Moments of click-evoked otoacoustic emissions in human ears: group delay and spread, instantaneous frequency and bandwidth

A point-wise artifact rejection method for estimating transient-evoked otoacoustic emissions and their group delay

Due to its low intensity, measurement of transient-evoked otoacoustic emission (TEOAE) requires repeated stimulation. When any acoustic artifact occurs, an entire click interval is typically abandoned. Here, a point-wise artifact rejection strategy is proposed, and it partially preserves the data when artifacts occur in an interval. At the noisiest setting (-46 dB signal-to-noise ratio) the proposed strategy retains four times more data and thereby reduces the root mean square signal estimation error by over 60%. Consequently, the group delay can be calculated more accurately. These findings might facilitate TEOAE measurement at home or in other noisy environments in the future.

Reflection-Source Emissions Evoked with Clicks and Frequency Sweeps: Comparisons Across Levels

According to coherent reflection theory, otoacoustic emissions (OAE) evoked with clicks (clicked-evoked, CE) or tones (stimulus frequency, SF) originate via the same mechanism. We test this hypothesis in gerbils by investigating the similarity of CE- and SFOAEs across a wide range of stimulus levels. The results show that OAE transfer functions measured in response to clicks and sweeps have nearly equivalent time-frequency characteristics, particularly at low stimulus levels. At high stimulus levels, the two OAE types are more dissimilar, reflecting the different dynamic properties of the evoking stimulus. At mid to high stimulus levels, time-frequency analysis reveals contributions from at least two OAE source components of varying latencies. Interference between these components explains the emergence of strong spectral microstructure. Time-frequency filtering based on mean basilar-membrane (BM) group delays (τ_BM) shows that late-latency OAE components (latency ~ 1.6τ_BM) dominate at low stimulus intensities and exhibit highly compressive growth with increasing stimulus intensity. In contrast, early-latency OAE components (~ 0.7τ_BM) are small at low stimulus levels but can come to dominate the overall response at higher intensities. Although the properties of long-latency OAEs are consistent with an origin via coherent reflection near the peak of the traveling wave, the generation place and/or mechanisms responsible for the early-latency OAE components warrant further investigation. Because their delay remains in constant proportion to τ_BM across sound intensity, long-latency OAEs, whether evoked with tones or clicks, can be used to predict characteristics of cochlear processing, such as the sharpness of frequency tuning, even at high stimulus levels.

Click evoked middle ear muscle reflex: Spectral and temporal aspects

This study describes a time series-based method of middle ear muscle reflex (MEMR) detection using bilateral clicks. Although many methods can detect changes in the otoacoustic emissions evoking stimulus to monitor the MEMR, they do not discriminate between true MEMR-mediated vs artifactual changes in the stimulus. We measured MEMR in 20 young clinically normal hearing individuals using 1-s-long click trains presented at six levels (65 to 95 dB peak-to-peak sound pressure level in 6 dB steps). Changes in the stimulus levels over the 1 s period were well-approximated by two-term exponential functions. The magnitude of ear canal pressure changes due to MEMR increased monotonically as a function of click level but non-monotonically with frequency when separated into 1/3 octave wide bands between 1 and 3.2 kHz. MEMR thresholds estimated using this method were lower than that obtained from a clinical tympanometer in ∼94% of the participants. A time series-based method, along with statistical tests, may provide additional confidence in detecting the MEMR. MEMR effects were smallest at 2 kHz, between 1 and 3.2 kHz, which may provide avenues for minimizing the MEMR influence while measuring other responses (e.g., the medial olivocochlear reflex).

Medial olivocochlear reflex effects on amplitude growth functions of long- and short-latency components of click-evoked otoacoustic emissions in humans

Functional outcomes of medial olivocochlear reflex (MOCR) activation, such as improved hearing in background noise and protection from noise damage, involve moderate to high sound levels. Previous noninvasive measurements of MOCR in humans focused primarily on otoacoustic emissions (OAEs) evoked at low sound levels. Interpreting MOCR effects on OAEs at higher levels is complicated by the possibility of the middle-ear muscle reflex and by components of OAEs arising from different locations along the length of the cochlear spiral. We overcame these issues by presenting click stimuli at a very slow rate and by time-frequency windowing the resulting click-evoked (CE)OAEs into short-latency (SL) and long-latency (LL) components. We characterized the effects of MOCR on CEOAE components using multiple measures to more comprehensively assess these effects throughout much of the dynamic range of hearing. These measures included CEOAE amplitude attenuation, equivalent input attenuation, phase, and slope of growth functions. Results show that MOCR effects are smaller on SL components than LL components, consistent with SL components being generated slightly basal of the characteristic frequency region. Amplitude attenuation measures showed the largest effects at the lowest stimulus levels, but slope change and equivalent input attenuation measures did not decrease at higher stimulus levels. These latter measures are less commonly reported and may provide insight into the variability in listening performance and noise susceptibility seen across individuals.NEW & NOTEWORTHY The auditory efferent system, operating at moderate to high sound levels, may improve hearing in background noise and provide protection from noise damage. We used otoacoustic emissions to measure these efferent effects across a wide range of sound levels and identified level-dependent and independent effects. Previous reports have focused on level-dependent measures. The level-independent effects identified here may provide new insights into the functional relevance of auditory efferent activity in humans.

