Stimulus-frequency Otoacoustic Emissions Research Articles

Human Medial Olivocochlear Reflex: Effects as Functions of Contralateral, Ipsilateral, and Bilateral Elicitor Bandwidths

Animal studies have led to the view that the acoustic medial olivocochlear (MOC) efferent reflex provides sharply tuned frequency-specific feedback that inhibits cochlear amplification. To determine if MOC activation is indeed narrow band, we measured the MOC effects in humans elicited by 60-dB sound pressure level (SPL) contralateral, ipsilateral, and bilateral noise bands as a function of noise bandwidth from 1/2 to 6.7 octaves. MOC effects were quantified by the change in stimulus frequency otoacoustic emissions from 40 dB SPL probe tones near 0.5, 1, and 4 kHz. In a second experiment, the noise bands were centered 2 octaves below probe frequencies near 1 and 4 kHz. In all cases, the MOC effects increased as elicitor bandwidth increased, with the effect saturating at about 4 octaves. Generally, the MOC effects increased as the probe frequency decreased, opposite expectations based on MOC innervation density in the cochlea. Bilateral-elicitor effects were always the largest. The ratio of ipsilateral/contralateral effects depended on elicitor bandwidth; the ratio was large for narrow-band probe-centered elicitors and approximately unity for wide-band elicitors. In another experiment, the MOC effects from increasing elicitor bandwidths were calculated from measurements of the MOC effects from adjacent half-octave noise bands. The predicted bandwidth function agreed well with the measured bandwidth function for contralateral elicitors, but overestimated it for ipsilateral and bilateral elicitors. Overall, the results indicate that (1) the MOC reflexes integrate excitation from almost the entire cochlear length, (2) as elicitor bandwidth is increased, the excitation from newly stimulated cochlear regions more than overcomes the reduced excitation at frequencies in the center of the elicitor band, and (3) contralateral, ipsilateral, and bilateral elicitors show MOC reflex spatial summation over most of the cochlea, but ipsilateral spatial summation is less additive than contralateral.

Comparing two proposed measures of cochlear mechanical filter bandwidth based on stimulus frequency otoacoustic emissions

It has been hypothesized that the sharpness of the cochlear mechanical filter is related to two measures based on stimulus frequency otoacoustic emissions (SFOAEs). The first is the group delay of the SFOAE; the second is the bandwidth of the SFOAE two-tone suppression isoinput tuning characteristic. A corollary of this is that natural variability in cochlear mechanical bandwidth within a population would lead to a positive correlation between these two SFOAE-based measures of tuning within that population. To test this prediction, SFOAE group delay and SFOAE two-tone suppression isoinput tuning characteristics were measured in a sample of 16 audiometrically normal subjects. Contrary to the prediction, no statistically significant correlation was found between the two bandwidth measures. Cochlear model simulations were used to aid the interpretation of this result. These suggested that a positive correlation between the two measures is expected, but that it may well be too weak to detect with the given sample size, due to the influence on the SFOAE measures of random inhomogeneities in basilar membrane impedance.

Use of stimulus-frequency otoacoustic emissions to investigate efferent and cochlear contributions to temporal overshoot.

Behavioral threshold for a tone burst presented in a long-duration noise masker decreases as the onset of the tone burst is delayed relative to masker onset. The threshold difference between detection of early- and late-onset tone bursts is called overshoot. Although the underlying mechanisms are unclear, one hypothesis is that overshoot occurs due to efferent suppression of cochlear nonlinearity [von Klitzing, R., and Kohlrausch, A. (1994). J. Acoust. Soc. Am. 95, 2192-2201]. This hypothesis was tested by using overshoot conditions to elicit stimulus-frequency otoacoustic emissions (SFOAEs), which provide a physiological measure of cochlear nonlinearity. SFOAE and behavioral thresholds were estimated using a modified maximum-likelihood yes-no procedure. The masker was a 400-ms "frozen" notched noise. The signal was a 20-ms, 4-kHz tone burst presented at 1 or 200 ms after the noise onset. Behavioral overshoot results replicated previous studies, but no overshoot was observed in SFOAE thresholds. This suggests that either efferent suppression of cochlear nonlinearity is not involved in overshoot, or a SFOAE threshold estimation procedure based on stimuli similar to those used to study behavioral overshoot is not sensitive enough to measure the effect.

