Influence of primary frequencies ratio on distortion product otoacoustic emissions amplitude. II. Interrelations between multicomponent DPOAEs, tone-burst-evoked OAEs, and spontaneous OAEs

Distortion product otoacoustic emissions (DPOAEs) are used widely in humans to assess cochlear function. It is well known that 2f1-f2 DPOAE amplitude increases as the f2/f1 ratio increases from 1.0 to about 1.20, and then decreases as the f2/f1 ratio increases above 1.20, showing an amplitude ratio function, which is thought to be related to cochlear filtering properties. Different lower sideband DPOAEs are believed to show the same amplitude ratio functions as the 2f1-f2 DPOAE, with a magnitude peak situated at a constant DPOAE frequency relative to f2. More recently, several studies have suggested the involvement of a DPOAE component coming from its own distortion product place as well as the DPOAE component coming from the f2 place. To investigate DPOAE generation sites and the importance of the DPOAE frequency place, amplitude ratio functions of 2f1-f2, 3f1-2f2, 4f1-3f2 and 2f2-f1, 3f2-2f1, 4f2-3f1 DPOAE components have been systematically studied in 18 normally hearing subjects, using an f2 fixed, f1 sweep method, and an f1 fixed, f2 sweep method, at ten different f2 frequencies. Results show a dependency of the distortion magnitude peak on f2 frequency for each lower sideband DPOAE, and a small frequency shift of the distortion peak for the high order lower sideband DPOAE components. Strong correlation between the different lower sideband DPOAE amplitude were obtained, whether they were recorded with the same f1 (and a different f2) or with the same f2 (and a different f1), suggesting that lower side-band DPOAE amplitude does not depend on small variations in the f2 frequency. Moreover, correlations between DPOAE amplitude and tone-burst evoked otoacoustic emissions (TBOAEs) are highly significant for TBOAEs centered at the f2 frequency and at 1/2 octave below the f2 frequency, suggesting some degree of importance of the cochlear status at frequencies below f2 in DPOAE amplitude. Subjects presenting spontaneous otoacoustic emissions showed a greater lower sideband DPOAE amplitude recorded for low f2/f1 ratios, and a distortion magnitude peak shifted towards higher frequencies. The best correlation between upper sideband DPOAE amplitude and lower sideband DPOAE amplitude occurred for lower sideband DPOAEs generated by an f2 frequency 1/2 octave to 1 octave below the primaries used to generate upper sideband DPOAEs, suggesting a site of generation basal to f2 for the upper sideband DPOAEs. Correlations between TBOAE amplitude and upper sideband DPOAE amplitude agreed with a site of upper sideband DPOAE generation basal to f2, and which would move with the DPOAE frequency itself.     

Ipsilateral distortion product otoacoustic emission (2f1-f2) suppression in children with sensorineural hearing loss

Distortion product otoacoustic emission (DPOAE) ipsilateral suppression has been applied to study cochlear function and maturation in laboratory animals and humans. Although DPOAE suppression appears to be sensitive to regions of specialized cochlear function and to cochlear immaturity, it is not known whether it reflects permanent cochlear damage, i.e., sensorineural hearing loss (SNHL), in a reliable and systematic manner in humans. Eight school-aged children with mild-moderate SNHL and 20 normal-hearing children served as subjects in this study. DPOAE (2f1-f2) suppression data were collected at four f2 frequencies (1500, 3000, 4000, and 6000 Hz) using moderate-level primary tones. Features of the DPOAE iso-suppression tuning curves and suppression growth were analyzed for both subject groups. Results show that DPOAE suppression tuning curves from hearing-impaired subjects can be reliably recorded. DPOAE suppression tuning curves were generally normal in appearance and shape for six out of eight hearing-impaired subjects but showed subtle abnormalities in at least one feature. There was not one single trend or pattern of abnormality that characterized all hearing-impaired subjects. The most prominent patterns of abnormality included: broadened tuning, elevated tip, and downward shift of tip frequency. The unique patterns of atypical DPOAE suppression in subjects with similar audiograms may suggest different patterns of underlying sensory cell damage. This speculation warrants further investigation.     

