Interpretation of distortion product otoacoustic emission measurements. I. Two stimulus tones

Minima in cubic distortion-product otoacoustic emission input/output functions due to distributed primary sources

Input/output (I/O) functions of distortion-product otoacoustic emissions (DPOAEs) may contain sudden amplitude minima (notches) although they are measured in animals with a negligible reflection source. We measured DPOAEs in humans and analyzed the data by time-frequency filtering to decompose the nonlinear-distortion and coherent-reflection components of DPOAE. The presented I/O functions of the nonlinear-distortion component contain notches. We suggest that because these notches are present only in the nonlinear-distortion component, they result from destructive interference between distortion-product wavelets coming from the primary generation region. Simulations conducted with a nonlinear cochlear model showed qualitative similarities with the presented experimental results.

The influence of distributed source regions in the formation of the nonlinear distortion component of cubic distortion-product otoacoustic emissions

Distortion product otoacoustic emissions (DPOAEs) are evoked by two stimulus tones with frequency f₁ and f₂ of ratio f₂/f₁  in the range between approximately 1.05 and 1.4. This study theoretically and experimentally analyzes the cubic 2f₁-f₂ DPOAE for different stimulus levels of one of the tones while the other is constant. Simulations for f₂/f₁ of 1.2 and moderate stimulus levels (30-70 dB sound pressure level) indicate that cubic distortion products are generated along a relatively large length of the basilar membrane, the extent of which increases with stimulus level. However, apical from the place of maximum nonlinear force, the wavelets generated by these distributed sources mutually cancel. Therefore, although the spatial extent of the primary DPOAE sources broadens with increasing stimulus level (up to 1.5 oct), the basilar-membrane region contributing to the DPOAE signal is relatively narrow (0.6 oct) and level independent. The observed dependence of DPOAE amplitude on stimulus level can be well-approximated by a point source at the basilar-membrane place where the largest distortion product (maximum of the nonlinear force) is generated. Onset and offset of the DPOAE signal may contain amplitude overshoots (complexities), which are in most cases asymmetrical. Two-tone suppression was identified as the main cause of these onset and offset complexities. DPOAE measurements in two normal-hearing subjects support the level dependence of the steady-state DPOAE amplitude and the asymmetry in the onset and offset responses predicted by the theoretical analysis.

Utility of auditory steady-state and brainstem responses in age-related hearing loss in rats

CONCLUSIONS

The results support the idea that auditory steady-state response (ASSR) is a more accurate test for studying age-related hearing loss (ARHL) in Sprague-Dawley rats. Differences in the rat middle ear may explain the variations of the click properties, with a displacement of the energy toward the 8 and 10 kHz frequencies compared with humans.

OBJECTIVES

The purpose of this study was to evaluate ARHL in older and younger Sprague-Dawley rats using auditory clicks and tone burst with auditory brainstem response (ABR), in addition to ASSR.

METHODS

This was a prospective cohort study with 50 animals divided into 5 groups based on their age in months. A total of 100 registers were elicited from each one of the 3 auditory measurements systems in an electrically shielded, double-walled, sound-treated cabin. Nine frequencies, from 0.5 to 16 kHz were analyzed with the auditory steady-state response and compared with the results elicited by the clicks and tone-burst ABR.

RESULTS

Comparisons between the different frequencies showed lower thresholds in those frequencies below 2 kHz, independently of their age in months. The ARHL was detected by each one of the three auditory measurement systems, but with lower thresholds with the ASSR test. Finally, auditory clicks showed better correlations with 8 and 10 kHz elicited by ASSR, which was different to what was expected, based on human studies.

Influence of ketamine-xylazine anaesthesia on cubic and quadratic high-frequency distortion-product otoacoustic emissions

Ketamine is a dissociative anaesthetic, analgesic drug as well as an N-methyl-D-aspartate receptor antagonist and has been reported to influence otoacoustic emission amplitudes. In the present study, we assess the effect of ketamine-xylazine on high-frequency distortion-product otoacoustic emissions (DPOAE) in the bat species Carollia perspicillata, which serves as model for sensitive high-frequency hearing. Cubic DPOAE provide information about the nonlinear gain of the cochlear amplifier, whereas quadratic DPOAE are used to assess the symmetry of cochlear amplification and potential efferent influence on the operating state of the cochlear amplifier. During anaesthesia, maximum cubic DPOAE levels can increase by up to 35 dB within a medium stimulus level range from 35 to 60 dB SPL. Close to the -10 dB SPL threshold, at stimulus levels below about 20-30 dB SPL, anaesthesia reduces cubic DPOAE amplitudes and raises cubic DPOAE thresholds. This makes DPOAE growth functions steeper. Additionally, ketamine increases the optimum stimulus frequency ratio which is indicative of a reduction of cochlear tuning sharpness. The effect of ketamine on cubic DPOAE thresholds becomes stronger at higher stimulus frequencies and is highly significant for f2 frequencies above 40 kHz. Quadratic DPOAE levels are increased by up to 25 dB by ketamine at medium stimulus levels. In contrast to cubic DPOAEs, quadratic DPOAE threshold changes are variable and there is no significant loss of sensitivity during anaesthesia. We discuss that ketamine effects could be caused by modulation of middle ear function or a release from ipsilateral efferent modulation that mainly affects the gain of cochlear amplification.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Time-domain demonstration of distributed distortion-product otoacoustic emission components

