Distortion Product Otoacoustic Emissions Fine Structure Research Articles

The Effect of Stimuli Level on Distortion Product Otoacoustic Emission in Normal Hearing Adults.

The goal of this study is to compare three of the most commonly used primary-level relation paradigms (i.e., Scissors, Boys Town 'Optimal', and Equal-Level) in generation of distortion product otoacoustic emissions (DPOAEs) in normal hearing adults. The generator and reflection components were extracted from DPOAEs in each paradigm. The generator and reflection component levels and input/output (I/O) functions were compared across paradigms and primary-tone levels. The results showed a different I/O function growth behavior across frequency and levels among paradigms. The Optimal paradigm showed a systematic change in the generator and reflection component levels and I/O slopes across primary levels among subjects. Moreover, the levels and slopes in the Optimal paradigm were more distinct across levels with less variations across frequency leading to a systematic change in the DPOAE fine structure across levels. The I/O functions were found to be more sensitive to the selected paradigm; especially the I/O function for the reflection component. The I/O functions of the reflection components showed large variability across frequencies due to different frequency shifts in their microstructure depending on the paradigm. The findings of this study suggested the Optimal paradigm as the proper primary-level relation to study cochlear amplification/compression. The findings of this study shows that care needs to be taken in comparing the findings of different studies that generated DPOAEs with a different level-relation paradigm.

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.

A clinical comparison of DPOAE fine structure reduction methods

Objective To evaluate two real-time methods for reducing distortion product otoacoustic emission (DPOAE) fine structure in terms of DPOAE amplitude and fine structure depth. Design A prospective, repeated-measures design was used to assess DPOAE characteristics in response to a conventional stimulation method (Conv.), as well as for methods implementing either a generic suppressor tone (Supp.) or frequency modulation of the f2 primary tone (FM). Study sample Eighty-three young adults (58 females) between the ages of 20 and 34 years with normal hearing completed testing for this study. Results Use of the Conv. and FM methods resulted in consistently higher DPOAE levels relative to the Supp. method, with average advantages of 6 and 5 dB, respectively. For all methods, increased fine structure depth was observed for stimulation with lower level (25−45 dB SPL) and lower frequency (1000−3000 Hz) primary tones. Finally, use of the Supp. and FM methods resulted in significantly decreased fine structure depth relative to the Conv. method. Conclusion Through frequency modulation of the f2 primary tone, it was possible to reduce the depth of fine structure across a clinically meaningful range of stimulation levels and frequencies without concomitant reduction in DPOAE amplitude.

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.

Early Indices of Reduced Cochlear Function in Young Adults with Type-1 Diabetes Revealed by DPOAE Fine Structure

The relationship between type-1 diabetes mellitus (DM) and cochlear dysfunction remains inconclusive. The purpose of this study was to examine otoacoustic emissions (OAEs) in normal-hearing young adults with type-1 DM as compared with matched controls and identify potential covariates influencing OAE findings. Cross-sectional study. N = 40 young adults aged 18-28 years including individuals with type-1 DM (n = 20) and age-gender matched controls (n = 20) with normal hearing sensitivity. Measures of pure-tone threshold sensitivity and OAEs, including distortion product otoacoustic emissions (DPOAEs), transient evoked OAEs, and DPOAE fine structure, were compared between groups. Covariates such as noise exposure and DM-related factors (e.g., duration of disease, glycated hemoglobin levels) were considered. Statistical analysis included analysis of variance and linear regression. Measures of hearing sensitivity and auditory function in both groups were comparable for all assays, except DPOAE fine structure. A reduced number of fine structure peaks and component amplitudes were found in the type-1 diabetes DM group with the primary difference in the reflection component. The results indicate that reduced cochlear function in young adults with type-1 DM can be revealed using DPOAE fine structure, suggesting potential clinical applications of DPOAE fine structure in early identification of cochlear pathology. Potential factors underlying these findings are discussed.

Cochlear Mechanisms and Otoacoustic Emission Test Performance.