Efferent-induced shifts in synchronized-spontaneous-otoacoustic-emission magnitude and frequency

Synchronized-spontaneous otoacoustic emissions (SSOAEs) present as slow-decaying emission energy that persists after the transient-evoked otoacoustic emission (TEOAE). SSOAEs possess high amplitudes and signal-to-noise ratios, making them potentially ideal candidates to assay the medial-olivocochlear reflex (MOCR). The current work quantified MOCR-induced changes to SSOAEs over a 36-dB stimulus level range and compared MOCR effects between TEOAE- and SSOAE-based assays. Otoacoustic emissions were evoked using band limited clicks from 52 to 88 dB peak sound pressure level (pSPL) with and without contralateral-acoustic stimulation (CAS) in 25 normal-hearing, female adults. The CAS was 50-dB sound pressure level (SPL) broadband noise and served to activate the MOCR. The number of SSOAEs increased with the stimulus level through approximately 70 dB pSPL. The presentation of CAS resulted in fewer SSOAEs. SSOAEs exhibited compressive growth and approached saturation for stimulus levels of 70 dB pSPL. The primary effects of CAS were a reduction in the SSOAE magnitude and an upward shift in the SSOAE frequency. These changes were not strongly affected by the stimulus level. Time-domain analysis of the SSOAE revealed an increase in the CAS-induced magnitude shift during the decay portion of the SSOAE. Compared to CAS-induced TEOAE magnitude shifts, SSOAE magnitude shifts were typically 2-3 dB larger. Findings support SSOAEs as a means to assay the MOCR.

Twin study of neonatal transient-evoked otoacoustic emissions

Our knowledge of which physiological mechanisms shape transient evoked otoacoustic emissions (TEOAEs) is incomplete, although thousands of TEOAEs are recorded each day as part of universal newborn hearing-screening (UNHS). TEOAE heritability may explain some of the large TEOAE variability observed in neonates, and give insights into the TEOAE generators and modulators, and why TEOAEs are generally larger in females and right ears. The aim was to estimate TEOAE heritability and describe ear and sex effects in a consecutive subset of all twins that passed UNHS at the same occasion at two hospitals during a six-year period (more than 30 000 neonates screened in total). TEOAEs were studied and TEOAE level correlations compared in twin sets of same-sex (SS, 302 individual twins, 151 twin pairs) and opposite-sex (OS, 152 individual twins, 76 twin pairs). A mathematical model was used to estimate and compare monozygotic (MZ) and dizygotic (DZ) intra-twin pair TEOAE level correlations, based on the data from the SS and OS twin sets. For both SS and OS twin pairs TEOAE levels were significantly higher in right ears and females, compared to left ears and males, as previously demonstrated in young adult twins and large groups of neonates. Neonatal females in OS twin pairs did not demonstrate masculinized TEOAEs, as has been demonstrated for OAEs in young adult females in OS twin pairs. The within-twin pair TEOAE level correlations were higher for SS twin pairs than for OS twin pairs, whereas the within-pair correlation coefficients could not be distinguished from zero when twins were randomly paired. These results reflect heredity as a key factor in TEOAE level variability. Additionally, the estimated MZ within-twin pair TEOAE level correlations were higher than those for DZ twin pairs. The heritability estimates reached up to 100% TEOAE heritability, which is numerically larger than previous estimates of about 75% in young adult twins.

