Understanding degraded speech leads to perceptual gating of a brainstem reflex in human listeners.

The ability to navigate "cocktail party" situations by focusing on sounds of interest over irrelevant, background sounds is often considered in terms of cortical mechanisms. However, subcortical circuits such as the pathway underlying the medial olivocochlear (MOC) reflex modulate the activity of the inner ear itself, supporting the extraction of salient features from auditory scene prior to any cortical processing. To understand the contribution of auditory subcortical nuclei and the cochlea in complex listening tasks, we made physiological recordings along the auditory pathway while listeners engaged in detecting non(sense) words in lists of words. Both naturally spoken and intrinsically noisy, vocoded speech-filtering that mimics processing by a cochlear implant (CI)-significantly activated the MOC reflex, but this was not the case for speech in background noise, which more engaged midbrain and cortical resources. A model of the initial stages of auditory processing reproduced specific effects of each form of speech degradation, providing a rationale for goal-directed gating of the MOC reflex based on enhancing the representation of the energy envelope of the acoustic waveform. Our data reveal the coexistence of 2 strategies in the auditory system that may facilitate speech understanding in situations where the signal is either intrinsically degraded or masked by extrinsic acoustic energy. Whereas intrinsically degraded streams recruit the MOC reflex to improve representation of speech cues peripherally, extrinsically masked streams rely more on higher auditory centres to denoise signals.

A Variable-Stimulus Distortion Product Otoacoustic Emission Screening Method to Match Cochlear Place-Specific Properties.

OBJECTIVES: Distortion product otoacoustic emissions (DPOAEs) are a popular screening tool for hearing loss in specific populations (e.g., newborns). Current screening protocols use stimulus conditions that are agnostic to local mechanical properties of the cochlea and are also limited to a narrow frequency range. We have recently reported locally optimized stimulus frequency ratio and level combinations for recording DPOAEs up to stimulus frequencies of 19 kHz. In normally functioning cochlea, optimized stimuli improved the signal to noise ratios and allowed the registration of higher DPOAE levels, especially at higher frequencies. The purpose of this study was to evaluate the clinical performance of these physiologically motivated, locally appropriate, stimulus parameters for a screening application to identify the presence of hearing loss. DESIGN: Subjects were 24 adults with sensorineural hearing loss and 31 adults with normal hearing. The cubic DPOAE was measured and analyzed up to frequencies of 16 kHz using a range of stimulus conditions. Receiver operating characteristic curves were used to identify stimulus combinations most sensitive to screening for hearing loss. RESULTS: Receiver operating characteristic curves demonstrated improved test efficacy for hearing loss detection when using stimulus frequency ratios and levels that are frequency-dependent and consistent with known mechanical properties of the cochlea. CONCLUSIONS: We propose a new DPOAE recording paradigm (variable-stimuli DP) using stimuli aligned to local cochlear properties which may improve early and accurate detection of decline in cochlear function.

Risk factors associated with occupational noise-induced hearing loss in the Hispanic community health study/study of Latinos: A cross-sectional epidemiologic investigation.

The purpose of this study was to estimate the prevalence of occupational noise exposure and risk factors of occupational noise-induced hearing loss (NIHL) in Hispanic/Latino adults included in the baseline wave of the Hispanic Community Health Study/Study of Latinos collected from 2008 to 2011. Sequential multiple linear regression modeled the relationship between occupational NIHL (defined as a 3-, 4-, 6-kHz pure-tone average [PTA]) and occupation type, self-reported noise exposure, cardiovascular disease (CVD) risk score, and hearing protective device (HPD) use. The final model controlled for sex, age, and recreational noise exposure. Among 12,851 included participants, approximately 40% (n = 5036) reported occupational noise exposure "Sometimes" (up to 50% of the time) or "Frequently" (75-100% of the time). In the final fitted model, longest-held occupation and CVD risk were associated with poorer hearing. Specifically, those in non-skilled, service, skilled, and military/police/other job categories had between 2.07- and 3.29-dB worse PTA than professional/office workers. Additionally, a shift in the CVD risk score category from low to medium was associated with a 2.25- and 8.20-dB worse PTA for medium and high CVD risk, respectively. Age and sex were also significantly associated with poorer hearing, such that men presented with 6.08 dB worse PTA than women, and for every one-year increase in age, PTA increased by 0.62 dB (ps < .001). No interactions were seen between noise*sometimes or frequent exposure to other ototoxic agents and PTA (ps = .33 & .92, respectively). The prevalence of occupational noise exposure was high in this cross-sectional investigation of adults from Hispanic/Latino backgrounds. Findings contribute to the extant literature by demonstrating that risk factors for occupational NIHL in adults from varying Hispanic/Latino backgrounds are consistent with those of other previously studied groups.

