Transient-evoked stimulus-frequency and distortion-product otoacoustic emissions in normal and impaired ears

Transient-evoked stimulus-frequency otoacoustic emissions (SFOAEs), recorded using a nonlinear differential technique, and distortion-product otoacoustic emissions (DPOAEs) were measured in 17 normal-hearing and 10 hearing-impaired subjects using pairs of tone pips (pp), gated tones (gg), and for DPOAEs, continuous and gated tones (cg). Temporal envelopes of stimulus and OAE waveforms were obtained by narrow-band filtering at the stimulus or DP frequency. Mean SFOAE latencies in normal ears at 2.7 and 4.0 kHz decreased with increasing stimulus level and were larger at 4.0 kHz than latencies in impaired ears. Equivalent auditory filter bandwidths were calculated as a function of stimulus level from SFOAE latencies by assuming that cochlear transmission is minimum phase. DPOAE latencies varied less with level than SFOAE latencies. The ppDPOAEs often had two (or more) peaks separated in time with latencies consistent with model predictions for distortion and reflection components. Changes in ppDPOAE latency with level were sometimes explained by a shift in relative amplitudes of distortion and reflection components. The pp SFOAE SPL within the main spectral lobe of the pip stimulus was higher for normal ears in the higher-frequency half of the pip than the lower-frequency half, which is likely an effect of basilar membrane two-tone suppression.     

Input-output functions for stimulus-frequency otoacoustic emissions in normal-hearing adult ears

Input-output (I/O) functions for stimulus-frequency (SFOAE) and distortion-product (DPOAE) otoacoustic emissions were recorded in 30 normal-hearing adult ears using a nonlinear residual method. SFOAEs were recorded at half octaves from 500-8000 Hz in an L1=L2 paradigm with L2=0 to 85 dB SPL, and in a paradigm with L1 fixed and L2 varied. DPOAEs were elicited with primary levels of Kummer et al. [J. Acoust. Soc. Am. 103, 3431-3444 (1998)] at f2 frequencies of 2000 and 4000 Hz. Interpretable SFOAE responses were obtained from 1000-6000 Hz in the equal-level paradigm. SFOAE levels were larger than DPOAEs levels, signal-to-noise ratios were smaller, and I/O functions were less compressive. A two-slope model of SFOAE I/O functions predicted the low-level round-trip attenuation, the breakpoint between linearity and compression, and compressive slope. In ear but not coupler recordings, the noise at the SFOAE frequency increased with increasing level (above 60 dB SPL), whereas noise at adjacent frequencies did not. This suggests the existence of a source of signal-dependent noise producing cochlear variability, which is predicted to influence basilar-membrane motion and neural responses. A repeatable pattern of notched SFOAE I/O functions was present in some ears, and explained using a two-source mechanism of SFOAE generation.     

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.     

Comparison of distortion-product otoacoustic emission growth rates and slopes of forward-masked psychometric functions

Slopes of forward-masked psychometric functions (FM PFs) were compared with distortion-product otoacoustic emission (DPOAE) input/output (I/O) parameters at 1 and 6 kHz to test the hypothesis that these measures provide similar estimates of cochlear compression. Implicit in this hypothesis is the assumption that both DPOAE I/O and FM PF slopes are functionally related to basilar-membrane (BM) response growth. FM PF-slope decreased with signal level, but this effect was reduced or reversed with increasing hearing loss; there was a trend of decreasing psychometric function (PF) slope with increasing frequency, consistent with greater compression at higher frequencies. DPOAE I/O functions at 6 kHz exhibited an increase in the breakpoint of a two-segment slope as a function of hearing loss with a concomitant decrease in the level of the distortion product (L(d)). Results of the comparison between FM PF and DPOAE I/O parameters revealed only a weak correlation, suggesting that one or both of these measures may provide unreliable information about BM compression.     

Distortion product otoacoustic emission input/output functions in normal-hearing and hearing-impaired human ears.

