Experimental confirmation of the two-source interference model for the fine structure of distortion product otoacoustic emissions

Sources of DPOAEs revealed by suppression experiments, inverse fast Fourier transforms, and SFOAEs in impaired ears

DPOAE sources are modeled by intermodulation distortion generated near the f2 place and a reflection of this distortion near the DP place. In a previous paper, inverse fast Fourier transforms (IFFTs) of DPOAE filter functions in normal ears were consistent with this model [Konrad-Martin et al., J. Acoust. Soc. Am. 109, 2862-2879 (2001)]. In the present article, similar measurements were made in ears with specific hearing-loss configurations. It was hypothesized that hearing loss at f2 or DP frequencies would influence the relative contributions to the DPOAE from the corresponding basilar membrane places, and would affect the relative magnitudes of SFOAEs at frequencies equal to f2 and fDP. DPOAEs were measured with f2 = 4 kHz, f1 varied, and a suppressor near fDP. L2 was 25-55 dB SPL (L1 = L2 + 10 dB). SFOAEs were measured at f2 and at 2.7 kHz (the average fDP produced by the f1 sweep) for stimulus levels of 20-60 dB SPL. SFOAE results supported predictions of the pattern of amplitude differences between SFOAEs at 4 and 2.7 kHz for sloping losses, but did not support predictions for the rising- and flat-loss categories. Unsuppressed IFFTs for rising losses typically had one peak. IFFTs for flat or sloping losses typically have two or more peaks; later peaks were more prominent in ears with sloping losses compared to normal ears. Specific predictions were unambiguously supported by the results for only four of ten cases, and were generally supported in two additional cases. Therefore, the relative contributions of the two DPOAE sources often were abnormal in impaired ears, but not always in the predicted manner.

The latency of evoked otoacoustic emissions: its relation to hearing loss and auditory evoked potentials

The latencies of transient evoked otoacoustic emissions (TEOAE), distortion product otoacoustic emissions (DPOAE) and auditory brainstem responses (ABR) have been studied in 173 patients with cochlear hearing impairment. No systematic or significant correlation between the OAE latency and the hearing loss at the corresponding frequency could be found, whereas the ABR latencies increase by a small but significant amount with increasing hearing loss. This can be explained by a broadening of the excited area and its shifting towards a more apical position if the hair cell population in the basal turn is depleted. Because of the considerable intersubject variability of OAE latencies the data do not allow to decide whether they are not influenced by cochlear hearing loss or whether an effect is merely not observable. The large variability is attributed to the interindividual differences of OAE generation time and return delay whereas the travel time of the primary wave is less variable as can be seen from the low variability of ABR latencies.

Origin of cubic difference tones generated by high-intensity stimuli: effect of ischemia and auditory fatigue on the gerbil cochlea

Cubic difference tone (CDT) otoacoustic emissions are thought to arise from the feedback loop allowing outer hair cells to enhance the sensitivity and tuning of the organ of Corti. The existence of residual CDTs during complete cochlear ischemia is therefore disturbing. That stimulus intensities must exceed 50-60 dB SPL for residual CDTs to be recorded and for level notches to be present in CDT growth functions is often cited as evidence for a two-component, "active/passive" model: one component, the residual one, would originate from a passive, hardly vulnerable mechanism and thus be unsuitable for hearing screening purposes. This model was probed in gerbil ears after complete interruption of the cochlear blood flow. Cochlear potentials and CDTs were controlled simultaneously through continuous monitoring of CDT level and phase for 50 and 60 dB SPL stimuli and group-delay measurements. After a clear initial decay, CDT levels elicited at 60 dB SPL plateaued for several minutes at about 20 dB below initial level, and when early level notches were observed, CDT phase changes remained minor. The CDT group delays decreased by less than 30%. Later CDT level notches were associated with sharp phase reversals but the similarity between CDT characteristics before and after a notch was hardly consistent with a two-component interpretation. When mild sound overexposure (pure tone, 90-95 dB SPL, 15-30 min) had been performed prior to ischemia, little or no ischemic CDT came from the frequency bands where auditory fatigue had been detected (within 1 kHz), irrespective of the stimulus intensity. It suggests that instead of being passive, residual ischemic CDTs were vulnerable and produced according to a near-normal tonotopy by the same mechanisms that were sensitive to auditory fatigue. All the results lined up with a simple feedback model of cochlear function assuming a single CDT source related to mechano-electrical transduction in outer hair cells. More parsimonious than a two-component model, it posits that although early stages of ischemia dramatically impair the overall performance of the cochlea, the nonlinear mechanical stages responsible for the existence of CDTs keep working albeit at higher intensities.