Influence of primary frequencies ratio on distortion product otoacoustic emissions amplitude. I. Intersubject variability and consequences on the DPOAE-gram

Distortion product otoacoustic emissions (DPOAEs) are used widely in humans to assess cochlear function. The standard procedure consists of recording the 2f1-f2 DPOAE amplitude as a function of the f2 frequency, using a fixed f2/f1 ratio (DPOAE-gram), close to 1.20. DPOAE amplitude, as recorded in the DPOAE-gram, shows a wide range of values in normal-hearing subjects, which can impair the predictive value of the DPOAE-gram for hearing thresholds. This study is aimed at comparing intersubject variability in 2f1-f2 DPOAE amplitude according to three paradigms: a fixed f2/f1 ratio, such as the DPOAE-gram, a variable ratio DPOAE-gram (f2/f1 adapted to frequency) and an "optimum" DPOAE-gram, where the f2/f1 is adapted both to subject and frequency. The 2f1-f2 DPOAE amplitude has been investigated on 18 normally hearing subjects at ten different f2 frequencies (from 0.75 to 6 kHz), using an f2 fixed, f1 sweep paradigm, and allowed to define, for each frequency, the f2/f1 ratio giving the greatest 2f1-f2 DPOAE amplitude (or optimum ratio). Results showed a large intersubject variability of the optimum ratio, especially at frequencies below 1.5 kHz, and a significant decrease of the optimum ratio with frequency. The optimum DPOAE-gram was underestimated by up to 5.8 dB on average (up to 14.9 dB for an individual subject) by the fixed ratio DPOAE-gram, and by up to 3 dB on average (up to 10.6 dB for an individual subject) by the variable ratio DPOAE-gram. Intersubject variability was slightly but significantly reduced in the optimum DPOAE-gram versus the fixed-ratio DPOAE-gram. Lastly, correlations between tone-burst evoked otoacoustic emission (TBOAE) amplitudes and maximum DPOAE amplitudes were significantly greater than correlations between TBOAE amplitudes and fixed-ratio DPOAE amplitudes.     

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.     

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.     

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.     

Distortion product otoacoustic emissions provide clues hearing mechanisms in the frog ear

2f1-f2 and 2 f2-f1 distortion product otoacoustic emissions (DPOAEs) were recorded from both ears of male and female Rana pipiens pipiens and Rana catesbeiana. The input-output (I/O) curves obtained from the amphibian papilla (AP) of both frog species are analogous to I/O curves recorded from mammals suggesting that, similarly to the mammalian cochlea, there may be an amplification process present in the frog AP. DPOAE level dependence on L1-L2 is different from that in mammals and consistent with intermodulation distortion expectations. Therefore, if a mechanical structure in the frog inner ear is functioning analogously to the mammalian basilar membrane, it must be more broadly tuned. DPOAE audiograms were obtained for primary frequencies spanning the animals' hearing range and selected stimulus levels. The results confirm that DPOAEs are produced in both papillae, with R. catesbeiana producing stronger emissions than R. p. pipiens. Consistent with previously reported sexual dimorphism in the mammalian and anuran auditory systems, females of both species produce stronger emissions than males. Moreover, it appears that 2 f1-f2 in the frog is generated primarily at the DPOAE frequency place, while 2 f2-f1 is generated primarily at a frequency place around the primaries. Regardless of generation place, both emissions within the AP may be subject to the same filtering mechanism, possibly the tectorial membrane.     

Pure-tone threshold estimation from extrapolated distortion product otoacoustic emission I/O-functions in normal and cochlear hearing loss ears

A new method for direct pure-tone threshold estimation from input/output functions of distortion product otoacoustic emissions (DPOAEs) in humans is presented. Previous methods use statistical models relating DPOAE level to hearing threshold including additional parameters e.g., age or slope of DPOAE I/O-function. Here we derive a DPOAE threshold from extrapolated DPOAE I/O-functions directly. Cubic 2 f1-f2 distortion products and pure-tone threshold at f2 were measured at 51 frequencies between f2=500 Hz and 8 kHz at up to ten primary tone levels between L2=65 and 20 dB SPL in 30 normally hearing and 119 sensorineural hearing loss ears. Using an optimized primary tone level setting (L1 = 0.4L2 + 39 dB) that accounts for the nonlinear interaction of the two primaries at the DPOAE generation site at f2, the pressure of the 2 f1-f2 distortion product pDP is a linear function of the primary tone level L2. Linear regression yields correlation coefficients higher than 0.8 in the majority of the DPOAE I/O-functions. The linear behavior is sufficiently fulfilled for all frequencies in normal and impaired hearing. This suggests that the observed linear functional dependency is quite general. Extrapolating towards pDP=0 yields the DPOAE threshold for L2. There is a significant correlation between DPOAE threshold and pure-tone threshold (r=0.65, p<0.001). Thus, the DPOAEs that reflect the functioning of an essential element of peripheral sound processing enable a reliable estimation of cochlear hearing threshold up to hearing losses of 50 dBHL without any statistical data.     