Jittering stimulus onset attenuates short-latency, synchronized-spontaneous otoacoustic emission energy

Synchronized-spontaneous otoacoustic emissions (SSOAEs) are slow-decaying otoacoustic emissions (OAEs) that persist up to several hundred milliseconds following presentation of a transient stimulus. If the inter-stimulus interval is sufficiently short, SSOAEs will contaminate the stimulus window of the adjacent epoch. In medial-olivocochlear reflex (MOCR) assays, SSOAE contamination can present as a change in the stimulus between quiet and noise conditions, since SSOAEs are sensitive to MOCR activation. Traditionally, a change in the stimulus between MOCR conditions implicates acoustic reflex activation by the contralateral noise; however, this interpretation is potentially confounded by SSOAEs. This study examined the utility of jittering stimulus onset to desynchronize and cancel short-latency SSOAE energy. Transient-evoked (TE) OAEs and SSOAEs were measured from 39 subjects in contralateral-quiet and -noise conditions. Clicks were presented at fixed and quasi-random intervals (by introducing up to 8 ms of jitter). For the fixed-interval condition, spectral differences in the stimulus window between quiet and noise conditions mirrored those in the SSOAE analysis window, consistent with SSOAE contamination. In contrast, spectral differences stemming from SSOAEs were attenuated and/or absent in the stimulus window for the jitter conditions. The use of jitter did not have a statistically significant effect on either TEOAE level or the estimated MOCR.

Medial olivocochlear reflex effects on synchronized spontaneous otoacoustic emissions

This study characterized medial olivocochlear (MOC) reflex activity on synchronized spontaneous otoacoustic emissions (SSOAEs) as compared to transient-evoked otoacoustic emissions (TEOAEs) in normal-hearing adults. Using two time windows, changes in TEOAE and SSOAE magnitude and phase due to a MOC reflex elicitor were quantified from 1 to 4 kHz. In lower frequency bands, changes in TEOAE and SSOAE magnitude were significantly correlated and were significantly larger for SSOAEs. Changes in TEOAE and SSOAE phase were not significantly different, nor were they significantly correlated. The larger effects on SSOAE magnitude may improve the sensitivity for detecting the MOC reflex.

Chirp-Evoked Otoacoustic Emissions and Middle Ear Absorbance for Monitoring Ototoxicity in Cystic Fibrosis Patients

OBJECTIVES

The goal of this study was to investigate the use of transient-evoked otoacoustic emissions (TEOAEs) and middle ear absorbance measurements to monitor auditory function in patients with cystic fibrosis (CF) receiving ototoxic medications. TEOAEs were elicited with a chirp stimulus using an extended bandwidth (0.71 to 8 kHz) to measure cochlear function at higher frequencies than traditional TEOAEs. Absorbance over a wide bandwidth (0.25 to 8 kHz) provides information on middle ear function. The combination of these time-efficient measurements has the potential to identify early signs of ototoxic hearing loss.

DESIGN

A longitudinal study design was used to monitor the hearing of 91 patients with CF (median age = 25 years; age range = 15 to 63 years) who received known ototoxic medications (e.g., tobramycin) to prevent or treat bacterial lung infections. Results were compared to 37 normally hearing young adults (median age = 32.5 years; age range = 18 to 65 years) without a history of CF or similar treatments. Clinical testing included 226-Hz tympanometry, pure-tone air-conduction threshold testing from 0.25 to 16 kHz and bone conduction from 0.25 to 4 kHz. Experimental testing included wideband absorbance at ambient and tympanometric peak pressure and TEOAEs in three stimulus conditions: at ambient pressure and at tympanometric peak pressure using a chirp stimulus with constant incident pressure level across frequency and at ambient pressure using a chirp stimulus with constant absorbed sound power across frequency.

RESULTS

At the initial visit, behavioral audiometric results indicated that 76 of the 157 ears (48%) from patients with CF had normal hearing, whereas 81 of these ears (52%) had sensorineural hearing loss for at least one frequency. Seven ears from four patients had a confirmed behavioral change in hearing threshold for ≥3 visits during study participation. Receiver operating characteristic curve analyses demonstrated that all three TEOAE conditions were useful for distinguishing CF ears with normal hearing from ears with sensorineural hearing loss, with an area under the receiver operating characteristic curve values ranging from 0.78 to 0.92 across methods for frequency bands from 2.8 to 8 kHz. Case studies are presented to illustrate the relationship between changes in audiometric thresholds, TEOAEs, and absorbance across study visits. Absorbance measures permitted identification of potential middle ear dysfunction at 5.7 kHz in an ear that exhibited a temporary hearing loss.

CONCLUSIONS

The joint use of TEOAEs and absorbance has the potential to explain fluctuations in audiometric thresholds due to changes in cochlear function, middle ear function, or both. These findings are encouraging for the joint use of TEOAE and wideband absorbance objective tests for monitoring ototoxicity, particularly, in patients who may be too ill for behavioral hearing tests. Additional longitudinal studies are needed in a larger number of CF patients receiving ototoxic drugs to further evaluate the clinical utility of these measures in an ototoxic monitoring program.