This study aims to determine the impact of controlling cochlear-source mechanism on the accuracy with which auditory status is identified using otoacoustic emissions (OAEs) in two groups of subjects with normal hearing (NH) and subjects with mild to moderate hearing loss. Data were collected from 212 subjects with NH and with mild to moderate hearing loss who fell into two categories based on a distortion product OAE (DPOAE) screening protocol: the uncertain-identification group (where errors were likely) and the certain-identification group (where errors were unlikely). DPOAE fine-structure patterns were recorded at intervals surrounding f2 = 1, 2 and 4 kHz (f2/f1 ratio = 1.22), with L2 = 35, 45, and 55 dB SPL (L1/L2 ratio = 10 dB). The discrete cosine transform was used to smooth fine structure, limiting the source contribution to the distortion source only. Reflection-source OAEs were also recorded using amplitude-modulated stimulus frequency OAEs (AM-SFOAE). Area under the relative operating characteristic (AROC) curve was used to quantify test accuracy when the source contribution was controlled versus the condition where both sources contribute. Additionally, failure rate, fixed at 5% for NH ears, as a function of behavioral-threshold category was evaluated. When data for the entire subject group were examined, reducing the reflection-source contribution to the DPOAE did not result in better test performance than the best control condition at any frequency tested. When the subjects with NH were restricted to those with confirmed fine structure, AROC analyses indicated that reducing the reflection-source contribution resulted in several small increases in the accuracy (2%-5%) with which auditory status was identified relative to the best control condition. This improvement was observed for the lowest stimulus levels (i.e., L2 = 35 or 45 dB SPL). In this subset of subjects, distortion-source DPOAEs resulted in more accurate identification of mild hearing loss for a fixed false-positive rate of 5% in NH ears at lower L2's, conditions with poor accuracy in the larger group of subjects. The impact of controlling the source contribution on the identification of moderate losses was less clear in the reduced subject group, with some conditions where the distortion-source DPOAE was more accurate than the control condition and other conditions where there was no change. There was no evidence that reflection-source AM-SFOAEs more accurately identified ears with hearing loss when compared to any of the DPOAE conditions in either the large or reduced group of subjects. While improvements in test accuracy were observed for some subjects and some conditions (e.g., mild hearing losses and low stimulus levels in the reduced subset of subjects), these results suggest that restricting cochlear source contribution by "smoothing" DPOAE fine structure is not expected to improve DPOAE test accuracy in a general population of subjects. Likewise, recording reflection-source OAEs using the AM-SFOAE technique would not be expected to more accurately identify hearing status compared to mixed- or single-source DPOAEs.

The Effect of Primary Levels and Frequencies on the Contralateral Suppression of Distortion Product Otoacoustic Emission.

Background and ObjectivesChanges in distortion product otoacoustic emission (DPOAE) caused by contralateral suppression (CS) allow the function of the auditory efferent system to be evaluated. Parameters affording maximum CS are preferred in terms of clinical application. Our objective was to evaluate the effects of primary levels and frequencies on DPOAE-mediated CS.Subjects and MethodsSixteen subjects with normal hearing participated. DPOAEs were recorded with and without contralateral acoustic stimulation; we delivered broadband noise of 65 dB SPL at f2 frequencies between 1,000 Hz and 6,727 Hz, at 8 pt/octave. The L2 was varied between 40 dB SPL and 80 dB SPL in 10-dB steps.ResultsL2 did not significantly affect DPOAE-mediated CS. Higher L2 levels significantly reduced the fine structure depth of both the baseline and suppressed DPOAE datasets. The amount of CS was greatly affected by the f2 frequency; lower and higher frequency ranges afforded significantly stronger suppression than did mid-frequencies within the studied range.ConclusionsOur findings suggest that DPOAE CS should be measured over a wide range of frequencies as the amount of CS seems to be highly dependent on f2. The use of a higher L2 level may be optimal when it is sought to evoke strong DPOAE-mediated suppression while simultaneously minimizing DPOAE fine structure. Our findings may assist in optimization of clinical procedures evaluating the integrity of the auditory efferent system.