Testing coherent reflection in chinchilla: Auditory-nerve responses predict stimulus-frequency emissions

Coherent-reflection theory explains the generation of stimulus-frequency and transient-evoked otoacoustic emissions by showing how they emerge from the coherent "backscattering" of forward-traveling waves by mechanical irregularities in the cochlear partition. Recent published measurements of stimulus-frequency otoacoustic emissions (SFOAEs) and estimates of near-threshold basilar-membrane (BM) responses derived from Wiener-kernel analysis of auditory-nerve responses allow for comprehensive tests of the theory in chinchilla. Model predictions are based on (1) an approximate analytic expression for the SFOAE signal in terms of the BM traveling wave and its complex wave number, (2) an inversion procedure that derives the wave number from BM traveling waves, and (3) estimates of BM traveling waves obtained from the Wiener-kernel data and local scaling assumptions. At frequencies above 4 kHz, predicted median SFOAE phase-gradient delays and the general shapes of SFOAE magnitude-versus-frequency curves are in excellent agreement with the measurements. At frequencies below 4 kHz, both the magnitude and the phase of chinchilla SFOAEs show strong evidence of interference between short- and long-latency components. Approximate unmixing of these components, and association of the long-latency component with the predicted SFOAE, yields close agreement throughout the cochlea. Possible candidates for the short-latency SFOAE component, including wave-fixed distortion, are considered. Both empirical and predicted delay ratios (long-latency SFOAE delay/BM delay) are significantly less than 2 but greater than 1. Although these delay ratios contradict models in which SFOAE generators couple primarily into cochlear compression waves, they are consistent with the notion that forward and reverse energy propagation in the cochlea occurs predominantly by means of traveling pressure-difference waves. The compelling overall agreement between measured and predicted delays suggests that the coherent-reflection model captures the dominant mechanisms responsible for the generation of reflection-source otoacoustic emissions.

Two-tone suppression of stimulus frequency otoacoustic emissions in human ears with sensorineural hearing loss

Two-tone suppression (2TS) of stimulus frequency otoacoustic emissions (SFOAEs) recorded in normal-functioning human ears is a noninvasive measure of cochlear nonlinearity. This study investigates 2TS SFOAE response properties in 63 subjects with sensorineural hearing loss and normal middle-ear function. SFOAEs were measured using tones at frequencies (fp) from 0.5 to 8 kHz and levels from 40-60 dB SPL. For interpreting SFOAEs, hearing status was classified at each frequency according to degree of hearing loss, with losses ranging from within normal limits to 95 dB HL. SFOAE suppression was measured as a function of suppressor frequency (fs) and level (Ls), with results analyzed in terms of a SFOAE suppression tuning curve. A tip-to-tail difference (TTD) was defined between the Ls needed to produce a criterion decrement in SFOAE level at fs one octave below fp and the Ls needed to produce a criterion decrement near fp. The TTD estimates cochlear-amplifier gain in normal-functioning ears, but its properties in impaired ears based on SFOAEs have not been examined. Results will be analyzed as a function of the hearing status to assess the relative impact of outer-hair cell dysfunction in the SFOAE source region at tonotopic place. [Work supported by NIH.]

