Carolina Abdala

Objectives: To use otoacoustic emissions (OAEs) for the serial monitoring of hearing or the tracking of hearing loss, test–retest reliability must be known, and normal OAE level variation must be established. Past studies have defined the reliability of distortion-product OAEs (DPOAEs) evoked with stimuli calibrated in a conventional manner (in-the-ear, dB sound pressure level), which can produce inaccuracies due to ear-canal standing waves. In the present study, we make within-subject comparisons of test reliability for conventionally measured 2 f ₁ - f ₂ DPOAEs compared with DPOAEs evoked by stimulus tones calibrated in dB forward pressure level (dB FPL) to mitigate the effect of standing waves. We also define the test–retest reliability of the stimulus-frequency OAE (SFOAE). Design: Conventional DPOAEs evoked by discrete tones calibrated in dB SPL as well as DPOAEs and SFOAEs evoked by swept tones calibrated in dB FPL were measured in the same ear of 20 adult subjects every 2 mo over the course of 1 yr. OAE test reliability was assessed for each OAE paradigm by calculating absolute level differences, standard errors of the measurement, and 95% normative shift limits. Comparisons in reliability were made between conventional DPOAEs and those using dB FPL calibration and between DPOAEs and SFOAEs, both evoked by stimuli calibrated in dB FPL. Results: Results indicate that overall DPOAEs evoked by swept tones calibrated in dB FPL have significantly better test–retest reliability than do conventional DPOAEs. Swept-tone DPOAEs also showed better reliability than swept-tone SFOAEs at mid-to-high frequencies, though at low frequencies test-to-test variation was comparable. In a subgroup of 5 subjects showing mild declines in audiometric hearing during the study, we found that DPOAEs and SFOAEs evoked by stimuli calibrated in dB FPL detected most instances of audiometric decline while conventional DPOAEs did not. Conclusions: FPL-based calibration of stimulus level reduces test-to-test variation in DPOAE level compared with conventional in-the-ear SPL methods. By creating a tighter distribution of natural or typical OAE level fluctuations on repeated testing, forward pressure calibration appears to improve the serial monitoring of OAEs and could provide earlier detection of slight hearing changes. Our preliminary look at this question should be further explored with a large group of individuals showing confirmed changes in hearing over time.

Measuring and analyzing both nonlinear-distortion and linear-reflection otoacoustic emissions (OAEs) combined creates what we have termed a “joint-OAE profile.” Here, we test whether these two classes of emissions have different sensitivities to hearing loss and whether our joint-OAE profile can detect mild-moderate hearing loss better than conventional OAE protocols have. 2f1-f2 distortion-product OAEs and stimulus-frequency OAEs were evoked with rapidly sweeping tones in 300 normal and impaired ears. Metrics included OAE amplitude for fixed-level stimuli as well as slope and compression features derived from OAE input/output functions. Results show that mild-moderate hearing loss impacts distortion and reflection emissions differently. Clinical decision theory was applied using OAE metrics to classify all ears as either normal-hearing or hearing-impaired. Our best OAE classifiers achieved 90% or better hit rates (with false positive rates of 5%–10%) for mild hearing loss, across a nearly five-octave range. In summary, results suggest that distortion and reflection emissions have distinct sensitivities to hearing loss, which supports the use of a joint-OAE approach for diagnosis. Results also indicate that analyzing both reflection and distortion OAEs together to detect mild hearing loss produces outstanding accuracy across the frequency range, exceeding that achieved by conventional OAE protocols.

Otoacoustic emissions (OAEs) arise from one (or a combination) of two basic generation mechanisms in the cochlea: nonlinear distortion and linear reflection. As a result of having distinct generation processes, these two classes of emissions may provide non-redundant information about hair-cell integrity and show distinct sensitivities to cochlear pathology. Here, we characterize the relationship between reflection and distortion emissions in normal hearers across a broad frequency and stimulus-level space using novel analysis techniques. Furthermore, we illustrate the promise of this approach in a small group of individuals with mild-moderate hearing loss. A “joint-OAE profile” was created by measuring interleaved swept-tone stimulus-frequency OAEs (SFOAEs) and 2f₁-f₂ distortion-product OAEs (DPOAEs) in the same ears using well-considered parameters. OAE spectra and input/output functions were calculated across five octaves. Using our specific recording protocol and analysis scheme, SFOAEs in normal hearers had higher levels than did DPOAEs, with the most pronounced differences occurring at the highest stimulus levels. Also, SFOAE compression occurred at higher stimulus levels (than did DPOAE compression) and its growth in the compressed region was steeper. The diagnostic implications of these findings and the influence of the measurement protocol on both OAEs (and on their relationship) are discussed.

