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.     

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.     

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.     

Evidence for two discrete sources of 2f1-f2 distortion-product otoacoustic emission in rabbit: I. Differential dependence on stimulus parameters.

The results of studies of the physiological vulnerability of distortion-product otoacoustic emissions (DPOAEs) suggest that the DPOAE at 2f1-f2 in vertebrate ears is generated by more than one source. The principal aims of the present study were to provide independent evidence for the existence of more than one DPOAE source, and to determine the contributions of each to the ear-canal 2f1-f2 signal. To accomplish these aims, specific stimulus parameters were separately and systematically varied to provide detailed parametric information regarding 2f1-f2 DPOAE amplitude and phase in normal ears of awake rabbits. The findings indicate that two discrete sources, demonstrating differential dependence on stimulus parameters, dominate the generation of the 2f1-f2 DPOAE. One source of distortion is dominant above 60-70 dB SPL at moderate primary-frequency separations, and at all stimulus levels when the primary tones are closely spaced. The other source is dominant below 60-70 dB SPL at moderate primary-frequency separations, and may be dominant at all stimulus levels when the primary tones are widely separated in frequency. The results suggest that by varying stimulus parameters, it may be possible to independently study the two generator mechanisms.     

Amplitude and phase of distortion product otoacoustic emissions in the guinea pig in an (f1 ,f2) area study

Lower sideband distortion product otoacoustic emissions (DPOAEs), measured in the ear canal upon stimulation with two continuous pure tones, are the result of interfering contributions from two different mechanisms, the nonlinear distortion component and the linear reflection component. The two contributors have been shown to have a different amplitude and, in particular, a different phase behavior as a function of the stimulus frequencies. The dominance of either component was investigated in an extensive (f1 ,f2) area study of DPOAE amplitude and phase in the guinea pig, which allows for both qualitative and quantitative analysis of isophase contours. Making a minimum of additional assumptions, simple relations between the direction of constant phase in the (f ,f2) plane and the group delays in f1-sweep, f2-sweep, and fixed f2/f1 paradigms can be derived, both for distortion (wave-fixed) and reflection (place-fixed) components. The experimental data indicate the presence of both components in the lower sideband DPOAEs, with the reflection component as the dominant contributor for low f2/f1 ratios and the distortion component for intermediate ratios. At high ratios the behavior cannot be explained by dominance of either component.     

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.     

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.     

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.     

Investigating the wave-fixed and place-fixed origins of the 2f(1)-f(2) distortion product otoacoustic emission within a micromechanical cochlear model

The 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) arises within the cochlea due to the nonlinear interaction of two stimulus tones (f(1) and f(2)). It is thought to comprise contributions from a wave-fixed source and a place-fixed source. The generation and transmission of the 2f(1)-f(2) DPOAE is investigated here using quasilinear solutions to an elemental model of the human cochlea with nonlinear micromechanics. The micromechanical parameters and nonlinearity are formulated to match the measured response of the cochlea to single- and two-tone stimulation. The controlled introduction of roughness into the active micromechanics of the model allows the wave- and place-fixed contributions to the DPOAE to be studied separately. It is also possible to manipulate the types of nonlinear suppression that occur within the quasilinear model to investigate the influence of stimulus parameters on DPOAE generation. The model predicts and explains a variety of 2f(1)-f(2) DPOAE phenomena: The dependence of emission amplitude on stimulus parameters, the weakness of experiments designed to quantify cochlear amplifier gain, and the predominant mechanism which gives rise to DPOAE fine structure. In addition, the model is used to investigate the properties of the wave-fixed source and how these properties are influenced by the stimulus parameters.     

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.     

In search of basal distortion product generators

The 2f1-f2 distortion product otoacoustic emission (DPOAE) is thought to arise primarily from the complex interaction of components that come from two different cochlear locations. Such distortion has its origin in the nonlinear interaction on the basilar membrane of the excitation patterns resulting from the two stimulus tones, f1 and f2. Here we examine the spatial extent of initial generation of the 2f1-f2 OAE by acoustically traumatizing the base of the cochlea and so eliminating the contribution of the basal region of the cochlea to the generation of 2f1-f2. Explicitly, amplitude-modulated, or continuously varying in level, stimulus tones with f2/f1= 1.2 and f2 =8000-8940 Hz were used to generate the 2f1-f2 DPOAE in guinea pig before and after acoustically traumatizing the basal region of the cochlea (the origin of any basal-to-f2 distortion product generators). It was found, based on correlation analysis, that there does not appear to be a basal-to-f2 distortion product generation mechanism contributing significantly to the guinea pig 2f1-f2 OAE up to L1 = 80 dB sound pressure level (SPL).     