DPOAE input/output (I/O) functions were measured at 7f2 frequencies (1 to 8 kHz; f2/f1 = 1.22) over a range of levels (-5 to 95 dB SPL) in normal-hearing and hearing-impaired human ears. L1-L2 was level dependent in order to produce the largest 2f1-f2 responses in normal ears. System distortion was determined by collecting DP data in six different acoustic cavities. These data were used to derive a multiple linear regression model to predict system distortion levels. The model was tested on cochlear-implant users and used to estimate system distortion in all other ears. At most but not all f2's, measurements in cochlear implant ears were consistent with model predictions. At all f2 frequencies, the ears with normal auditory thresholds produced I/O functions characterized by compressive nonlinear regions at moderate levels, with more rapid growth at low and high stimulus levels. As auditory threshold increased, DPOAE threshold increased, accompanied by DPOAE amplitude reductions, notably over the range of levels where normal ears showed compression. The slope of the I/O function was steeper in impaired ears. The data from normal-hearing ears resembled direct measurements of basilar membrane displacement in lower animals. Data from ears with hearing loss showed that the compressive region was affected by cochlear damage; however, responses at high levels of stimulation resembled those observed in normal ears.     

Evidence for basal distortion-product otoacoustic emission components

Distortion-product otoacoustic emissions (DPOAEs) were measured with traditional DP-grams and level/phase (L/P) maps in rabbits with either normal cochlear function or unique sound-induced cochlear losses that were characterized as either low-frequency or notched configurations. To demonstrate that emission generators distributed basal to the f(2) primary-tone contribute, in general, to DPOAE levels and phases, a high-frequency interference tone (IT) was presented at 1/3 of an octave (oct) above the f(2) primary-tone, and DPOAEs were re-measured as "augmented" DP-grams (ADP-grams) and L/P maps. The vector difference between the control and augmented functions was then computed to derive residual DP-grams (RDP-grams) and L/P maps. The resulting RDP-grams and L/P maps, which described the DPOAEs removed by the IT, supported the notion that basal DPOAE components routinely contribute to the generation of standard measures of DPOAEs. Separate experiments demonstrated that these components could not be attributed to the effects of the 1/3-oct IT on f(2), or DPOAEs generated by the addition of a third interfering tone. These basal components can "fill in" the lesion estimated by the commonly employed DP-gram. Thus, ADP-grams more accurately reveal the pattern of cochlear damage and may eventually lead to an improved DP-gram procedure.     

Characterizing distortion-product otoacoustic emission components across four species

Distortion-product otoacoustic emissions (DPOAEs) were measured as level/phase (L/P) maps in humans, rabbits, chinchillas, and rats with and without an interference tone (IT) placed either near the 2f(1)-f(2) DPOAE frequency place (f(dp)) or at one-third of an octave above the f(2) primary tone (1/3-oct IT). Vector differences between with and without IT conditions were computed to derive a residual composed of the DPOAE components removed by the IT. In humans, a DPOAE component could be extracted with the expected steep phase gradient indicative of reflection emissions by ITs near f(dp). In the laboratory species, ITs near f(dp) failed to produce any conclusive evidence for reflection components. For all species, 1/3-oct ITs extracted large DPOAE components presumably generated at or basal to the IT-frequency place that exhibited both distortion- and reflection-like phase properties. Together, these findings suggested that basal distortion components could assume reflection-like phase behavior when the assumptions of cochlear-scaling symmetry, the basis for shallow phase gradients for constant f(2)/f(1) ratio sweeps, are violated. The present results contradict the common belief that DPOAE components associated with steep or shallow phase slopes are unique signatures for reflection emissions arising from f(dp) or distortion emissions generated near f(2), respectively.     

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.     