Testing coherent reflection in chinchilla

Coherent-reflection theory explains the generation of stimulus-frequency and transient-evoked otoacoustic emissions by showing how they emerge from the coherent “backscattering” of forward-traveling waves by mechanical irregularities in the cochlear partition. Recent published measurements of stimulus-frequency otoacoustic emissions (SFOAEs) and estimates of near-threshold basilar-membrane (BM) responses derived from Wiener-kernel analysis of auditory-nerve responses allow for comprehensive tests of the theory in chinchilla. Model predictions are based on: (1) an approximate analytic expression for the SFOAE signal in terms of the BM traveling wave and its complex wave number; (2) an inversion procedure that derives the wave number from BM traveling waves; and (3) estimates of BM traveling waves obtained from the Wiener-kernel data and local scaling assumptions. At frequencies above 4 kHz, predicted median SFOAE phase-gradient delays are in excellent agreement with the SFOAE measurements. At frequencies below 4 kHz, chinchilla SFOAEs show strong evidence of interference between short- and long-latency components. Approximate unmixing of these components, and association of the long-latency component with the predicted SFOAE, yields close agreement throughout the cochlea.

Stimulus-frequency otoacoustic emission: Measurements in humans and simulations with an active cochlear model

An efficient method for measuring stimulus-frequency otoacoustic emissions (SFOAEs) was developed incorporating (1) stimulus with swept frequency or level and (2) the digital heterodyne analysis. SFOAEs were measured for 550-1450 Hz and stimulus levels of 32-62 dB sound pressure level in eight normal human adults. The mean level, number of peaks, frequency spacing between peaks, phase change, and energy-weighted group delays of SFOAEs were determined. Salient features of the human SFOAEs were stimulated with an active cochlear model containing spatially low-pass filtered irregularity in the impedance. An objective fitting procedure yielded an optimal set of model parameters where, with decreasing stimulus level, the amount of cochlear amplification and the base amplitude of the irregularity increased while the spatial low-pass cutoff and the slope of the spatial low-pass filter decreased. The characteristics of the human cochlea were inferred with the model. In the model, an SFOAE consisted of a long-delay component originating from irregularity in a traveling-wave peak region and a short-delay component originating from irregularity in regions remote from the peak. The results of this study should be useful both for understanding cochlear function and for developing a clinical method of assessing cochlear status.

Two-tone suppression of stimulus frequency otoacoustic emissions.

Stimulus frequency otoacoustic emissions (SFOAEs) measured using a suppressor tone in human ears are analogous to two-tone suppression responses measured mechanically and neurally in mammalian cochleae. SFOAE suppression was measured in 24 normal-hearing adults at octave frequencies (f(p)=0.5-8.0 kHz) over a 40 dB range of probe levels (L(p)). Suppressor frequencies (f(s)) ranged from -2.0 to 0.7 octaves re: f(p), and suppressor levels ranged from just detectable suppression to full suppression. The lowest suppression thresholds occurred for "best" f(s) slightly higher than f(p). SFOAE growth of suppression (GOS) had slopes close to one at frequencies much lower than best f(s), and shallow slopes near best f(s), which indicated compressive growth close to 0.3 dBdB. Suppression tuning curves constructed from GOS functions were well defined at 1, 2, and 4 kHz, but less so at 0.5 and 8.0 kHz. Tuning was sharper at lower L(p) with an equivalent rectangular bandwidth similar to that reported behaviorally for simultaneous masking. The tip-to-tail difference assessed cochlear gain, increasing with decreasing L(p) and increasing f(p) at the lowest L(p) from 32 to 45 dB for f(p) from 1 to 4 kHz. SFOAE suppression provides a noninvasive measure of the saturating nonlinearities associated with cochlear amplification on the basilar membrane.

Comparing stimulus-frequency otoacoustic emissions measured by compression, suppression, and spectral smoothing