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

OBJECTIVES: 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. DESIGN: 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. RESULTS: 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. CONCLUSIONS: 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.

Optimizing distortion product otoacoustic emission recordings in normal-hearing ears by adopting cochlear place-specific stimuli.

Distortion product otoacoustic emissions (DPOAEs) provide a window into active cochlear processes and have become a popular clinical and research tool. DPOAEs are commonly recorded using stimulus with fixed presentation levels and frequency ratio irrespective of the test frequency. However, this is inconsistent with the changing mechanical properties of the cochlear partition from the base to the apex that lend specific frequency-dependent spatial properties to the cochlear traveling wave. Therefore, the frequency and level characteristics between the stimulus tones should also need to be adjusted as a function of frequency to maintain optimal interaction between them. The goal of this investigation was to establish a frequency-specific measurement protocol guided by local cochlear mechanics. A broad stimulus parameter space extending up to 20 kHz was explored in a group of normal-hearing individuals. The stimulus frequency ratio yielding the largest 2f1-f2 DPOAE level changed as a function of frequency and stimulus level. Specifically, for a constant stimulus level, the frequency ratio producing the largest DPOAE level decreased with increasing frequency. Similarly, at a given f2 frequency, the stimulus frequency ratio producing the largest DPOAE level became wider as stimulus level increased. These results confirm and strengthen our current understanding of DPOAE generation in the normally functioning cochlea and expand our understanding to previously unexamined higher frequencies. These data support the use of frequency- and level-specific stimulus frequency ratios to maximize DPOAE generation.

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.

Factors Associated With Self-Perceived Hearing Handicap in Adults From Hispanic/Latino Background: Findings From the Hispanic Community Health Study/Study of Latinos.

OBJECTIVES: We sought to determine what factors, including acculturation (language and social contact preferences), were associated with self-perceived hearing handicap among adults from Hispanic/Latino background. We utilized the Aday-Andersen behavioral model of health services utilization to frame our hypotheses that predisposing characteristics (age, sex, education, city of residence, Hispanic/Latino background, and acculturation), enabling resources (annual income and current health insurance coverage), and need (measured hearing loss and self-reported hearing loss) would be related to clinically-significant self-perceived hearing handicap as measured by the Hearing Handicap Inventory - Screening (HHI-S) version. DESIGN: We analyzed baseline data collected from 2008 to 2011 as part of the multisite Hispanic Community Health Study/Study of Latinos. Data were from 6585 adults with hearing loss (defined by a worse-ear 500, 1000, 2000, and 4000 Hz pure-tone average [PTA] of ≥25 dB HL and/or a 4000, 6000, and 8000 Hz high-frequency PTA of ≥25 dB HL) aged 18 to 74 years from various Hispanic/Latino backgrounds. We conducted a series of multivariable logistic regression models examining the roles of independent variables of interest representing predisposing, enabling, and need indicators on the occurrence of clinically-significant self-perceived hearing handicap (e.g., HHI-S score > 8). RESULTS: Among included participants, 953 (14.5%) had an HHI-S score >8. The final model revealed significant associations between predisposing characteristics, enabling resources, need, and HHI-S outcome. Predisposing characteristics and need factors were associated with higher odds of reporting self-perceived hearing handicap (HHI-S score >8) including acculturation as measured by the Short Acculturation Scale for Hispanics (odds ratio [OR] = 1.28, 95% confidence interval [CI]: 1.09-1.50), female sex (OR = 1.72, 95% CI: 1.27-2.33), and poorer worse ear 500, 1000, 2000, and 4000 Hz PTA (OR = 1.02, 95% CI: 1.01-1.03); suggesting that a 5-decibel increase in a person's PTA was consistent with 10% higher odds of a HHI-S score of >8. Greater enabling resources were associated with lower odds of reporting clinically-significant self-perceived hearing handicap: compared with individuals with income <$10,000/year, the multivariable-adjusted OR among individuals with income $40,000 to $7500/year was 0.55 (95% CI: 0.33-0.89) and among individuals with income >$75,000/year was 0.28 (95% CI: 0.13-0.59]; p-trend < 0.0001). CONCLUSIONS: Our findings suggest there are associations between predisposing, enabling and need variables consistent with the Aday-Andersen model and self-perceived hearing handicap among adults from Hispanic/Latino background. The influence of language and culture on perceived hearing loss and associated handicap is complex, and deserves more attention in future studies. Our findings warrant further investigation into understanding the role of language and language access in hearing health care utilization and outcomes, as the current body of literature is small and shows mixed outcomes.