High frequency transient-evoked otoacoustic emission measurements using chirp and click stimuli

Transient-evoked otoacoustic emissions (TEOAEs) at high frequencies are a non-invasive physiological test of basilar membrane mechanics at the basal end, and have clinical potential to detect risk of hearing loss related to outer-hair-cell dysfunction. Using stimuli with constant incident pressure across frequency, TEOAEs were measured in experiment 1 at low frequencies (0.7-8 kHz) and high frequencies (7.1-14.7 kHz) in adults with normal hearing up to 8 kHz and varying hearing levels from 9 to 16 kHz. In combination with click stimuli, chirp stimuli were used with slow, medium and fast sweep rates for which the local frequency increased or decreased with time. Chirp TEOAEs were transformed into equivalent click TEOAEs by inverse filtering out chirp stimulus phase, and analyzed similarly to click TEOAEs. To improve detection above 8 kHz, TEOAEs were measured in experiment 2 with higher-level stimuli and longer averaging times. These changes increased the TEOAE signal-to-noise ratio (SNR) by 10 dB. Slower sweep rates were investigated but the elicited TEOAEs were detected in fewer ears compared to faster rates. Data were acquired in adults and children (age 11-17 y), including children with cystic fibrosis (CF) treated with ototoxic antibiotics. Test-retest measurements revealed satisfactory repeatability of high-frequency TEOAE SNR (median of 1.3 dB) and coherence synchrony measure, despite small test-retest differences related to changes in forward and reverse transmission in the ear canal. The results suggest the potential use of such tests to screen for sensorineural hearing loss, including ototoxic loss. Experiment 2 was a feasibility study to explore TEOAE test parameters that might be used in a full-scale study to screen CF patients for risk of ototoxic hearing loss.

Analyzing transient-evoked otoacoustic emissions by concentration of frequency and time

The linear part of transient evoked otoacoustic emission (TEOAE) is thought to be generated via coherent reflection near the characteristic place of constituent wave components. Because of the tonotopic organization of the cochlea, high frequency emissions return earlier than low frequencies; however, due to the random nature of coherent reflection, the instantaneous frequency (IF) and amplitude envelope of TEOAEs both fluctuate. Multiple reflection components and synchronized spontaneous emissions can further make it difficult to extract the IF by linear transforms. This paper proposes to model TEOAEs as a sum of intrinsic mode-type functions and analyze it by a nonlinear-type time-frequency (T-F) analysis technique called concentration of frequency and time (ConceFT). When tested with synthetic otoacoustic emission signals with possibly multiple oscillatory components, the present method is able to produce clearly visualized traces of individual components on the T-F plane. Further, when the signal is noisy, the proposed method is compared with existing linear and bilinear methods in its accuracy for estimating the fluctuating IF. Results suggest that ConceFT outperforms the best of these methods in terms of optimal transport distance, reducing the error by 10% to 21% when the signal to noise ratio is 10 dB or below.

Assessing Sensorineural Hearing Loss Using Various Transient-Evoked Otoacoustic Emission Stimulus Conditions

OBJECTIVES

An important clinical application of transient-evoked otoacoustic emissions (TEOAEs) is to evaluate cochlear outer hair cell function for the purpose of detecting sensorineural hearing loss (SNHL). Double-evoked TEOAEs were measured using a chirp stimulus, in which the stimuli had an extended frequency range compared to clinical tests. The present study compared TEOAEs recorded using an unweighted stimulus presented at either ambient pressure or tympanometric peak pressure (TPP) in the ear canal and TEOAEs recorded using a power-weighted stimulus at ambient pressure. The unweighted stimulus had approximately constant incident pressure magnitude across frequency, and the power-weighted stimulus had approximately constant absorbed sound power across frequency. The objective of this study was to compare TEOAEs from 0.79 to 8 kHz using these three stimulus conditions in adults to assess test performance in classifying ears as having either normal hearing or SNHL.

DESIGN

Measurements were completed on 87 adult participants. Eligible participants had either normal hearing (N = 40; M F = 16 24; mean age = 30 years) or SNHL (N = 47; M F = 20 27; mean age = 58 years), and normal middle ear function as defined by standard clinical criteria for 226-Hz tympanometry. Clinical audiometry, immittance, and an experimental wideband test battery, which included reflectance and TEOAE tests presented for 1-min durations, were completed for each ear on all participants. All tests were then repeated 1 to 2 months later. TEOAEs were measured by presenting the stimulus in the three stimulus conditions. TEOAE data were analyzed in each hearing group in terms of the half-octave-averaged signal to noise ratio (SNR) and the coherence synchrony measure (CSM) at frequencies between 1 and 8 kHz. The test-retest reliability of these measures was calculated. The area under the receiver operating characteristic curve (AUC) was measured at audiometric frequencies between 1 and 8 kHz to determine TEOAE test performance in distinguishing SNHL from normal hearing.

