Distortion Product Otoacoustic Emissions Components Research Articles

Reliable Long-Term Serial Evaluation of Cochlear Function Using Pulsed Distortion-Product Otoacoustic Emissions: Analyzing Levels and Pressure Time Courses.

To date, there is no international standard on how to use distortion-product otoacoustic emissions (DPOAEs) in serial measurements to accurately detect changes in the function of the cochlear amplifier due, for example, to ototoxic therapies, occupational noise, or the development of regenerative therapies. The use of clinically established standard DPOAE protocols for serial monitoring programs appears to be hampered by multiple factors, including probe placement and calibration effects, signal-processing complexities associated with multiple sites of emission generation as well as suboptimal selection of stimulus parameters. Pulsed DPOAEs were measured seven times within 3 months for f2 = 1 to 14 kHz and L2 = 25 to 80 dB SPL in 20 ears of 10 healthy participants with normal hearing (mean age = 32.1 ± 9.7 years). L1 values were computed from individual optimal-path parameters derived from the corresponding individual DPOAE level map in the first test session. Three different DPOAE metrics for evaluating the functional state of the cochlear amplifier were investigated with respect to their test-retest reliability: (1) the interference-free, nonlinear-distortion component level (LOD), (2) the time course of the DPOAE-envelope levels, LDP(t), and (3) the squared, zero-lag correlation coefficient () between the time courses of the DPOAE-envelope pressures, pDP(t), measured in two sessions. The latter two metrics include the two main DPOAE components and their state of interference. Collated over all sessions and frequencies, the median absolute difference for LOD was 1.93 dB and for LDP(t) was 2.52 dB; the median of was 0.988. For the low (f2 = 1 to 3 kHz), mid (f2 = 4 to 9 kHz), and high (f2 = 10 to 14 kHz) frequency ranges, the test-retest reliability of LOD increased with increasing signal to noise ratio (SNR). On the basis of the knowledge gained from this study on the test-retest reliability of pulsed DPOAE signals and the current literature, we propose a DPOAE protocol for future serial monitoring applications that takes into account the following factors: (1) separation of DPOAE components, (2) use of individually optimal stimulus parameters, (3) SNR of at least 15 dB, (4) accurate pressure calibration, (5) consideration of frequency- and level-dependent test-retest reliabilities and corresponding reference ranges, and (6) stimulus levels L2 that are as low as possible with sufficient SNR to capture the nonlinear functional state of the cochlear amplifier operating at its highest gain.

Descriptive Characterization of High-Frequency Distortion Product Otoacoustic Emission Source Components in Children.

Distortion product otoacoustic emissions (DPOAEs) provide an objective assessment of cochlear function and are used for serial ototoxicity monitoring in pediatric cancer patients. DPOAEs are modeled as having distortion (near f2) and reflection (near 2f1-f2) component sources, and developmental changes are observed in these components' relative strengths in infants compared with adults. However, little is known about source component strengths in childhood or at extended high frequencies (EHFs; > 8 kHz). Thus, the purpose of this study was to describe the effects of age and stimulus frequency on DPOAE components in children. DPOAEs were collected with varied frequency ratios (f2/f1 = 1.1-1.25) for a wide range of frequencies (2-16 kHz) in 39 younger (3-6 years) and 41 older (10-12 years) children with constant levels (L1/L2) of 65/50 dB SPL. A depth-compensated simulator sound pressure level method of calibration was employed. A time waveform representation of the results across various ratios was created to estimate peak pressures and latencies of each DPOAE component. Estimated peak pressures of DPOAE components revealed the greatest differences in DPOAE sources between children occurring at the highest frequencies tested, where the peak pressure of both components was largest for younger compared with older children. Latency differences between the children were only noted at higher frequencies for the distortion component. These results suggest that DPOAE levels decrease with age and reflection emissions are vulnerable to cochlear change. This work guides optimization of protocols for pediatric ototoxicity monitoring, whereby including EHF otoacoustic emissions is clearly warranted and choosing to isolate DPOAE sources may prove beneficial. https://doi.org/10.23641/asha.23669214.