Stimulus-frequency otoacoustic emissions (SFOAEs) have been measured in several different ways, including (1) nonlinear compression, (2) two-tone suppression, and (3) spectral smoothing. Each of the three methods exploits a different cochlear phenomenon or signal-processing technique to extract the emission. The compression method makes use of the compressive growth of emission amplitude relative to the linear growth of the stimulus. The emission is defined as the complex difference between ear-canal pressure measured at one intensity and the rescaled pressure measured at a higher intensity for which the emission is presumed negligible. The suppression method defines the SFOAE as the complex difference between the ear-canal pressure measured with and without a suppressor tone at a nearby frequency. The suppressor tone is presumed to substantially reduce or eliminate the emission. The spectral smoothing method involves convolving the complex ear-canal pressure spectrum with a smoothing function. The analysis exploits the differing latencies of stimulus and emission and is equivalent to windowing in the corresponding latency domain. Although the three methods are generally assumed to yield identical emissions, no equivalence has ever been established. This paper compares human SFOAEs measured with the three methods using procedures that control for temporal drifts, contamination of the calibration by evoked emissions, and other potential confounds. At low stimulus intensities, SFOAEs measured using all three methods are nearly identical. At higher intensities, limitations of the procedures contribute to small differences, although the general spectral shape and phase of the three SFOAEs remain similar. The near equivalence of SFOAEs measured by compression, suppression, and spectral smoothing indicates that SFOAE characteristics are not mere artifacts of measurement methodology.

Measurement of the Distribution of Medial Olivocochlear Acoustic Reflex Strengths Across Normal-Hearing Individuals via Otoacoustic Emissions

A clinical test for the strength of the medial olivocochlear reflex (MOCR) might be valuable as a predictor of individuals at risk for acoustic trauma or for explaining why some people have trouble understanding speech in noise. A first step in developing a clinical test for MOCR strength is to determine the range and variation of MOCR strength in a research setting. A measure of MOCR strength near 1 kHz was made across a normal-hearing population (N = 25) by monitoring stimulus-frequency otoacoustic emissions (SFOAEs) while activating the MOCR with 60 dB SPL wideband contralateral noise. Statistically significant MOCR effects were measured in all 25 subjects; but not all SFOAE frequencies tested produced significant effects within the time allotted. To get a metric of MOCR strength, MOCR-induced changes in SFOAEs were normalized by the SFOAE amplitude obtained by two-tone suppression. We found this "normalized MOCR effect" varied across frequency and time within the same subject, sometimes with significant differences between measurements made as little as 40 Hz apart or as little as a few minutes apart. Averaging several single-frequency measures spanning 200 Hz in each subject reduced the frequency- and time-dependent variations enough to produce correlated measures indicative of the true MOCR strength near 1 kHz for each subject. The distribution of MOCR strengths, in terms of SFOAE suppression near 1 kHz, across our normal-hearing subject pool was reasonably approximated by a normal distribution with mean suppression of approximately 35% and standard deviation of approximately 12%. The range of MOCR strengths spanned a factor of 4, suggesting that whatever function the MOCR plays in hearing (e.g., enhancing signal detection in noise, reducing acoustic trauma), different people will have corresponding differences in their abilities to perform that function.

Near equivalence of human click-evoked and stimulus-frequency otoacoustic emissions

Otoacoustic emissions (OAEs) evoked by broadband clicks and by single tones are widely regarded as originating via different mechanisms within the cochlea. Whereas the properties of stimulus-frequency OAEs (SFOAEs) evoked by tones are consistent with an origin via linear mechanisms involving coherent wave scattering by preexisting perturbations in the mechanics, OAEs evoked by broadband clicks (CEOAEs) have been suggested to originate via nonlinear interactions among the different frequency components of the stimulus (e.g., intermodulation distortion). The experiments reported here test for bandwidth-dependent differences in mechanisms of OAE generation. Click-evoked and stimulus-frequency OAE input/output transfer functions were obtained and compared as a function of stimulus frequency and intensity. At low and moderate intensities human CEOAE and SFOAE transfer functions are nearly identical. When stimulus intensity is measured in "bandwidth-compensated" sound-pressure level (cSPL), CEOAE and SFOAE transfer functions have equivalent growth functions at fixed frequency and equivalent spectral characteristics at fixed intensity. This equivalence suggests that CEOAEs and SFOAEs are generated by the same mechanism. Although CEOAEs and SFOAEs are known by different names because of the different stimuli used to evoke them, the two OAE "types" are evidently best understood as members of the same emission family.