The effect of stimulus-frequency ratio on distortion product otoacoustic emission components

A detailed measurement of distortion product otoacoustic emission (DPOAE) fine structure was used to extract estimates of the two major components believed to contribute to the overall DPOAE level in the ear canal. A fixed-ratio paradigm was used to record DPOAE fine structure from three normal-hearing ears over a range of 400 Hz for 12 different stimulus-frequency ratios between 1.053 and 1.36 and stimulus levels between 45 and 75 dB SPL. Inverse Fourier transforms of the amplitude and phase data were filtered to extract the early component from the generator region of maximum stimulus overlap and the later component reflected from the characteristic frequency region of the DPOAE. After filtering, the data were returned to the frequency domain to evaluate the impact of the stimulus-frequency ratio and stimulus level on the relative levels of the components. Although there were significant differences between data from different ears some consistent patterns could be detected. The component from the overlap region of the stimulus tones exhibits a bandpass shape, with the maximum occurring at a ratio of 1.2. The mean data from the DPOAE characteristic frequency region also exhibits a bandpass shape but is less sharply tuned and exhibits greater variety across ears and stimulus levels. The component from the DPOAE characteristic frequency region is dominant at ratios narrower than approximately 1.1 (the transition varies between ears). The relative levels of the two components are highly variable at ratios greater than 1.3 and highly dependent on the stimulus level. The reflection component is larger at all ratios at the lowest stimulus level tested (45/45 dB SPL). We discuss the factors shaping DPOAE-component behavior and some cursory implications for the choice of stimulus parameters to be used in clinical protocols.     

Distortion product otoacoustic emission suppression tuning curves in normal-hearing and hearing-impaired human ears

Distortion product otoacoustic emission (DPOAE) suppression measurements were made in 20 subjects with normal hearing and 21 subjects with mild-to-moderate hearing loss. The probe consisted of two primary tones (f2, f1), with f2 held constant at 4 kHz and f2/f1 = 1.22. Primary levels (L1, L2) were set according to the equation L1 = 0.4 L2 + 39 dB [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)], with L2 ranging from 20 to 70 dB SPL (normal-hearing subjects) and 50-70 dB SPL (subjects with hearing loss). Responses elicited by the probe were suppressed by a third tone (f3), varying in frequency from 1 octave below to 1/2 octave above f2. Suppressor level (L3) varied from 5 to 85 dB SPL. Responses in the presence of the suppressor were subtracted from the unsuppressed condition in order to convert the data into decrements (amount of suppression). The slopes of the decrement versus L3 functions were less steep for lower frequency suppressors and more steep for higher frequency suppressors in impaired ears. Suppression tuning curves, constructed by selecting the L3 that resulted in 3 dB of suppression as a function of f3, resulted in tuning curves that were similar in appearance for normal and impaired ears. Although variable, Q10 and Q(ERB) were slightly larger in impaired ears regardless of whether the comparisons were made at equivalent SPL or equivalent sensation levels (SL). Larger tip-to-tail differences were observed in ears with normal hearing when compared at either the same SPL or the same SL, with a much larger effect at similar SL. These results are consistent with the view that subjects with normal hearing and mild-to-moderate hearing loss have similar tuning around a frequency for which the hearing loss exists, but reduced cochlear-amplifier gain.     

Growth of suppression in humans based on distortion-product otoacoustic emission measurements

Distortion-product otoacoustic emissions (DPOAEs) were used to describe suppression growth in normal-hearing humans. Data were collected at eight f(2) frequencies ranging from 0.5 to 8 kHz for L(2) levels ranging from 10 to 60 dB sensation level. For each f(2) and L(2) combination, suppression was measured for nine or eleven suppressor frequencies (f(3)) whose levels varied from -20 to 85 dB sound pressure level (SPL). Suppression grew nearly linearly when f(3) ≈ f(2), grew more rapidly for f(3)?< f(2), and grew more slowly for f(3)?> f(2). These results are consistent with physiological and mechanical data from lower animals, as well as previous DPOAE data from humans, although no previous DPOAE study has described suppression growth for as wide a range of frequencies and levels. These trends were evident for all f(2) and L(2) combinations; however, some exceptions were noted. Specifically, suppression growth rate was less steep as a function of f(3) for f(2) frequencies ≤ 1 kHz. Thus, despite the qualitative similarities across frequency, there were quantitative differences related to f(2), suggesting that there may be subtle differences in suppression for frequencies above 1 kHz compared to frequencies below 1 kHz.     