An additional source of distortion-product otoacoustic emissions from perturbation of nonlinear force by reflection from inhomogeneities

The basilar membrane in the cochlea can be modeled as an array of fluid coupled segments driven by stapes vibration and by the undamping nonlinear force simulating cochlear amplification. If stimulated with two tones, the model generates additional tones due to nonlinear distortion. These distortion products (DPs) can be transmitted into the ear canal and produce distortion-product otoacoustic emissions (DPOAEs) known to be generated in the healthy ear of various vertebrates. This study presents a solution for DPs in a two-dimensional nonlinear cochlear model with cochlear roughness-small irregularities in the impedance along the basilar membrane, which may produce additional DPs due to coherent reflection. The solution allows for decomposition of various sources of DPs in the model. In addition to the already described nonlinear-distortion and coherent-reflection mechanisms of DP generation, this study identifies a long-latency DPOAE component due to perturbation of nonlinear force. DP wavelets that are coherently reflected due to impedance irregularities travel toward the stapes across the primary generation region of DPs and there evoke perturbation of the nonlinear undamping force. The ensuing DP wavelets have opposite phase to the wavelets arising from coherent reflection, which results in partial cancellation of the coherent-reflection DP wavelets.

Distortion Product Otoacoustic Emission Component Behavior as a Function of Primary Frequency Ratio and Primary Level.

Distortion product otoacoustic emissions (DPOAEs) are composed of distortion and reflection components. Much is known about the influence of the stimulus frequency ratio (f 2 /f 1 ) on the overall/composite DPOAE level. However, the influence of f 2 /f 1 on individual DPOAE components is not as well examined. The goals of this pilot study were to systematically evaluate the effects of f 2 /f 1 on DPOAE components in clinically normal-hearing young adult ears. To extend the limited reports in the literature, this examination was carried out over an extended frequency range using two stimulus-level combinations. DPOAEs were recorded from seven normal-hearing, young adult ears for f 2 frequencies between 0.75 and 16 kHz over a range of f 2 /f 1 using two stimulus-level combinations. The distortion (DPOAE D ) and reflection (DPOAE R ) components were separated using an inverse fast Fourier transform algorithm. Optimal ratios for the composite DPOAE and DPOAE components were determined from smoothed versions of level versus ratio functions in each case. The optimal ratio for the composite DPOAE level increased with stimulus level and decreased as a function of frequency above 1 kHz. The optimal ratios for the DPOAE components followed a similar trend, decreasing with increasing frequency. The optimal ratio for DPOAE D was generally higher than that for DPOAE R . The overall level for DPOAE D was greater than that of DPOAE R , both decreasing with increasing frequency. DPOAE R , but not DPOAE D , became unrecordable above the noise floor at the higher frequencies. DPOAE components behave similarly but not identically as a function of f 2 /f 1 . The ear canal DPOAE is generally dominated by DPOAE D . The behavior of DPOAE D as a function of f 2 /f 1 is entirely consistent with known properties of cochlear mechanics. The behavior of DPOAE R is more variable across ears, perhaps reflective of the increased number of parameters that influence its final form. Attempting to use an f 2 /f 1 that would allow a greater bias of the ear canal DPOAE toward one component or the other does not appear to be practical.

The influence of self-reported noise exposure on 2ƒ1-ƒ2 distortion product otoacoustic emission level, fine structure, and components in a normal-hearing population.

