Vulnerability and adaptation of distortion product otoacoustic emissions to endocochlear potential variation

Influence of tinnitus, lifetime noise exposure, and firearm use on hearing thresholds, distortion product otoacoustic emissions, and their relative metric

Distortion product otoacoustic emissions (DPOAEs) and hearing thresholds (HTs) are widely used to evaluate auditory physiology. DPOAEs are sensitive to cochlear amplification processes, while HTs are additionally dependent upon inner hair cells, synaptic junctions, and the auditory nervous system. These distinctions between DPOAEs and HTs might help differentially diagnose auditory dysfunctions. This study aims to differentially diagnose auditory dysfunctions underlying tinnitus, firearm use, and high lifetime noise exposure (LNE) using HTs, DPOAEs, and a derived metric comparing HTs and DPOAEs, in a sample containing overlapping subgroups of 133 normal-hearing young adults (56 with chronic tinnitus). A structured interview was used to evaluate LNE and firearm use. Linear regression was used to model the relationship between HTs and DPOAEs, and their regression residuals were used to quantify their relative agreement. Participants with chronic tinnitus showed significantly elevated HTs, yet DPOAEs remained comparable to those without tinnitus. In contrast, firearm users revealed elevated HTs and significantly lower DPOAEs than predicted from HTs. High LNE was associated with elevated HTs and a proportional decline in DPOAEs, as predicted from HTs. We present a theoretical model to interpret the findings, which suggest neural (or synaptic) dysfunction underlying tinnitus and disproportional mechanical dysfunction underlying firearm use.

Reliability of Serological Prestin Levels in Humans and its Relation to Otoacoustic Emissions, a Functional Measure of Outer Hair Cells

OBJECTIVES

Serological biomarkers, common to many areas of medicine, have the potential to inform on the health of the human body and to give early warning of risk of compromised function or illness before symptoms are experienced. Serological measurement of prestin, a motor protein uniquely produced and expressed in outer hair cells, has recently been identified as a potential biomarker to inform on the health of the cochlea. Before any test can be introduced into the clinical toolkit, the reproducibility of the measurement when repeated in the same subject must be considered. The primary objective of this study is to outline the test-retest reliability estimates and normative ranges for serological prestin in healthy young adults with normal hearing. In addition, we examine the relation between serum prestin levels and otoacoustic emissions (OAEs) to compare this OHC-specific protein to the most common measure of OHC function currently used in hearing assessments.

DESIGN

We measured prestin levels serologically from circulating blood in 34 young adults (18 to 24 years old) with clinically normal pure-tone audiometric averages at five different timepoints up to six months apart (average intervals between measurements ranged from <1 week to 7 weeks apart). To guide future studies of clinical populations, we present the standard error of the measurement, reference normative values, and multiple measures of reliability. Additionally, we measured transient evoked OAEs at the same five timepoints and used correlation coefficients to examine the relation between OAEs and prestin levels (pg/mL).

RESULTS

Serum prestin levels demonstrated good to excellent reliability between and across the five different time points, with correlation coefficients and intraclass correlations >0.8. Across sessions, the average serum prestin level was 250.20 pg/mL, with a standard error of measurement of 7.28 pg/mL. Moreover, positive correlations (generally weak to moderate) were found between prestin levels and OAE magnitudes and signal-to-noise ratios.

CONCLUSIONS

Findings characterize serum prestin in healthy young adults with normal hearing and provide initial normative data that may be critical to interpreting results from individuals with sensorineural hearing loss. Our results demonstrate reliability of serum prestin levels in a sample of normal-hearing young adults across five test sessions up to 6 months apart, paving the way for testing larger samples to more accurately estimate test-retest standards for clinical protocols, including those involving serial monitoring. The positive correlations between serum prestin and OAE levels, although weak to moderate, reinforce that the source of serum prestin is likely the outer hair cells in the inner ear, but also that serum prestin and OAEs each may also index aspects of biologic function not common to the other.

Manipulation of the Endocochlear Potential Reveals Two Distinct Types of Cochlear Nonlinearity

The mammalian hearing organ, the cochlea, contains an active amplifier to boost the vibrational response to low level sounds. Hallmarks of this active process are sharp location-dependent frequency tuning and compressive nonlinearity over a wide stimulus range. The amplifier relies on outer hair cell (OHC)-generated forces driven in part by the endocochlear potential, the ∼+80 mV potential maintained in scala media, generated by the stria vascularis. We transiently eliminated the endocochlear potential in vivo by an intravenous injection of furosemide and measured the vibrations of different layers in the cochlea's organ of Corti using optical coherence tomography. Distortion product otoacoustic emissions were also monitored. After furosemide injection, the vibrations of the basilar membrane lost the best frequency (BF) peak and showed broad tuning similar to a passive cochlea. The intra-organ of Corti vibrations measured in the region of the OHCs lost the BF peak and showed low-pass responses but retained nonlinearity. This strongly suggests that OHC electromotility was operating and being driven by nonlinear OHC current. Thus, although electromotility is presumably necessary to produce a healthy BF peak, the mere presence of electromotility is not sufficient. The BF peak recovered nearly fully within 2 h, along with the recovery of odd-order distortion product otoacoustic emissions. The recovery pattern suggests that physical shifts in operating condition are a critical step in the recovery process.

Divergent Auditory Nerve Encoding Deficits Between Two Common Etiologies of Sensorineural Hearing Loss

Speech intelligibility can vary dramatically between individuals with similar clinically defined severity of hearing loss based on the audiogram. These perceptual differences, despite equal audiometric-threshold elevation, are often assumed to reflect central-processing variations. Here, we compared peripheral-processing in auditory nerve (AN) fibers of male chinchillas between two prevalent hearing loss etiologies: metabolic hearing loss (MHL) and noise-induced hearing loss (NIHL). MHL results from age-related reduction of the endocochlear potential due to atrophy of the stria vascularis. MHL in the present study was induced using furosemide, which provides a validated model of age-related MHL in young animals by reversibly inhibiting the endocochlear potential. Effects of MHL on peripheral processing were assessed using Wiener-kernel (system identification) analyses of single AN fiber responses to broadband noise, for direct comparison to previously published AN responses from animals with NIHL. Wiener-kernel analyses show that even mild NIHL causes grossly abnormal coding of low-frequency stimulus components. In contrast, for MHL the same abnormal coding was only observed with moderate to severe loss. For equal sensitivity loss, coding impairment was substantially less severe with MHL than with NIHL, probably due to greater preservation of the tip-to-tail ratio of cochlear frequency tuning with MHL compared with NIHL rather than different intrinsic AN properties. Differences in peripheral neural coding between these two pathologies-the more severe of which, NIHL, is preventable-likely contribute to individual speech perception differences. Our results underscore the need to minimize noise overexposure and for strategies to personalize diagnosis and treatment for individuals with sensorineural hearing loss.SIGNIFICANCE STATEMENT Differences in speech perception ability between individuals with similar clinically defined severity of hearing loss are often assumed to reflect central neural-processing differences. Here, we demonstrate for the first time that peripheral neural processing of complex sounds differs dramatically between the two most common etiologies of hearing loss. Greater processing impairment with noise-induced compared with an age-related (metabolic) hearing loss etiology may explain heightened speech perception difficulties in people overexposed to loud environments. These results highlight the need for public policies to prevent noise-induced hearing loss, an entirely avoidable hearing loss etiology, and for personalized strategies to diagnose and treat sensorineural hearing loss.