Distortion-product otoacoustic emissions (DPOAEs) were measured in rabbits as time waveforms by employing a phase-rotation technique to cancel all components in the final average, except the 2f1-f2 DPOAE. Subsequent filtering allowed the DPOAE waveform to be clearly visualized in the time domain. In most conditions, f2 was turned off for 6 ms, which produced a gap so that the DPOAE was no longer generated. These procedures allowed the DPOAE onset as well as the decay during the gap to be observed in the time domain. DPOAEs were collected with L1 = L2 = 65-dB sound pressure level primary-tone levels for f2/f1 ratios from 1.25 to 1.01 in 0.02 steps. Findings included the appearance of complex onsets and decays for the DPOAE time waveforms as the f2/f1 ratio was decreased and the DPOAE level was reduced. These complexities were unaffected by interference tones (ITs) near the DPOAE frequency place (fdp), but could be removed by ITs presented above f2, which also increased DPOAE levels. Similar outcomes were observed when DPOAEs were measured at a sharp notch in the DPOAE level as a function of the f2 primary tone frequency, i.e., DP-gram. Both findings were consistent with the hypothesis that the DPOAE-ratio function, and some notches in the DP-gram, are caused by interactions of distributed DPOAE components with unique phases.

The mouse cochlea expresses a local hypothalamic-pituitary-adrenal equivalent signaling system and requires corticotropin-releasing factor receptor 1 to establish normal hair cell innervation and cochlear sensitivity

Cells of the inner ear face constant metabolic and structural stress. Exposure to intense sound or certain drugs destroys cochlea hair cells, which in mammals do not regenerate. Thus, an endogenous stress response system may exist within the cochlea to protect it from everyday stressors. We recently described the existence of corticotropin-releasing factor (CRF) in the mouse cochlea. The CRF receptor type 1 (CRFR1) is considered the primary and canonical target of CRF signaling, and systemically it plays an essential role in coordinating the body-wide stress response via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Here, we describe an essential role for CRFR1 in auditory system development and function, and offer the first description of a complete HPA equivalent signaling system resident within the cochlea. To reveal the role of CRFR1 activation in the cochlea, we have used mice carrying a null ablation of the CRFR1 gene. CRFR1(-/-) mice exhibited elevated auditory thresholds at all frequencies tested, indicating reduced sensitivity. Furthermore, our results suggest that CRFR1 has a developmental role affecting inner hair cell morphology and afferent and efferent synapse distribution. Given the role of HPA signaling in maintaining local homeostasis in other tissues, the presence of a cochlear HPA signaling system suggests important roles for CRFR1 activity in setting cochlear sensitivity, perhaps both neural and non-neural mechanisms. These data highlight the complex pleiotropic mechanisms modulated by CRFR1 signaling in the cochlea.

Quantitative estimation of minor conductive hearing loss with distortion product otoacoustic emissions in the guinea pig

Subclinical conductive hearing losses (CHLs) can affect otoacoustic emissions and therefore limit their potential in the assessment of the cochlear function. Theoretical considerations to estimate a minor CHL from DPOAE measurements [Kummer et al. (2006). HNO 54, 457-467] are evaluated experimentally. They are based on the fact, that the level difference of the stimulus tones L(1) and L(2) for optimal excitation of the inner ear is given by L(1)=aL(2)+b. A CHL is presumed to attenuate both L(1) and L(2) to the same extent such that excitation of the inner ear is no longer optimal. From the change of L(1) that is necessary to restore optimal excitation of the inner ear and thus to produce maximal DPOAE levels, the CHL can be estimated. In 10 guinea pig ears an experimental CHL was produced, quantified by determination of compound action potential (CAP) thresholds at 8 kHz (CHL(CAP)) and estimated from DPOAE measurements at 8 kHz (CHL(DPOAE)). CHLs up to 12 dB could be assessed. CHL(DPOAE) correlated well with CHL(CAP) (R=0.741, p=0.0142). Mean difference between CHL(DPOAE) and CHL(CAP) was 4.2±2.6 dB. Estimation of minor CHL from DPOAE measurements might help to increase the diagnostic value of DPOAEs.