Distortion product otoacoustic emissions (DPOAEs) offer an outcome measure to consider for clinical detection and monitoring outer hair cell dysfunction as a result of noise exposure. This investigation detailed DPOAE characteristics and behavioral hearing thresholds up to 20 kHz to identify promising metrics for early detection of cochlear dysfunction. In a sample of normal-hearing individuals with and without self-reported noise exposure, the DPOAE and hearing threshold measures, as assessed by two questions, were examined. The effects on various auditory measures in individuals aged 10-65 years old with clinically normal/near-normal hearing through 4 kHz were evaluated. Individuals reporting occupational noise exposures (n = 84) and recreational noise exposures (n = 46) were compared to age-matched nonexposed individuals. The hearing thresholds and DPOAE level, fine structure, and component characteristics for the full frequency bandwidth were examined. The data suggest that the DPOAE levels measured using a range of stimulus levels hold clinical utility while fine structure characteristics offer limited use. Under carefully calibrated conditions, the extension to frequencies beyond 8 kHz in combination with various stimulus levels holds clinical utility. Moreover, this work supports the potential utility of the distortion product place component level for revealing differences in cochlear function due to self-reported, casual noise exposure that are not observable in behavioral hearing thresholds.

Derivation of input-output functions from distortion-product otoacoustic emission level maps.

Distortion-product otoacoustic emissions (DPOAEs) emerge from the cochlea when elicited with two tones of frequencies f1 and f2. DPOAEs mainly consist of two components, a nonlinear-distortion and a coherent-reflection component. Input-output (I/O) functions of DPOAE pressure at the cubic difference frequency, fDP=2f1-f2, enable the computation of estimated distortion-product thresholds (EDPTs), offering a noninvasive approach to estimate auditory thresholds. However, wave interference between the DPOAE components and suboptimal stimulus-level pairs reduces the accuracy of EDPTs. Here, the amplitude P of the nonlinear-distortion component is extracted from short-pulse DPOAE time signals. DPOAE level maps representing the growth behavior of P in L1,L2 space are recorded for 21 stimulus-level pairs and 14 frequencies with f2=1 to 14 kHz (f2/f1=1.2) from 20 ears. Reproducing DPOAE growth behavior using a least-squares fit approach enables the derivation of ridge-based I/O functions from model level maps. Objective evaluation criteria assess the fit results and provide EDPTs, which correlate significantly with auditory thresholds (p < 0.001). In conclusion, I/O functions derived from model level maps provide EDPTs with high precision but without the need of predefined optimal stimulus-level pairs.

Observations of Distortion Product Otoacoustic Emission Components in Adults With Hearing Loss.

Distortion product otoacoustic emissions (DPOAEs) measured in the ear canal are composed of OAEs generated by at least two mechanisms coming from different places in the cochlea. Otoacoustic emission (OAE) models hypothesize that reduction of cochlear gain will differentially impact the components. The purpose of the current experiment was to provide preliminary data about DPOAE components in adults with hearing loss in relation to OAE models and explore whether evaluation of the relative amplitudes of generator and reflection components can enhance identification of hearing loss. DPOAEs were measured from 45 adult ears; 21 had normal hearing (≤15 dB HL) and 24 with mild-to-severe sensorineural hearing loss (>15 dB HL). The higher frequency primary (f2) was swept logarithmically between 1500 and 6000 Hz, and f2/f1 was 1.22. The two equal-level primaries varied from 55 to 75 dB SPL in 5 dB steps. The swept primary procedure permitted the measurement of the amplitude and phase of the DPOAE fine structure and the extraction of the two major components (generator and reflection) by varying the predicted delays of the analysis windows. DPOAE fine structure was reduced or absent in ears with hearing loss. DPOAE generator and reflection components were lower in ears with hearing loss than those with normal hearing, especially for the reflection component. Significant correlations were found between the generator component and hearing threshold but not between reflection levels and hearing threshold. Most ears with normal hearing had both components, but only a small number of ears with hearing loss had both components. The reflection component is not recordable or low in level in ears with hearing loss, explaining the reduced or absent DPOAE fine structure. DPOAE generator components are also lower in level in ears with hearing loss than in ears without hearing loss. In ears that had both measurable generator and reflection components, the relationship between the two did not depend on the presence or absence of hearing loss. Because reflection components are not measurable in many ears with hearing thresholds >15 dB HL, stimuli that evoke other types of reflection emissions, such as stimulus-frequency or long-latency transient-evoked emissions, should be explored in conjunction with DPOAE generator components.