Slow oscillatory changes of DPOAE magnitude and phase after exposure to intense low-frequency sounds

Sensitive sound detection within the mammalian cochlea is performed by hair cells surrounded by cochlear fluids. Maintenance of cochlear fluid homeostasis and tight regulation of intracellular conditions in hair cells are crucial for the auditory transduction process but can be impaired by intense sound stimulation. After a short, intense low-frequency sound, the cochlea shows the previously described "bounce phenomenon," which manifests itself as slow oscillatory changes of hearing thresholds and otoacoustic emissions. In this study, distortion product otoacoustic emissions (DPOAEs) were recorded after Mongolian gerbils were exposed to intense low-frequency sounds (200 Hz, 100 dB SPL) with different exposure times up to 1 h. After all sound exposure durations, a certain percentage of recordings (up to 80% after 1.5-min-long exposure) showed oscillatory DPOAE changes, similar to the bounce phenomenon in humans. Changes were quite uniform with respect to size and time course, and they were independent from sound exposure duration. Changes showed states of hypo- and hyperactivity with either state preceding the other. The direction of changes was suggested to depend on the static position of the cochlear operating point. As assessed with DPOAEs, no indication for a permanent damage after several or long exposure times was detected. We propose that sensitivity changes occur due to alterations of the mechanoelectrical transduction process of outer hair cells. Those alterations could be induced by different challenged homeostatic processes with slow electromotility of outer hair cells being the most plausible source of the bounce phenomenon. NEW & NOTEWORTHY Low-frequency, high-intensity sound can cause slowly cycling activity changes in the mammalian cochlea. We examined the effect of low-frequency sound duration on the degree of these alterations. We found that cochlear changes showed a stereotypical biphasic pattern independent of sound exposure duration, but the probability that significant changes occurred decreased with increasing sound duration. Despite exposure durations of up to 1 h, no permanent or transient impairments of the cochlea were detected.

Adaptation of Cochlear Amplification to Low Endocochlear Potential

Endocochlear potential (EP) is essential for cochlear amplification by providing the voltage source needed to drive outer hair cell (OHC) transducer current, which leads to OHC electromechanical force. An early study using furosemide to reversibly reduce EP showed that distortion product otoacoustic emissions (DPOAEs) recovered before EP. This indicated that cochlear amplification may be able to adjust to a new, lower EP. To investigate the mechanism of this adjustment, the extracellular OHC voltage, which we term local cochlear microphonic (LCM), was measured simultaneously with DPOAE and EP while using intraperitoneal (IP) and intravenous injection of furosemide to reversibly reduce EP. With IP injection, the DPOAEs recovered fully, whereas the EP was reduced, but LCM showed a similar time course as EP. The DPOAEs failed to accurately report the variation of cochlear amplification. With intravenous injection, for which both reduction and recovery of EP are known to occur relatively quickly compared to IP, the cochlear amplification observed in LCM could attain nearly full or even full recovery with reduced EP. This showed the cochlea has an ability to adjust to diminished operating condition. Furthermore, the cochlear amplifier and EP recovered with different time courses: cochlear amplification just started to recover after the EP was nearly fully recovered and stabilized. Using a Boltzmann model and the second harmonic of the LCM to estimate the mechanoelectric transducer channel operating point, we found that the recovery of cochlear amplification occurred with recentering of the operating point.

The aging cochlea: Towards unraveling the functional contributions of strial dysfunction and synaptopathy

Strial dysfunction is commonly observed as a key consequence of aging in the cochlea. A large body of animal research, especially in the quiet-aged Mongolian gerbil, shows specific histopathological changes in the cochlear stria vascularis and the putatively corresponding effects on endocochlear potential and auditory nerve responses. However, recent work suggests that synaptopathy, or the loss of inner hair cell-auditory nerve fiber synapses, also presents as a consequence of aging. It is now believed that the loss of synapses is the earliest age-related degenerative event. The present review aims to integrate classic and novel research on age-related pathologies of the inner ear. First, we summarize current knowledge on age-related strial dysfunction and synaptopathy. We describe how these cochlear pathologies fit into the categories for presbyacusis, as first defined by Schuknecht in the '70s. Further, we discuss how strial dysfunction and synaptopathy affect sound coding by the auditory nerve and how they can be experimentally induced to study their specific contributions to age-related hearing deficits. As such, we aim to give an overview of the current literature on age-related cochlear pathologies and hope to inspire further research on the role of cochlear aging in age-related hearing deficits.

Furosemide Administered before Noise Exposure can Protect the Ear

Objectives

We assessed the effect of furosemide administration on noise-induced hearing loss. This drug reversibly elevates the auditory threshold by inducing a temporary reduction of the endocochlear potential and thereby suppresses the cochlear amplifier and active cochlear mechanics.

Methods

Mice were given a single injection of furosemide 30 minutes before exposure to 113 dB sound pressure level broadband noise. Control animals received saline solution. Furosemide was administered in other mice after the noise exposure. Auditory threshold shifts were assessed by recording auditory nerve brain stem evoked response (ABR) thresholds to broadband clicks.

Results

The mean ABR threshold in the group injected with furosemide and exposed to temporary threshold shift (TTS)-producing noise was elevated by 20.4 ± 12.3 dB, and that in the saline control group was elevated by 35.4 ± 18.3 dB (p < 0.02). The mean threshold elevations in the group injected with furosemide and exposed to permanent threshold shift (PTS)-producing noise and in the PTS saline control group were 15.0 ± 10.3 dB and 27.0 ± 12.7 dB, respectively (p < 0.01). Similar results were obtained when the PTS was assessed with an 8-kHz tone burst ABR. There was no significant difference in the PTS between mice given a single injection of furosemide and those given saline solution after the noise; this finding shows that furosemide is not acting as an antioxidant.

Conclusions

It appears that reversible hearing threshold elevation as a result of furosemide administration before noise exposure can reduce the TTS and PTS. This finding provides insight into the mechanism of noise-induced hearing loss.

Direct administration of 2-Hydroxypropyl-Beta-Cyclodextrin into guinea pig cochleae: Effects on physiological and histological measurements

2-Hydroxypropyl-Beta-Cyclodextrin (HPβCD) can be used to treat Niemann-Pick type C disease, Alzheimer's disease, and atherosclerosis. But, a consequence is that HPβCD can cause hearing loss. HPβCD was recently found to be toxic to outer hair cells (OHCs) in the organ of Corti. Previous studies on the chronic effects of in vivo HPβCD toxicity did not know the intra-cochlear concentration of HPβCD and attributed variable effects on OHCs to indirect drug delivery to the cochlea. We studied the acute effects of known HPβCD concentrations administered directly into intact guinea pig cochleae. Our novel approach injected solutions through pipette sealed into scala tympani in the cochlear apex. Solutions were driven along the length of the cochlear spiral toward the cochlear aqueduct in the base. This method ensured that therapeutic levels were achieved throughout the cochlea, including those regions tuned to mid to low frequencies and code speech vowels and background noise. A wide variety of measurements were made. Results were compared to measurements from ears treated with the HPβCD analog methyl-β-cyclodextrin (MβCD), salicylate that is well known to attenuate the gain of the cochlear amplifier, and injection of artificial perilymph alone (controls). Histological data showed that OHCs appeared normal after treatment with a low dose of HPβCD, and physiological data was consistent with attenuation of cochlear amplifier gain and disruption of non-linearity associated with transferring acoustic sound into neural excitation, an origin of distortion products that are commonly used to objectively assess hearing and hearing loss. A high dose of HPβCD caused sporadic OHC losses and markedly affected all physiologic measurements. MβCD caused virulent destruction of OHCs and physiologic responses. Toxicity of HPβCD to OHC along the cochlear length is variable even when a known intra-cochlear concentration is administered, at least for the duration of our acute studies.