Distortion product otoacoustic emission test performance when both 2f1-f2 and 2f2-f1 are used to predict auditory status

The objective of this study was to determine whether distortion product otoacoustic emission (DPOAE) test performance, defined as its ability to distinguish normal-hearing ears from those with hearing loss, can be improved by examining response and noise amplitudes at 2 f1-f2 and 2f2-f1 simultaneously. In addition, there was interest in knowing whether measurements at both DPs and for several primary frequency pairs can be used in a multivariate analysis to further optimize test performance. DPOAE and noise amplitudes were measured at 2f1-f2 and 2 f2-f1 for 12 primary levels (L2 from 10 to 65 dB SPL in 5-dB steps) and 9 pairs of primary frequencies (0.5 to 8 kHz in 1/2-octave steps). All data were collected in a sound-treated room from 70 subjects with normal hearing and 80 subjects with hearing loss. Subjects had normal middle-ear function at the time of the DPOAE test, based on standard tympanometric measurements. Measurement-based stopping rules were used such that the test terminated when the noise floor around the 2 f1-f2 DP was < or = -30 dB SPL or after 32 s of artifact-free averaging, whichever occurred first. Data were analyzed using clinical decision theory in which relative operating characteristics (ROC) curves were constructed and areas under the ROC curves were estimated. In addition, test performance was assessed by selecting the criterion value that resulted in a sensitivity of 90% and determining the specificity at that criterion value. Data were analyzed using traditional univariate comparisons, in which predictions about auditory status were based only on data obtained when f2 = audiometric frequency. In addition, multivariate analysis techniques were used to determine whether test performance can be optimized by using many variables to predict auditory status. As expected, DPOAEs were larger for 2f1-f2 compared to 2 f2-f1 in subjects with normal hearing. However, noise amplitudes were smaller for 2f2-f1, but this effect was restricted to the lowest f2 frequencies. A comparison of signal-to-noise ratios (SNR) within normal-hearing ears showed that the 2f1-f2 DP was more frequently characterized by larger SNRs compared to 2f2-f1. However, there were several subjects in whom 2f2-f1 produced a larger SNR. ROC curve areas and specificities for a fixed sensitivity increased only slightly when data from both DPs were used to predict auditory status. Multivariate analyses, in which the inputs included both DPs for several primary frequency pairs surrounding each audiometric frequency, produced the highest areas and specificities. Thus, DPOAE test performance was improved slightly by examining data at two DP frequencies simultaneously. This improvement was achieved at no additional cost in terms of test time. When measurements at both DPs were combined with data obtained for several primary frequency pairs and then analyzed in a multivariate context, the best test performance was achieved. Excellent test performance (ROC) curve areas >0.95% and specificities >92% at all frequencies, including 500 Hz, were achieved for these conditions. Although the results described should be validated on an independent set of data, they suggest that the accuracy with which DPOAE measurements identify auditory status can be improved with multivariate analyses and measurements at multiple DPs.     

Distortion product otoacoustic emissions and basilar membrane vibration in the 6-9 kHz region of sensitive chinchilla cochleae

Distortion product otoacoustic emissions (DPOAEs) and basilar membrane (BM) vibration were measured simultaneously in the 6-9 kHz region of chinchilla cochleae. BM-Input-Output functions in a two-tone paradigm behaved similarly to DPOAEs for the 2f1-f2 component, nonmonotonic growth with the intensity of the lower frequency primary and a notch in the functions around 60 dB SPL. Ripples in frequency functions occur in both BM and OAE curves as a function of the distortion frequency. Optimum f2/f1 ratios for DPOAE generation are near 1.2. The slope of phase curves indicates that for low f2f1(<1.1) the emission source is the place location while for f2f1>1.1 the relative constancy of the phase function suggests that the place is the nonlinear region of f2, i.e., the wave location. Magnitudes of the DPOAEs increase rapidly above 60 dB SPL suggesting a different source or mechanism at high levels. This is supported by the observation that the high level DPOAE and BM-DP responses remain for a considerable period postmortem.     