Distributed sources as a cause of abrupt amplitude decrease in cubic distortion-product otoacoustic emissions at high stimulus intensities.

The amplitudes of distortion-product otoacoustic emissions (DPOAEs) may abruptly decrease even though the stimulus level is relatively high. These notches observed in the DPOAE input/output functions or distortion-product grams have been hypothesized to be due to destructive interference between wavelets generated by distributed sources of the nonlinear-distortion component of DPOAEs. In this paper, simulations with a smooth cochlear model and its analytical solution support the hypothesis that destructive interference between individual wavelets may lead to the amplitude notches and explain the cause for onset and offset amplitude overshoots in the DPOAE signal measured for intensity pairs in the notches.

Modeling the dependence of the distortion product otoacoustic emission response on primary frequency ratio.

When measured as a function of primary frequency ratio r = f2/f1, using a constant f2, distortion product otoacoustic emission (DPOAE) response demonstrates a bandpass shape, previously interpreted as the evidence for a cochlear "second filter." In this study, an alternate, interference-based explanation, previously advanced in variants, is forwarded on the basis of experimental data along with numerical and analytical solutions of nonlinear and linear cochlear models. The decrease of the DPOAE response with increasing and decreasing ratios is explained by a diminishing "overlap" generation region and the onset of negative interference among wavelets of different phase, respectively. In this paper, the additional quantitative hypothesis is made that negative interference becomes the dominant effect when the spatial width of the generation (overlap) region exceeds half a wavelength of the DPOAE wavelets. Therefore, r is predicted to be optimal when this condition is matched. Additionally, the minimum on the low-ratio side of the DPOAE curve is predicted to occur as the overlap region width equals one wavelength. As the width of the overlap region depends on both tuning and ratio, while wavelength depends on tuning only, an experimental method for estimating tuning from either the width of the pass band or the optimal ratio of the DPOAE vs. ratio curve has been theoretically formulated and evaluated using numerical simulations. A linear model without the possibility of nonlinear suppression is shown to reasonably approximate data from human subjects at low ratios reinforcing the relevance of the proposed negative interference effect. The different dependence of the distortion and reflection DPOAE components on r as well as the nonmonotonic behavior of the distortion component observed at very low ratios are also in agreement with this interpretation.

Comparison of time-domain source-separation techniques for short-pulse distortion-product otoacoustic emissions.

Distortion-product otoacoustic emissions (DPOAEs) are presumed to consist mainly of two components, a nonlinear-distortion component and a coherent-reflection component. Wave interference between these two components reduces the accuracy of DPOAEs when used to evaluate cochlear function. Here, short tone pulses are utilized to record DPOAE signals in normal-hearing subjects. DPOAE components are extracted from recordings at discrete frequencies using two different techniques in the time domain. The extracted DPOAE components are compared to recordings obtained with conventional, continuous primary tones.

Time-domain analysis of distortion-product otoacoustic emissions using a hydrodynamic cochlea model

Abstract As a by-product of nonlinear amplification in the cochlea, the inner ear emits sound waves in response to two tones with different frequencies. These sound waves are measurable in the ear canal as distortion-product otoacoustic emissions (DPOAEs). DPOAEs putatively consist of two components emerging at different locations in the cochlea. Wave interference between the two components limits the accuracy of DPOAEs as a noninvasive measure of cochlear function. Using short stimulus pulses instead of continuous stimuli, the two DPOAE components can be separated in the time domain due to their different latencies. The present work utilizes a nonlinear hydrodynamic cochlea model to simulate short-pulse DPOAEs in the time domain. When adding irregularities to the mechanical parameters of the model, the simulated DPOAE signals show two distinguishable components and long-lasting beat tones, similar to band-pass filtered experimental data from normal-hearing human subjects. The model results suggest that the beat tones can occur solely due to interference of the coherent-reflection component with the fading nonlinear-distortion component.