The potential use of low-frequency tones to locate regions of outer hair cell loss

Current methods used to diagnose cochlear hearing loss are limited in their ability to determine the location and extent of anatomical damage to various cochlear structures. In previous experiments, we have used the electrical potential recorded at the round window -the cochlear response (CR) -to predict the location of damage to outer hair cells in the gerbil. In a follow-up experiment, we applied 10 mM ouabain to the round window niche to reduce neural activity in order to quantify the neural contribution to the CR. We concluded that a significant proportion of the CR to a 762 Hz tone originated from phase-locking activity of basal auditory nerve fibers, which could have contaminated our conclusions regarding outer hair cell health. However, at such high concentrations, ouabain may have also affected the responses from outer hair cells, exaggerating the effect we attributed to the auditory nerve. In this study, we lowered the concentration of ouabain to 1 mM and determined the physiologic effects on outer hair cells using distortion-product otoacoustic emissions. As well as quantifying the effects of 1 mM ouabain on the auditory nerve and outer hair cells, we attempted to reduce the neural contribution to the CR by using near-infrasonic stimulus frequencies of 45 and 85 Hz, and hypothesized that these low-frequency stimuli would generate a cumulative amplitude function (CAF) that could reflect damage to hair cells in the apex more accurately than the 762 stimuli. One hour after application of 1 mM ouabain, CR amplitudes significantly increased, but remained unchanged in the presence of high-pass filtered noise conditions, suggesting that basal auditory nerve fibers have a limited contribution to the CR at such low frequencies.

Influence of ketamine-xylazine anaesthesia on cubic and quadratic high-frequency distortion-product otoacoustic emissions

Ketamine is a dissociative anaesthetic, analgesic drug as well as an N-methyl-D-aspartate receptor antagonist and has been reported to influence otoacoustic emission amplitudes. In the present study, we assess the effect of ketamine-xylazine on high-frequency distortion-product otoacoustic emissions (DPOAE) in the bat species Carollia perspicillata, which serves as model for sensitive high-frequency hearing. Cubic DPOAE provide information about the nonlinear gain of the cochlear amplifier, whereas quadratic DPOAE are used to assess the symmetry of cochlear amplification and potential efferent influence on the operating state of the cochlear amplifier. During anaesthesia, maximum cubic DPOAE levels can increase by up to 35 dB within a medium stimulus level range from 35 to 60 dB SPL. Close to the -10 dB SPL threshold, at stimulus levels below about 20-30 dB SPL, anaesthesia reduces cubic DPOAE amplitudes and raises cubic DPOAE thresholds. This makes DPOAE growth functions steeper. Additionally, ketamine increases the optimum stimulus frequency ratio which is indicative of a reduction of cochlear tuning sharpness. The effect of ketamine on cubic DPOAE thresholds becomes stronger at higher stimulus frequencies and is highly significant for f2 frequencies above 40 kHz. Quadratic DPOAE levels are increased by up to 25 dB by ketamine at medium stimulus levels. In contrast to cubic DPOAEs, quadratic DPOAE threshold changes are variable and there is no significant loss of sensitivity during anaesthesia. We discuss that ketamine effects could be caused by modulation of middle ear function or a release from ipsilateral efferent modulation that mainly affects the gain of cochlear amplification.

Hearing dummies: individualized computer models of hearing impairment

Objective: Our aim was to explore the usage of individualized computer models to simulate hearing loss based on detailed psychophysical assessment and to offer hypothetical diagnoses of the underlying pathology.

DESIGN

Individualized computer models of normal and impaired hearing were constructed and evaluated using the psychophysical data obtained from human listeners. Computer models of impaired hearing were generated to reflect the hypothesized underlying pathology (e.g. dead regions, outer hair cell dysfunction, or reductions in endocochlear potential). These models were evaluated in terms of their ability to replicate the original patient data.

STUDY SAMPLE

Auditory profiles were measured for two normal and five hearing-impaired listeners using a battery of three psychophysical tests (absolute thresholds, frequency selectivity, and compression).

RESULTS

The individualized computer models were found to match the data. Useful fits to the impaired profiles could be obtained by changing only a single parameter in the model of normal hearing. Sometimes, however, it was necessary to include an additional dead region.

CONCLUSION

The creation of individualized computer models of hearing loss can be used to simulate auditory profiles of impaired listeners and suggest hypotheses concerning the underlying peripheral pathology.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Time-domain demonstration of distributed distortion-product otoacoustic emission components

Distortion-product otoacoustic emissions (DPOAEs) were measured in rabbits as time waveforms by employing a phase-rotation technique to cancel all components in the final average, except the 2f1-f2 DPOAE. Subsequent filtering allowed the DPOAE waveform to be clearly visualized in the time domain. In most conditions, f2 was turned off for 6 ms, which produced a gap so that the DPOAE was no longer generated. These procedures allowed the DPOAE onset as well as the decay during the gap to be observed in the time domain. DPOAEs were collected with L1 = L2 = 65-dB sound pressure level primary-tone levels for f2/f1 ratios from 1.25 to 1.01 in 0.02 steps. Findings included the appearance of complex onsets and decays for the DPOAE time waveforms as the f2/f1 ratio was decreased and the DPOAE level was reduced. These complexities were unaffected by interference tones (ITs) near the DPOAE frequency place (fdp), but could be removed by ITs presented above f2, which also increased DPOAE levels. Similar outcomes were observed when DPOAEs were measured at a sharp notch in the DPOAE level as a function of the f2 primary tone frequency, i.e., DP-gram. Both findings were consistent with the hypothesis that the DPOAE-ratio function, and some notches in the DP-gram, are caused by interactions of distributed DPOAE components with unique phases.

Energy extraction from the biologic battery in the inner ear

Endocochlear potential (EP) is a battery-like electrochemical gradient found in and actively maintained by the inner ear. Here we demonstrate that the mammalian EP can be used as a power source for electronic devices. We achieved this by designing an anatomically sized, ultra-low quiescent-power energy harvester chip integrated with a wireless sensor capable of monitoring the EP itself. Although other forms of in vivo energy harvesting have been described in lower organisms, and thermoelectric, piezoelectric and biofuel devices are promising for mammalian applications, there have been few, if any, in vivo demonstrations in the vicinity of the ear, eye and brain. In this work, the chip extracted a minimum of 1.12 nW from the EP of a guinea pig for up to 5 h, enabling a 2.4 GHz radio to transmit measurement of the EP every 40-360 s. With future optimization of electrode design, we envision using the biologic battery in the inner ear to power chemical and molecular sensors, or drug-delivery actuators for diagnosis and therapy of hearing loss and other disorders.