RESULTS

Mean TEOAE SNR was ≥8.7 dB for normal-hearing ears and ≤6 dB for SNHL ears for all three stimulus conditions across all frequencies. Mean test-retest reliability of TEOAE SNR was ≤4.3 dB for both hearing groups across all frequencies, although it was generally less (≤3.5 dB) for lower frequencies (1 to 4 kHz). AUCs were between 0.85 and 0.94 for all three TEOAE conditions at all frequencies, except for the ambient TEOAE condition at 2 kHz (0.82) and for all TEOAE conditions at 5.7 kHz with AUCs between 0.78 and 0.81. Power-weighted TEOAE AUCs were significantly higher (p < 0.05) than ambient TEOAE AUCs at 2 and 2.8 kHz, as was the TPP TEOAE AUC at 2.8 kHz when using CSM as the classifier variable.

CONCLUSIONS

TEOAEs evaluated in an ambient condition, at TPP and in a power-weighted stimulus condition, had good test performance in identifying ears with SNHL based on SNR and CSM in the frequency range from 1 to 8 kHz and showed good test-retest reliability. Power-weighted TEOAEs showed the best test performance at 2 and 2.8 kHz. These findings are encouraging as a potential objective clinical tool to identify patients with cochlear hearing loss.

Otoacoustic emissions from ears with spontaneous activity behave differently to those without: Stronger responses to tone bursts as well as to clicks

It has been reported that both click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) have higher amplitudes in ears that possess spontaneous otoacoustic emissions (SOAEs). The general aim of the present study was to investigate whether the presence of spontaneous activity in the cochlea affected tone-burst evoked otoacoustic emissions (TBOAEs). As a benchmark, the study also measured growth functions of CEOAEs. Spontaneous activity in the cochlea was measured by the level of synchronized spontaneous otoacoustic emissions (SSOAEs), an emission evoked by a click but closely related to spontaneous otoacoustic emissions (SOAEs, which are detectable without any stimulus). Measurements were made on a group of 15 adults whose ears were categorized as either having recordable SSOAEs or no SSOAEs. In each ear, CEOAEs and TBOAEs were registered at frequencies of 0.5, 1, 2, and 4 kHz, and input/output functions were measured at 40, 50, 60, 70, and 80 dB SPL. Global and half-octave-band values of response level and latency were estimated. Our main finding was that in ears with spontaneous activity, TBOAEs had higher levels than in ears without. The difference was more apparent for global values, but were also seen with half-octave-band analysis. Input/output functions had similar growth rates for ears with and without SSOAEs. There were no significant differences in latencies between TBOAEs from ears with and without SSOAEs, although latencies tended to be longer for lower stimulus levels and lower stimulus frequencies. When TBOAE levels were compared to CEOAE levels, the latter showed greater differences between recordings from ears with and without SSOAEs. Although TBOAEs reflect activity from a more restricted cochlear region than CEOAEs, at all stimulus frequencies their behavior still depends on whether SSOAEs are present or not.

Pressurized transient otoacoustic emissions measured using click and chirp stimuli

Transient-evoked otoacoustic emission (TEOAE) responses were measured in normal-hearing adult ears over frequencies from 0.7 to 8 kHz, and analyzed with reflectance/admittance data to measure absorbed sound power and the tympanometric peak pressure (TPP). The mean TPP was close to ambient. TEOAEs were measured in the ear canal at ambient pressure, TPP, and fixed air pressures from 150 to -200 daPa. Both click and chirp stimuli were used to elicit TEOAEs, in which the incident sound pressure level was constant across frequency. TEOAE levels were similar at ambient and TPP, and for frequencies from 0.7 to 2.8 kHz decreased with increasing positive and negative pressures. At 4-8 kHz, TEOAE levels were larger at positive pressures. This asymmetry is possibly related to changes in mechanical transmission through the ossicular chain. The mean TEOAE group delay did not change with pressure, although small changes were observed in the mean instantaneous frequency and group spread. Chirp TEOAEs measured in an adult ear with Eustachian tube dysfunction and TPP of -165 daPa were more robust at TPP than at ambient. Overall, results demonstrate the feasibility and clinical potential of measuring TEOAEs at fixed pressures in the ear canal, which provide additional information relative to TEOAEs measured at ambient pressure.