Bias due to noise in otoacoustic emission measurements

Measurements of otoacoustic emission (OAE) magnitude are often made at low signal/noise ratios (SNRs) where measurement noise generates bias and variability errors that have led to the misinterpretation of OAE data. To gain an understanding for these errors and their effects, a two part investigation was carried out. First, the nature of OAE measurement noise was investigated using human data from 50 stimulus-frequency OAE experiments involving medial olivocochlear reflex (MOCR) activation. The noise was found to be reasonably approximated by circular Gaussian noise. Furthermore, when bias errors were taken into account, measurement variability was not found to be affected by MOCR activation as had been previously reported. Second, to quantify the errors circular Gaussian noise produces for different methods of OAE magnitude estimation for distortion-product, stimulus-frequency, and spontaneous OAEs, simulated OAE measurements were analyzed via four different magnitude estimation methods and compared. At low SNRs (below -6 dB), estimators involving Rice probability density functions produced less biased estimates of OAE magnitudes than conventional estimation methods, and less total rms error-particularly for spontaneous OAEs. They also enabled the calculation of probability density functions for OAE magnitudes from experimental data.

New procedure for measuring stimulus frequency otoacoustic emissions

Stimulus frequency otoacoustic emissions (SFOAE) are generated by single tones. This is both an advantage and a disadvantage. The advantage is that they are more likely to be primarily generated from a more limited region of the cochlea than other types of OAE. The disadvantage is that when tone pulses are used, they occur at the same frequency and time as the stimulus, making it very difficult to separate the OAE from the signal, unless one uses suppression. The SFOAE is generated in the cochlea and has to travel along the basilar membrane before being measured in the ear canal. Cochlear travel times mean that the SFOAE is always delayed relative to the stimulus. The delay is frequency dependent. We have modified the continuously sweeping tone procedure developed in our lab for evaluating DPOAE to separate the stimulus and SFOAE. The LSF procedure was modified to permit dynamic fitting of the delay of a given signal component permitting the extraction of the time varying SFOAE.

Stimulus frequency otoacoustic emissions in the Northern leopard frog, Rana pipiens pipiens: Implications for inner ear mechanics

Otoacoustic emissions (OAEs) are weak sounds that originate from the inner ear which are traditionally classified/named based on their evoking stimulus. Recently, it has been argued that such a classification, at least for mammals, misrepresents the underlying mechanisms of emission-generation. As an alternative classification, it has been suggested to recognize that OAEs arise either via nonlinear distortion or linear coherent reflection. For non-mammalian vertebrates, data on evoked OAEs that arise via the latter mechanism are largely missing. Here, we present the first measurements of stimulus frequency OAEs (SFOAEs), which are emissions thought to arise via linear coherent reflection, from an amphibian (the Northern leopard frog, Rana pipiens pipiens). Their properties as a function of the evoking stimulus frequencies and levels are described and subsequently compared with the previously reported properties of distortion product OAEs (DPOAEs) from the same frog species.

Use of stimulus-frequency otoacoustic emission latency and level to investigate cochlear mechanics in human ears.

Stimulus frequency otoacoustic emission (SFOAE) sound pressure level (SPL) and latency were measured at probe frequencies from 500 to 4000 Hz and probe levels from 40 to 70 dB SPL in 16 normal-hearing adult ears. The main goal was to use SFOAE latency estimates to better understand possible source mechanisms such as linear coherent reflection, nonlinear distortion, and reverse transmission via the cochlear fluid, and how those sources might change as a function of stimulus level. Another goal was to use SFOAE latencies to noninvasively estimate cochlear tuning. SFOAEs were dominated by the reflection source at low stimulus levels, consistent with previous research, but neither nonlinear distortion nor fluid compression become the dominant source even at the highest stimulus level. At each stimulus level, the SFOAE latency was an approximately constant number of periods from 1000 to 4000 Hz, consistent with cochlear scaling symmetry. SFOAE latency decreased with increasing stimulus level in an approximately frequency-independent manner. Tuning estimates were constant above 1000 Hz, consistent with simultaneous masking data, but in contrast to previous estimates from SFOAEs.