Influence of acute hyperglycemia on otoacoustic emissions and the medial olivocochlear reflex

Stimulus-frequency (SF) otoacoustic emission (OAE) amplitude and the amplitude of medial olivocochlear (MOC) inhibition of SF OAEs for ipsilateral, contralateral and bilateral MOC reflex elicitors were recorded in six subjects with type 2 diabetes during a glucose tolerance test (GTT). Five of the six subjects were tested twice for a total of 11 trials and three subjects were tested in a control experiment. During the GTT experiment, the subjects' blood glucose was elevated from a euglycemic level below 150 mg/dL to a hyperglycemic level above 160 mg/dL following the consumption of a bolus of 80 g of sugar. A subset of three subjects were tested in a control experiment during which SF OAE and MOC reflex measurements were made while blood sugar levels remained constant within the euglycemic region. Mean SF OAE amplitudes were elevated following glucose consumption. A statistically significant increase in MOC inhibition amplitude was observed during elevated sugar levels for the 11 GTT trials. Maximum inhibition occurred about an hour after glucose consumption when blood glucose levels peaked. Results indicate that acute hyperglycemia influences efferent control of the cochlea in people with type 2 diabetes.

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

Distortion-product otoacoustic emission (DPOAE) input/output (I/O) functions were measured in 322 ears of 176 subjects at as many as 8 f(2) frequencies per ear for a total of 1779 I/O functions. The f(2) frequencies ranged from 0.7 to 8 kHz in half-octave steps. Behavioral thresholds (BTs) at the f(2) frequencies ranged from -5 to 60 dB hearing loss (HL). Both linear-pressure and nonlinear, two-slope functions were fitted to the data. The two-slope function describes I/O compression as output-controlled self-suppression. Most I/O functions (96%) were better fitted by the two-slope method. DPOAE thresholds based on each method were used to predict BTs. Compared to estimates based on linear-pressure functions, individual BTs predicted from DPOAE thresholds based on the two-slope model had lower residual error and accounted for more variance. Another advantage of the two-slope method is that it provides an estimate of response growth rate (RGR) that is not tied to threshold. At all frequencies, the median low-level RGR (across I/O functions of the same f(2) and BT) usually increased as BT increased, while high-level compression decreased. The observed characteristics of DPOAE I/O functions are consistent with the loss of cochlear compression that is typically associated with mild-to-moderate HL.

Accuracy of velocity distortion product otoacoustic emissions for estimating mechanically based hearing loss

Distortion product otoacoustic emissions (DPOAEs) measured as vibration of the human eardrum have been successfully used to estimate hearing threshold. The estimates have proved more accurate than similar methods using sound-pressure DPOAEs. Nevertheless, the estimation accuracy of the new technique might have been influenced by endogenous noise, such as heart beat, breathing and swallowing. Here, we investigate in an animal model to what extent the accuracy of the threshold estimation technique using velocity-DPOAEs might be improved by reducing noise sources. Velocity-DPOAE I/O functions were measured in normal and hearing-impaired anaesthetized guinea pigs. Hearing loss was either conductive or induced by furosemide injection. The estimated distortion product threshold (EDPT) obtained by extrapolation of the I/O function to the abscissa was found to linearly correlate with the compound action potential threshold at the f(2) frequency, provided that furosemide data were excluded. The standard deviation of the linear regression fit was 6 dB as opposed to 8 dB in humans, suggesting that this accuracy should be achievable in humans with appropriate improvement of signal-to-noise ratio. For the furosemide animals, the CAP threshold relative to the regression line provided an estimate of the functional loss of the inner hair cell system. For mechanical losses in the middle ear and/or cochlear amplifier, DPOAEs measured as velocity of the umbo promise an accuracy of hearing threshold estimation comparable to classical audiometry.

Source of level dependent minima in rabbit distortion product otoacoustic emissions

Sharp level dependent minima (commonly called nulls or notches) in the distortion product otoacoustic emissions (DPOAEs) have been postulated to be due to two different mechanisms. It is shown here that the level dependent nulls in rabbit 2f(1)-f(2) DPOAEs carry the signature of the mixing of a third order nonlinear term with a fifth order nonlinear term. This suggests that the minima are not due to the mixing of signals from two different physical sites of origin, but rather are due to the nature of the nonlinearity itself. Model simulations show that null production is indifferent to several properties of nonlinear input/output functions.

Sensitive response to low-frequency cochlear distortion products in the auditory midbrain

During auditory stimulation with several frequency components, distortion products (DPs) are generated as byproduct of nonlinear cochlear amplification. After generated, DP energy is reemitted into the ear channel where it can be measured as DP otoacoustic emission (DPOAE), and it also induces an excitatory response at cochlear places related to the DP frequencies. We measured responses of 91 inferior colliculus (IC) neurons in the gerbil during two-tone stimulation with frequencies well above the unit's receptive field but adequate to generate a distinct distortion product (f2-f1 or 2f1-f2) at the unit's characteristic frequency (CF). Neuronal responses to DPs could be accounted for by the simultaneously measured DPOAEs for DP frequencies >1.3 kHz. For DP frequencies <1.3 kHz (n = 25), there was a discrepancy between intracochlear DP magnitude and DPOAE level, and most neurons responded as if the intracochlear DP level was significantly higher than the DPOAE level in the ear channel. In 12% of those low-frequency neurons, responses to the DPs could be elicited even if the stimulus tone levels were below the threshold level of the neuron at CF. High intracochlear f2-f1 and 2f1-f2 DP-levels were verified by cancellation of the neuronal DP response with a third phase-adjusted tone stimulus at the DP frequency. A frequency-specific reduction of middle ear gain at low frequencies is possibly involved in the reduction of DPOAE level. The results indicate that pitch-related properties of complex stimuli may be produced partially by high intracochlear f2-f1 distortion levels.

Influence of contralateral acoustic stimulation on the quadratic distortion product f2-f1 in humans

Contralateral acoustic stimulation is known to activate the medial olivocochlear system which is capable of modulating the amplification process in the outer hair cells of the inner ear. We investigated the influence of different levels of contralateral broadband noise on distortion product otoacoustic emissions in humans, with a particular focus on the quadratic distortion product at f2-f1. The primary stimulus frequency ratio was optimized to yield maximum f2-f1 level. While the cubic distortion product at 2f1-f2 was not significantly affected during contralateral noise stimulation, the level of f2-f1 was reduced by up to 4.8dB on average (maximum: 10.1dB), with significant suppression occurring for noise levels as low as 40dB SPL. In addition, a significant phase lead was observed. Quadratic distortions are minimal at a symmetrical position of the transfer function of the cochlear amplifier. The observed sensitivity of f2-f1 to contralateral noise stimulation could hence be resulting from a shift of the operating state and/or a change in the gain of the cochlear amplification due to contralateral induced efferent modulation of the outer hair cell properties.

Determining the cause of hearing loss: differential diagnosis using a comparison of audiometric and otoacoustic emission responses

OBJECTIVE

The purpose of this study was to further investigate the possibility of developing noninvasive methods of differential diagnosis of hearing disorders through the study of experimental animals with induced lesions. In particular, it was desired to compare distortion product otoacoustic emission (DPOAE) responses and auditory brain stem response (ABR) thresholds in Mongolian gerbils having either acoustic or strial damage, using as a reference the same responses measured in a control group of normal young adult gerbils. The goal was to evaluate the potential clinical application of this approach to determining the dominant contribution to sensorineural hearing loss in individual human subjects.