Objective: As humans age, compressive nonlinearity—a hallmark of healthy cochlear function—changes. The nonlinear distortion-component of the distortion product otoacoustic emission (DPOAE) provides a noninvasive gauge of cochlear nonlinearity. Earlier published work has suggested that weakened nonlinearity begins in middle age; the current work extends this investigation into the eight decade of life using advanced DPOAE data collection and analysis methods as well as multiple metrics of nonlinearity, including a test of loudness scaling. Design: The 2f ₁−f ₂ DPOAE was recorded in 20 young adults, 25 middle-aged adults and 32 older adults from f ₂ = 0.78 to 9.4 kHz with primary tones (f ₂/f ₁ = 1.22) swept upward at a rate of 0.5 octave/sec. Only frequencies with audiometric thresholds ≤20 dB HL were included in the analysis and to the extent possible, ears were audiometrically matched to eliminate hearing threshold as a contributing factor to the observed age effects. Input/output functions were generated for the separated distortion-component of the DPOAE to probe compressive nonlinearity of the cochlea, and ipsilateral suppression of the DPOAE was conducted to probe two-tone suppression. To investigate the perceptual effects of weakening nonlinearity on loudness perception, the same subjects performed categorical loudness scaling. Age effects on both DPOAE and loudness scaling variables were assessed, and correlations were conducted between key OAE and perceptual metrics. Results: Age × Frequency ANOVAs revealed that the compression knee of the DPOAE I/O function occurred at higher stimulus levels in both groups of older adults compared to young adults, suggesting an expanded linear range with aging; also, the compressive slope (growth beyond the knee point) was steeper in older-adults compared to young adults. These results were most notable at high frequencies. ANOVAs including age and auditory threshold as factors confirmed that the age effect observed was independent of threshold. Additionally, in smaller subsets of subjects with audiometrically matched data, these same trends persisted, further ruling out hearing threshold as an influential factor. The growth of DPOAE ipsilateral suppression was shallower near 4 kHz in middle-aged and older adults compared to young adults and elevated suppression thresholds were observed. Results of categorical loudness scaling showed steeper growth of loudness for older adults and, at fixed sensation levels (dB SL), the older-adult group rated tones as louder than did their young-adult counterparts, suggesting abnormal loudness growth and perception. Several correlations between the compression knee of the DPOAE I/O function and key metrics of loudness scaling were significant and accounted for up to one-third of the variance. Conclusions: Results indicate that the aging cochlea begins to show weakened nonlinearity in middle age and it progressively weakens further into senescence. The perceptual impact of weakened nonlinearity during aging is manifested as abnormal loudness judgments; that is, in older-adult ears, a tone considered comfortable or medium in young-adult ears can be considered loud. The biophysical origin of this weakened nonlinearity is not known. It is hypothesized to reflect aging-related damage to, or loss of, outer hair cells and their stereocilia. More work is warranted to better define the perceptual impact of a linearized cochlear response in older adults and to consider how this deficit might impact the fitting of hearing aids and other intervention strategies.

Although anatomical development of the cochlear duct is thought to be complete by term birth, human newborns continue to show postnatal immaturities in functional measures such as otoacoustic emissions (OAEs). Some of these OAE immaturities are no doubt influenced by incomplete maturation of the external and middle ears in infants; however, the observed prolongation of distortion-product OAE phase-gradient delays in newborns cannot readily be explained by conductive factors. This functional immaturity suggests that the human cochlea at birth may lack fully adult-like traveling-wave motion. In this study, we analyzed temporal-bone sections at the light microscopic level in newborns and adults to quantify dimensions and geometry of cochlear structures thought to influence the mechanical response of the cochlea. Contrary to common belief, results show multiple morphological immaturities along the length of the newborn spiral, suggesting that important refinements in the size and shape of the sensory epithelium and associated structures continue after birth. Specifically, immaturities of the newborn basilar membrane and organ of Corti are consistent with a more compliant and less massive cochlear partition, which could produce longer DPOAE delays and a shifted frequency-place map in the neonatal ear.