Evidence for a bipolar change in distortion product otoacoustic emissions during contralateral acoustic stimulation in humans

The aim of this study was to investigate the activity of the medial olivocochlear (MOC) efferents during contralateral (CAS) and ipsilateral acoustic stimulation (IAS) by recording distortion product otoacoustic emission (DPOAE) suppression and DPOAE adaptation in humans. The main question was: do large bipolar changes in DPOAE level (transition from enhancement to suppression) also occur in humans when changing the primary tone level within a small range as described by Maison and Liberman for guinea pigs [J. Neurosci. 20, 4701-4707 (2000)]? In the present study, large bipolar changes in DPOAE level (14 dB on average across subjects) were found during CAS predominantly at frequencies where dips in the DPOAE fine structure occurred. Thus, effects of the second DPOAE source might be responsible for the observed bipolar effect. In contrast, comparable effects were not found during IAS as was reported in guinea pigs. Reproducibility of CAS DPOAEs was better than that for IAS DPOAEs. Thus, contralateral DPOAE suppression is suggested to be superior to ipsilateral DPOAE adaptation with regard to measuring the MOC reflex strength and for evaluating the vulnerability of the cochlea to acoustic overexposure in a clinical context.     

One source for distortion product otoacoustic emissions generated by low- and high-level primaries

Distortion product otoacoustic emissions (DPOAE) elicited by tones below 60-70 dB sound pressure level (SPL) are significantly more sensitive to cochlear insults. The vulnerable, low-level DPOAE have been associated with the postulated active cochlear process, whereas the relatively robust high-level DPOAE component has been attributed to the passive, nonlinear macromechanical properties of the cochlea. However, it is proposed that the differences in the vulnerability of DPOAEs to high and low SPLs is a natural consequence of the way the cochlea responds to high and low SPLs. An active process boosts the basilar membrane (BM) vibrations, which are attenuated when the active process is impaired. However, at high SPLs the contribution of the active process to BM vibration is small compared with the dominating passive mechanical properties of the BM. Consequently, reduction of active cochlear amplification will have greatest effect on BM vibrations and DPOAEs at low SPLs. To distinguish between the "two sources" and the "single source" hypotheses we analyzed the level dependence of the notch and corresponding phase discontinuity in plots of DPOAE magnitude and phase as functions of the level of the primaries. In experiments where furosemide was used to reduce cochlear amplification, an upward shift of the notch supports the conclusion that both the low- and high-level DPOAEs are generated by a single source, namely a nonlinear amplifier with saturating I/O characteristic.     

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.     

Modeling the growth rate of distortion product otoacoustic emissions by active nonlinear oscillators

In this work, growth-rate curves of the 2 f1-f2 distortion product otoacoustic emission (DPOAE) are analyzed in a population of 30 noise exposed subjects, including both normal-hearing and hearing impaired subjects. A particular embedded limit-cycle oscillator equation is used to model the cochlear resonant response at the cochlear places of the primary and secondary tone frequencies (f2 and 2 f1-f2). The parameters of the oscillator equation can be directly interpreted in terms of effectiveness of the cochlear feedback mechanisms associated with the active filter amplification. A two-sources paradigm is included in the model, in agreement with experimental evidence and with the assumptions of more detailed full cochlear models based on the transmission line formalism. According to this paradigm, DPOAEs are nonlinearly generated at the cochlear place that is resonant at frequency f2, and coherently reflected at the 2 f1-f2 place. The analysis shows that the model, which had been previously used to describe the relaxation dynamics of transient evoked otoacoustic emissions (TEOAEs), also correctly predicts the observed growth rate of the DPOAE response as a function of the primary tones amplitude. A significant difference is observed between normal and impaired ears. The comparison between the growth rate curves at different frequencies provides information about the dependence of cochlear tuning on frequency.     

Physiopathological significance of distortion-product otoacoustic emissions at 2f1-f2 produced by high- versus low-level stimuli

Distortion product otoacoustic emissions emitted by the cochlea at 2f1-f2 in response to pairs of pure tones at f1 and f2 (DPOAE) form a class of otoacoustic emissions and as such, are viewed as a reliable tool for screening outer hair cell (OHC) dysfunctions on a pass/fail basis. However, the persistence of residual DPOAEs from impaired cochleae at high stimulus levels has suggested that above 60-70 dB SPL, instead of reflecting "active" cochlear motion, DPOAEs might represent another "passive" modality: they would thus become unsuitable for analyzing cochlear function. The present work reports the consequences on high- vs low-level DPOAEs of three types of cochlear impairments involving OHCs: progressive OHC degeneration of genetic origin in CD1 mice, complete cochlear ischemia in gerbils, and furosemide injection vs ischemia-reperfusion in gerbils. An alternative to the "active-passive" model was used wherein regardless of stimulus level, cubic DPOAEs are produced by N (probably OHC-borne) nonlinear elements driven by input I and modulated by a function F3 of their operating point o; thus, DPOAE proportional to NI3F3(o). When OHCs degenerated, thereby implying a decrease of N, DPOAE levels also decreased regardless of the stimulus level up to 80 dB SPL, in line with the previous formula but at variance with the prediction of the active-passive concept. Instead of affecting N, the other two experiments impaired the efficiency of the cochlear feedback loop as a result of its electrical drive being decreased by strial dysfunction. As it is well accepted that the impaired basilar-membrane motion, although greatly reduced at low levels, tends to catch up with a normal one at higher levels, it was assumed the same was true with I so that DPOAE levels had to be, and indeed were little affected at high levels while plummeting at low levels, without any need for invoking two modalities for DPOAE generation. Finally, comparisons of furosemide vs ischemia effects revealed additional influences on DPOAEs, possibly accounted for by function F3(o). These results lead to the proposal that although high-level DPOAEs are expected to be poor audiometric indicators, they seem well adapted to assessing the functional integrity of nonlinear elements in OHCs, i.e., presumably their mechanoelectrical transduction channels.     