Sources of distortion product otoacoustic emissions revealed by suppression experiments and inverse fast Fourier transforms in normal ears

Primary and secondary sources combine to produce the 2f1-f2 distortion product otoacoustic emission (DPOAE) measured in the ear canals of humans. DPOAEs were obtained in nine normal-hearing subjects using a fixed-f2 paradigm in which f1 was varied. The f2 was 2 or 4 kHz, and absolute and relative primary levels were varied. Data were obtained with and without a third tone (f3) placed 15.6 Hz below 2f1-f2. The level of f3 was varied in order to suppress the stimulus frequency otoacoustic emission (SFOAE) coming from the 2f1-f2 place. These data were converted from the complex frequency domain into an equivalent time representation using an inverse fast Fourier transform (IFFT). IFFTs of unsuppressed DPOAE data were characterized by two or more peaks. Relative amplitudes of these peaks depended on overall primary level and on primary-level differences. The suppressor eliminated later peaks, but early peaks remained relatively unaltered. Results are interpreted to mean that the DPOAE measured in humans includes components from the f2 place (intermodulation distortion) and DP place (in the form of a SFOAE). These findings build on previous work by providing evidence that multiple peaks in the IFFT are due to a secondary source at the DP place.

Amplitude of distortion product otoacoustic emissions in the guinea pig in f(1)- and f(2)-sweep paradigms

The amplitude versus frequency relations of distortion product otoacoustic emissions (DPOAEs) were studied in the guinea pig, using both the f(1)- and the f(2)-sweep paradigms to vary the primary frequency separation. The amplitude of the DPOAEs 2f(1)-f(2), 3f(1)-2f(2), 4f(1)-3f(2), and 2f(2)-f(1), plotted as a function of DP frequency, exhibited a bandpass structure. The separation of the primaries for which the DPOAE level is maximum is referred to as the optimum ratio f(2)/f(1). For the lower sideband DPOAEs (f(dp)<f(1), f(2)), the optimum ratio varies non-monotonically with the primary frequency region. At an f(2) around 4.4 kHz, the optimum ratio for 2f(1)-f(2) reaches a maximum of about 1.46 while elsewhere it is in the more commonly found 1.2-1.3 range. The width of the amplitude profiles was studied by determining their Q(10 dB). The f(2)-sweep yielded significantly larger Q(10 dB) than f(1)-sweep, for the lower sideband DPOAEs. The amplitude versus frequency functions of the lower sideband DPOAEs approximately line up. Upon closer inspection, however, with f(1)-sweep the 2f(1)-f(2) DPOAE has its maximum at a slightly smaller DP frequency than the higher order DPOAEs. With f(2)-sweep, on the contrary, the 2f(1)-f(2) tends to peak at a higher DP frequency than the other lower sideband distortion products. When the amplitude is considered as a function of the ratio between f(dp) and f(2), the difference between f(1)- and f(2)-sweep with respect to the width and the alignment of the amplitude functions disappears. The amplitude profiles of the lower sideband DPOAEs are a function of the DPOAE frequency f(dp) relative to f(2).