Efferent modulation of pre-neural and neural distortion products

Distortion product otoacoustic emissions (DPOAEs) and distortion product frequency following responses (DPFFRs) are respectively pre-neural and neural measurements associated with cochlear nonlinearity. Because cochlear nonlinearity is putatively linked to outer hair cell electromotility, DPOAEs and DPFFRs may provide complementary measurements of the human medial olivocochlear (MOC) reflex, which directly modulates outer hair cell function. In this study, we first quantified MOC reflex-induced DPOAE inhibition at spectral fine structure peaks in 22 young human adults with normal hearing. The f1 and f2 tone pairs producing the largest DPOAE fine structure peak for each subject were then used to evoke DPFFRs with and without MOC reflex activation to provide a related neural measure of efferent inhibition. We observed significant positive relationships between DPOAE fine structure peak inhibition and inhibition of DPFFR components representing neural phase locking to f2 and 2f1-f2, but not f1. These findings may support previous observations that the MOC reflex inhibits DPOAE sources differentially. That these effects are maintained and represented in the auditory brainstem suggests that the MOC reflex may exert a potent influence on subsequent subcortical neural representation of sound.

Distortion-product otoacoustic emission fine structure as an indicator of combination-tone-mediated masking release

During simultaneous masking, when the probe (fp) is higher in frequency than the masker (fm), the cubic difference tone (CDT; fcdt = 2fm-fp) can provide masking release. Given their shared origin, the 2f1-f2 distortion-product otoacoustic emission (DPOAE) may be a physiological correlate of CDT-mediated masking release [McFadden et al. (2012). J. Acoust. Soc. Am. 132, 968–983]. To test this hypothesis, masking release and DPOAEs were measured in normal-hearing listeners for 2 and 4 kHz probes. Probe threshold was measured in the presence of a masker (fp/fm = 1.2) and masker-suppressor pair. The suppressor was designed to mask detection of the CDT. Masking release was calculated as the change in masked-probe threshold resulting from presentation of the suppressor. DPOAEs were measured for an f2 (f2/f1 = 1.2) between 1.8 and 2 kHz (6-Hz intervals) and between 3.6 and 4.4 kHz (12-Hz intervals). Masking release was associated with DPOAE fine structure. Listeners without masking release lacked DPOAE fine struc...

Estimates of cochlear compression using distortion product otoacoustic emissions and growth of forward masking.

The authors investigated the relationship between behavioral and physiologic estimates of cochlear compression. Cochlear compression was estimated in distortion product otoacoustic emission (DPOAE) fine structure minima and maxima near 4 kHz. The composite DPOAE response and separated generator and reflection components yielded three estimates in four young adults with normal hearing. DPOAE estimates were compared to behavioral compression estimates derived using a growth of forward masking (GOFM) paradigm. The DPOAE primary tone f2 and GOFM signal were identical and selected individually based on placement in a DPOAE fine structure minimum. Across participants, DPOAE compression estimates derived from the generator component were most similar to estimates derived from the GOFM paradigm and did not vary with DPOAE fine structure. These results suggest that the generator component may provide a quick, reliable estimate of cochlear compression in humans. This may prove useful in populations that cannot give behavioral responses.

Distortion product otoacoustic emission fine-structure: An insight into the ear asymmetries

Context: Use of distortion product otoacoustic emission (DPOAE) as a measure of hearing sensitivity is common in clinical practice. However, use of a more informative DPOAE fine-structure has been limited due to non-reliability of DPOAE fine-structure. Aim: The current study was aimed at testing the interaural differences between the DPOAE fine-structure across three age groups. Settings and Design: Acoustically treated room with a calibrated dual channel audiometer (Orbiter 922) along with TDH-39 headphones and B-71 bone vibrator. GSI Tympstar was used for tympanometry and acoustic reflex measurements while 'ILO V6' OAE analyzer was used for recording of DPOAE and DPOAE fine-structure. Material and Methods : A total of 98 subjects with normal peripheral hearing sensitivity were tested for DPOAE fine-structure. The three age groups consisted of young (8-18 years; n = 50), middle aged (30-40 years; n = 30), and elderly (50-60 years; n = 18). DPOAE fine-structure was studied at 8 points per octave on a total of 25 frequencies from 1000 to 8000 Hz. Statistical Analysis Used: Repeated measure analysis of variance. Results: There was a significant difference (P < 0.05) in the amplitudes at frequencies between 2000 and 3000 Hz. This was evident irrespective of the age groups. There was also a decrease in DPOAE amplitude in elderly group. Conclusions: Interaural asymmetry can be attributed as a reason to these findings and it occurred mostly in the speech perception frequencies. Reduced amplitude of DPAOE in the elderly group can be attributed to presbycusis.