Comparing the optimal signal conditions for recording cubic and quadratic distortion product otoacoustic emissions

Odd- and even-order distortion products (DPs), evoked by two primary tones (f(1),f(2),f(1)<f(2)), represent different aspects of cochlear nonlinearity. The cubic and quadratic difference tones (CDT 2f(1)-f(2) and QDT f(2)-f(1)) are prominent representatives of the odd and even DPs. Distortion product otoacoustic emissions (DPOAEs) were measured within a primary level (L(1),L(2)) space over a wide range of f(2)f(1) ratios to compare the optimal signal conditions for these DPs. For CDT, the primary level difference decreased as L(1) increased with a rate proportional to the f(2)f(1) ratio. Moreover, the optimal ratio increased with L(1). A set of two formulas is proposed to describe the optimal signal conditions. However, for a given level of a primary, increasing the other tone level could maximize the QDT amplitude. The frequency ratio at the maximal QDT was about 1.3 and quite constant across different primary levels. A notch was found in the QDT amplitude at the f(2)f(1) ratio of about 1.22-1.25. These opposite behaviors suggest that the optimal recording conditions are different for CDT and QDT due to the different aspects in the cochlear nonlinearity. Optimizing the DPOAE recordings could improve the reliability in clinical or research practices.

Cochlear nonlinearity in normal-hearing subjects as inferred psychophysically and from distortion-product otoacoustic emissions

The aim was to investigate the correlation between compression exponent, compression threshold, and cochlear gain for normal-hearing subjects as inferred from temporal masking curves (TMCs) and distortion-product otoacoustic emission (DPOAEs) input-output (I/O) curves. Care was given to reduce the influence of DPOAE fine structure on the DPOAE I/O curves. A high correlation between compression exponent estimates obtained with the two methods was found at 4 kHz but not at 0.5 and 1 kHz. One reason is that the DPOAE I/O curves show plateaus or notches that result in unexpectedly high compression estimates. Moderately high correlation was found between compression threshold estimates obtained with the two methods, although DPOAE-based values were around 7 dB lower than those based on TMCs. Both methods show that compression exponent and threshold are approximately constant across the frequency range from 0.5 to 4 kHz. Cochlear gain as estimated from TMCs was found to be approximately 16 dB greater at 4 than at 0.5 kHz. In conclusion, DPOAEs and TMCs may be used interchangeably to infer precise individual nonlinear cochlear characteristics at 4 kHz, but it remains unclear that the same applies to lower frequencies.

Evaluation of cochlear hearing disorders: normative distortion product otoacoustic emission measurements

OBJECTIVE

The overall objective was to begin the investigation, in humans, of distortion product otoacoustic emission measurements, which are intended to be part of a diagnostic protocol being developed. This protocol, designed to distinguish among different cochlear hearing disorders, has been tested to date only through lesion studies in the gerbil (Mills, Ear and Hearing, 27, 508-525, 2006). To be applied successfully to human subjects, it was required that parameters and procedures for emission measurements be found, which resulted in sufficiently small intersubject variability in normal subjects, among other requirements. To attain these objectives, measurements of particular otoacoustic emission responses were made in a reference group of young adults having excellent hearing.

DESIGN

Twenty young adults (age 18 to 24 yr; 40 ears) comprised the subject group, with equal numbers of men and women. Inclusion criteria included hearing thresholds of 10 dB HL or better in both ears at all frequencies (0.5, 1, 2, 3, 4, 6, and 8 kHz), plus a tympanometric peak response located within +/-30 daPa of ambient pressure in both ears. The otoacoustic emission stimulus consisted of two tones (frequencies f1 and f2) varied in level using 5-dB steps with the lower-frequency stimulus level always 10 dB greater than that of the higher-frequency stimulus. The emission isoresponse threshold was defined to be the stimulus level required to obtain an emission amplitude of -10 dB SPL. Another potential measure was defined to be the emission amplitude at the highest stimulus levels routinely tested. Mean emission amplitudes and thresholds were determined for f2 frequencies equal to audiometric frequencies from 1 to 8 kHz, using two different stimulus frequency ratios, f2/f1=1.21 and 1.28.

RESULTS

One result of the study was the derivation of reference standards for the emission threshold level, similar to the HL scale for pure-tone thresholds. For use in diagnosis, the optimal measure was found to be the otoacoustic emission threshold for the stimulus frequency ratio f2/f1=1.21 and for f2 frequencies from 1 to 6 kHz. The f2 frequency of 8 kHz seemed less useful because the emission had a relatively high mean threshold. For frequencies 1 to 6 kHz, the variances were adequately small: 95% of the emission thresholds fell within +/-13 dB of the mean at each frequency, a variability only slightly larger than that for the gerbil. Finally, even within the 10-dB HL limit, responses showed a slight trend for increased emission thresholds with increased auditory threshold. Only at 8 kHz was the amount of covariance important, however, with the relationship between emission and auditory thresholds strongest for men.

CONCLUSIONS

Emission reference standards can be developed by testing a group of young adults with excellent hearing. The diagnostic procedure previously proposed on the basis of gerbil lesion studies may be adapted with relatively little modification for use in human subjects. However, validity of the test and specific numerical results for human subjects remain to be firmly established for the purpose of distinguishing among different cochlear disorders.

Determining the cause of hearing loss: differential diagnosis using a comparison of audiometric and otoacoustic emission responses

OBJECTIVE

The purpose of this study was to further investigate the possibility of developing noninvasive methods of differential diagnosis of hearing disorders through the study of experimental animals with induced lesions. In particular, it was desired to compare distortion product otoacoustic emission (DPOAE) responses and auditory brain stem response (ABR) thresholds in Mongolian gerbils having either acoustic or strial damage, using as a reference the same responses measured in a control group of normal young adult gerbils. The goal was to evaluate the potential clinical application of this approach to determining the dominant contribution to sensorineural hearing loss in individual human subjects.