DPOAE group delays versus electrophysiological measures of cochlear delay in normal human ears

Group delays of 2 f1-f2 distortion product otoacoustic emissions (DPOAEs) were determined using both f1- and f2-sweep paradigms in 24 normal-hearing subjects. These DPOAE group delays were studied in comparison with cochlear delays estimated from derived band VIIIth nerve compound action potentials (CAPs) and auditory brainstem responses (ABRs) in the same subjects. The center frequencies of the derived bands in the electrophysiological experiment were matched with the f2-frequencies in the DPOAE recording to ensure that DPOAEs and derived CAPs and ABRs were generated at the same places along the cochlear partition, thus allowing for a direct comparison. The degree to which DPOAE group delays are larger in the f2- than in the f1-sweep paradigm is consistent with a theoretical analysis of the so-called wave-fixed model. Both DPOAE group delays are highly correlated with CAP- and ABR-derived measures of cochlear delay. The principal result of this study is that "roundtrip" DPOAE group delay in the f1-sweep paradigm is exactly twice as large as the neural estimate of the "forward" cochlear delay. The interpretation of this notion in the context of cochlear wave propagation properties and DPOAE-generating mechanisms is discussed.

Wave and place fixed DPOAE maps of the human ear

Human intermodulation distortion product otoacoustic emissions (DPOAE) can be a mixture of low and high latency components. They have different level, phase, and suppression characteristics, which indicate that emissions arise both from the frequency region of the primary tones directly and indirectly via the DP frequency place. Which component dominates the measured DPOAE in the ear canal depends on the stimulus parameters, especially the frequency ratio, f2/f1. Interference between the two emissions adds complexity to measurements of DPOAE. The behavior and even existence of whichever emission route is lower in level often cannot directly be deduced from the raw DPOAE data because the other emission covers it. It is therefore not known whether both emissions are present for all stimulus parameters or whether the trends seen in each emission when they are the dominant emission route continue under stimulus conditions when they are not dominant. In this study, the two DPOAE components are separated by a post-processing method. Previously, maps of raw DPOAE data against f2/f1 and DP frequency have been obtained. To separate the components, sets of data consisting of f2/f1 sweeps were transformed by an inverse Fourier transform into the time domain. The low and high latency components appeared as two distinct peaks because of their different phase gradients. These peaks were separated by windowing in the time domain and two frequency domain maps were reconstructed, representing the low and high latency DPOAEs. It was found that the low latency component of the 2 f1-f2 DP was only emitted strongly with f2/f1 between approximately 1.1 and 1.3. The removal of the high latency component revealed the low ratio edge of this region, at which the level falls sharply. However, the low latency emission has been traced at reduced amplitude over a wide range of stimulus parameters. Although previously only observed at small frequency ratios, the high latency component was found to be present widely in the lower sideband, its level reducing slowly at larger f2/f1. Its phase behavior changes in the lower sideband, being approximately constant with DP frequency at small ratios of f2/f1, but deviating from this at wider ratios. These results support the hypothesis that a DPOAE component which propagates to and is re-emitted from the DP frequency place (place fixed emission) is present across a wide parameter range. However, for all but the close primary condition the lower sideband DPOAE is dominated by direct emission from the region of f2 and f1 wave interaction (wave fixed emission). A simple transmission line model is presented to illustrate how the observed DPOAE maps can arise on the basis of this hypothesis.

Distortion-product source unmixing: a test of the two-mechanism model for DPOAE generation

This paper tests key predictions of the "two-mechanism model" for the generation of distortion-product otoacoustic emissions (DPOAEs). The two-mechanism model asserts that lower-sideband DPOAEs constitute a mixture of emissions arising not simply from two distinct cochlear locations (as is now well established) but, more importantly, by two fundamentally different mechanisms: nonlinear distortion induced by the traveling wave and linear coherent reflection off pre-existing micromechanical impedance perturbations. The model predicts that (1) DPOAEs evoked by frequency-scaled stimuli (e.g., at fixed f2/f1) can be unmixed into putative distortion- and reflection-source components with the frequency dependence of their phases consistent with the presumed mechanisms of generation; (2) The putative reflection-source component of the total DPOAE closely matches the reflection-source emission (e.g., low level stimulus-frequency emission) measured at the same frequency under similar conditions. These predictions were tested by unmixing DPOAEs into components using two completely different methods: (a) selective suppression of the putative reflection source using a third tone near the distortion-product frequency and (b) spectral smoothing (or, equivalently, time-domain windowing). Although the two methods unmix in very different ways, they yield similar DPOAE components. The properties of the two DPOAE components are consistent with the predictions of the two-mechanism model.