Synchronized Spontaneous Otoacoustic Emissions Provide a Signal-to-Noise Ratio Advantage in Medial-Olivocochlear Reflex Assays

Detection of medial olivocochlear-induced (MOC) changes to transient-evoked otoacoustic emissions (TEOAE) requires high signal-to-noise ratios (SNR). TEOAEs associated with synchronized spontaneous (SS) OAEs exhibit higher SNRs than TEOAEs in the absence of SSOAEs, potentially making the former well suited for MOC assays. Although SSOAEs may complicate interpretation of MOC-induced changes to TEOAE latency, recent work suggests SSOAEs are not a problem in non-latency-dependent MOC assays. The current work examined the potential benefit of SSOAEs in TEOAE-based assays of the MOC efferents. It was hypothesized that the higher SNR afforded by SSOAEs would permit detection of smaller changes to the TEOAE upon activation of the MOC reflex. TEOAEs were measured in 24 female subjects in the presence and absence of contralateral broadband noise. Frequency bands with and without SSOAEs were identified for each subject. The prevalence of TEOAEs and statistically significant MOC effects were highest in frequency bands that also contained SSOAEs. The median TEOAE SNR in frequency bands with SSOAEs was approximately 8 dB higher than the SNR in frequency bands lacking SSOAEs. After normalizing by TEOAE amplitude, MOC-induced changes to the TEOAE were similar between frequency bands with and without SSOAEs. Smaller MOC effects were detectable across a subset of the frequency bands with SSOAEs, presumably due to a higher TEOAE SNR. These findings demonstrate that SSOAEs are advantageous in assays of the MOC reflex.

Comparing otoacoustic emissions evoked by chirp transients with constant absorbed sound power and constant incident pressure magnitude

Human ear-canal properties of transient acoustic stimuli are contrasted that utilize measured ear-canal pressures in conjunction with measured acoustic pressure reflectance and admittance. These data are referenced to the tip of a probe snugly inserted into the ear canal. Promising procedures to calibrate across frequency include stimuli with controlled levels of incident pressure magnitude, absorbed sound power, and forward pressure magnitude. An equivalent pressure at the eardrum is calculated from these measured data using a transmission-line model of ear-canal acoustics parameterized by acoustically estimated ear-canal area at the probe tip and length between the probe tip and eardrum. Chirp stimuli with constant incident pressure magnitude and constant absorbed sound power across frequency were generated to elicit transient-evoked otoacoustic emissions (TEOAEs), which were measured in normal-hearing adult ears from 0.7 to 8 kHz. TEOAE stimuli had similar peak-to-peak equivalent sound pressure levels across calibration conditions. Frequency-domain TEOAEs were compared using signal level, signal-to-noise ratio (SNR), coherence synchrony modulus (CSM), group delay, and group spread. Time-domain TEOAEs were compared using SNR, CSM, instantaneous frequency and instantaneous bandwidth. Stimuli with constant incident pressure magnitude or constant absorbed sound power across frequency produce generally similar TEOAEs up to 8 kHz.

Profiles of Stimulus-Frequency Otoacoustic Emissions from 0.5 to 20 kHz in Humans

The characteristics of human otoacoustic emissions (OAEs) have not been thoroughly examined above the standard audiometric frequency range (>8 kHz). This is despite the fact that deterioration of cochlear function often starts at the basal, high-frequency end of the cochlea before progressing apically. Here, stimulus-frequency OAEs (SFOAEs) were obtained from 0.5 to 20 kHz in 23 young, audiometrically normal female adults and three individuals with abnormal audiograms, using a low-to-moderate probe level of 36 dB forward pressure level (FPL). In audiometrically normal ears, SFOAEs were measurable at frequencies approaching the start of the steeply sloping high-frequency portion of the audiogram (∼12-15 kHz), though their amplitudes often declined substantially above ∼7 kHz, rarely exceeding 0 dB SPL above 8 kHz. This amplitude decline was typically abrupt and occurred at a frequency that was variable across subjects and not strongly related to the audiogram. In contrast, certain ears with elevated mid-frequency thresholds but regions of normal high-frequency sensitivity could possess surprisingly large SFOAEs (>10 dB SPL) above 7 kHz. When also measured, distortion-product OAEs (DPOAEs) usually remained stronger at higher stimulus frequencies and mirrored the audiogram more closely than SFOAEs. However, the high-frequency extent of SFOAE and DPOAE responses was similar when compared as a function of the response frequency, suggesting that middle ear transmission may be a common limiting factor at high frequencies. Nevertheless, cochlear factors are more likely responsible for complexities observed in high-frequency SFOAE spectra, such as abrupt amplitude changes and narrowly defined response peaks above 10 kHz, as well as the large responses in abnormal ears. These factors may include altered cochlear reflectivity due to subtle damage or the reduced spatial extent of the SFOAE generation region at the cochlear base. The use of higher probe levels is necessary to further evaluate the characteristics and potential utility of high-frequency SFOAE measurements.