DESIGN

DPOAE input-output functions and ABR thresholds were measured over a wide range of stimulus frequencies for three groups: (1) a reference group of normal young adult gerbils; (2) a group in which acoustic damage had been induced 2 wk earlier; (3) a group in which damage to the stria vascularis was induced by a series of furosemide injections. The responses in the experimental groups relative to the normal means were compared to determine which combinations of responses were effective in discriminating between animals with different lesions. Three measures were evaluated in detail: the ABR threshold, the emission threshold at a criterion emission amplitude, and the emission amplitude at a high stimulus level.

RESULTS

Considering cases with significant hearing loss (ABR thresholds elevated by 20 dB or more), the best method for distinguishing between the two lesions involved a two-dimensional plot comparing emission and ABR thresholds at the same stimulus frequencies. Acoustic damage cases were found in a broad region where the emission and ABR thresholds were roughly equal, whereas strial damage cases were found in a narrower region where the emission threshold was about 0.4 times the ABR threshold (both in dB). These two cases were compared with a third case introduced by definition, that is, damage to inner hair cell or neural systems resulting in an increase in audiometric threshold but no change in emission responses (e.g., auditory neuropathy). The responses for these three cases were found to lie in different regions of the two-dimensional plot comparing emission and ABR thresholds, provided only that ABR thresholds were elevated 20 dB or more. This diagram also revealed cases of preclinical acoustic damage, in which the ABR threshold was shifted less than 20 dB but where the emission threshold was significantly elevated.

CONCLUSIONS

The results clearly demonstrate the possibility of developing a clinical method of noninvasive differential diagnosis of hearing loss. The method demonstrated was to add to a standard audiometric evaluation the measurement of DPOAE growth functions over the range of frequencies where these emissions were relatively easy to measure and consistent. The DPOAE stimulus frequencies were chosen to match the audiometric frequencies, and the corresponding emission and audiometric thresholds were compared on a threshold-threshold plot for each individual at a number of stimulus frequencies. Responses in different regions in this plot were found to correspond to different types of sensorineural hearing loss.

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.

L1,L2 maps of distortion-product otoacoustic emissions from a moth ear with only two auditory receptor neurons

The tympanal organ of the moth Empyreuma affinis emits physiologically vulnerable distortion-product otoacoustic emissions. To assess the nature of underlying mechanical nonlinearities, we measured L1,L2 maps by varying both stimulus levels. Two types of maps were found: (1) Maps containing dominant islands centered at the L1=L2 diagonal as it is typical for saturating nonlinearities that can be described by Boltzmann functions. In contrast to maps published for mammals and frogs, the shape of such islands includes sharp ridges at L1 or L2 levels close to 70 dB sound pressure level. This could be produced by a strongly asymmetric operating point of the respective transfer functions, consistent with the fact that the auditory sensory cells are not hair cells but primary mechanoreceptors with a single cilium. The saturating map components could be selectively reduced by acoustic suppression. (2) Maps where separated islands were less conspicuous but in which the dominant feature consisted of contour lines which were orthogonal to the L1=2L2 diagonal and could be generated by an expansive nonlinearity. Maps showing strong islands were found for f2 frequencies between 26.7 and 45 kHz, maps without strong islands for f2 between 42 and 57.5 kHz. This suggests a frequency-dependent change regarding the involved mechanical nonlinearities.

Distortion product otoacoustic emission (DPOAE) in tinnitus patients

The aim of this study was to determine the effect of tinnitus (experiment I) and the combined effect of tinnitus and sensorineural hearing loss (experiment II) on the distortion product otoacoustic emission (DPOAE) for two age groups of tinnitus patients. Tinnitus patients with normal earing, along with normal-hearing control subjects, participated in experiment I. They were divided into two age groups, below 50 and above 50 years. Experiment I showed that the DPOAE levels in the tinnitus patients were lower than those in the normal-hearing (nontinnitus) subjects. The differences depended on the frequency and the age of the patients, suggesting the confounding influence of presbyacusis. The second group of tinnitus patients with increasing and notch-like hearing loss participated in experiment II. They were also divided into two age groups, below 50 and above 50 years. The data from experiment II showed that DPOAE activity well reflects the increasing and notch-like hearing loss functions up to about 40 dB HL. The effect of age on the DPOAE level was clearly noted only for the tinnitus patients with clinically normal-hearing thresholds and was ambiguous for the tinnitus patients with hearing loss.