The short-wave model and waves in two directions

In the region where a sinusoidal wave in the cochlea reaches its maximum amplitude, the long-wave (or one-dimensional) model of the cochlea is deficient. In this region a short-wave model is more appropriate. However, in its current form, the short-wave model supports only waves in one direction. Therefore, it cannot cope with reflection effects associated with, e.g., inhomogeneities. Theoretical explorations of creation and internal reflection of otoacoustic emissions have almost exclusively been based on the long-wave model. In this article the road is paved for future explorations on a generalized form of the short-wave model, one that supports forward as well as backward waves, and thus can include internal reflections.

Interrelations among distortion-product phase-gradient delays: their connection to scaling symmetry and its breaking

Distortion-product-otoacoustic-emission (DPOAE) phase-versus-frequency functions and corresponding phase-gradient delays have received considerable attention because of their potential for providing information about mechanisms of emission generation, cochlear wave latencies, and characteristics of cochlear tuning. The three measurement paradigms in common use (fixed-f1, fixed-f2, and fixed-f2/f1) yield significantly different delays, suggesting that they depend on qualitatively different aspects of cochlear mechanics. In this paper, theory and experiment are combined to demonstrate that simple phenomenological arguments, which make no detailed mechanistic assumptions concerning the underlying cochlear mechanics, predict relationships among the delays that are in good quantitative agreement with experimental data obtained in guinea pigs. To understand deviations between the simple theory and experiment, a general equation is found that relates the three delays for any deterministic model of DPOAE generation. Both model-independent and exact, the general relation provides a powerful consistency check on the measurements and a useful tool for organizing and understanding the structure in DPOAE phase data (e.g., for interpreting the relative magnitudes and intensity-dependencies of the three delays). Analysis of the general relation demonstrates that the success of the simple, phenomenological approach can be understood as a consequence of the mechanisms of emission generation and the approximate local scaling symmetry of cochlear mechanics. The general relation is used to quantify deviations from scaling manifest in the measured phase-gradient delays; the results indicate that deviations from scaling are typically small and that both linear and nonlinear mechanisms contribute significantly to these deviations. Intensity-dependent mechanisms contributing to deviations from scaling include cochlear-reflection and wave-interference effects associated with the mixing of distortion- and reflection-source emissions (as in DPOAE fine structure). Finally, the ratio of the fixed-f1 and fixed-f2 phase-gradient delays is shown to follow from the choice of experimental paradigm and, in the scaling limit, contains no information about cochlear physiology whatsoever. These results cast considerable doubt on the theoretical basis of recent attempts to use relative DPOAE phase-gradient delays to estimate the bandwidths of peripheral auditory filters.

Modeling the combined effects of basilar membrane nonlinearity and roughness on stimulus frequency otoacoustic emission fine structure

A theoretical framework for describing the effects of nonlinear reflection on otoacoustic emission fine structure is presented. The following models of cochlear reflection are analyzed: weak nonlinearity, distributed roughness, and a combination of weak nonlinearity and distributed roughness. In particular, these models are examined in the context of stimulus frequency otoacoustic emissions (SFOAEs). In agreement with previous studies, it is concluded that only linear cochlear reflection can explain the underlying properties of cochlear fine structures. However, it is shown that nonlinearity can unexpectedly, in some cases, significantly modify the level and phase behaviors of the otoacoustic emission fine structure, and actually enhance the pattern of fine structures observed. The implications of these results on the stimulus level dependence of SFOAE fine structure are also explored.

On the relationships between the fixed-f1, fixed-f2, and fixed-ratio phase derivatives of the 2f1-f2 distortion product otoacoustic emission

For primary frequency ratios, f2/f1, in the range 1.1-1.3, the fixed-f1 ("f2-sweep") phase derivative of the 2f1-f2 distortion product otoacoustic emission (DPOAE) is larger than the fixed-f2("f1-sweep") one. It has been proposed by some researchers that part or all of the difference between these delays may be attributed to the so-called cochlear filter "build-up" or response time in the DPOAE generation region around the f2 tonotopic site. The analysis of an approximate theoretical expression for the DPOAE signal [Talmadge et al., J. Acoust. Soc. Am. 104, 1517-1543 (1998)] shows that the contributions to the phase derivatives associated with the cochlear filter response is small. It is also shown that the difference between the phase derivatives can be qualitatively accounted for by assuming the approximate scale invariance of cochlear mechanics. The effects of DPOAE fine structure on the phase derivative are also explored, and it is found that the interpretation of the phase derivative in terms of the phase variation of a single DPOAE component can be quite problematic.