Evidence for two discrete sources of 2f1-f2 distortion-product otoacoustic emission in rabbit. II: Differential physiological vulnerability.

In a previous report, it was shown that, in normal rabbit ears, the amplitude and phase of 2f1-f2 distortion-product otoacoustic emissions (DPOAEs) elicited by low-level (< 60-70 dB SPL) stimuli display a differential dependence on stimulus parameters to those evoked by high-level (> 60-70 dB SPL) stimuli, indicating differences in the underlying generation mechanisms. In the present study, the physiological vulnerability of DPOAEs in each of the two 2f1-f2 DPOAE-response regions identified on the basis of differential parametric properties, was characterized. Thus emissions evoked using stimulus levels from 45-75 dB SPL were measured over time upon: (1) induction of lethal anoxia, (2) acute injection of ethacrynic acid, and (3) acute injection of ethacrynic acid 2 h after a single administration of gentamicin. The DPOAEs evoked by low-level stimuli (45 dB SPL) were abolished within 3-4 min of induction of anoxia, whereas DPOAEs evoked by high-level stimuli (75 dB SPL) were unchanged in this period. The high-level emissions decreased with a complex time course postmortem, and demonstrated behaviors, including evidence of susceptibility to fatigue, suggesting a dependence upon a cochlear energy supply. Low-level DPOAEs could be temporarily abolished, with complete recovery, by an acute administration of ethacrynic acid that had little effect on high-level DPOAEs. Treatment with the gentamicin and ethacrynic-acid combination, which would be expected to produce widespread hair-cell damage, eliminated low-level DPOAEs, and greatly reduced high-level emissions. In combination with previously published data, these findings strongly suggest that low- and high-level 2f1-f2 DPOAEs arise from discrete sources. The data are consistent with the proposal that the low-level DPOAE source is an active, micromechanical process, but suggest that the proposed origin of high-level DPOAEs exclusively in the passive macromechanics of the cochlear partition may be incorrect. The elimination of both low- and high-level DPOAEs revealed the presence of a third, residual 2f1-f2 DPOAE component, approximately 75-80 dB below the stimulus-tone levels, that may reflect the true passive-distortion response of the cochlea.     

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.     

Acoustic distortion products in humans: systematic changes in amplitudes as a function of f2/f1 ratio

The effects of primary-tone separation on the amplitude of distortion-product emissions (DPEs) at the 2f1-f2 frequency were systematically examined in ten ears of five subjects. All individuals had normal hearing and middle-ear function based upon standard clinical measures. Acoustic-distortion products were elicited at 1, 2.5, and 4 kHz by equilevel primaries at 65, 75, and 85 dB SPL, while f2/f1 ratios were varied in 0.02 increments from 1.01-1.41 (4 kHz), 1.01-1.59 (2.5 kHz), or 1.01-1.79 (1 kHz). A principal outcome reflected in the detailed structure of both average and individual ratio functions was a nonmonotonic change in DPE amplitude as the ratio of f2/f1 increased. Despite the presence of amplitude nonmonotonicities, there was clearly a region of f1 and f2 separation that generated a maximum DPE. The effects of primary-tone separation on DPE amplitudes were systematically related to DPE frequency and primary-tone level. For all three levels of stimulation, the f2/f1 ratio was inversely related to DPE frequency. Thus larger ratios reflecting a greater separation of f1 and f2 were more effective in generating DPEs at 1 kHz rather than at 4 kHz. The optimal ratio for 2.5 kHz fell at an intermediate value. Conversely, acoustic distortion-product amplitude as a function of primary-tone level was directly related to the frequency separation of the primary tones. Regardless of the frequency region of the primary tones, smaller f2/f1 ratios were superior in generating DPEs in response to 65-dB stimuli, whereas larger ratios elicited bigger DPEs with primaries at 75 and 85 dB SPL. Within any specific stimulus-parameter combination, individual variability in DPE amplitude was noted. When all stimulus conditions describing the variations in frequency and level were considered, an f2/f1 ratio of 1.22 was most effective in maximizing DPE amplitude.