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.

Extraction of otoacoustic distortion product sources using pulse basis functions

Distortion product otoacoustic emissions (DPOAEs) acquired in normal-hearing subjects show considerable variation in amplitude with varying frequency. This is known as DPOAE fine structure. It is widely accepted that fine structure results from wave interference from two DPOAE sources, a non-linear generation component and a coherent reflection component. Here a method is presented that decomposes short-pulse DPOAE recordings into pulse basis functions and enables the quantification of both source components in the time domain, independent of their relative phase and at low cost of measurement time. Input-output functions utilizing the extracted primary-source component are analyzed.

Two-source interference as the major reason for auditory-threshold estimation error based on DPOAE input–output functions in normal-hearing subjects

Fine structure in the frequency response of distortion product otoacoustic emissions (DPOAEs) can severely limit the usefulness of DPOAEs in estimating auditory thresholds. Here, fine structure is removed by extracting the primary-source DPOAE component using the onset-decomposition technique (Vetešník et al., 2009) and auditory threshold estimates are compared to those obtained from DPOAEs in response to conventional, continuous two-tone stimulation. Auditory thresholds are predicted using the estimated distortion product thresholds (EDPTs), obtained from linear regression of input–output (I/O) functions of DPOAE pressure amplitude versus second-tone stimulus level (Boege and Janssen, 2002). The accuracy of the auditory-threshold predictions is derived by comparison with measured auditory thresholds. The parameters of the two primary stimulus tones of frequency f1 and f2 and levels of L1 and L2 are chosen as: f2/f1 = 1.2 with 1.5 ≤ f2 ≤ 2.5 kHz, and L1 = 0.4L2 + 39 dB SPL, with 25 ≤ L2 ≤ 65 dB SPL. Data are from 12 normal-hearing subjects with profound DPOAE fine structure. 255 DPOAE I/O functions were measured for each of the two DPOAE paradigms. An EDPT value was accepted as reliable if: 1) the squared correlation coefficient, r2 ≥ 0.8, 2) the regression slope, sI/O ≥ 0.2 μPa/dB, and 3) the standard deviation of the EDPT, σEDPT ≤ 10 dB. The proportion of rejected I/O functions was 8% for onset-decomposition DPOAEs, and 25% for continuous-tone DPOAEs. Removal of data points from the saturation region of the DPOAE I/O function by an automated algorithm reduced the rejection rate, to zero for onset-decomposition DPOAEs, but to only 13% for continuous-tone DPOAEs. In the absence of saturated DPOAE responses, auditory thresholds were predicted with standard deviation of only 4 dB for onset-decomposition DPOAEs, but 12 dB for continuous-tone DPOAEs. In summary, by extracting the primary-source component of the DPOAE by the method of onset-decomposition it is possible to predict human auditory threshold with hitherto unattainable accuracy.