Changes in 2f1 - f2 acoustic distortion products in humans during quinine-induced cochlear dysfunction

Quinine is a suitable model substance for the study of otoacoustic emissions (OAEs) as it reversibly affects the outer hair cells, thus reducing sensitivity, frequency-selectivity and various forms of OAEs. The aim of this experiment was to study quinine-induced changes in the input/output (I/O) function of 2f1 - f2 distortion product OAE (DPOAE; f2/f1 = 1.22; 750-6,000 Hz). Six volunteers with normal hearing (26-39 years old) were intravenously infused to achieve pseudostable quinine plasma concentrations (approximately12 microM) inducing an average pure-tone threshold (PTT; 750-6,000 Hz) shift of 18 dB (5-30 dB) (frequency-independent and reversible). The mean quinine-induced DPOAE shift increased continuously with decreasing equal-level primary tones, e.g. from 1.0 dB at 70 dB sound pressure level (SPL) (n = 42) to 10.5 dB (n = 22) at 40 dB SPL (pooled data, no frequency dependence). According to recruitment, the mean slope of the DPOAE I/O function (at 30-60 dB SPL) increased from 0.86 to 1.35 dB/dB. The lack of correlation between shifts in DPOAE and PTT is in stark contrast to the excellent correlation reported between shifts in transient evoked OAE detection threshold and its corresponding psychoacoustic threshold. The highly vulnerable spontaneous OAEs, in combination with the less vulnerable DPOAEs, fit into a recently proposed taxonomic classification for OAEs.

Group delays of distortion product otoacoustic emissions: relating delays measured with f1- and f2-sweep paradigms

A theoretical analysis is presented of group delays of distortion product otoacoustic emissions (DPOAEs) measured with the phase-gradient method. The aim of the analysis is to clarify the differences in group delays D1 and D2, obtained using the f1- and the f2-sweep paradigms, respectively, and the dependence of group delays on the order of the DPOAE. Two models are considered, the place-fixed and the wave-fixed models. While in the former model the generation place is assumed to be invariant with both f1- and f2-sweeps, in the latter model the shift of generation place is fully accounted for. By making a simple local approximation of the cochlear scale invariance, a mathematical conversion from phase-place to phase-frequency gradients is incorporated in the wave-fixed model. Under the assumption that the DPOAE (as recorded at the best f2/f1 ratio) is dominated by the contribution from the generation site and not by, e.g., reflection components, the analysis leads to simple expressions for the ratio and difference between D1 and D2. Validation of the models against experimental data indicates that lower sideband DPOAEs (2f1-f2, 3f1-2f2, 4f1-3f2) are most consistent with the wave-fixed model. Upper sideband components (2f2-f1), in contrast, are not properly described by either the place-fixed or the wave-fixed model, independent whether DPOAE generation is assumed to originate at the f2 or at the more basally located f(dp) characteristic place.

Fine structure and multicomponents of the electrically evoked otoacoustic emission in gerbil

Like the acoustically evoked distortion product otoacoustic emissions (DPOAE), the amplitude spectrum of the extracochlear electrically evoked otoacoustic emission (EEOAE) also shows peaks and valleys, which are termed the fine structure (FS) of the EEOAE. The hypothesis that the FS of the EEOAE is generated by multiple wave interactions in the cochlea is investigated by examining the relationship between the FS and the multiple-delay components of the EEOAE. The bulla of the gerbil was exposed using a ventral surgical approach. One pole of a bipolar electrode was placed in the round window niche, and the other pole on the surface of the first cochlear turn. A microphone was used to measure electrically evoked sound pressure change in the ear canal. A recently developed multicomponent analysis method was used to detect the EEOAE multiple delays. It was found that the FS is the spectral representation of the multiple-delay components. The relative power of a prominent long delay component (LDC) shows a negative relationship to the electrical stimulus level. Both the FS and the LDC were abolished by intravenous furosemide. Reconstructed signals showed that mathematical removal of the EEOAE LDC also completely eliminated the FS. These data demonstrate that the FS and the EEOAE multicomponents are properties of normal cochlear mechanics in a healthy ear and that the FS is a manifestation of the multicomponents. The findings in this study strongly indicate that the FS of the EEOAE evoked by extracochlear electrical stimulation is generated by wave interaction in the cochlea. The similarity between the EEOAE FS and the DPOAE FS suggests that they may share the same mechanism.