Distortion-product otoacoustic emission spectra and high-resolution audiometry in noise-induced hearing loss

Distortion product otoacoustic emissions (DPOAE) elicited by 60dB SPL pure tones at f1 and f2 were collected at 2f1-f2, in 1/10th octave steps, in a sample of 36 ears from 27 patients with noise-induced hearing loss (NIHL). They were analyzed in the frequency domain against the outcome of high-resolution pure-tone audiometry, performed with the help of a Békésy sweep-frequency automatic audiometer. The characteristics of DPOAE level plots as a function of frequency (the so-called DP-grams), relative to the DPOAE levels of a control age-matched group, were compared to their alleged counterparts on the audiograms, i.e., the lower and upper frequency boundaries of the interval with hearing loss. Ears with NIHL split into two subgroups, one (n = 25) with a notch in the DP-gram such that its lower boundary matched the lower limit of the audiometric notch (linear regression with a slope of 0.91, r(2) = 0.644, p < 0.001). Likewise, when it existed, its upper boundary matched its upper counterpart on the audiogram (linear regression with a slope of 0.96, r(2) = 0.89, p < 0.001). In this respect, DP-grams performed better than transient-evoked OAE spectra, which exhibited poor correlations with audiogram patterns. The second subgroup (n = 11) exhibited normal DPOAEs at all frequencies despite audiometric losses similar to those of the first subgroup. In all cases, DPOAE levels were poor predictors of the degree of hearing losses. It is hypothesized that NIHL in the second subgroup involves inner hair cells or auditory neurons, instead of outer hair cells in the first subgroup. Provided NIHL affected outer hair cells, DP-grams provided a comparatively accurate predictor of the spectral extent of hearing loss.

Characteristics of distortion product otoacoustic emissions in the frog from L1,L2 maps

For a given set of stimulus frequencies (f1 ,f2), the level of distortion product otoacoustic emissions (DPOAEs) varies with the levels of the stimulus tones. By variation of the stimulus levels, L1,L2-maps for DPOAEs can be constructed. Here, we report on L1 ,L2-maps for DPOAEs from the frog ear. In general, these maps were similar to those obtained from the mammalian cochlea. We found a conspicuous difference between the equal-level contour lines for low-level and high-level DPOAEs, which could be modeled by a saturating and an expansive nonlinearity, respectively. The transition from the high-level to the low-level response was accompanied by a DPOAE phase-change, which increased from 0 to pi rad with increasing frequency. These results suggest that in the frog low-level and high-level DPOAEs are generated by separate nonlinear mechanisms. Also, there was a conspicuous difference in the growth of the low-level emissions from the two anuran auditory papillae. In the basilar papilla, this growth was expansive for the lowest stimulus levels and saturated for intermediate levels. This is consistent with the behavior of a Boltzman nonlinearity. In the amphibian papilla this growth was compressive, suggesting the additional effect of a compressive amplification mechanism on the generation of DPOAEs.

Haplo-insufficiency revealed in deafwaddler mice when tested for hearing loss and ataxia

The auditory and vestibular systems rely on the plasma membrane calcium ATPase, isoform 2 (PMCA2) to extrude calcium that enters the stereocilia during transduction. Mutations in the gene encoding this protein result in recessive sensorineural deafness and ataxia in the deafwaddler mouse. In this study, we report the identification of a new allele of deafwaddler, dfw(3j). This allele contains a 4-nucleotide deletion resulting in a frame-shift and predicted truncation of PMCA2. No protein is detected in dfw(3j) homozygotes. To examine the dependence of auditory and vestibular function on PMCA2 activity, we compared dfw(3j) with another functional null allele, dfw(2j), and the partial loss-of-function allele, dfw. All mice studied were in the good-hearing CBA/CaJ background. Heterozygotes of either functional null allele displayed highly significant hearing loss by auditory-evoked brainstem responses relative to controls (P < 0.0001), particularly at high frequencies (> 24 kHz). Ataxia was also apparent in these mice on an accelerating rotarod (P < 0.05). In contrast, +/dfw mice were not measurably different from controls in either behavioral test. dfw/dfw mice were deaf, but showed less ataxia than dfw(2j)/dfw(2j) or dfw(3j)/dfw(3j) mice. These results demonstrate that hearing loss and ataxia are dependent on gene dosage and PMCA2 dysfunction.

Level dependence of distortion product otoacoustic emissions in the leopard frog, Rana pipiens pipiens

The inner ear of frogs holds two papillae specialized in detecting airborne sound, the amphibian papilla (AP) and the basilar papilla (BP). We measured input-output (I/O) curves of distortion product otoacoustic emissions (DPOAEs) from both papillae, and compared their properties. As in other vertebrates, DPOAE I/O curves showed two distinct segments, separated by a notch or kneepoint. The slope of the low-level segment was conspicuously different between the AP and the BP. For DPOAE I/O curves from the AP, slopes were < or = 1 dB/dB, similar to what is found in mammals, birds and some lizards. For DPOAE I/O curves from the BP these slopes were much steeper (approximately 2 dB/dB). Slopes found at high stimulus levels were similar in the AP and the BP (approximately 2 dB/dB). This quantitative difference between the low-level slopes for DPOAEs from the AP and the BP may signify the involvement of different mechanisms in low-level DPOAE generation for the two papillae, respectively.

