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.

A DPOAE assessment of outer hair cell integrity in ears with age-related hearing loss.

Distortion product otoacoustic emissions (DPOAEs) were used to assess outer hair cell (OHC) integrity in human ears with age-related hearing loss. Sound pressure measurements were made in the ear canal over the stimulus range 40-90 dB SPL (L2), with L1 = 0.45*L2 + 44 with F2 = 2 and 3 or 4 kHz. Model-generated DPOAE I/O functions were fit to DPOAE data to quantify the contribution of loss of nonlinearity (OHC loss) to the hearing loss. Results suggest OHC loss as a contributing cause of age-related hearing, regardless of audiogram configuration. It seems likely that OHC and strial pathology co-exist in ears with AHL.

Within- and Across-Subject Variability of Repeated Measurements of Medial Olivocochlear-Induced Changes in Transient-Evoked Otoacoustic Emissions.

OBJECTIVES: Measurement of changes in transient-evoked otoacoustic emissions (TEOAEs) caused by activation of the medial olivocochlear reflex (MOCR) may have clinical applications, but the clinical utility is dependent in part on the amount of variability across repeated measurements. The purpose of this study was to investigate the within- and across-subject variability of these measurements in a research setting as a step toward determining the potential clinical feasibility of TEOAE-based MOCR measurements. DESIGN: In 24 normal-hearing young adults, TEOAEs were elicited with 35 dB SL clicks and the MOCR was activated by 35 dB SL broadband noise presented contralaterally. Across a 5-week span, changes in both TEOAE amplitude and phase evoked by MOCR activation (MOC shifts) were measured at four sessions, each consisting of four independent measurements. Efforts were undertaken to reduce the effect of potential confounds, including slow drifts in TEOAE amplitude across time, activation of the middle-ear muscle reflex, and changes in subjects' attentional states. MOC shifts were analyzed in seven 1/6-octave bands from 1 to 2 kHz. The variability of MOC shifts was analyzed at the frequency band yielding the largest and most stable MOC shift at the first session. Within-subject variability was quantified by the size of the standard deviations across all 16 measurements. Across-subject variability was quantified as the range of MOC shift values across subjects and was also described qualitatively through visual analyses of the data. RESULTS: A large majority of MOC shifts in subjects were statistically significant. Most subjects showed stable MOC shifts across time, as evidenced by small standard deviations and by visual clustering of their data. However, some subjects showed within- and across-session variability that could not be explained by changes in hearing status, middle ear status, or attentional state. Simulations indicated that four baseline measurements were sufficient to predict the expected variability of subsequent measurements. However, the measured variability of subsequent MOC shifts in subjects was often larger than expected (based on the variability present at baseline), indicating the presence of additional variability at subsequent sessions. CONCLUSIONS: Results indicated that a wide range of within- and across-subject variability of MOC shifts was present in a group of young normal-hearing individuals. In some cases, very large changes in MOC shifts (e.g., 1.5 to 2 dB) would need to occur before one could attribute the change to either an intervention or pathology, rather than to measurement variability. It appears that MOC shifts, as analyzed in the present study, may be too variable for clinical use, at least in some individuals. Further study is needed to determine the extent to which changes in MOC shifts can be reliably measured across time for clinical purposes.

Medial olivocochlear-induced transient-evoked otoacoustic emission amplitude shifts in individual subjects.

Activation of the medial olivocochlear reflex (MOCR) can be assessed indirectly using transient-evoked otoacoustic emissions (TEOAEs). The change in TEOAE amplitudes when the MOCR is activated (medial olivocochlear (MOC) shift) has most often been quantified as the mean value in groups of subjects. The usefulness of MOC shift measurements may be increased by the ability to quantify significant shifts in individuals. This study used statistical resampling to quantify significant MOC shifts in 16 subjects. TEOAEs were obtained using transient stimuli containing energy from 1 to 10 kHz. A nonlinear paradigm was used to extract TEOAEs. Transient stimuli were presented at 30 dB sensation level (SL) with suppressor stimuli presented 12 dB higher. Contralateral white noise, used to activate the MOCR, was presented at 30 dB SL and was interleaved on and off in 30-s intervals during a 7-min recording period. Confounding factors of middle ear muscle reflex and slow amplitude drifts were accounted for. TEOAEs were analyzed in 11 1/3-octave frequency bands. The statistical significance of each individual MOC shift was determined using a bootstrap procedure. The minimum detectable MOC shifts ranged from 0.10 to 3.25 dB and were highly dependent on signal-to-noise ratio at each frequency. Subjects exhibited a wide range of magnitudes of significant MOC shifts in the 1.0-3.2-kHz region (median = 1.94 dB, range = 0.34-6.51 dB). There was considerable overlap between the magnitudes of significant and nonsignificant shifts. While most subjects had significant MOC shifts in one or more frequency bands below 4 kHz, few had significant shifts in all of these bands. Above 4 kHz, few significant shifts were seen, but this may have been due to lower signal-to-noise ratios. The specific frequency bands containing significant shifts were variable across individuals. Further work is needed to determine the clinical usefulness of examining MOC shifts in individuals.