DESIGN

DPOAE input-output functions and ABR thresholds were measured over a wide range of stimulus frequencies for three groups: (1) a reference group of normal young adult gerbils; (2) a group in which acoustic damage had been induced 2 wk earlier; (3) a group in which damage to the stria vascularis was induced by a series of furosemide injections. The responses in the experimental groups relative to the normal means were compared to determine which combinations of responses were effective in discriminating between animals with different lesions. Three measures were evaluated in detail: the ABR threshold, the emission threshold at a criterion emission amplitude, and the emission amplitude at a high stimulus level.

RESULTS

Considering cases with significant hearing loss (ABR thresholds elevated by 20 dB or more), the best method for distinguishing between the two lesions involved a two-dimensional plot comparing emission and ABR thresholds at the same stimulus frequencies. Acoustic damage cases were found in a broad region where the emission and ABR thresholds were roughly equal, whereas strial damage cases were found in a narrower region where the emission threshold was about 0.4 times the ABR threshold (both in dB). These two cases were compared with a third case introduced by definition, that is, damage to inner hair cell or neural systems resulting in an increase in audiometric threshold but no change in emission responses (e.g., auditory neuropathy). The responses for these three cases were found to lie in different regions of the two-dimensional plot comparing emission and ABR thresholds, provided only that ABR thresholds were elevated 20 dB or more. This diagram also revealed cases of preclinical acoustic damage, in which the ABR threshold was shifted less than 20 dB but where the emission threshold was significantly elevated.

CONCLUSIONS

The results clearly demonstrate the possibility of developing a clinical method of noninvasive differential diagnosis of hearing loss. The method demonstrated was to add to a standard audiometric evaluation the measurement of DPOAE growth functions over the range of frequencies where these emissions were relatively easy to measure and consistent. The DPOAE stimulus frequencies were chosen to match the audiometric frequencies, and the corresponding emission and audiometric thresholds were compared on a threshold-threshold plot for each individual at a number of stimulus frequencies. Responses in different regions in this plot were found to correspond to different types of sensorineural hearing loss.

Long term effects of BAPTA in scala media on cochlear function

BAPTA was iontophoresed or allowed to diffuse into the scala media of the first turn of the guinea pig cochlea via pipettes inserted through the round window and basilar membrane. Cochlear action potential (CAP) thresholds for basal turn frequencies were elevated, scala media cochlear microphonic in response to a 207Hz tone were drastically reduced and the distortion products 2f1-f2 and f2-f2 in response to primaries set at 18 and 21.6kHz were eliminated or severely reduced. The animals were recovered and the above measurements repeated between 24 and 240h after the application of BAPTA. In all animals thresholds for basal turn frequencies remained elevated, and the distortion components were severely reduced. The endolymphatic potential (EP), measured through the basilar membrane on recovery, was not significantly different from the values measured before BAPTA was applied. If the effect of BAPTA, in lowering endolymphatic Ca(2+) concentration, is restricted to the destruction of tip links, as has been shown in many other preparations, then these results suggest that this effect has permanent consequences, either because the tip links failed to regenerate or because their destruction precipitated the degeneration of OHCs. These results may have a bearing on the mechanisms behind permanent threshold shift.

Modeling distortion product otoacoustic emission input/output functions using segmented regression

Distortion product otoacoustic emissions (DPOAEs) are low-level acoustic signals, the detection of which involves extraction from a background of noise. Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] described a method for modeling the presence and growth of these responses. While improving growth function parameter estimation, this technique excludes a significant fraction of the data (especially low-level responses), and relies on ad hoc model fit acceptance criteria. The statistical difficulties associated with these limitations are described, and a weighted segmented linear regression model that avoids them is proposed. A simple test is presented for the presence of DPOAE growth. This technique is compared to that of Boege and Janssen in a dataset of 9 556 input/output (I/O) functions collected over 4 years on 866 ears from 379 construction apprentices and 63 age-matched controls. Comparisons are made on the entire dataset and within audiometric hearing loss categories. Segmented regression avoids the statistical pitfalls of the previous method, allows estimation of the threshold and slope of auditory response on a far greater number of I/O functions, and improves estimation of these parameters in this dataset. The potential for this method to yield more sensitive metrics of hearing function and compromise is discussed.

A model of real time monitoring of the cochlear function during an induced local ischemia

The aim of this study was to investigate the utility of distortion product otoacoustic emissions (DPOAEs) in intraoperative monitoring (IM) of cochlear ischemic episodes in animals during internal auditory artery (IAA) compression. The IAA was exposed using the posterior fossa approach and then compressed for 3 and 5 min intervals to effect ischemia. DPOAE amplitudes and phases were measured at 4, 8, and 12 kHz geometric mean frequency (GMF). In each monitored ear, laser-Doppler cochlear blood flow (CBF) was measured. All IAA compressions resulted in rapid decrease of DPOAE amplitude and CBF, with simultaneous DPOAE phase increase. DPOAE phase changes were found to increase consistently within several seconds of IAA compression, while corresponding DPOAE amplitudes changed more slowly, with up to 30-40 s delays. Following IAA release, DPOAEs at 12 kHz GMF were characterized by longer delays in returning to baseline than those measured at lower frequencies. In some cases, CBF did not return to baseline. In this animal model, DPOAEs were found to be sensitive measures of cochlear function during transient cochlear ischemic episodes, suggesting the utility of DPOAE monitoring of auditory function during surgery of cerebello-pontine angle tumors.

Distortion product otoacoustic emission (DPOAE) in tinnitus patients

The aim of this study was to determine the effect of tinnitus (experiment I) and the combined effect of tinnitus and sensorineural hearing loss (experiment II) on the distortion product otoacoustic emission (DPOAE) for two age groups of tinnitus patients. Tinnitus patients with normal earing, along with normal-hearing control subjects, participated in experiment I. They were divided into two age groups, below 50 and above 50 years. Experiment I showed that the DPOAE levels in the tinnitus patients were lower than those in the normal-hearing (nontinnitus) subjects. The differences depended on the frequency and the age of the patients, suggesting the confounding influence of presbyacusis. The second group of tinnitus patients with increasing and notch-like hearing loss participated in experiment II. They were also divided into two age groups, below 50 and above 50 years. The data from experiment II showed that DPOAE activity well reflects the increasing and notch-like hearing loss functions up to about 40 dB HL. The effect of age on the DPOAE level was clearly noted only for the tinnitus patients with clinically normal-hearing thresholds and was ambiguous for the tinnitus patients with hearing loss.

Characteristics of distortion product otoacoustic emissions in the frog from L1,L2 maps

For a given set of stimulus frequencies (f1 ,f2), the level of distortion product otoacoustic emissions (DPOAEs) varies with the levels of the stimulus tones. By variation of the stimulus levels, L1,L2-maps for DPOAEs can be constructed. Here, we report on L1 ,L2-maps for DPOAEs from the frog ear. In general, these maps were similar to those obtained from the mammalian cochlea. We found a conspicuous difference between the equal-level contour lines for low-level and high-level DPOAEs, which could be modeled by a saturating and an expansive nonlinearity, respectively. The transition from the high-level to the low-level response was accompanied by a DPOAE phase-change, which increased from 0 to pi rad with increasing frequency. These results suggest that in the frog low-level and high-level DPOAEs are generated by separate nonlinear mechanisms. Also, there was a conspicuous difference in the growth of the low-level emissions from the two anuran auditory papillae. In the basilar papilla, this growth was expansive for the lowest stimulus levels and saturated for intermediate levels. This is consistent with the behavior of a Boltzman nonlinearity. In the amphibian papilla this growth was compressive, suggesting the additional effect of a compressive amplification mechanism on the generation of DPOAEs.