Modeling the temporal behavior of distortion product otoacoustic emissions

The temporal behavior of the 2f1-f2 distortion product otoacoustic emission is theoretically investigated for the case in which the lower frequency (f1) primary tone is on continuously, and the higher frequency (f2) one is pulsed on and off [e.g., Talmadge et al., J. Acoust. Soc. Am. 105, 275-292 (1999)]. On physical grounds, this behavior is expected to be characterized by various group delays associated with the propagation of (1) the f2 cochlear primary wave between the cochlear base and the primary distortion product generation region around x2 (the f2 tonotopic place), and (2) the 2f1-f2 cochlear distortion product (DP) waves between the cochlear base, the primary generation region of the distortion product, and the region around the 2f1-f2 tonotopic place where the generated apical moving DP wave is reflected toward the cochlear base [e.g., Talmadge et al., J. Acoust. Soc. Am. 104, 1517-1543 (1998)]. An approximate analytic expression is obtained for this behavior from the analysis of the Fourier integral representation of the auditory peripheral response to the primary stimuli. This expression also approximately describes the transient build-up of the components of different latencies in terms of the damping properties of the cochlear partition. It is shown that considerable caution must be applied in attempting to relate phase derivatives of the distortion product otoacoustic emissions for steady state stimuli and the physical time delays which are associated with the temporal behavior of a distortion product emission in the case of a pulsed primary.

Evidence for multiple DPOAE components based upon group delay of the 2f(1)-f(2) distortion in the gerbil

The cochlear delay of the 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) was measured using the phase gradient method. With a constant f(2) and swept f(1), the resulting phase change of 2f(1)-f(2) was used to calculate the group delay for f(2) frequencies from 1 to 60 kHz. For f(2) frequencies between 2 and 60 kHz, the group delays were between 2.2 and 0.11 ms and continuously decreased for increasing f(2) and for increasing primary stimulus levels. For f(2) frequencies below 2 kHz, the group delay decreased to around 1 ms and was largely independent of stimulus level. The ratio curves resulting from the f(1) sweeps for high frequencies (f(2)16 kHz) displayed the typical mammalian shape with a peak in the level of 2f(1)-f(2) for a larger primary frequency separation (f(2)/f(1)1.15) and decreasing 2f(1)-f(2) level for smaller primary separation. In addition to this typical level maximum, for f(2) frequencies from about 1.8 to 16 kHz, the ratio curves displayed a second component in the form of an increase in the level of 2f(1)-f(2) for small primary separation at higher primary levels (level of f(2)30 dB SPL). For f(2) frequencies below 1.8 kHz, only the second component and no typical ratio peak as for higher f(2) could be observed and the associated group delay was always close to 0.8 ms. Several possible causes for this behavior are discussed, including different modes of DPOAE generation and modulation as well as changes in the nature of mechanical processing from base to apex in the gerbil cochlea. To evaluate the relative sensitivity of non-linear cochlear mechanics, an iso-distortion threshold curve was constructed from acoustical growth functions of the 2f(1)-f(2) DPOAE at optimum primary separation, by plotting the level of f(2) sufficient to evoke a distortion of -10 dB SPL as a function of f(2)2.5 kHz but failed to reflect the sensitivity for lower frequencies. This may be a consequence of more linear frequency processing in the apex.