Mechanisms of generation of the 2f2-f1 distortion product otoacoustic emission in humans

The 2f1-f2 distortion product otoacoustic emission (DPOAE) is considered to consist of two components in normally hearing ears, one having constant phase with changing DP frequency (wave fixed) and one having an increasing phase lag with increasing frequency (place fixed). The aim was to identify the wave-fixed and place-fixed components of both 2f1-f2 and 2f2-f1 DPs, and, in particular, to show whether a wave-fixed 2f2-f1 DP exists in normally hearing adults. DPOAE recordings were made in 20 ears of normally hearing young adults. Four frequency ratios were used and recording entailed fixed frequency-ratio sweeps. A separation into wave-fixed and place-fixed components was carried out using a time-window separation method. A method for estimating the noise floor after data processing was developed. Results confirmed the presence of wave-fixed and place-fixed components for 2f1-f2, consistent with previous studies. Both components were also present for 2f2-f1 in virtually all subjects. This latter finding conflicts with current models of DPOAE generation, and so a modified model is proposed. Unlike the 2f1-f2 emission, which has a wave-fixed component that is strongly dependent on the frequency ratio, neither component of the 2f2-f1 emission showed such a dependence. The proposed model explains these findings in terms of the overlap of the primary frequency traveling waves.     

Hybrid measurement of auditory steady-state responses and distortion product otoacoustic emissions using an amplitude-modulated primary tone

A maximum auditory steady-state response (ASSR) amplitude is yielded when the ASSR is elicited by an amplitude-modulated tone (f(c)) with a fixed modulation frequency (f(m) = 40 Hz), whereas the maximum distortion product otoacoustic emission (DPOAE) level is yielded when the DPOAE is elicited using a fixed frequency ratio of the primary tones (f2/f1 = 1.2). When eliciting the DPOAE and ASSR by the same tone pair, optimal stimulation is present for either DPOAE or ASSR and thus adequate simultaneous DPOAE/ASSR measurement is not possible across test frequency f2 or f(c), respectively. The purpose of the present study was to determine whether the ASSR and DPOAE can be measured simultaneously without notable restrictions using a DPOAE stimulus setting in which one primary tone is amplitude modulated. A DPOAE of frequency 2f1-f2 and ASSR of modulation frequency 41 Hz were measured in ten normal hearing subjects at a test frequency between 0.5 and 8 kHz (f2 = f(c)). The decrease in the DPOAE level and the loss in ASSR amplitude during hybrid mode stimulation amounted, on average, to only 2.60 dB [standard deviation (SD) = 1.38 dB] and 1.83 dB (SD = 2.38 dB), respectively. These findings suggest simultaneous DPOAE and ASSR measurements to be feasible across all test frequencies when using a DPOAE stimulus setting where the primary tone f2 is amplitude modulated.     

Group delays of distortion product otoacoustic emissions in the guinea pig.

This paper presents a comprehensive set of experimental data on group delays of distortion product otoacoustic emissions (DPOAEs) in the guinea pig. Group delays of the DPOAEs with frequencies 2f1-f2, 3f1-2f2, 4f1-3f2, and 2f2-f1 were measured with the phase gradient method. Both the f1- and the f2-sweep paradigm were used. Differences between the two sweep paradigms were investigated for the four DPOAEs, as well as the group delay differences between the DPOAEs. Analysis revealed larger group delays with the f2-sweep paradigm, but only for the lower sideband DPOAEs (with fdp < f1,f2). For the lower sideband cubic distortion product 2f1-f2, the f2-sweep delays were a factor of 1.17-1.54 larger than the f1-sweep delays, depending on frequency. The upper sideband DPOAE 2f2-f1 showed no significant difference between f1- and f2-sweep group delays, except for the highest and lowest f2 frequencies. Comparing the group delays of the DPOAEs for each sweep paradigm separately, equal group delays were found for all four DPOAEs measured with the f1-sweep. With the f2-sweep paradigm on the other hand, the group delays of the three lower sideband DPOAEs occurred to be larger than the group delays of the upper sideband DPOAE 2f2-f1. A tentative interpretation of the data in the context of proposed explanatory hypotheses on DPOAE group delays is given.     

Breaking away: violation of distortion emission phase-frequency invariance at low frequencies

The phase versus frequency function of the distortion product otoacoustic emission (DPOAE) at 2f(1) - f(2) is approximately invariant at frequencies above 1.5 kHz in human subjects when recorded with a constant f(2)/f(1). However, a secular break from this invariance has been observed at lower frequencies where the phase-gradient becomes markedly steeper. Apical DPOAEs, such as 2f(1)?- f(2), are known to contain contributions from multiple sources. This experiment asked whether the phase behavior of the ear canal DPOAE at low frequencies is driven by the phase of the component from the distortion product (DP) region at 2f(1)?- f(2), which exhibits rapid phase accumulation. Placing a suppressor tone close in the frequency to 2f(1)?- f(2) reduced the contribution of this component to the ear canal DPOAE in normal-hearing adult human ears. When the contribution of this component was reduced, the phase behavior of the ear canal DPOAE was not altered, suggesting that the breaking from DPOAE phase invariance at low frequencies is an outcome of apical-basal differences in cochlear mechanics. The deviation from DPOAE phase invariance appears to be a manifestation of the breaking from approximate scaling symmetry in the human cochlear apex.     