Time-course of the human medial olivocochlear reflex

The time-course of the human medial olivocochlear reflex (MOCR) was measured via its suppression of stimulus-frequency otoacoustic emissions (SFOAEs) in nine ears. MOCR effects were elicited by contralateral, ipsilateral or bilateral wideband acoustic stimulation. As a first approximation, MOCR effects increased like a saturating exponential with a time constant of 277+/-62 ms, and decayed exponentially with a time constant of 159+/-54 ms. However, in ears with the highest signal-to-noise ratios (4/9), onset time constants could be separated into "fast," tau= approximately 70 ms, "medium," tau = approximately 330 ms, and "slow," tau = approximately 25 s components, and there was an overshoot in the decay like an under-damped sinusoid. Both the buildup and decay could be modeled as a second order differential equation and the differences between the buildup and decay could be accounted for by decreasing one coefficient by a factor of 2. The reflex onset and offset delays were both approximately 25 ms. Although changing elicitor level over a 20 dB SPL range produced a consistent systematic change in response amplitude, the time course did not show a consistent dependence on elictor level, nor did the time-courses of ipsilaterally, contralaterally, and bilaterally activated MOCR responses differ significantly. Given the MOCR's time-course, it is best suited to operate on acoustic changes that persist for 100's of milliseconds.

Simultaneous Measurement of Noise-Activated Middle-Ear Muscle Reflex and Stimulus Frequency Otoacoustic Emissions

Otoacoustic emissions serve as a noninvasive probe of the medial olivocochlear (MOC) reflex. Stimulus frequency otoacoustic emissions (SFOAEs) elicited by a low-level probe tone may be the optimal type of emission for studying MOC effects because at low levels, the probe itself does not elicit the MOC reflex [Guinan et al. (2003) J. Assoc. Res. Otolaryngol. 4:521]. Based on anatomical considerations, the MOC reflex activated by ipsilateral acoustic stimulation (mediated by the crossed olivocochlear bundle) is predicted to be stronger than the reflex to contralateral stimulation. Broadband noise is an effective activator of the MOC reflex; however, it is also an effective activator of the middle-ear muscle (MEM) reflex, which can make results difficult to interpret. The MEM reflex may be activated at lower levels than measured clinically, and most previous human studies have not explicitly included measurements to rule out MEM reflex contamination. The current study addressed these issues using a higher-frequency SFOAE probe tone to test for cochlear changes mediated by the MOC reflex, while simultaneously monitoring the MEM reflex using a low-frequency probe tone. Broadband notched noise was presented ipsilaterally at various levels to elicit probe-tone shifts. Measurements are reported for 15 normal-hearing subjects. With the higher-frequency probe near 1.5 kHz, only 20% of subjects showed shifts consistent with an MOC reflex in the absence of an MEM-induced shift. With the higher-frequency probe near 3.5 kHz, up to 40% of subjects showed shifts in the absence of an MEM-induced shift. However, these responses had longer time courses than expected for MOC-induced shifts, and may have been dominated by other cochlear processes, rather than MOC reflex. These results suggest caution in the interpretation of effects observed using ipsilaterally presented acoustic activators intended to excite the MOC reflex.

Audiometric predictions using stimulus-frequency otoacoustic emissions and middle ear measurements.