Interpretability of the audiogram by audiologists and physician non-specialists.

OBJECTIVE: The audiogram is frequently used by hearing specialists communicate test results to non-specialists, such as physicians. However, it relies on uncommon terminology and interpreting unusual graphical elements to extract information. In this study, we examine whether the audiogram can be accurately interpreted by specialists and non-specialists. DESIGN: We used an online questionnaire to assess the interpretability of an audiogram by audiologists and primary-care physicians. Participants viewed a sample audiogram and submitted their answers via an online survey system. STUDY SAMPLE: We recruited actively practicing primary care physicians (n = 100) and actively practicing audiologists (n = 67). We only accepted respondents from the United States. RESULTS: The audiogram was not easily interpreted by physicians, with a median score of 4/9. Fewer than 25% could accurately report a threshold correctly. Audiologists were more accurate than physicians (median score 7/9, Wilcoxon two-sample p < 0.001, r = 0.648). CONCLUSIONS: The audiogram is difficult for non-specialists to interpret. Clinicians are advised to supplement or supplant the audiogram in interprofessional communication. The development of tools to facilitate interprofessional communication between audiologists and physicians could have positive effects on physician awareness of hearing loss, and even downstream influences on patient behaviour and outcomes.

Cochlear tuning estimates from level ratio functions of distortion product otoacoustic emissions.

Objective: Distortion product otoacoustic emission (DPOAE) levels plotted as a function of stimulus frequency ratio demonstrate a bandpass shape. This bandpass shape is narrower at higher frequencies compared to lower frequencies and thus has been thought to be related to cochlear mechanical tuning.Design: However, the frequency- and level-dependence of these functions above 8 kHz is largely unknown. Furthermore, how tuning estimates from these functions are related to behavioural tuning is not fully understood.Study Sample: From experiment 1, we report DPOAE level ratio functions (LRF) from seven normal-hearing, young-adults for f2 = 0.75-16 kHz and two stimulus levels of 62/52 and 52/37 dB FPL. We found that LRFs became narrower as a function of increasing frequency and decreasing level.Results: Tuning estimates from these functions increased as expected from 1-8 kHz. In experiment 2, we compared tuning estimates from DPOAE LRF to behavioural tuning in 24 normal-hearing, young adults for 1 and 4 kHz and found that behavioural tuning generally predicted DPOAE LRF estimated tuning.Conclusions: Our findings suggest that DPOAE LRFs generally reflect the tuning profile consistent with basilar membrane, neural, and behavioural tuning. However, further investigations are warranted to fully determine the use of DPOAE LRF as a clinical measure of cochlear tuning.

Health Literacy and Self-Reported Hearing Aid Use in the Health and Retirement Study.