Frequency specificity of distortion-product otoacoustic emissions produced by high-level tones despite inefficient cochlear electromechanical feedback

Distortion product otoacoustic emissions (DPOAEs) are thought to stem from the outer hair cells (OHCs) around the normally narrow place tuned to the primary tone stimuli. They are thus said to be frequency-specific: their local absence should accurately pinpoint local OHC damage. Yet the influence of impaired tuning on DPOAE frequency specificity is poorly documented. Mice with local damage to OHCs were examined. Their DPOAEs were frequency-specific in that audiometric notches were accurately tracked. The same cochleae were further impaired by ischemia or furosemide injection inducing strial dysfunction with flat loss of sensitivity and tuning, while the preexisting pattern of damaged OHCs remained unaltered. Despite the loss of cochlear activity, DPOAEs produced by high-level (> or =70 dB SPL) primaries remained large in about the same interval where they had been initially normal, i.e., that with nondamaged OHCs, albeit with a slight frequency shift, of -1.1 kHz on average. Thus, the ability of DPOAEs to map structurally intact OHCs cannot be a mere consequence of cochlear tuning as it largely persists in its absence. The key element for this correct mapping is likely part of intact OHC structures (e.g., stereocilia bundles) and must have some tuning of its own.

Relationship of neural and otoacoustic emission thresholds during endocochlear potential development in the gerbil

Distortion product otoacoustic emissions and auditory brainstem responses (ABRs) were measured in neonatal gerbils at three ages: at 15-16 days after birth (dab), near the onset of hearing when the endocochlear potential (EP) is known to be still immature; at 22 dab, when the EP first reaches mature levels; and at 30 dab. Comparing individual 15-16 dab animals to the 22 dab group, ABR threshold changes were typically larger than those for cubic distortion tone (CDT, 2f1-f2) emission thresholds which were, in turn, larger than those for the simple difference tone (DT, f2-f1). In contrast, from 22 to 30 dab there were no important changes in CDT or DT emission thresholds. Observed threshold-change relationships were very similar to those found in differential diagnosis investigations, where the EP was experimentally decreased using a chronic furosemide application. Therefore, most of the change in cochlear function over the two week period studied could be attributed to the maturation of EP during the first week. Model calculations further show that relative changes in CDT and DT emission thresholds are compatible with a movement of the operating point of the cochlear amplifier toward its symmetrical "central" point as the EP reaches mature levels.

Evaluation of cochlear function in an acute endolymphatic hydrops model in the guinea pig by measuring low-level DPOAEs

During and after microinjection of artificial endolymph into scala media of the guinea pig, the 2f1- f2 -DPOAE at 4.5 kHz generated by low-level primaries was recorded. Reproducible changes were measured when 1.1 microl of artificial endolymph was injected at a rate of 1.65 nl/s (1.53-1.83). This volume corresponds with an acute endolymphatic hydrops of 23%. After the onset of injection the inner ear pressure immediately increased to a mean higher level of 22 Pa, whereas the 2f1- f2 -amplitude and -phase did not change for about 1 min. Thereafter, the amplitude decreased 2.6 dB (+/- 0.7) on average and slowly regained almost its initial value, with recovery frequently starting within the period of injection. In an attempt to explain the observed changes in 2f1- f2 -amplitude the basilar membrane displacement towards scala tympani at the 2f1- f2 generation site is estimated to be 19 nm for a 1.1 microl increase of endolymph volume. A small deflection of the outer hair cell stereocilia and as a consequence a change in cell conductance may explain the 2f1- f2 -amplitude changes. However, the precise mechanism of cochlear function change caused by endolymph volume increase (hydrops) remains to be elucidated.

Metabolic presbycusis: differential changes in auditory brainstem and otoacoustic emission responses with chronic furosemide application in the gerbil

Auditory characteristics of metabolic or strial presbycusis were investigated using an animal model in which young adult Mongolian gerbils ( Meriones unguiculates) were implanted with an osmotic pump supplying furosemide continuously to the round window. This model causes chronic lowering of the endocochlear potential (EP) and results in auditory responses very similar to those seen in quiet-aged gerbils (Schmiedt et al., J. Neurosci. 22:9643-9650, 2002). Auditory function was examined up to one week post-implant by measurement of auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). Emission "threshold" was defined as the stimulus level required to reach a criterion emission amplitude. Comparing all responses on a "threshold-shift diagram," where emission threshold increases were plotted versus ABR threshold increases, the following results were obtained: (1) On average, the increase of the emission threshold was about 55% of the increase in ABR threshold, with comparatively little scatter. (2) The main dysfunction in metabolic presbycusis appears to be a decrease in the gain of the cochlear amplifier, combined with an additional, smaller increase in neural threshold, both effects caused by a chronically low EP. (3) For ABR threshold increases over 20 dB, the points for the chronic low-EP condition were largely separate from those previously found for permanent acoustic damage. The threshold-shift diagram therefore provides a method for noninvasive differential diagnosis of two common hearing dysfunctions.