Indications of different distortion product otoacoustic emission mechanisms from a detailed f1,f2 area study

The primary site of generation on the basilar membrane for the 2f1-f2 distortion product (DP) is generally considered to be near where the higher-frequency stimulus tone peaks. This site has also been shown to be a source of DP otoacoustic emission (DPOAE) in the ear canal, but a second source of emission is known to exist in the region of the DP frequency place. The DPOAE phase versus frequency gradient provides a means of investigating the emission mechanisms. "Wave-fixed" and "place-fixed" mechanisms have been proposed to account for the very different phase gradients found depending on whether the 2f1-f2 DPOAE is evoked by a small or large stimulus-frequency ratio. DPOAE phase versus frequency gradients can be investigated either by sweeping f1,f2 or by sweeping both frequencies maintaining a constant frequency ratio. Each manipulation gives only a partial description of DP behavior. In this study, the place-fixed/ wave-fixed dichotomy is analyzed using extensive 2f1-f2 and 2f2-f1 DP stimulus-frequency sweep data presented on matrices of f1 vs f2 and f2/f1 ratio versus DP frequency. These show how the DPs are related and provide a more complete picture of 2 f1-f2 and 2f2-f1 DPOAE phase and amplitude versus frequency behavior. The phase data contain evidence for a systematic variation in the proportions of wave- and place-fixed emission. The results suggest that 2f1-f2 DPOAEs with a wide stimulus frequency ratio are wave fixed, while all other DPOAEs are place fixed. A transition occurs within the 2 f1-f2 DP data region at a frequency ratio of about f2/f1 = 1.1. The 2f1-f2 DP and 2 f2-f1 DP phase behavior is continuous across the f2/f1 = 1 boundary. As the 2 f2-f1 DP generation region must be strongly influenced by the DP frequency place, the results imply that the place-fixed component of the 2 f1-f2 DP is also linked to its frequency place. A similar pattern was obtained with the 3f1-2f2 and 3f2-2f1 DPs. The results support the following model: For the limited set of stimulus conditions that gives rise to 2 f1-f2 wave-fixed emissions, DP energy is largely generated in the f2 region and is emitted directly. All other DPOAEs are place-fixed emissions, and while nonlinearity within the f2 stimulus envelope remains the generator, the DP is not directly emitted but travels apically until it is re-emitted basally via a separate reflection mechanism in the region of the DP place.

Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. II. Fine structure for different shapes of cochlear hearing loss

Distortion product otoacoustic emissions (DPOAE) were recorded from eight human subjects with mild to moderate cochlear hearing loss, using a frequency spacing of 48 primary pairs per octave and at a level L1 = L2 = 60 dBSPL and with a fixed ratio f2/f1. Subjects with different shapes of hearing thresholds were selected. They included subjects with near-normal hearing within only a limited frequency range, subjects with a notch in the audiogram, and subjects with a mild to moderate high-frequency loss. If the primaries were located in a region of normal or near-normal hearing, but DP frequencies were located in a region of raised thresholds, the distortion product 2 f1-f2 was still observable, but the DP fine structure disappeared. If the DP frequencies fell into a region of normal thresholds, fine structure was preserved as long as DPOAE were generated, even in cases of mild hearing loss in the region of the primaries. These experimental results give further strong evidence that, in addition to the initial source in the primary region, there is a second source at the characteristic place of fDP. Simulations in a nonlinear and active computer model for DPOAE generation indicate different generation mechanisms for the two components. The disappearance of DPOAE fine structure might serve as a more sensitive indicator of hearing impairment than the consideration of DP level alone.

Evidence for the distortion product frequency place as a source of distortion product otoacoustic emission (DPOAE) fine structure in humans. I. Fine structure and higher-order DPOAE as a function of the frequency ratio f2/f1