Level dependence of distortion-product otoacoustic emissions measured at high frequencies in humans

Given that high-frequency hearing is most vulnerable to cochlear pathology, it is important to characterize distortion-product otoacoustic emissions (DPOAEs) measured with higher-frequency stimuli in order to utilize these measures in clinical applications. The purpose of this study was to explore the dependence of DPOAE amplitude on the levels of the evoking stimuli at frequencies greater than 8 kHz, and make comparisons with those data that have been extensively measured with lower-frequency stimuli. To accomplish this, DPOAE amplitudes were measured at six different f2 frequencies (2, 5, 10, 12, 14, and 16 kHz), with a frequency ratio (f2/f1) of 1.2, at five fixed levels (30 to 70 dB SPL) of one primary (either f1 or f2), while the other primary was varied in level (30 to 70 dB SPL). Generally, the level separation between the two primary tones (L1 > L2) generating the largest DPOAE amplitude (referred to as the "optimal level separation") decreased as the level of the fixed primary increased. Additionally, the optimal level separation was frequency dependent, especially at the lower fixed primary tone levels ( < or = 50 dB SPL). In agreement with previous studies, the DPOAE level exhibited greater dependence on L1 than on L2.

Level dependence of distortion product otoacoustic emissions in the leopard frog, Rana pipiens pipiens

The inner ear of frogs holds two papillae specialized in detecting airborne sound, the amphibian papilla (AP) and the basilar papilla (BP). We measured input-output (I/O) curves of distortion product otoacoustic emissions (DPOAEs) from both papillae, and compared their properties. As in other vertebrates, DPOAE I/O curves showed two distinct segments, separated by a notch or kneepoint. The slope of the low-level segment was conspicuously different between the AP and the BP. For DPOAE I/O curves from the AP, slopes were < or = 1 dB/dB, similar to what is found in mammals, birds and some lizards. For DPOAE I/O curves from the BP these slopes were much steeper (approximately 2 dB/dB). Slopes found at high stimulus levels were similar in the AP and the BP (approximately 2 dB/dB). This quantitative difference between the low-level slopes for DPOAEs from the AP and the BP may signify the involvement of different mechanisms in low-level DPOAE generation for the two papillae, respectively.

Relationship of neural and otoacoustic emission thresholds during endocochlear potential development in the gerbil

Distortion product otoacoustic emissions and auditory brainstem responses (ABRs) were measured in neonatal gerbils at three ages: at 15-16 days after birth (dab), near the onset of hearing when the endocochlear potential (EP) is known to be still immature; at 22 dab, when the EP first reaches mature levels; and at 30 dab. Comparing individual 15-16 dab animals to the 22 dab group, ABR threshold changes were typically larger than those for cubic distortion tone (CDT, 2f1-f2) emission thresholds which were, in turn, larger than those for the simple difference tone (DT, f2-f1). In contrast, from 22 to 30 dab there were no important changes in CDT or DT emission thresholds. Observed threshold-change relationships were very similar to those found in differential diagnosis investigations, where the EP was experimentally decreased using a chronic furosemide application. Therefore, most of the change in cochlear function over the two week period studied could be attributed to the maturation of EP during the first week. Model calculations further show that relative changes in CDT and DT emission thresholds are compatible with a movement of the operating point of the cochlear amplifier toward its symmetrical "central" point as the EP reaches mature levels.

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.

The influence of transducer operating point on distortion generation in the cochlea

Distortion generated by the cochlea can provide a valuable indicator of its functional state. In the present study, the dependence of distortion on the operating point of the cochlear transducer and its relevance to endolymph volume disturbances has been investigated. Calculations have suggested that as the operating point moves away from zero, second harmonic distortion would increase. Cochlear microphonic waveforms were analyzed to derive the cochlear transducer operating point and to quantify harmonic distortions. Changes in operating point and distortion were measured during endolymph manipulations that included 200-Hz tone exposures at 115-dB SPL, injections of artificial endolymph into scala media at 80, 200, or 400 nl/min, and treatment with furosemide given intravenously or locally into the cochlea. Results were compared with other functional changes that included action potential thresholds at 2.8 or 8 kHz, summating potential, endocochlear potential, and the 2 f1-f2 and f2-f1 acoustic emissions. The results demonstrated that volume disturbances caused changes in the operating point that resulted in predictable changes in distortion. Understanding the factors influencing operating point is important in the interpretation of distortion measurements and may lead to tests that can detect abnormal endolymph volume states.

Metabolic presbycusis: differential changes in auditory brainstem and otoacoustic emission responses with chronic furosemide application in the gerbil

Auditory characteristics of metabolic or strial presbycusis were investigated using an animal model in which young adult Mongolian gerbils ( Meriones unguiculates) were implanted with an osmotic pump supplying furosemide continuously to the round window. This model causes chronic lowering of the endocochlear potential (EP) and results in auditory responses very similar to those seen in quiet-aged gerbils (Schmiedt et al., J. Neurosci. 22:9643-9650, 2002). Auditory function was examined up to one week post-implant by measurement of auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). Emission "threshold" was defined as the stimulus level required to reach a criterion emission amplitude. Comparing all responses on a "threshold-shift diagram," where emission threshold increases were plotted versus ABR threshold increases, the following results were obtained: (1) On average, the increase of the emission threshold was about 55% of the increase in ABR threshold, with comparatively little scatter. (2) The main dysfunction in metabolic presbycusis appears to be a decrease in the gain of the cochlear amplifier, combined with an additional, smaller increase in neural threshold, both effects caused by a chronically low EP. (3) For ABR threshold increases over 20 dB, the points for the chronic low-EP condition were largely separate from those previously found for permanent acoustic damage. The threshold-shift diagram therefore provides a method for noninvasive differential diagnosis of two common hearing dysfunctions.

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.

Degeneration of sensory outer hair cells following pharmacological blockade of cochlear KCNQ channels in the adult guinea pig

In the inner ear, hair cell function is inextricably linked with intracellular potassium homeostasis. KCNQ potassium channels may play an important role by preventing accumulation of potassium in the hair cells. Linopirdine, a tool useful in targeting native or heterologous KCNQ channels, was used to study the role of KCNQ channels in the guinea pig cochlea. When perfused into intact cochlea, linopirdine transiently increases the summating potential and endocochlear potential, suggesting that it alters K+ homeostasis. The concomitant decrease in cochlear microphonic potential and distortion product otoacoustic emission amplitude indicates that linopirdine has an effect on the outer hair cells (OHCs). To determine the pathological consequences of the inhibition of cochlear KCNQ channels, we developed a hearing loss model based on a chronic intracochlear perfusion of linopirdine via an osmotic minipump. Ultrastructural analysis reveals that KCNQ channel blockade leads to OHC degeneration. Together, these results demonstrate that KCNQ channels, most probably of the KCNQ4 subtype, are crucial for the function and survival of sensory OHCs. Clinically, KCNQ4 channel dysfunction is known to be associated with the DFNA2 form of nonsyndromic dominant deafness. Our study shows that OHC KCNQ4 dysfunction could contribute to the early (40dB) hearing loss, but not for the profound deafness observed at the final stage of this disease.