Input-output functions of the nonlinear-distortion component of distortion-product otoacoustic emissions in normal and hearing-impaired human ears.

Distortion-product otoacoustic emissions (DPOAEs) arise in the cochlea in response to two tones with frequencies f1 and f2 and mainly consist of two components, a nonlinear-distortion and a coherent-reflection component. Wave interference between these components limits the accuracy of DPOAEs when evaluating the function of the cochlea with conventional continuous stimulus tones. Here, DPOAE components are separated in the time domain from DPOAE signals elicited with short stimulus pulses. The extracted nonlinear-distortion components are used to derive estimated distortion-product thresholds (EDPTs) from semi-logarithmic input-output (I/O) functions for 20 normal-hearing and 21 hearing-impaired subjects. I/O functions were measured with frequency-specific stimulus levels at eight frequencies f2 = 1,…, 8 kHz (f2/f1 = 1.2). For comparison, DPOAEs were also elicited with continuous primary tones. Both acquisition paradigms yielded EDPTs, which significantly correlated with behavioral thresholds (p < 0.001) and enabled derivation of estimated hearing thresholds (EHTs) from EDPTs using a linear regression relationship. DPOAE-component separation in the time domain significantly reduced the standard deviation of EHTs compared to that derived from continuous DPOAEs (p < 0.01). In conclusion, using frequency-specific stimulus levels and DPOAE-component separation increases the reliability of DPOAE I/O functions for assessing cochlear function and estimating behavioral thresholds.

Synergistic effects of noise and hand-arm vibration on distortion product otoacoustic emissions in healthy subjects

An experimental study has been performed on the effect of exposure to noise (N), vibration (V), and combined noise and vibration (NV), on the distortion product otoacoustic emission (DPOAE) level. The aim of the study is investigating the possible synergistic effect of noise and vibration, increasing the risk of damage to the hearing function due to noise only.Twelve normal-hearing volunteers were randomly exposed to hand arm vibration (HAV) only, noise only, and HAV and noise simultaneously. The exposure consisted of five consecutive sessions interspersed with quiet periods. DPOAEs were recorded during the quiet intervals following each exposure session, and during recovery, and compared to their baseline level, and among different test treatments. An ad hoc acquisition system developed in Labview was used to record high frequency-resolution DPOAE spectra in a suitably short time. Time-frequency filtering, based on the wavelet transform, was used to separate the DPOAE components coming from different generation mechanisms.The three test treatments, N, V, and NV, yielded significantly different results. The condition affecting most significantly all DPOAE component levels was the exposure to vibration only, causing a statistically significant increase of the signal level during all the exposure periods, with a decreasing trend during recovery. Also the exposure to noise only, N, caused a DPOAE level increase, statistically significant for the long latency (LL) DPOAE component. Only when exposure to noise was in combination with vibration, the zero-latency (ZL) component was clearly suppressed during the exposure. The results suggest a synergistic adverse effect of noise and vibration on the cochlear function. Relevance to industryThe simultaneous exposure to noise and hand arm vibration is a very common industrial scenario. The risk that exposure to mild noise levels could interfere with the exposure to vibration, enhancing the adverse effect on the hearing function, should be considered to optimize prevention strategies at the workplace.

Changes in the Compressive Nonlinearity of the Cochlea During Early Aging: Estimates From Distortion OAE Input/Output Functions.