Critical experiments were performed in order to validate the two-source hypothesis of distortion product otoacoustic emissions (DPOAE) generation. Measurements of the spectral fine structure of DPOAE in response to stimulation with two sinusoids have been performed with normal-hearing subjects. The dependence of fine-structure patterns on the frequency ratio f2/f1 was investigated by changing f1 or f2 only (fixed f2 or fixed f1 paradigm, respectively), and by changing both primaries at a fixed ratio and looking at different order DPOAE. When f2/f1 is varied in the fixed ratio paradigm, the patterns of 2 f1-f2 fine structure vary considerably more if plotted as a function of f2 than as a function of fDP. Different order distortion products located at the same characteristic place on the basilar membrane (BM) show similar patterns for both, the fixed-f2 and fDP paradigms. Fluctuations in DPOAE level up to 20 dB can be observed. In contrast, the results from a fixed-fDP paradigm do not show any fine structure but only an overall dependence of DP level on the frequency ratio, with a maximum for 2f1-f2 at f2/f1 close to 1.2. Similar stimulus configurations used in the experiments have also been used for computer simulations of DPOAE in a nonlinear and active model of the cochlea. Experimental results and model simulations give strong evidence for a two-source model of DPOAE generation: The first source is the initial nonlinear interaction of the primaries close to the f2 place. The second source is caused by coherent reflection from a re-emission site at the characteristic place of the distortion product frequency. The spectral fine structure of DPOAE observed in the ear canal reflects the interaction of both these sources.

Suppression and enhancement of distortion-product otoacoustic emissions by interference tones above f(2). I. Basic findings in rabbits

The present study measured interference-response areas (IRAs) for distortion-product otoacoustic emissions (DPOAEs) at 2f(1)-f(2), 3f(1)-2f(2), and 2f(2)-f(1). The IRAs were obtained in either awake or anesthetized rabbits, or in anesthetized guinea pigs and mice, by sweeping the frequencies and levels of an interference tone (IT) around a set of f(1) and f(2) primary tones, at several fixed frequencies and levels, while plotting the effects of the IT on DPOAE level. An unexpected outcome was the occurrence of regions of suppression and/or enhancement of DPOAE level when the IT was at a frequency slightly less than to more than an octave above f(2). The IRA of the 2f(1)-f(2) DPOAE typically displayed a high-frequency (HF) lobe of suppression, while the 2f(2)-f(1) emission often exhibited considerable amounts of enhancement. Moreover, for the 2f(2)-f(1) DPOAE, when enhancement was absent, its IRA usually tuned to a region above f(2). Whether or not suppression/enhancement was observed depended upon primary-tone level and frequency separation, as well as on the relative levels of the two primaries. Various physiological manipulations involving anesthesia, eighth-nerve section, diuretic administration, or pure-tone overstimulation showed that these phenomena were of cochlear origin, and were not dependent upon the acoustic reflex or cochlear-efferent activity. The aftereffects of applying diuretics or over-exposures revealed that suppression/enhancement required the presence of sensitive, low-level DPOAE-generator sources. Additionally, suppression/enhancement were general effects in that, in addition to rabbits, they were also observed in mice and guinea pigs. Further, corresponding plots of DPOAE phase often revealed areas of differing phase change in the vicinity of the primary tones as compared to regions above f(2). These findings, along with the effects of tonal exposures designed to fatigue regions above f(2), and instances in which DPOAE level was dependent upon the amount of suppression/enhancement, suggested that the interactions of two DPOAE-generator sources contributed, in some manner, to these phenomena.

Evoked otoacoustic emissions arise by two fundamentally different mechanisms: a taxonomy for mammalian OAEs

Otoacoustic emissions (OAEs) of all types are widely assumed to arise by a common mechanism: nonlinear electromechanical distortion within the cochlea. In this view, both stimulus-frequency (SFOAEs) and distortion-product emissions (DPOAEs) arise because nonlinearities in the mechanics act as "sources" of backward-traveling waves. This unified picture is tested by analyzing measurements of emission phase using a simple phenomenological description of the nonlinear re-emission process. The analysis framework is independent of the detailed form of the emission sources and the nonlinearities that produce them. The analysis demonstrates that the common assumption that SFOAEs originate by nonlinear distortion requires that SFOAE phase be essentially independent of frequency, in striking contradiction with experiment. This contradiction implies that evoked otoacoustic emissions arise by two fundamentally different mechanisms within the cochlea. These two mechanisms (linear reflection versus nonlinear distortion) are described and two broad classes of emissions--reflection-source and distortion-source emissions--are distinguished based on the mechanisms of their generation. The implications of this OAE taxonomy for the measurement, interpretation, and clinical use of otoacoustic emissions as noninvasive probes of cochlear function are discussed.