Maturation of cochlear nonlinearity as measured by distortion product otoacoustic emission suppression growth in humans

The growth of distortion product otoacoustic emission (DPOAE) suppression follows a systematic, frequency-dependent pattern. The pattern is consistent with direct measures of basilar-membrane response growth, psychoacoustic measures of masking growth, and measures of neural rate growth. This pattern has its basis in the recognized nonlinear properties of basilar-membrane motion and, as such, the DPOAE suppression growth paradigm can be applied to human neonates to study the maturation of cochlear nonlinearity. The objective of this experiment was to investigate the maturation of human cochlear nonlinearity and define the time course for this maturational process. Normal-hearing adults, children, term-born neonates, and premature neonates, plus a small number of children with sensorineural hearing loss, were included in this experiment. DPOAE suppression growth was measured at two f2 frequencies (1500 and 6000 Hz) and three primary tone levels (55-45, 65-55, and 75-65 dB SPL). Slope of DPOAE suppression growth, as well as an asymmetry ratio (to compare slope for suppressor tones below and above f2 frequency), were generated. Suppression threshold was also measured in all subjects. Findings indicate that both term-born neonates and premature neonates who have attained term-like age, show non-adult-like DPOAE suppression growth for low-frequency suppressor tones. These age effects are most evident at f2 = 6000 Hz. In neonates, suppression growth is shallower and suppression thresholds are elevated for suppressor tones lower in frequency than f2. Additionally, the asymmetry ratio is smaller in neonates, indicating that the typical frequency-dependent pattern of suppression growth is not present. These findings suggest that an immaturity of cochlear nonlinearity persists into the first months of postnatal life. DPOAE suppression growth examined for a small group of hearing-impaired children also showed abnormalities.     

Origin of the bell-like dependence of the DPOAE amplitude on primary frequency ratio.

For low and medium sound pressure levels (SPLs), the amplitude of the distortion product otoacoustic emission (DPOAE) recorded from guinea pigs at the 2f1-f2 frequency is maximal when f2/f1 approximately 1.23 and decreases for lower and higher f2/f1 ratios. The high-ratio slope of the DPOAE dependence on the ratio of the primary frequencies might be anticipated since the f1 amplitude at the f2 place is expected to decrease for higher f2/f1 ratios. The low-ratio slope of the dependence at low and medium SPLs of the primaries is actually one slope of a notch. The DPOAE amplitude recovers from the notch when the f2/f1 ratio is further reduced. In two-dimensional space formed by the f2/f1 ratio, and the levels of the primaries, the notch is continuous and has a level-dependent phase transition. The notch is identical to that seen in DPOAE growth functions. Similar notches and phase transitions were observed for high-order and high-frequency DPOAEs. Theoretical analysis reveals that a single saturating nonlinearity is capable of generating similar amplitude notch and phase transition when the f2/f1 ratio is decreased because of the increase in f1 amplitude at the DPOAE generation place (f2 place). The difference between the DPOAE recorded from guinea pigs and humans is discussed in terms of different position of the operating point of the DPOAE generating nonlinearity.     

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.     

Suppression of distortion product otoacoustic emissions and hearing threshold

A distortion product otoacoustic emission (DPOAE) suppression tuning curve (STC) shows the minimum level of suppressor tone that is required to reduce DPOAE level by a fixed amount, as a function of suppressor frequency. Several years ago, Mills [J. Acoust. Soc. Am. 103, 507-523 (1998)] derived, theoretically, an approximately linear relationship between the tip-to-tail suppressor level difference on a DPOAE STC, and the gain of the cochlear amplifier, defined as the maximum increase in the active over the passive basilar membrane (BM) response. In this paper, preliminary data from adult human subjects are presented that establish a correlation between this tip-to-tail DPOAE STC difference and the threshold of hearing, the latter measured at the frequency of the f2 primary tone. Assuming that both suppression and the DPOAE are by-products of active, nonlinear BM dynamics, the above result suggests that threshold elevation in mild levels of hearing loss may be attributed, in part, to a reduction of cochlear amplifier gain, which is detectable with the suppression paradigm.