Comparisons of transient evoked otoacoustic emissions using chirp and click stimuli

Transient-evoked otoacoustic emission (TEOAE) responses (0.7-8 kHz) were measured in normal-hearing adult ears using click stimuli and chirps whose local frequency increased or decreased linearly with time over the stimulus duration. Chirp stimuli were created by allpass filtering a click with relatively constant incident pressure level over frequency. Chirp TEOAEs were analyzed as a nonlinear residual signal by inverse allpass filtering each chirp response into an equivalent click response. Multi-window spectral and temporal averaging reduced noise levels compared to a single-window average. Mean TEOAE levels using click and chirp stimuli were similar with respect to their standard errors in adult ears. TEOAE group delay, group spread, instantaneous frequency, and instantaneous bandwidth were similar overall for chirp and click conditions, except for small differences showing nonlinear interactions differing across stimulus conditions. These results support the theory of a similar generation mechanism on the basilar membrane for both click and chirp conditions based on coherent reflection within the tonotopic region. TEOAE temporal fine structure was invariant across changes in stimulus level, which is analogous to the intensity invariance of click-evoked basilar-membrane displacement data.

Estimating cochlear tuning dependence on stimulus level and frequency from the delay of otoacoustic emissions

An objective technique based on the time-frequency analysis of otoacoustic emissions is proposed to get fast and stable estimates of cochlear tuning. Time-frequency analysis allows one to get stable measurements of the delay/frequency function, which is theoretically expected to be a function of cochlear tuning. Theoretical considerations and numerical solutions of a nonlinear cochlear model suggest that the average phase-gradient delay of the otoacoustic emission single-reflection components, weighted, for each frequency, by the amplitude of the corresponding wavelet coefficients, approximately scales as the square root of the cochlear quality factor. The application of the method to human stimulus-frequency and transient-evoked otoacoustic emissions shows that tuning decreases approximately by a factor of 2, as the stimulus level increases by 30 dB in a moderate stimulus level range. The results also show a steady increase of tuning with increasing frequency, by a factor of 2 between 1 and 5 kHz. This last result is model-dependent, because it relies on the assumption that cochlear scale-invariance breaking is only due to the frequency dependence of tuning. The application of the method to the reflection component of distortion product otoacoustic emissions, separated using time-frequency filtering, is complicated by the necessity of effectively canceling the distortion component.

Salient features of otoacoustic emissions are common across tetrapod groups and suggest shared properties of generation mechanisms

Otoacoustic emissions (OAEs) are faint sounds generated by healthy inner ears that provide a window into the study of auditory mechanics. All vertebrate classes exhibit OAEs to varying degrees, yet the biophysical origins are still not well understood. Here, we analyzed both spontaneous (SOAE) and stimulus-frequency (SFOAE) otoacoustic emissions from a bird (barn owl, Tyto alba) and a lizard (green anole, Anolis carolinensis). These species possess highly disparate macromorphologies of the inner ear relative to each other and to mammals, thereby allowing for novel insights into the biomechanical mechanisms underlying OAE generation. All ears exhibited robust OAE activity, and our chief observation was that SFOAE phase accumulation between adjacent SOAE peak frequencies clustered about an integral number of cycles. Being highly similar to published results from human ears, we argue that these data indicate a common underlying generator mechanism of OAEs across all vertebrates, despite the absence of morphological features thought essential to mammalian cochlear mechanics. We suggest that otoacoustic emissions originate from phase coherence in a system of coupled oscillators, which is consistent with the notion of "coherent reflection" but does not explicitly require a mammalian-type traveling wave. Furthermore, comparison between SFOAE delays and auditory nerve fiber responses for the barn owl strengthens the notion that most OAE delay can be attributed to tuning.

Influence of the stimulus presentation rate on medial olivocochlear system assays

Click evoked otoacoustic emissions (CEOAEs) are commonly used both in research and clinics to assay the medial olivocochlear system (MOC). Clicks presented at rates >50 Hz in the contralateral ear have previously been reported to evoke contralateral MOC activity. However, in typical MOC assays, clicks are presented in the ipsilateral ear in conjunction with MOC elicitor (noise) in the contralateral ear. The effect of click rates in such an arrangement is currently unknown. A forward masking paradigm was used to emulate typical MOC assays to elucidate the influence of ipsilateral click presentation rates on MOC inhibition of CEOAEs in 28 normal hearing adults. Influence of five click rates (20.83, 25, 31.25, 41.67, and 62.5 Hz) presented at 55 dB peSPL was tested. Results indicate that click rates as low as 31.25 Hz significantly enhance contralateral MOC inhibition, possibly through the activation of ipsilateral and binaural MOC neurons with potential contributions from the middle ear muscle reflex. Therefore, click rates ≤25 Hz are recommended for use in MOC assays, at least for 55 dB peSPL click level.