The level-dependent growth of distortion product otoacoustic emissions (DPOAEs) provides an indirect metric of cochlear compressive nonlinearity. Recent evidence suggests that aging reduces nonlinear distortion emissions more than those associated with linear reflection. Therefore, in this study, we generate input/output (I/O) functions from the isolated distortion component of the DPOAE to probe the effects of early aging on the compressive nonlinearity of the cochlea. Thirty adults whose ages ranged from 18 to 64 years participated in this study, forming a continuum of young to middle-age subjects. When necessary for analyses, subjects were divided into a young-adult group with a mean age of 21 years, and a middle-aged group with a mean age of 52 years. All young-adult subjects and 11 of the middle-aged subjects had normal hearing; 4 middle-aged ears had slight audiometric threshold elevation at mid-to-high frequencies. DPOAEs (2f1 - f2) were recorded using primary tones swept upward in frequency from 0.5 to 8 kHz, and varied from 25 to 80 dB sound pressure level. The nonlinear distortion component of the total DPOAE was separated and used to create I/O functions at one-half octave intervals from 1.3 to 7.4 kHz. Four features of OAE compression were extracted from a fit to these functions: compression threshold, range of compression, compression slope, and low-level growth. These values were compared between age groups and correlational analyses were conducted between OAE compression threshold and age with audiometric threshold controlled. Older ears had reduced DPOAE amplitude compared with young-adult ears. The OAE compression threshold was elevated at test frequencies above 2 kHz in the middle-aged subjects by 19 dB (35 versus 54 dB SPL), thereby reducing the compression range. In addition, middle-aged ears showed steeper amplitude growth beyond the compression threshold. Audiometric threshold was initially found to be a confound in establishing the relationship between compression and age; however, statistical analyses allowed us to control its variance. Correlations performed while controlling for age differences in high-frequency audiometric thresholds showed significant relationships between the DPOAE I/O compression threshold and age: Older subjects tended to have elevated compression thresholds compared with younger subjects and an extended range of monotonic growth. Cochlear manifestations of nonlinearity, such as the DPOAE, weaken during early aging, and DPOAE I/O functions become linearized. Commensurate changes in high-frequency audiometric thresholds are not sufficient to fully explain these changes. The results suggest that age-related changes in compressive nonlinearity could produce a reduced dynamic range of hearing, and contribute to perceptual difficulties in older listeners.

Level dependence of the nonlinear-distortion component of distortion-product otoacoustic emissions in humans.

Distortion-product otoacoustic emissions (DPOAEs) emerge when presenting two primary tones with different frequencies f1 and f2 to the cochlea and are commonly used in diagnosis and research to evaluate the functional state of the cochlea. Optimal primary-tone stimulus levels accounting for the different level dependencies of the traveling-wave amplitudes of the two primary tones near the f2-tonotopic place on the basilar membrane are often used to maximize DPOAE amplitudes. However, parameters defining the optimal levels can be affected by wave interference between the nonlinear-distortion and coherent-reflection components of the DPOAE. Here, the components were separated in the time domain using a pulsed stimulus paradigm and optimal levels determined. Based on the amplitude dependence of the nonlinear-distortion components on primary-tone stimulus levels, level parameters yielding maximum DPOAE amplitudes were derived for six normal-hearing adults and compared to data recorded with continuous two-tone stimulation. The level parameters resulting from analysis of the nonlinear-distortion components show dependence on stimulus frequency and small standard deviations. DPOAE input/output functions derived for optimal levels exhibit larger slopes, wider dynamic range and less variability across subjects than those derived for conventional stimulus and analysis conditions, potentially increasing their reliability and sensitivity for assessing cochlea function.

Negative Middle Ear Pressure and Composite and Component Distortion Product Otoacoustic Emissions.