Influence of in situ, sound-level calibration on distortion-product otoacoustic emission variability

Standing waves can cause errors during in-the-ear calibration of sound pressure level (SPL), affecting both stimulus magnitude and distortion-product otoacoustic emission (DPOAE) level. Sound intensity level (SIL) and forward pressure level (FPL) are two measurements theoretically unaffected by standing waves. SPL, SIL, and FPL in situ calibrations were compared by determining sensitivity of DPOAE level to probe-insertion depth (deep and "shallow") for a range of stimulus frequencies (1-8 kHz) and levels (20-60 dB). Probe-insertion depth was manipulated with the intent to shift the frequencies with standing-wave minima at the emission probe, introducing variability during SPL calibration. The absolute difference in DPOAE level between insertions was evaluated after correcting for an incidental change caused by the effect of ear-canal impedance on the emission traveling from the cochlea. A three-way analysis of variance found significant main effects for stimulus level, stimulus frequency, and calibration method, as well as significant interactions involving calibration method. All calibration methods exhibited changes in DPOAE level due to the insertion depth, especially above 4 kHz. However, SPL demonstrated the greatest changes across all stimulus levels for frequencies above 2 kHz, suggesting that SIL and FPL provide more consistent measurements of DPOAEs for frequencies susceptible to standing-wave calibration errors.     

Changes in amplitude and phase of distortion-product otoacoustic emission fine-structure and separated components during efferent activation

Medial olivocochlear (MOC) efferent fibers synapse directly on the outer hair cells (OHCs). Efferent activation evoked by contralateral acoustic stimulation (CAS) will affect OHC amplification and subsequent measures of distortion-product otoacoustic emissions (DPOAEs). The aim of this study was to investigate measures of total and separated DPOAEs during efferent activation. Efferent activation produces both suppression and enhancement of the total DPOAE level. Level enhancements occurred near fine-structure minima and were associated with consistent MOC evoked upward shifts in DPOAE fine-structure frequency. Examination of the phase of the separated components revealed that frequency shifts stemmed from increasing phase leads of the reflection component during CAS, while the generator component phase was nearly invariant. Separation of the two DPOAE components responsible for the fine-structure revealed more consistent reduction of the levels of both components. Using vector subtraction (which takes into account both level and phase) to estimate the changes in the unseparated DPOAE provided consistent evidence of DPOAE suppression. Including phase information provided a more sensitive, valid and consistent estimate of CAS function even if one does not know the position of the DPOAE in the fine-structure.     

Temporal aspects of suppression in distortion-product otoacoustic emissions

This study examined the time course of cochlear suppression using a tone-burst suppressor to measure decrement of distortion-product otoacoustic emissions (DPOAEs). Seven normal-hearing subjects with ages ranging from 19 to 28 yr participated in the study. Each subject had audiometric thresholds ≤ 15 dB HL [re ANSI (2004) Specifications for Audiometers] for standard octave and inter-octave frequencies from 0.25 to 8 kHz. DPOAEs were elicited by primary tones with f(2)?= 4.0 kHz and f(1)?= 3.333 kHz (f(2)/f(1)?= 1.2). For the f(2), L(2) combination, suppression was measured for three suppressor frequencies: One suppressor below f(2) (3.834 kHz) and two above f(2) (4.166 and 4.282 kHz) at three levels (55, 60, and 65 dB SPL). DPOAE decrement as a function of L(3) for the tone-burst suppressor was similar to decrements obtained with longer duration suppressors. Onset- and setoff- latencies were ≤ 4 ms, in agreement with previous physiological findings in auditory-nerve fiber studies that suggest suppression results from a nearly instantaneous compression of the waveform. Persistence of suppression was absent for the below-frequency suppressor (f(3)?= 3.834 kHz) and was ≤ 3 ms for the two above-frequency suppressors (f(3)?= 4.166 and 4.282 kHz).     