Differential responses to acoustic damage and furosemide in auditory brainstem and otoacoustic emission measures

Characteristics of distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) were measured in Mongolian gerbil before and after the introduction of two different auditory dysfunctions: (1) acoustic damage with a high-intensity tone, or (2) furosemide intoxication. The goal was to find emission parameters and measures that best differentiated between the two dysfunctions, e.g., at a given ABR threshold elevation. Emission input-output or "growth" functions were used (frequencies f1 and f2, f2/f1 = 1.21) with equal levels, L1 = L2, and unequal levels, with L1 = L2 + 20 dB. The best parametric choice was found to be unequal stimulus levels, and the best measure was found to be the change in the emission threshold level, delta x. The emission threshold was defined as the stimulus level required to reach a criterion emission amplitude, in this case -10 dB SPL. (The next best measure was the change in emission amplitude at high stimulus levels, specifically that measured at L1 x L2 = 90 x 70 dB SPL.) For an ABR threshold shift of 20 dB or more, there was essentially no overlap in the emission threshold measures for the two conditions, sound damage or furosemide. The dividing line between the two distributions increased slowly with the change in ABR threshold, delta ABR, and was given by delta x(t) = 0.6 delta ABR + 8 dB. For a given delta ABR, if the shift in emission threshold was more than the calculated dividing line value, delta x(t), the auditory dysfunction was due to acoustic damage, if less, it was due to furosemide.

Physiopathological significance of distortion-product otoacoustic emissions at 2f1-f2 produced by high- versus low-level stimuli

Distortion product otoacoustic emissions emitted by the cochlea at 2f1-f2 in response to pairs of pure tones at f1 and f2 (DPOAE) form a class of otoacoustic emissions and as such, are viewed as a reliable tool for screening outer hair cell (OHC) dysfunctions on a pass/fail basis. However, the persistence of residual DPOAEs from impaired cochleae at high stimulus levels has suggested that above 60-70 dB SPL, instead of reflecting "active" cochlear motion, DPOAEs might represent another "passive" modality: they would thus become unsuitable for analyzing cochlear function. The present work reports the consequences on high- vs low-level DPOAEs of three types of cochlear impairments involving OHCs: progressive OHC degeneration of genetic origin in CD1 mice, complete cochlear ischemia in gerbils, and furosemide injection vs ischemia-reperfusion in gerbils. An alternative to the "active-passive" model was used wherein regardless of stimulus level, cubic DPOAEs are produced by N (probably OHC-borne) nonlinear elements driven by input I and modulated by a function F3 of their operating point o; thus, DPOAE proportional to NI3F3(o). When OHCs degenerated, thereby implying a decrease of N, DPOAE levels also decreased regardless of the stimulus level up to 80 dB SPL, in line with the previous formula but at variance with the prediction of the active-passive concept. Instead of affecting N, the other two experiments impaired the efficiency of the cochlear feedback loop as a result of its electrical drive being decreased by strial dysfunction. As it is well accepted that the impaired basilar-membrane motion, although greatly reduced at low levels, tends to catch up with a normal one at higher levels, it was assumed the same was true with I so that DPOAE levels had to be, and indeed were little affected at high levels while plummeting at low levels, without any need for invoking two modalities for DPOAE generation. Finally, comparisons of furosemide vs ischemia effects revealed additional influences on DPOAEs, possibly accounted for by function F3(o). These results lead to the proposal that although high-level DPOAEs are expected to be poor audiometric indicators, they seem well adapted to assessing the functional integrity of nonlinear elements in OHCs, i.e., presumably their mechanoelectrical transduction channels.

Solvent ototoxicity in the rat and guinea pig

There is clear evidence that aromatic solvents can disrupt the auditory system in humans and animals. As far as animal models are concerned, solvent-induced hearing loss seems to be species-dependent. Indeed, most published data have been obtained with the rat, which shows mid-frequency cochlear deficits, whereas the guinea pig does not show any permanent hearing loss after solvent exposure. In the current investigation, the effects of two solvents, toluene (600 ppm) and styrene (1000 ppm), were studied in both Long-Evans rats and pigmented guinea pigs exposed 6 h/day for 5 consecutive days. Cochlear function was tested by using distortion product otoacoustic emissions (DPOAE) measured prior to the solvent exposure, 20 min after the end of the exposure and successively at 2 and 4 weeks post-exposure. In addition to cochlear testing, solvent concentrations in blood and urinary metabolites were measured. A cochlear histological analysis was performed at the end of the experiment. No decrease in DPOAE amplitude was observed in the guinea pig, even immediately following the end of exposure. The rat model showed severe disruption of auditory function and cochlear pathology, whereas the guinea pig had no disruption of DPOAE or cochlear pathological alterations. Therefore, the vulnerability of the cochlear function was strictly dependent on the species. As expected, an important difference in the styrene concentration in blood was observed: the solvent concentrations were fourfold higher in the rat than in the guinea pig. Therefore, it is clear that a pharmacokinetic or an uptake difference might explain the difference in susceptibility observed between the two species. Moreover, the metabolism pathways of the solvents were different depending on the species. Attempts to explain differences of vulnerability between the rat and guinea pig are addressed in the present paper.

Effects of furosemide applied chronically to the round window: a model of metabolic presbyacusis

Hearing thresholds in elderly humans without a history of noise exposure commonly show a profile of a flat loss at low frequencies coupled with a loss that increases with frequency above approximately 2 kHz. This profile and the relatively robust distortion product otoacoustic emissions that are found in elderly subjects challenge the common belief that age-related hearing loss (presbyacusis) is based primarily on sensory-cell disorders. Here, we examine a model of presbyacusis wherein the endocochlear potential (EP) is reduced by means of furosemide applied chronically to one cochlea of a young gerbil. The model results in an EP that is reduced from 90 to approximately 60 mV, a value often seen in quiet-aged gerbils, with no concomitant loss of hair cells. Resulting measures of cochlear and neural function are quantitatively similar to those seen in aging gerbils and humans, e.g., a flat threshold loss at low frequencies with a high-frequency roll-off of approximately -8.4 dB/octave. The effect of the EP on neural thresholds can be parsimoniously explained by the known gain characteristics of the cochlear amplifier as a function of cochlear location: in the apex, amplification is limited to approximately 20 dB, whereas in the base, the gain can be as high as 60 dB. At high frequencies, amplification is directly proportional to the EP on an approximately 1 dB/mV basis. This model suggests that the primary factor in true age-related hearing loss is an energy-starved cochlear amplifier that results in a specific audiogram profile.

Interpretation of standard distortion product otoacoustic emission measurements in light of the complete parametric response

Emission characteristics (at 2f1-f2) are measured in Mongolian gerbil as a function of the independent variation of all four stimulus parameters, the frequencies (f1 and f2) and the intensities (L1 and L2) of the two stimulus tones. The main five-dimensional display chosen is a logarithmic grid of frequencies, where for each frequency pair there is a contour map of the emission amplitude as a function of the two stimulus levels. The feature which leads to the greatest complexity in the proper interpretation of emission responses is the widespread presence of "notches" in these contour maps. Notches are lines of relative minima in the emission amplitude, and are found at either: (1) constant L1, but only in regions where L1 > L2; or (2) at constant L2, only where L2 > or = L1. Notches are not found at any other orientations, and are associated with emission phase shifts of about 180 degrees as the notch line is traversed. These notch characteristics are explained by phase cancellation in a simple cochlear amplifier model in which there is a change, as a function of the stimulus level alone, of relevant characteristics of the cochlear response to a single tone. Only one mechanism of emission generation is required to explain the observed patterns, i.e., there is no need to invoke different "active" and "passive" mechanisms. Unless properly accounted for, the presence of notches adversely affects all of the standard emission measurements, i.e., all methods which cover a restricted parameter set such as DPgrams, input-output or "growth" functions, and frequency ratio functions. Conversely, because the notch location appears approximately invariant in the cochlea, notches potentially make it possible to use certain emission growth functions to estimate forward and reverse middle-ear transfer functions.