Distortion product otoacoustic emissions and auditory evoked potentials in the hedgehog tenrec, Echinops telfairi

The hedgehog tenrec, Echinops telfairi, has certain basal mammalian features, like a cloaca and a sparsely differentiated brain with smooth cerebral hemispheres. The peripheral auditory capabilities of this species were investigated by means of distortion product otoacoustic emissions (DPOAE). For comparison, we determined auditory evoked potentials (AEP) in the inferior colliculus and the auditory cortex. Both methods show that the auditory range of E. telfairi extends well into ultrasonic frequencies, with a region of highest sensitivity at around 16 kHz. The total auditory range spans about 4 octaves at 40 dB SPL. The low-frequency limit of auditory processing is found at frequencies of about 2-3 kHz. The DPOAE and the AEP thresholds of E. telfairi do not run fully parallel in the high-frequency range. For a threshold value of 40 dB SPL, cochlear mechanical thresholds as measured with DPOAE extend up to 50 kHz, whereas neuronal thresholds reach the high-frequency limit at about 30 kHz. Frequency tuning, as assessed from DPOAE suppression tuning curves, was low to moderate with Q(10 dB) values ranging from 1.7 to 8. The lack of discontinuity in the group delay (derived from DPOAE measurements) reveals that cochlear frequency representation is tonotopic without any region of specialized mechanical tuning.

Differential responses to acoustic damage and furosemide in auditory brainstem and otoacoustic emission measures

Characteristics of distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs) were measured in Mongolian gerbil before and after the introduction of two different auditory dysfunctions: (1) acoustic damage with a high-intensity tone, or (2) furosemide intoxication. The goal was to find emission parameters and measures that best differentiated between the two dysfunctions, e.g., at a given ABR threshold elevation. Emission input-output or "growth" functions were used (frequencies f1 and f2, f2/f1 = 1.21) with equal levels, L1 = L2, and unequal levels, with L1 = L2 + 20 dB. The best parametric choice was found to be unequal stimulus levels, and the best measure was found to be the change in the emission threshold level, delta x. The emission threshold was defined as the stimulus level required to reach a criterion emission amplitude, in this case -10 dB SPL. (The next best measure was the change in emission amplitude at high stimulus levels, specifically that measured at L1 x L2 = 90 x 70 dB SPL.) For an ABR threshold shift of 20 dB or more, there was essentially no overlap in the emission threshold measures for the two conditions, sound damage or furosemide. The dividing line between the two distributions increased slowly with the change in ABR threshold, delta ABR, and was given by delta x(t) = 0.6 delta ABR + 8 dB. For a given delta ABR, if the shift in emission threshold was more than the calculated dividing line value, delta x(t), the auditory dysfunction was due to acoustic damage, if less, it was due to furosemide.

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.

Effects of furosemide applied chronically to the round window: a model of metabolic presbyacusis

Hearing thresholds in elderly humans without a history of noise exposure commonly show a profile of a flat loss at low frequencies coupled with a loss that increases with frequency above approximately 2 kHz. This profile and the relatively robust distortion product otoacoustic emissions that are found in elderly subjects challenge the common belief that age-related hearing loss (presbyacusis) is based primarily on sensory-cell disorders. Here, we examine a model of presbyacusis wherein the endocochlear potential (EP) is reduced by means of furosemide applied chronically to one cochlea of a young gerbil. The model results in an EP that is reduced from 90 to approximately 60 mV, a value often seen in quiet-aged gerbils, with no concomitant loss of hair cells. Resulting measures of cochlear and neural function are quantitatively similar to those seen in aging gerbils and humans, e.g., a flat threshold loss at low frequencies with a high-frequency roll-off of approximately -8.4 dB/octave. The effect of the EP on neural thresholds can be parsimoniously explained by the known gain characteristics of the cochlear amplifier as a function of cochlear location: in the apex, amplification is limited to approximately 20 dB, whereas in the base, the gain can be as high as 60 dB. At high frequencies, amplification is directly proportional to the EP on an approximately 1 dB/mV basis. This model suggests that the primary factor in true age-related hearing loss is an energy-starved cochlear amplifier that results in a specific audiogram profile.

Interpretation of standard distortion product otoacoustic emission measurements in light of the complete parametric response

Emission characteristics (at 2f1-f2) are measured in Mongolian gerbil as a function of the independent variation of all four stimulus parameters, the frequencies (f1 and f2) and the intensities (L1 and L2) of the two stimulus tones. The main five-dimensional display chosen is a logarithmic grid of frequencies, where for each frequency pair there is a contour map of the emission amplitude as a function of the two stimulus levels. The feature which leads to the greatest complexity in the proper interpretation of emission responses is the widespread presence of "notches" in these contour maps. Notches are lines of relative minima in the emission amplitude, and are found at either: (1) constant L1, but only in regions where L1 > L2; or (2) at constant L2, only where L2 > or = L1. Notches are not found at any other orientations, and are associated with emission phase shifts of about 180 degrees as the notch line is traversed. These notch characteristics are explained by phase cancellation in a simple cochlear amplifier model in which there is a change, as a function of the stimulus level alone, of relevant characteristics of the cochlear response to a single tone. Only one mechanism of emission generation is required to explain the observed patterns, i.e., there is no need to invoke different "active" and "passive" mechanisms. Unless properly accounted for, the presence of notches adversely affects all of the standard emission measurements, i.e., all methods which cover a restricted parameter set such as DPgrams, input-output or "growth" functions, and frequency ratio functions. Conversely, because the notch location appears approximately invariant in the cochlea, notches potentially make it possible to use certain emission growth functions to estimate forward and reverse middle-ear transfer functions.