Effects of middle-ear immaturity on distortion product otoacoustic emission suppression tuning in infant ears

Distortion product otoacoustic emission (DPOAE) measures of cochlear function, including DPOAE suppression tuning curves and input/output (I/O) functions, are not adultlike in human infants. These findings suggest the cochlear amplifier might be functionally immature in newborns. However, many noncochlear factors influence DPOAEs and must be considered. This study examines whether age differences in DPOAE I/O functions recorded from infant and adult ears reflect maturation of ear-canal/middle-ear function or cochlear mechanics. A model based on linear middle-ear transmission and nonlinear cochlear generation was developed to fit the adult DPOAE I/O data. By varying only those model parameters related to middle-ear transmission (and holding cochlear parameters at adult values), the model successfully fitted I/O data from infants at birth through age 6 months. This suggests that cochlear mechanics are mature at birth. The model predicted an attenuation of stimulus energy through the immature ear canal and middle ear, and evaluated whether immaturities in forward transmission could explain the differences consistently observed between infant and adult DPOAE suppression. Results show that once the immaturity was compensated for by providing infants with a relative increase in primary tone level, DPOAE suppression tuning at f2= 6000 Hz was similar in adults and infants.     

Reliability of distortion-product otoacoustic emissions and their relation to loudness

The reliability of distortion-product otoacoustic emission (DPOAE) measurements and their relation to loudness measurements was examined in 16 normal-hearing subjects and 58 subjects with hearing loss. The level of the distortion product (L(d)) was compared across two sessions and resulted in correlations that exceeded 0.90. The reliability of DPOAEs was less when parameters from nonlinear fits to the input/output (I/O) functions were compared across visits. Next, the relationship between DPOAE I/O parameters and the slope of the low-level portion of the categorical loudness scaling (CLS) function (soft slope) was assessed. Correlations of 0.65, 0.74, and 0.81 at 1, 2, and 4 kHz were observed between CLS soft slope and combined DPOAE parameters. Behavioral threshold had correlations of 0.82, 0.83, and 0.88 at 1, 2, and 4 kHz with CLS soft slope. Combining DPOAEs and behavioral threshold provided little additional information. Lastly, a multivariate approach utilizing the entire DPOAE I/O function was used to predict the CLS rating for each input level (dB SPL). Standard error of the estimate when using this method ranged from 2.4 to 3.0 categorical units (CU), suggesting that DPOAE I/O functions can predict CLS measures within the CU step size used in this study (5).     

Cochlear compression estimates from measurements of distortion-product otoacoustic emissions

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.     

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.     

Level dependence of distortion-product otoacoustic emissions measured at high frequencies in humans

Given that high-frequency hearing is most vulnerable to cochlear pathology, it is important to characterize distortion-product otoacoustic emissions (DPOAEs) measured with higher-frequency stimuli in order to utilize these measures in clinical applications. The purpose of this study was to explore the dependence of DPOAE amplitude on the levels of the evoking stimuli at frequencies greater than 8 kHz, and make comparisons with those data that have been extensively measured with lower-frequency stimuli. To accomplish this, DPOAE amplitudes were measured at six different f2 frequencies (2, 5, 10, 12, 14, and 16 kHz), with a frequency ratio (f2/f1) of 1.2, at five fixed levels (30 to 70 dB SPL) of one primary (either f1 or f2), while the other primary was varied in level (30 to 70 dB SPL). Generally, the level separation between the two primary tones (L1 > L2) generating the largest DPOAE amplitude (referred to as the "optimal level separation") decreased as the level of the fixed primary increased. Additionally, the optimal level separation was frequency dependent, especially at the lower fixed primary tone levels ( < or = 50 dB SPL). In agreement with previous studies, the DPOAE level exhibited greater dependence on L1 than on L2.     

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.