Objective: The stimuli used to evoke otoacoustic emissions (OAEs) are typically calibrated based on the total SPL measured at the probe microphone. However, due to the acoustics of the ear-canal space (i.e., standing-wave interference), this method can underestimate the stimulus pressure reaching the tympanic membrane at certain frequencies. To mitigate this effect, stimulus calibrations based on forward pressure level (FPL) can be applied. Furthermore, the influence of ear-canal acoustics on measured OAE levels can be compensated by expressing them in emitted pressure level (EPL). To date, studies have used artificial shallow versus deep probe fits to assess the effects of calibration method on changes in probe insertion. In an attempt to better simulate a clinical setting, the combined effects of FPL calibration of stimulus level and EPL compensation of OAE level on response variability during routine (noncontrived) probe fittings were examined. Design: The distortion component of the distortion-product OAE (DPOAE) and the stimulus-frequency OAE (SFOAE) were recorded at low and moderate stimulus levels in 20 normal-hearing young-adult subjects across a five-octave range. In each subject, three different calibration approaches were compared: (1) the conventional SPL-based stimulus calibration with OAE levels expressed in SPL; (2) FPL stimulus calibration with OAEs expressed in SPL; and (3) FPL stimulus calibration with OAEs expressed in EPL. Test and retest measurements were obtained during the same session and, in a subset of subjects, several months after the initial test. The effects of these different procedures on the inter- and intra-subject variability of OAE levels were assessed across frequency and level. Results: There were no significant differences in the inter-subject variability of OAE levels across the three calibration approaches. However, there was a significant effect on OAE intra-subject variability. The FPL/EPL approach resulted in the overall lowest test-rest differences in DPOAE level for frequencies above 4 kHz, where standing-wave interference is strongest. The benefit was modest, ranging on average from 0.5 to 2 dB and was strongest at the lower stimulus level. SFOAE level variability did not show significant differences among the three procedures, perhaps due to insufficient signal-to-noise ratio and nonoptimized stimulus levels. Correlations were found between the short-term replicability of DPOAEs and the benefit derived from the FPL/EPL procedure: the more variable the DPOAE, the stronger the benefit conferred by the advanced calibration methods. Conclusions: Stimulus and response calibration procedures designed to mitigate the effects of standing-wave interference on both the stimulus and the OAE enhance the repeatability of OAE measurements and reduce their dependence on probe position, even when probe shifts are small. Modest but significant improvements in short-term test-retest repeatability were observed in the mid- to high-frequency region when using combined FPL/EPL procedures. The authors posit that the benefit will be greater in a more heterogeneous group of subjects and when different testers participate in the fitting and refitting of subjects, which is a common practice in the audiology clinic. The impact of calibration approach on OAE inter-subject variability was not significant, possibly due to a homogeneous subject population and because factors other than probe position are at play.

Several types of otoacoustic emissions have been characterized in newborns to study the maturational status of the cochlea at birth and to develop effective tests of hearing. The stimulus-frequency otoacoustic emission (SFOAE), a reflection-type emission elicited with a single low-level pure tone, is the least studied of these emissions and has not been comprehensively characterized in human newborns. The SFOAE has been linked to cochlear tuning and is sensitive to disruptions in cochlear gain (i.e., hearing loss) in adult subjects. In this study, we characterize SFOAEs evoked with rapidly sweeping tones in human neonates and consider the implications of our findings for human cochlear maturation. SFOAEs were measured in 29 term newborns within 72 hr of birth using swept tones presented at 2 oct/s across a four-octave frequency range (0.5–8 kHz); 20 normal-hearing young adults served as a control group. The prevalence of SFOAEs in newborns was as high as 90% (depending on how response “presence” was defined). Evidence of probe-tip leakage and abnormal ear-canal energy reflectance was observed in those ears with absent or unmeasurable SFOAEs. Results in the group of newborns with present stimulus-frequency emissions indicate that neonatal swept-tone SFOAEs are adult-like in morphology but have slightly higher amplitude compared with adults and longer SFOAE group delays. The origin of these nonadult-like features is probably mixed, including contributions from both conductive (ear canal and middle ear) and cochlear immaturities.