The goals of the study are to determine how well stimulus-frequency otoacoustic emissions (SFOAEs) identify hearing loss, classify hearing loss as mild or moderate-severe, and correlate with pure-tone thresholds in a population of adults with normal middle ear function. Other goals are to determine if middle ear function as assessed by wideband acoustic transfer function (ATF) measurements in the ear canal account for the variability in normal thresholds, and if the inclusion of ATFs improves the ability of SFOAEs to identify hearing loss and predict pure-tone thresholds. The total suppressed SFOAE signal and its corresponding noise were recorded in 85 ears (22 normal ears and 63 ears with sensorineural hearing loss) at octave frequencies from 0.5 to 8 kHz, using a nonlinear residual method. SFOAEs were recorded a second time in three impaired ears to assess repeatability. Ambient-pressure ATFs were obtained in all but one of these 85 ears and were also obtained from an additional 31 normal-hearing subjects in whom SFOAE data were not obtained. Pure-tone air and bone conduction thresholds and 226-Hz tympanograms were obtained on all subjects. Normal tympanometry and the absence of air-bone gaps were used to screen subjects for normal middle ear function. Clinical decision theory was used to assess the performance of SFOAE and ATF predictors in classifying ears as normal or impaired, and linear regression analysis was used to test the ability of SFOAE and ATF variables to predict the air conduction audiogram. The ability of SFOAEs to classify ears as normal or hearing impaired was significant at all test frequencies. The ability of SFOAEs to classify impaired ears as either mild or moderate-severe was significant at test frequencies from 0.5 to 4 kHz. SFOAEs were present in cases of severe hearing loss. SFOAEs were also significantly correlated with air conduction thresholds from 0.5 to 8 kHz. The best performance occurred with the use of the SFOAE signal-to-noise ratio as the predictor, and the overall best performance was at 2 kHz. The SFOAE signal-to-noise measures were repeatable to within 3.5 dB in impaired ears. The ATF measures explained up to 25% of the variance in the normal audiogram; however, ATF measures did not improve SFOAEs predictors of hearing loss except at 4 kHz. In common with other OAE types, SFOAEs are capable of identifying the presence of hearing loss. In particular, SFOAEs performed better than distortion-product and click-evoked OAEs in predicting auditory status at 0.5 kHz; SFOAE performance was similar to that of other OAE types at higher frequencies except for a slight performance reduction at 4 kHz. Because SFOAEs were detected in ears with mild to severe cases of hearing loss, they may also provide an estimate of the classification of hearing loss. Although SFOAEs were significantly correlated with hearing threshold, they do not appear to have clinical utility in predicting a specific behavioral threshold. Information on middle ear status as assessed by ATF measures offered minimal improvement in SFOAE predictions of auditory status in a population of normal and impaired ears with normal middle ear function. However, ATF variables did explain a significant fraction of the variability in the audiograms of normal ears, suggesting that audiometric thresholds in normal ears are partially constrained by middle ear function as assessed by ATF tests.

Delays of stimulus-frequency otoacoustic emissions and cochlear vibrations contradict the theory of coherent reflection filtering

When stimulated by tones, the ear appears to emit tones of its own, stimulus-frequency otoacoustic emissions (SFOAEs). SFOAEs were measured in 17 chinchillas and their group delays were compared with a place map of basilar-membrane vibration group delays measured at the characteristic frequency. The map is based on Wiener-kernel analysis of responses to noise of auditory-nerve fibers corroborated by measurements of vibrations at several basilar-membrane sites. SFOAE group delays were similar to, or shorter than, basilar-membrane group delays for frequencies >4 kHz and <4 kHz, respectively. Such short delays contradict the generally accepted "theory of coherent reflection filtering" [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018-2047 (1995)], which predicts that the group delays of SFOAEs evoked by low-level tones approximately equal twice the basilar-membrane group delays. The results for frequencies higher than 4 kHz are compatible with hypotheses of SFOAE propagation to the stapes via acoustic waves or fluid coupling, or via reverse basilar membrane traveling waves with speeds corresponding to the signal-front delays, rather than the group delays, of the forward waves. The results for frequencies lower than 4 kHz cannot be explained by hypotheses based on waves propagating to and from their characteristic places in the cochlea.

Stimulus-frequency otoacoustic emissions measured with amplitude-modulated suppressor tones

Stimulus-frequency otoacoustic emissions (SFOAEs) are typically derived as the difference in sound pressure in the ear canal with and without a suppressor tone added to the probe tone. A novel variation of this method applies a sinusoidal amplitude modulation (AM) to the suppressor tone, which causes the SFOAE to also be modulated. The AM-SFOAE can be separated from the probe frequency using spectral methods. AM-SFOAE measurements are described for four normal-hearing subjects using 6-Hz AM. Because the suppressor modulation is at a higher rate, the AM-SFOAE technique avoids the confounding influence of heartbeat, which also modulates the probe tone.