Distortion product otoacoustic emissions (DPOAEs), a by-product of normal outer hair cell function, are used in research and clinical settings to noninvasively test cochlear health. The composite DPOAE recorded in the ear canal is the result of interactions of at least two components: a nonlinear distortion component (the generator component) and a linear reflection component. Negative middle ear pressure (NMEP) results in the tympanic membrane being pulled inward and increases middle ear impedance. This influences both the forward travel of stimuli used to induce distortion products and the reverse travel of the emissions back to the ear canal. NMEP may therefore limit the effectiveness of DPOAEs in clinical settings. Twenty-six normal-hearing subjects were recruited, and eight were able to reliably and consistently induce NMEP using the Toynbee maneuver. Eight interleaved measures of tympanic peak pressure (TPP) were collected for each subject at normal pressure and NMEP. DPOAEs were then collected both when middle ear pressure was normal and during subject-induced NMEP. All measures were interleaved. Two primary tones were swept logarithmically across frequency (1 second per octave) from f1 = 410 to 6560 Hz and f2 = 500 to 8000 Hz (f1/f2 = 1.22), producing 2f1 - f2 DPOAEs from 320 to 5120 Hz. DPOAEs were collected at three equal-level primary level combinations (L65, L70, L75 dB SPL). Before composite and component DPOAE analysis, analysis of the f1 DPOAE primary confirmed that subjects had successfully induced NMEP. DPOAE and component magnitudes were separately analyzed using repeated measures analysis of variances with three factors, primary level (L65, L70, L75 dB SPL), middle ear pressure (normal pressure versus NMEP), and frequency (500 to 4000 Hz). Mean subject-induced NMEP ranged from -65 to -324 daPa. Changes in the magnitude (dB) of the primary tones used to induce the DPOAE provided a reliable indicator of subject-induced NMEP. Composite DPOAE and component levels were significantly affected by NMEP for all the frequencies tested. Changes were most clearly observed for the generator component with one subject demonstrating a mean decrease of 12 dB in magnitude during NMEP. Results were subject-specific, and there was a correlation between the degree of negative TPP induced and the amount of change in DPOAE level. Mean TPPs collected during NMEP ranged from -65 to -324 daPa and significantly affected composite DPOAE, generator, and reflection component levels. Changes in the magnitude of the swept-primaries as a function of frequency were used to confirm that NMEP had been successfully induced. The patterns of change in the composite DPOAEs were clearer and easier to interpret when the components of the DPOAE were separated with evaluation of the generator component alone.

Frequency-change in DPOAE evoked by 1 s/octave sweeping primaries in newborns and adults

Distortion product otoacoustic emissions (DPOAE) in newborns and adults were evoked by sweeping primaries up and down in frequency at 1 s/octave. Sweeping up and down in frequency resulted in changes in the amplitude vs. frequency functions of the composite DPOAE and its two major components. In addition, DPOAE component phases differed slightly between the up- and down-swept conditions. The changes in amplitude vs. frequency functions were quantified using a covariate correlation technique, yielding single-valued estimates of the magnitude of the frequency changes. Separate analyses were performed for the entire DPOAE frequency range and split into low and high frequency ranges. There were consistent changes in newborn and adult composite DPOAEs and reflection components, but not generator components. Adults had significant frequency changes in the composite DPOAE for all frequency ranges and in the reflection component for the entire frequency range. Newborns had significant frequency change in the reflection component for all frequency ranges. Differences in frequency change between adults and newborns may stem from developmental changes in cochlear processing. Alignment of the component phase differences between the up- and down-swept conditions resulted in elimination of frequency-change in reconstructed composite DPOAEs.

Stability of the Medial Olivocochlear Reflex as Measured by Distortion Product Otoacoustic Emissions

The purpose of this study was to assess the repeatability of a fine-resolution, distortion product otoacoustic emission (DPOAE)-based assay of the medial olivocochlear (MOC) reflex in normal-hearing adults. Data were collected during 36 test sessions from 4 normal-hearing adults to assess short-term stability and 5 normal-hearing adults to assess long-term stability. DPOAE level and phase measurements were recorded with and without contralateral acoustic stimulation. MOC reflex indices were computed by (a) noting contralateral acoustic stimulation-induced changes in DPOAE level (both absolute and normalized) at fine-structure peaks, (b) recording the effect as a vector difference, and (c) separating DPOAE components and considering a component-specific metric. Analyses indicated good repeatability of all indices of the MOC reflex in most frequency ranges. Short- and long-term repeatability were generally comparable. Indices normalized to a subject's own baseline fared best, showing strong short- and long-term stability across all frequency intervals. These results suggest that fine-resolution DPOAE-based measures of the MOC reflex measured at strategic frequencies are stable, and natural variance from day-to-day or week-to-week durations is small enough to detect between-group differences and possibly to monitor intervention-related success. However, this is an empirical question that must be directly tested to confirm its utility.