OBJECTIVES: Understanding the determinants of hearing aid use is important to improve the provision of hearing healthcare. Prior research has indicated that materials in the clinic and online, as well as audiologists' language during appointments, require a higher literacy level than most patients possess. We hypothesized that low health literacy is a barrier to entry in hearing healthcare, and therefore that health literacy would be positively correlated with the probability of hearing aid use. DESIGN: We performed retrospective analyses of the Health and Retirement Study, a longitudinal survey of American adults of retirement age. Objective health literacy was measured in different but overlapping subsamples using subsets of the Test of Functional Health Literacy in Adults and the Rapid Estimate of Adult Literacy in Medicine (n = 1240, n = 1586, and n = 2412). Subjective health literacy was assessed using a single-question screener in a larger sample (n = 8362). Separate discrete time models including common covariates of hearing aid use were constructed for each subsample. RESULTS: Objective health literacy measures did not correlate significantly with hearing aid use when age, gender, race/ethnicity, self-assessed hearing ability, and net income were included in the models. Subjective health literacy did correlate significantly with hearing aid use in the complete model, with lower subjective health literacy correlated with lower odds of reporting hearing aid use. CONCLUSIONS: Taken at face value, the results provide mixed evidence for a link between health literacy and hearing aid use. The results from the analysis of the largest sample, using the subjective health literacy measure, were consistent with the hypothesis that low health literacy is a barrier to hearing aid use. However, this was not supported by the analysis of the objective health literacy measures in these samples. Further research using full health literacy measurement tools and capturing other relevant variables would offer clarification on this conflict. The literacy level of clinical materials and conversation is a modifiable potential factor in hearing aid uptake, so further clinical and research consideration is warranted.

The Association Between Physiological Noise Levels and Speech Understanding in Noise.

OBJECTIVES: Traditionally, elevated hearing thresholds have been considered to be the main contributors to difficulty understanding speech in noise; yet, patients will often report difficulties with speech understanding in noise despite having audiometrically normal hearing. The purpose of this cross-sectional study was to critically evaluate the relationship of various metrics of auditory function (behavioral thresholds and otoacoustic emissions) on speech understanding in noise in a large sample of audiometrically normal-hearing individuals. DESIGN: Behavioral hearing thresholds, distortion product otoacoustic emission (DPOAE) levels, stimulus-frequency otoacoustic emission levels, and physiological noise (quantified using OAE noise floors) were measured from 921 individuals between 10 and 68 years of age with normal pure-tone averages. The quick speech-in-noise (QuickSIN) test outcome, quantified as the signal-to-noise ratio (SNR) loss, was used as the metric of speech understanding in noise. Principle component analysis (PCA) and linear regression modeling were used to evaluate the relationship between the measures of auditory function and speech in noise performance. RESULTS: Over 25% of participants exhibited mild or worse degree of SNR loss. PCA revealed DPOAE levels at 12.5 to 16 kHz to be significantly correlated with the variation in QuickSIN scores, although correlations were weak (R = 0.017). Out of all the metrics evaluated, higher levels of self-generated physiological noise accounted for the most variance in QuickSIN performance (R = 0.077). CONCLUSIONS: Higher levels of physiological noise were associated with worse QuickSIN performance in listeners with normal hearing sensitivity. We propose that elevated physiological noise levels in poorer speech in noise performers could diminish the effective SNR, thereby negatively impacting performance as seen by poorer QuickSIN scores.

CEDRA: A Tool to Help Consumers Assess Risk for Ear Disease.

This article introduces the Consumer Ear Disease Risk Assessment (CEDRA) tool. CEDRA is a brief questionnaire designed to screen for targeted ear diseases. It offers an opportunity for consumers to self-screen for disease before seeking a hearing device and may be used by clinicians to help their patients decide the appropriate path to follow in hearing healthcare. Here we provide highlights of previously published validation in the context of a more thorough description of CEDRA's development and implementation. CEDRA's sensitivity and specificity, using a cut-off score of 4 or higher, was 90% and 72%, respectively, relative to neurotologist diagnoses in the initial training sample used to create the scoring algorithm (n = 246). On a smaller independent test sample (n = 61), CEDRA's sensitivity and specificity were 76% and 80%, respectively. CEDRA has readability levels similar to many other patient-oriented questionnaires in hearing healthcare, and informal reports from pilot CEDRA-providers indicate that the majority of patients can complete it in less than 10 min. As the hearing healthcare landscape changes and provider intercession is no longer mandated, CEDRA provides a measure of safety without creating a barrier to access.

A Retrospective Estimate of Ear Disease Detection Using the "Red Flags" in a Clinical Sample.