Demonstrating True DPOAE Modulation by the Efferent Pathway

Objective: 1) To isolate the distortion product otoacoustic emission (DPOAE) components using an ipsilateral and individualized suppressor tone. 2) To demonstrate the modulation of DPOAEs by the medial olivocochlear efferent pathway once component interference has been eliminated. Method: DPOAE (2f1-f2 ) recordings using a frequency-sweep paradigm with ipsilateral suppressor tones and CAS were made on 8 normal-hearing subjects in a sound-treated audiology booth from January to February 2012. Estimates of DPOAE level and phase were obtained using fast Fourier transform and least-squares-fit techniques. Results: Fine structure depth was significantly reduced with the use of an ideal ipsilateral suppressor tone, demonstrating the effectiveness of the suppressor tone at separating DPOAE components. In addition, once the DPOAE components were separated to remove phase interference, MOC activation resulted in consistent suppression of DPOAE level as compared to nonseparated DPOAE level which showed enhancement as well as suppression. These results help to support the theory of DPOAE component interference leading to enhancements often seen with efferent activation and to demonstrate a more consistent way to view DPOAE modulation by the efferent pathway. Conclusion: This study demonstrates that DPOAE fine structure enhancement seen with activation of the MOC pathway is likely due to DPOAE component interference as opposed to true DPOAE modulation. In addition, the results show that DPOAE component isolation can be achieved by an ipsilateral suppressor tone, however it must be individualized.

Investigating the wave-fixed and place-fixed origins of the 2f1-f2distortion product otoacoustic emission within a micromechanical cochlear model

The 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) arises within the cochlea due to the nonlinear interaction of two stimulus tones (f(1) and f(2)). It is thought to comprise contributions from a wave-fixed source and a place-fixed source. The generation and transmission of the 2f(1)-f(2) DPOAE is investigated here using quasilinear solutions to an elemental model of the human cochlea with nonlinear micromechanics. The micromechanical parameters and nonlinearity are formulated to match the measured response of the cochlea to single- and two-tone stimulation. The controlled introduction of roughness into the active micromechanics of the model allows the wave- and place-fixed contributions to the DPOAE to be studied separately. It is also possible to manipulate the types of nonlinear suppression that occur within the quasilinear model to investigate the influence of stimulus parameters on DPOAE generation. The model predicts and explains a variety of 2f(1)-f(2) DPOAE phenomena: The dependence of emission amplitude on stimulus parameters, the weakness of experiments designed to quantify cochlear amplifier gain, and the predominant mechanism which gives rise to DPOAE fine structure. In addition, the model is used to investigate the properties of the wave-fixed source and how these properties are influenced by the stimulus parameters.

The Influence of Common Stimulus Parameters on Distortion Product Otoacoustic Emission Fine Structure

To determine whether common approaches to setting stimulus parameters influence the depth of fine structure present in the distortion product otoacoustic emission (DPOAE) response. Because the presence of fine structure has been suggested as a possible source of errors, if one of the common parametric approaches results in reduced fine-structure depth, it may be preferred over other approaches. DPOAE responses were recorded in a group of 21 subjects with normal hearing for 1/3-octave intervals surrounding 3 f2s (1, 2, and 4 kHz) at three L2s (30, 45, and 55 dB SPL). For each f2 and L2 combination, L1 and f2/f1 were set according to three commonly used parametric approaches. These included a simple approach, the approach recommended by Kummer et al., and the approach described by Johnson et al. These three approaches primarily differ in the recommended relationship between L1 and L2. For each parametric approach, DPOAE fine structure was evaluated by varying f2 in small steps. Differences in DPOAE level and DPOAE fine-structure depth across f2, L2, and the various stimulus parameters were evaluated using repeated-measures analysis of variance. As expected, significant variations in DPOAE level were observed across the three parametric approaches. For stimulus levels #45 dB SPL, the simple stimuli resulted in lower DPOAE levels than were observed for other approaches. An unexpected finding was that stimulus parameters developed by Johnson et al., which were believed to produce higher DPOAE levels than other approaches, produced the lowest DPOAE levels of the three approaches when f2 = 4 kHz. Significant differences in fine-structure depth were also observed. Greater fine-structure depth was observed with the simple parameters, although this effect was restricted to L2 # 45 dB SPL. When L2 = 55 dB SPL, all three parametric approaches resulted in equivalent fine-structure depth. A significant difference in fine-structure depth across the 3 f2s was also observed. The interval surrounding 2 kHz was associated with greater fine-structure depth than the intervals surrounding 1 and 4 kHz. The simple stimulus parameters resulted in more fine structure than the other parametric approaches; however, this effect was restricted to L2 # 45 dB SPL. At the moderate stimulus levels used in most clinical applications of DPOAEs (L2 = 55 dB SPL), all three approaches resulted in similar fine-structure depths. These findings suggest that manipulating stimulus parameters, particularly the L1, L2 relationship, is not an effective technique for reducing fine structure, except at the lowest stimulus levels, and that all the common parameters result in equivalent fine structure for moderate stimulus levels. These results also suggest that the stimulus parameters used in future studies of the clinical implications of fine structure may be relatively unimportant, unless stimulus levels #45 dB SPL will be evaluated.