Ouabain application to the round window of the gerbil cochlea: a model of auditory neuropathy and apoptosis

The physiological and morphological changes resulting from acute and chronic infusion of ouabain onto the intact round-window (RW) membrane were examined in the gerbil cochlea. Osmotic pumps fitted with cannulas allowed chronic (0.5-8 days) infusions of ouabain. Acute and short-term applications of ouabain (1-24 h) induced an increase in auditory-nerve compound action potential (CAP) thresholds at high frequencies with lower frequencies unaffected. The resulting threshold shifts were basically all (no response) or none (normal thresholds), with a sharp demarcation between high and low frequencies. Survival times of 2 days or greater after ouabain exposure resulted in complete auditory neuropathy with no CAP response present at any frequency. Distortion product otoacoustic emissions (DPOAEs) and the endocochlear potential (EP) were largely unaffected by the ouabain indicating normal function of the outer hair cells (OHC) and stria vascularis. One to 3 days after short-term applications, apoptosis was evident among the spiral ganglion neurons assessed both morphologically and with TdT-mediated dUTP-biotin nick end labeling (TUNEL). With 4-8 day survival times, most spiral ganglion cells were absent; however, a few cell bodies remained intact in many ganglia profiles. These surviving neurons had many of the characteristics of type II afferents. Our working hypothesis is that the ouabain induces a spreading depression among the type I ganglion cells by blocking the Na,K-ATPase pump. Because of the constant spike activity of these cells, the ouabain rapidly alters potassium concentrations within ([K+]i) and external to ([K+]o) the ganglion cells, thereby initiating an apoptotic cascade.

Evaluation of anesthesia effects in a rat animal model using otoacoustic emission protocols

Anesthesia effects on otoacoustic emission (OAE) recordings were evaluated in a group of 72 Sprague-Dawley rats (mean weight 225+/-20 gr). Two anesthesia dosages (high and normal) and two anesthetic protocols (ketamine-xylazine, ketamine-xylazine-atropine) were tested. Transient evoked OAE (TEOAE) and distortion product OAE (DPOAE) responses were recorded in 10 min intervals, for a total period of 60 min. Analyses of the data with repeated measure models indicated the following: (1) The animals receiving a high dose of anesthesia (cumulative dose 66.6 mg of ketamine and 13.2 mg of xylazine/kg of body weight) presented significant alterations of the TEOAE response level and the signal to noise ratio at 3.0 kHz; (2) the animals receiving a normal dose of ketamine-xylazine anesthesia (cumulative dose 50 mg of ketamine and 10 mg of xylazine/kg of body weight) presented TEOAE and DPOAE responses invariant in terms of time; (3) significant differences were observed in the DPOAE responses from animals anesthetized with ketamine-xylazine and ketamine-xylazine-atropine. The data support the hypothesis that the ketamine anesthesia OAE suppressing mechanism is related to middle-ear mechanics.

Deriving a cochlear transducer function from low-frequency modulation of distortion product otoacoustic emissions

In this paper, a new method is introduced to derive a cochlear transducer function from measuring distortion product otoacoustic emissions (DPOAEs). It is shown that the cubic difference tone (CDT, 2f1-f2) is produced from the odd-order terms of a power series that approximates a nonlinear function characterizing cochlear transduction. Exploring the underlying mathematical formulation, it is found that the CDT is proportional to the third derivative of the transduction function when the primary levels are sufficiently small. DPOAEs were measured from nine gerbils in response to two-tone signals biased by a low-frequency tone with different amplitudes. The CDT magnitude was obtained at the peak regions of the bias tone. The results of the experiment demonstrated that the shape of the CDT magnitudes as a function of bias levels was similar to the absolute value of the third derivative of a sigmoidal function. A second-order Boltzmann function was derived from curve fitting the CDT data with an equation that represents the third derivative of the Boltzmann function. Both the CDT-bias function and the derived nonlinear transducer function showed effects of primary levels. The results of the study indicate that the low-frequency modulated DPOAEs can be used to estimate the cochlear transducer function.

One source for distortion product otoacoustic emissions generated by low- and high-level primaries

Distortion product otoacoustic emissions (DPOAE) elicited by tones below 60-70 dB sound pressure level (SPL) are significantly more sensitive to cochlear insults. The vulnerable, low-level DPOAE have been associated with the postulated active cochlear process, whereas the relatively robust high-level DPOAE component has been attributed to the passive, nonlinear macromechanical properties of the cochlea. However, it is proposed that the differences in the vulnerability of DPOAEs to high and low SPLs is a natural consequence of the way the cochlea responds to high and low SPLs. An active process boosts the basilar membrane (BM) vibrations, which are attenuated when the active process is impaired. However, at high SPLs the contribution of the active process to BM vibration is small compared with the dominating passive mechanical properties of the BM. Consequently, reduction of active cochlear amplification will have greatest effect on BM vibrations and DPOAEs at low SPLs. To distinguish between the "two sources" and the "single source" hypotheses we analyzed the level dependence of the notch and corresponding phase discontinuity in plots of DPOAE magnitude and phase as functions of the level of the primaries. In experiments where furosemide was used to reduce cochlear amplification, an upward shift of the notch supports the conclusion that both the low- and high-level DPOAEs are generated by a single source, namely a nonlinear amplifier with saturating I/O characteristic.

Effects of age on the distortion product otoacoustic emission growth functions

Age-related hearing loss (presbycusis) is thought to result from age-related degeneration (aging) of the cochlea plus the cumulative effects of extrinsic damage (noise and other ototoxic agents) and intrinsic disorders (e.g. systemic diseases). Previous studies have implicated dysfunction of the hair cells (sensory presbycusis) as the principal mechanism of age-related hearing loss. However, recent evidence from quiet-reared gerbils suggests that cochlear aging results primarily from atrophy of the stria vascularis, which is associated with diminished endocochlear potential (EP), spiral ganglion atrophy, and a relatively flat audiometric loss, termed metabolic presbycusis. Because it is not currently possible to measure EP directly in the clinical setting, we wondered if cochlear metabolic dysfunction might be evidenced indirectly from existing clinical tests, specifically, the input-output (IO) growth function of the distortion product (DP) otoacoustic emissions in relation to behavioral hearing threshold levels (HTL). We anticipated finding discordance between the IO functions and HTL with either a greater decline with age in HTL than in IO functions if an age-related metabolic dysfunction of the cochlea was operant, or a greater loss of IO function than HTL if outer hair cell dysfunction was the dominant pathology. To address this supposition we analyzed existing auditory data from a large cohort of adults to determine the change with age in three aspects of the DP IO function: area under the curve, threshold, and slope. The analyses demonstrated a greater effect of age on HTL than on the DP IO measures. This effect supports the hypothesis that strial dysfunction is a substantive factor in cochlear aging. The etiology and mechanisms for this dysfunction are conjectural at present.