OBJECTIVES: The purpose of this study was to evaluate the specificity and sensitivity of two red flag protocols in detecting ear diseases associated with changes in hearing. DESIGN: The presence of red-flag symptoms was determined in a chart review of 307 adult patients from the Mayo Clinic Florida Departments of Otorhinolaryngology and Audiology. Participants formed a convenience sample recruited for a separate study. Neurotologist diagnosis was the criterion for comparisons. RESULTS: Of the 251 patient files retained for analysis, 191 had one or more targeted diseases and 60 had age- or noise-related hearing loss. Food and Drug Administration red flags sensitivity was 91% (confidence interval [CI], 86 to 95%) and specificity was 72% (CI, 59 to 83%). American Academy of Otolaryngology-Head and Neck Surgery red flags sensitivity was 98% (CI, 95 to 99%) and specificity was 20% (CI, 11 to 32%). CONCLUSIONS: Stakeholders must determine which diseases are meaningful contraindications for hearing aid use and whether these red-flag protocols have acceptable levels of sensitivity and specificity. As direct-to-consumer models of hearing devices increase, a disease detection method that does not require provider intercession would be useful.

A common microstructure in behavioral hearing thresholds and stimulus-frequency otoacoustic emissions.

Behavioral hearing thresholds and otoacoustic emission (OAE) spectra often exhibit quasiperiodic fluctuations with frequency. For behavioral and OAE responses to single tones-the latter referred to as stimulus-frequency otoacoustic emissions (SFOAEs)-this microstructure has been attributed to intracochlear reflections of SFOAE energy between its region of generation and the middle ear boundary. However, the relationship between behavioral and SFOAE microstructures, as well as their presumed dependence on the properties of the SFOAE-generation mechanism, have yet to be adequately examined. To address this, behavioral thresholds and SFOAEs evoked by near-threshold tones were compared in 12 normal-hearing female subjects. The microstructures observed in thresholds and both SFOAE amplitudes and delays were found to be strikingly similar. SFOAE phase accumulated an integer number of cycles between the frequencies of microstructure maxima, consistent with a dependence of microstructure periodicity on SFOAE propagation delays. Additionally, microstructure depth was correlated with SFOAE magnitude in a manner resembling that predicted by the intracochlear reflection framework, after assuming reasonable values of parameters related to middle ear transmission. Further exploration of this framework may yield more precise estimates of such parameters and provide insight into their frequency dependence.

Comparing the Accuracy and Speed of Manual and Tracking Methods of Measuring Hearing Thresholds.

OBJECTIVES: The reliability of hearing thresholds obtained using the standard clinical method (modified Hughson-Westlake) has been the focus of previous investigation given the potential for tester bias (). In recent years, more precise methods in laboratory studies have been used that control for sources of bias, often at the expense of longer test times. The aim of this pilot study was to compare test-retest variability and time requirement to obtain a full set of hearing thresholds (0.125 - 20 kHz) of the clinical modified Hughson-Westlake (manual) method with that of the automated, modified (single frequency) Békésy tracking method (). DESIGN: Hearing thresholds from 10 subjects (8 female) between 19 to 47 years old (mean = 28.3; SD = 9.4) were measured using two methods with identical test hardware and calibration. Thresholds were obtained using the modified Hughson-Westlake (manual) method and the Békésy method (tracking). Measurements using each method were repeated after one-week. Test-retest variability within each measurement method was computed across test sessions. Results from each test method as well as test time across methods were compared. RESULTS: Test-retest variability was comparable and statistically indistinguishable between the two test methods. Thresholds were approximately 5 dB lower when measured using the tracking method. This difference was not statistically significant. The manual method of measuring thresholds was faster by approximately 4 minutes. Both methods required less time (~ 2 mins) in the second session as compared to the first. CONCLUSION: Hearing thresholds obtained using the manual method can be just as reliable as those obtained using the tracking method over the large frequency range explored here (0.125 - 20 kHz). These results perhaps point to the importance of equivalent and valid calibration techniques that can overcome frequency dependent discrepancies, most prominent at higher frequencies, in the sound pressure delivered to the ear.

Working memory training to improve speech perception in noise across languages.