Effects of aspirin on distortion product fine structure: Interpreted by the two-source model for distortion product otoacoustic emissions generation

Distortion product otoacoustic emission (DPOAE) fine structure is due to the interaction of two major components coming from different places in the cochlea. One component is generated from the region of maximal overlap of the traveling waves generated by the two primaries and is attributed to nonlinear distortion (nonlinear component). The other component arises predominantly from the tonotopic region of the distortion product and is attributed to linear coherent reflection (reflection component). Aspirin (salicylate) ototoxicity can cause reversible hearing loss and reduces otoacoustic emission generation in the cochlea. The two components are expected to be affected differentially by cochlear health. Changes in DPOAE fine structure were recorded longitudinally in three subjects before, during, and after aspirin consumption. Full data sets were analyzed for two subjects, but only partial data could be analyzed from the third subject. Resulting changes in the two components of DPOAE fine structure revealed variability among subjects and differential effects on the two components. For low-intensity primaries, both components were reduced with the reflection component being more vulnerable. For high-intensity primaries, the nonlinear component showed little or no change, but the reflection component was always reduced.

Distortion product otoacoustic emission phase and component analysis in human newborns

Apical distortion product otoacoustic emissions (DPOAEs) are comprised of at least two components, as evidenced by the interference pattern of alternating maxima and minima known as fine structure. DPOAE fine structure is produced by the shifting phase relationship in the ear canal, between the generator and characteristic frequency (CF) component of the response. Each component arises from a different cochlear region and, according to theory, reflects a distinct generation mechanism. The analysis of DPOAE components and phase in newborns may provide a window into targeted aspects of cochlear physiology during development. 2f(1)-f(2) DPOAE fine structure was recorded from 15 adults and 14 newborns using a swept-tone technique. DPOAE group delay, as well as magnitude and phase of each component, was compared between age groups. Results show narrower fine structure spacing, a longer group delay (steeper phase gradient) in low frequencies, and a stronger relative contribution from the CF component in newborns. The prolonged group delay for low-frequency DPOAEs could indicate immature basilar membrane motion in the apex of the cochlea and warrants further investigation. The enhanced contribution from the CF component may have implications for clinical practice as well as for theories of cochlear maturation.

Contralateral acoustic stimulation alters the magnitude and phase of distortion product otoacoustic emissions

Activation of medial olivocochlear efferents through contralateral acoustic stimulation (CAS) has been shown to modulate distortion product otoacoustic emission (DPOAE) level in various ways (enhancement, reduction, or no change). The goal of this study was to investigate the effect of a range of CAS levels on DPOAE fine structure. The 2f(1)-f(2) DPOAE was recorded (f(2)/f(1)=1.22, L(1)=55 dB, and L(2)=40 dB) from eight normal-hearing subjects, using both a frequency-sweep paradigm and a fixed frequency paradigm. Contamination due to the middle ear muscle reflex was avoided by monitoring the magnitude and phase of a probe in the test ear and by monitoring DPOAE stimulus levels throughout testing. Results show modulations in both level and frequency of DPOAE fine structure patterns. Frequency shifts observed at DPOAE level minima could explain reports of enhancement in DPOAE level due to efferent activation. CAS affected the magnitude and phase of the DPOAE component from the characteristic frequency region to a greater extent than the component from the overlap region between the stimulus tones. This differential effect explains the occasional enhancement observed in DPOAE level as well as the frequency shift in fine structure patterns.