Basal contributions to short-latency transient-evoked otoacoustic emission components

The presence of short-latency (SL), less compressive-growing components in bandpass-filtered transient-evoked otoacoustic emission (TEOAE) waveforms may implicate contributions from cochlear regions basal to the tonotopic place. Recent empirical work suggests a region of SL generation between ∼1/5 and 1/10-octave basal to the TEOAE frequency's tonotopic place. However, this estimate may be biased to regions closer to the tonotopic place as the TEOAE extraction technique precluded measurement of components with latencies shorter than ∼5 ms. Using a variant of the non-linear, double-evoked extraction paradigm that permitted extraction of components with latencies as early as 1 ms, the current study empirically estimated the spatial-extent of the cochlear region contributing to 2 kHz SL TEOAE components. TEOAEs were evoked during simultaneous presentation of a suppressor stimulus, in order to suppress contributions to the TEOAE from different places along the cochlear partition. Three or four different-latency components of similar frequency content (∼2 kHz) were identified for most subjects. Component latencies ranged from 1.4 to 9.6 ms; latency was predictive of the component's growth rate and the suppressor frequency to which the component's magnitude was most sensitive to change. As component latency decreased, growth became less compressive and suppressor-frequency sensitivity shifted to higher frequencies. The shortest-latency components were most sensitive to suppressors approximately 3/5-octave higher than their nominal frequency of 2 kHz. These results are consistent with a distributed region of generation extending to approximately 3/5-octave basal to the TEOAE frequency's tonotopic place. The empirical estimates of TEOAE generation are similar to model-based estimates where generation of the different-latency components occurs through linear reflection from impedance discontinuities distributed across the cochlear partition.

Experimental evidence for the basal generation place of the short-latency transient-evoked otoacoustic emissions

Time-frequency analysis of the transient-evoked otoacoustic emission response was performed on a population of subjects affected by sensory-neural hearing loss characterized by a sharp audiometric profile, caused by firearm noise exposure (42 ears), and on a control population of normal-hearing subjects (84 ears). Time-frequency filtering permitted a careful evaluation of the relation between the audiometric profile and the spectral shape of the long- and short-latency otoacoustic components. Both filtered spectra closely follow the shape of the audiometric profile, with a frequency shift between them. The typical frequency shift was evaluated by averaging the otoacoustic spectra and the audiograms among groups of ears with the same cutoff frequency. Assuming that the otoacoustic emission source function depends on the local effectiveness of the cochlear amplifier, this experimental evidence suggests that the short-latency response is generated at a cochlear place displaced towards the base by about 0.5-1 mm with respect to the generation place of the long-latency component. The analysis of the control group demonstrates that, below 4 kHz, the observed effect is not dependent on the data acquisition and analysis procedure. These results confirm previous theoretical estimates and independent experimental evidence based on the measured latency difference between the two components.

Distortion-product otoacoustic emission reflection-component delays and cochlear tuning: estimates from across the human lifespan

A consistent relationship between reflection-emission delay and cochlear tuning has been demonstrated in a variety of mammalian species, as predicted by filter theory and models of otoacoustic emission (OAE) generation. As a step toward the goal of studying cochlear tuning throughout the human lifespan, this paper exploits the relationship and explores two strategies for estimating delay trends-energy weighting and peak picking-both of which emphasize data at the peaks of the magnitude fine structure. Distortion product otoacoustic emissions (DPOAEs) at 2f1-f2 were recorded, and their reflection components were extracted in 184 subjects ranging in age from prematurely born neonates to elderly adults. DPOAEs were measured from 0.5-4 kHz in all age groups and extended to 8 kHz in young adults. Delay trends were effectively estimated using either energy weighting or peak picking, with the former method yielding slightly shorter delays and the latter somewhat smaller confidence intervals. Delay and tuning estimates from young adults roughly match those obtained from SFOAEs. Although the match is imperfect, reflection-component delays showed the expected bend (apical-basal transition) near 1 kHz, consistent with a break in cochlear scaling. Consistent with other measures of tuning, the term newborn group showed the longest delays and sharpest tuning over much of the frequency range.