosis measured in the time and the frequency domain for each exposure, there is a clear indication that these statistical metrics are good predictors of the relative magnitude and frequency distribution of the acoustic trauma.

Measurement of cochlear basilar membrane traveling wave velocity by derived ABR

Auditory brainstem response (ABR) can be used to measure the basilar membrane traveling wave velocity (TWV). Traveling wave velocity was calculated from the latency difference between wave V of different derived ABR and the cochlear location distance between the appropriate derived band center frequency. The latency of wave V of derived ABR produced by 6 noise-masked ABR using high pass filtered noise and the location of the corresponding cochlear partition (distance from the stapes foot-plate) were measured, and five traveling wave velocities were estimated based on this parameter. Ten subjects with normal hearing, 7 patients with Meniere's disease, and 8 patients with sensorineural hearing loss were used in this study. The traveling wave velocity in the sensorineural hearing loss group was within normal limits at all frequencies, whereas the traveling wave velocity at 8 kHz in the Meniere group greatly exceeded that of the normal and sensorineural hearing loss group.