Previous research on distortion product otoacoustic emission (DPOAE) components has hinted at possible differences in the effect of aging on the two basic types of OAEs: those generated by a reflection mechanism in the cochlea and those created by nonlinear distortion (Abdala and Dhar in J Assoc Res Otolaryngol 13:403–421, 2012). This initial work led to the hypothesis that micromechanical irregularity (“roughness”) increases in the aging cochlea, perhaps as the result of natural tissue degradation. Increased roughness would boost the backscattering of traveling waves (i.e., reflection emissions) while minimally impacting DPOAEs. To study the relational effect of aging on both types of emissions and address our hypothesis of its origin, we measured reflection- and distortion-type OAEs in 77 human subjects aged 18–76 years. The stimulus-frequency OAE (SFOAE), a reflection emission, and the distortion component of the DPOAE, a nonlinear distortion emission, were recorded at multiple stimulus levels across a four-octave range in all ears. Although the levels of both OAE types decreased with age, the rate of decline in OAE level was consistently greater for DPOAEs than for SFOAEs; that is, SFOAEs are relatively preserved with advancing age. Multiple regression analyses and other controls indicate that aging per se, and not hearing loss, drives this effect. Furthermore, SFOAE generation was simulated using computational modeling to explore the origin of this result. Increasing the amount of mechanical irregularity with age produced an enhancement of SFOAE levels, providing support for the hypothesis that increased intra-cochlear roughness during aging may preserve SFOAE levels. The characteristic aging effect—relatively preserved reflection-emission levels combined with more markedly reduced distortion-emission levels—indicates that SFOAE magnitudes in elderly individuals depend on more than simply the gain of the cochlear amplifier. This relative pattern of OAE decline with age may provide a diagnostic marker for aging-related changes in the cochlea.

Stimulus-frequency otoacoustic emissions (SFOAEs) are reflection-source emissions, and are the least familiar and perhaps most underutilized otoacoustic emission. Here, normative SFOAE data are presented from a large group of 48 young adults at probe levels from 20 to 60 dB sound pressure level (SPL) across a four-octave frequency range to characterize the typical SFOAE and describe recent methodological advances that have made its measurement more efficient. In young-adult ears, SFOAE levels peaked in the low-to-mid frequencies at mean levels of ∼6–7 dB SPL while signal-to-noise ranged from 23 to 34 dB SPL and test-retest reliability was ±4 dB for 90% of the SFOAE data. On average, females had ∼2.5 dB higher SFOAE levels than males. SFOAE input/output functions showed near linear growth at low levels and a compression threshold averaging 35 dB SPL across frequency. SFOAE phase accumulated ∼32–36 cycles across four octaves on average, and showed level effects when converted to group delay: low-level probes produced longer SFOAE delays. A “break” in the normalized SFOAE delay was observed at 1.1 kHz on average, elucidating the location of the putative apical-basal transition. Technical innovations such as the concurrent sweeping of multiple frequency segments, post hoc suppressor decontamination, and a post hoc artifact-rejection technique were tested.

Otoacoustic emissions (OAEs) provide salient information about cochlear function and dysfunction. Two broad classes of emissions, linear reflection and nonlinear distortion, arise via distinct cochlear processes and hence, appear to provide independent information about cochlear health and hearing. Considered in combination, these two OAE types may characterize sensory hearing loss most effectively. In this study, the level-dependent growth of stimulus-frequency OAEs (a reflection-type emission) and distortion-product OAEs (a distortion-type emission) were measured in ten normal-hearing ears and eight ears with slight-to-moderate sensorineural hearing loss. Metrics of OAE strength and compression were derived from OAE input/output functions and then considered in a combined fashion. Results indicate that SFOAEs and DPOAEs differ significantly in their strength and compression features. When SFOAE and DPOAE metrics are displayed together on a two-dimensional plot, relatively well-defined data clusters describe their normative relationship. In hearing-impaired ears, this relationship is disrupted but not in a uniform way across ears; ears with similar audiograms showed differently altered joint-OAE profiles. Hearing loss sometimes affected only one OAE or one more than the other. Results suggest a joint-OAE profile is promising and warrants study in a large group of subjects with sensory hearing loss of varied etiologies.