Working memory capacity has been linked to performance on many higher cognitive tasks, including the ability to perceive speech in noise. Current efforts to train working memory have demonstrated that working memory performance can be improved, suggesting that working memory training may lead to improved speech perception in noise. A further advantage of working memory training to improve speech perception in noise is that working memory training materials are often simple, such as letters or digits, making them easily translatable across languages. The current effort tested the hypothesis that working memory training would be associated with improved speech perception in noise and that materials would easily translate across languages. Native Mandarin Chinese and native English speakers completed ten days of reversed digit span training. Reading span and speech perception in noise both significantly improved following training, whereas untrained controls showed no gains. These data suggest that working memory training may be used to improve listeners' speech perception in noise and that the materials may be quickly adapted to a wide variety of listeners.

Aging of the medial olivocochlear reflex and associations with speech perception.

The medial olivocochlear reflex (MOCR) modulates cochlear amplifier gain and is thought to facilitate the detection of signals in noise. High-resolution distortion product otoacoustic emissions (DPOAEs) were recorded in teens, young, middle-aged, and elderly adults at moderate levels using primary tones swept from 0.5 to 4 kHz with and without a contralateral acoustic stimulus (CAS) to elicit medial efferent activation. Aging effects on magnitude and phase of the 2f1-f2 DPOAE and on its components were examined, as was the link between speech-in-noise performance and MOCR strength. Results revealed a mild aging effect on the MOCR through middle age for frequencies below 1.5 kHz. Additionally, positive correlations were observed between strength of the MOCR and performance on select measures of speech perception parsed into features. The elderly group showed unexpected results including relatively large effects of CAS on DPOAE, and CAS-induced increases in DPOAE fine structure as well as increases in the amplitude and phase accumulation of DPOAE reflection components. Contamination of MOCR estimates by middle ear muscle contractions cannot be ruled out in the oldest subjects. The findings reiterate that DPOAE components should be unmixed when measuring medial efferent effects to better consider and understand these potential confounds.

Spontaneous otoacoustic emissions, threshold microstructure, and psychophysical tuning over a wide frequency range in humans.

Hearing thresholds have been shown to exhibit periodic minima and maxima, a pattern known as threshold microstructure. Microstructure has previously been linked to spontaneous otoacoustic emissions (SOAEs) and normal cochlear function. However, SOAEs at high frequencies (>4 kHz) have been associated with hearing loss or cochlear pathology in some reports. Microstructure would not be expected near these high-frequency SOAEs. Psychophysical tuning curves (PTCs), the expression of frequency selectivity, may also be altered by SOAEs. Prior comparisons of tuning between ears with and without SOAEs demonstrated sharper tuning in ears with emissions. Here, threshold microstructure and PTCs were compared at SOAE frequencies ranging between 1.2 and 13.9 kHz using subjects without SOAEs as controls. Results indicate: (1) Threshold microstructure is observable in the vicinity of SOAEs of all frequencies; (2) PTCs are influenced by SOAEs, resulting in shifted tuning curve tips, multiple tips, or inversion. High frequency SOAEs show a greater effect on PTC morphology. The influence of most SOAEs at high frequencies on threshold microstructure and PTCs is consistent with those at lower frequencies, suggesting that high-frequency SOAEs reflect the same cochlear processes that lead to SOAEs at lower frequencies.

Comparison of nine methods to estimate ear-canal stimulus levels.

The reliability of nine measures of the stimulus level in the human ear canal was compared by measuring the sensitivity of behavioral hearing thresholds to changes in the depth of insertion of an otoacoustic emission probe. Four measures were the ear-canal pressure, the eardrum pressure estimated from it and the pressure measured in an ear simulator with and without compensation for insertion depth. The remaining five quantities were derived from the ear-canal pressure and the Thévenin-equivalent source characteristics of the probe: Forward pressure, initial forward pressure, the pressure transmitted into the middle ear, eardrum sound pressure estimated by summing the magnitudes of the forward and reverse pressure (integrated pressure) and absorbed power. Two sets of behavioral thresholds were measured in 26 subjects from 0.125 to 20 kHz, with the probe inserted at relatively deep and shallow positions in the ear canal. The greatest dependence on insertion depth was for transmitted pressure and absorbed power. The measures with the least dependence on insertion depth throughout the frequency range (best performance) included the depth-compensated simulator, eardrum, forward, and integrated pressures. Among these, forward pressure is advantageous because it quantifies stimulus phase.

Characteristics of the 2f(1)-f(2) 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.