Changes in distortion product otoacoustic emission components after music exposure.

Young adults experience some type of recreational noise exposure on a daily basis; this includes using personal music (PM) systems with earphones. In most cases, this exposure is intermittent and the short-term effects of this exposure on the auditory system are becoming better understood. The purpose of this study was to examine the effects of one hour of music exposure using a PM system on distortion product otoacoustic emission (DPOAE) absolute levels and generator and characteristic frequency (CF) component levels. Young adults (n = 101) between 18-30 years with normal hearing participated listened to one hour of music through earphones. A second group of young adults (n = 21) served as controls and did not listen to music, but sat in the sound-treated room for one hour. Otoscopy, tympanometry, and a hearing screening (≤20 dB HL at 0.5, 1, 2, and 4 kHz) were completed in a randomly determined test ear. Preferred listening level, in dBA, was obtained and DPOAEs (2f1-f2) were measured between 1 and 6 kHz with stimulus levels fixed at L1,L2 = 55,40 dB SPL. Absolute DPOAE levels, along with generator and CF components levels were measured before and after each participant listened to one hour of music at their preferred level in a quiet setting. For data analyses, absolute DPOAE and generator and CF component levels were collapsed into 1/3rd octave bands centered around 1, 1.5, 2, 3, 4, and 6 kHz. Mean preferred listening level was 57.8 dBA, with males having a higher mean level of 61.1 dBA compared with females who had a mean level of 55.7 dBA. Females and males had negligible mean changes in absolute DPOAE levels at 1, 1.5, and 2 kHz, but males had 0.4-1 dB mean decreases after music at 3, 4, and 6 kHz compared to females, although not statistically significant. For DPOAE generator component data, females had small mean decreases for the two lower frequencies whereas males had mean decreases of 0.4-0.8 dB at 3, 4, and 6 kHz. Because of missing data, analyses of the CF component were limited, although females had small mean decreases at four frequencies while males had small mean decreases at two frequencies. There was no effect of listening to music on changes in DPOAEs but control participants had strong reliability (i.e., little or no change) after sitting in quiet for one hour. Young adults listened to music well below what would be considered hazardous. Because of the lower listening levels, DPOAEs showed very little change after music. Although there were no significant short-term changes in DPOAEs after music, the cumulative effect of this noise exposure is still unknown.

Characteristics of the 2f1-f2 distortion product otoacoustic emission in a normal hearing population

Distortion-product otoacoustic emission (DPOAE) fine structure and component characteristics are reported between 0.75 and 16 kHz in 356 clinically normal hearing human subjects ages 10 to 65 yr. Stimulus tones at 55/40, 65/55, and 75/75 dB SPL were delivered using custom designed drivers and a calibration method that compensated for the depth of insertion of the otoacoustic emission (OAE) probe in the ear canal. DPOAE fine structure depth and spacing were found to be consistent with previous reports with depth varying between 3 and 7 dB and average spacing ratios (f/Δf) between 15 and 25 depending on stimulus level and frequency. In general, fine structure depth increased with increasing frequency, likely due to a diminishing difference between DPOAE component levels. Fine structure spacing became wider with increasing age above 8 kHz. DPOAE components were extracted using the inverse fast Fourier transform method, adhering to a strict signal to noise ratio criterion for clearer interpretation. Component data from four age groups between 18 and 55 yr old were available for the stimulus levels of 75/75 dB SPL. The age groups could be differentiated with greater than 90% accuracy when using the level of the component presumed to originate from the DPOAE characteristic frequency place. This accuracy held even for frequencies at and below 4 kHz where the age groups exhibited similar average hearing thresholds.