Short-latency transient-evoked otoacoustic emissions as predictors of hearing status and thresholds.

Estimating audiometric thresholds using objective measures can be clinically useful when reliable behavioral information cannot be obtained. Transient-evoked otoacoustic emissions (TEOAEs) are effective for determining hearing status (normal hearing vs hearing loss), but previous studies have found them less useful for predicting audiometric thresholds. Recent work has demonstrated the presence of short-latency TEOAE components in normal-hearing ears, which have typically been eliminated from the analyses used in previous studies. The current study investigated the ability of short-latency components to predict hearing status and thresholds from 1-4 kHz. TEOAEs were measured in 77 adult ears with thresholds ranging from normal hearing to moderate sensorineural hearing loss. Emissions were bandpass filtered at center frequencies from 1 to 4 kHz. TEOAE waveforms were analyzed within two time windows that contained either short- or long-latency components. Waveforms were quantified by root-mean-square amplitude. Long-latency components were better overall predictors of hearing status and thresholds, relative to short-latency components. There were no significant improvements in predictions when short-latency components were included with long-latency components in multivariate analyses. The results showed that short-latency TEOAE components, as analyzed in the current study, were less predictive of both hearing status and thresholds from 1-4 kHz than long-latency components.

Detecting high-frequency hearing loss with click-evoked otoacoustic emissions.

In contrast to clinical click-evoked otoacoustic emission (CEOAE) tests that are inaccurate above 4-5 kHz, a research procedure measured CEOAEs up to 16 kHz in 446 ears and predicted the presence/absence of a sensorineural hearing loss. The behavioral threshold test that served as a reference to evaluate CEOAE test accuracy used a yes-no task in a maximum-likelihood adaptive procedure. This test was highly efficient between 0.5 and 12.7 kHz: Thresholds measured in 2 min per frequency had a median standard deviation (SD) of 1.2-1.5 dB across subjects. CEOAE test performance was assessed by the area under the receiver operating characteristic curve (AUC). The mean AUC from 1 to 10 kHz was 0.90 (SD=0.016). AUC decreased to 0.86 at 12.7 kHz and to 0.7 at 0.5 and 16 kHz, possibly due in part to insufficient stimulus levels. Between 1 and 12.7 kHz, the medians of the magnitude difference in CEOAEs and in behavioral thresholds were <4 dB. The improved CEOAE test performance above 4-5 kHz was due to retaining the portion of the CEOAE response with latencies as short as 0.3 ms. Results have potential clinical significance in predicting hearing status from at least 1 to 10 kHz using a single CEOAE response.

High-frequency click-evoked otoacoustic emissions and behavioral thresholds in humans.

Relationships between click-evoked otoacoustic emissions (CEOAEs) and behavioral thresholds have not been explored above 5 kHz due to limitations in CEOAE measurement procedures. New techniques were used to measure behavioral thresholds and CEOAEs up to 16 kHz. A long cylindrical tube of 8 mm diameter, serving as a reflectionless termination, was used to calibrate audiometric stimuli and design a wideband CEOAE stimulus. A second click was presented 15 dB above a probe click level that varied over a 44 dB range, and a nonlinear residual procedure extracted a CEOAE from these click responses. In some subjects (age 14-29 years) with normal hearing up to 8 kHz, CEOAE spectral energy and latency were measured up to 16 kHz. Audiometric thresholds were measured using an adaptive yes-no procedure. Comparison of CEOAE and behavioral thresholds suggested a clinical potential of using CEOAEs to screen for high-frequency hearing loss. CEOAE latencies determined from the peak of averaged, filtered temporal envelopes decreased to 1 ms with increasing frequency up to 16 kHz. Individual CEOAE envelopes included both compressively growing longer-delay components consistent with a coherent-reflection source and linearly or expansively growing shorter-delay components consistent with a distortion source. Envelope delays of both components were approximately invariant with level.