Modifications of a single saturating non-linearity account for post-onset changes in 2f1-f2 distortion product otoacoustic emission

2f1-f2 distortion product otoacoustic emissions (DPOAEs) were recorded from guinea pigs. DPOAEs showed complex time dependence at the onset of stimulation. The DPOAE, measured during the first 500 ms, can either decrease or increase at the onset depending on both the frequencies and levels of the primary tones. These changes are closely associated with amplitude minima (notches) of the DPOAE I/O functions. These notches are characteristic of DPOAE growth functions measured from guinea pigs for primary tones of 50-60-dB sound-pressure level (SPL). Apparent changes in the DPOAE amplitude occur because the notch shifts to higher levels of the primaries during the onset of stimulation. This shift of the notch to higher levels increases for lower f2/f1 ratios but does not exceed about 2 dB. DPOAE amplitude increases for a constant level of the primaries if the onset emission is situated at the low-level, falling slope of the notch. If the onset DPOAE is located on the high-level, rising slope of the notch, then the upward shift of the notch causes the emission either to decrease monotonically, or to decrease initially and then increase. By establishing that the 2f1-f2 onset changes reflect a shift in the growth-function notch, it is possible to predict the temporal behavior of DPOAEs in the two-dimensional space of the amplitude of the primaries and for their different frequency ratios.

Deriving a cochlear transducer function from low-frequency modulation of distortion product otoacoustic emissions

In this paper, a new method is introduced to derive a cochlear transducer function from measuring distortion product otoacoustic emissions (DPOAEs). It is shown that the cubic difference tone (CDT, 2f1-f2) is produced from the odd-order terms of a power series that approximates a nonlinear function characterizing cochlear transduction. Exploring the underlying mathematical formulation, it is found that the CDT is proportional to the third derivative of the transduction function when the primary levels are sufficiently small. DPOAEs were measured from nine gerbils in response to two-tone signals biased by a low-frequency tone with different amplitudes. The CDT magnitude was obtained at the peak regions of the bias tone. The results of the experiment demonstrated that the shape of the CDT magnitudes as a function of bias levels was similar to the absolute value of the third derivative of a sigmoidal function. A second-order Boltzmann function was derived from curve fitting the CDT data with an equation that represents the third derivative of the Boltzmann function. Both the CDT-bias function and the derived nonlinear transducer function showed effects of primary levels. The results of the study indicate that the low-frequency modulated DPOAEs can be used to estimate the cochlear transducer function.

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.

Components of the 2f(1)-f(2) distortion-product otoacoustic emission in a moth

The noctuoid moth Empyreuma affinis has a simple tympanal organ with only two receptor cells attached to the eardrum. As in vertebrates, the growth of distortion-product otoacoustic emissions (DPOAE) with increasing stimulus level is characterized by two distinct components. An initial increase of DPOAE level for f(2) levels in the range of 30-65 dB SPL is followed by a second steep growth of the DPOAE at f(2) levels above 65 dB SPL. Both components increase at a slope of about 2 dB/dB and the difference between both components was used to assess a mechanical gain of the tympanal organ of 17 dB (n=23). At around 65 dB SPL, a notch in the level function coincides with an abrupt phase change of up to 180 degrees. The sensitive component induced by f(2) levels below 65 dB SPL is selectively affected by application of ethyl ether and disappears more quickly than the high-level component during ongoing deterioration of the moth preparation.

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.

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.

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.

Frequency responses of two- and three-tone distortion product otoacoustic emissions in Mongolian gerbils

The frequency responses of distortion product otoacoustic emission (DPOAEs) were investigated in adult Mongolian gerbils. The main goal was to investigate in this species the extent to which DPOAE measurements might be useful in estimating cochlear frequency-tuning characteristics. Specifically, this study investigated the parameter space for generation of DPOAEs to determine those regions, if any, where the emission responses gave "simple" frequency responses, i.e., responses similar in form to typical neural responses. At the same time, it was desired to determine in this species the existence, extent, and nature of the more complex three-tone emission frequency responses as observed in some other species [e.g., Martin et al., Hearing Res. 136, 105-123 (1999)]. In the present work, two-tone frequency response curves (f2/f1 ratio functions) were obtained by varying the lower frequency, f1, while holding the f2 frequency and both amplitudes (L1, L2) constant. Only for frequencies, f2, near 8 kHz did the response at the emission frequency, 2 f1-f2, form a simple, relatively broad peak. At all lower frequencies, the two-tone frequency response curve was typically complex and composed of multiple peaks. In comparison, three-tone frequency responses were constructed by fixing the primary stimulus pair (f1, f2) and varying a third tone widely in frequency (f3) and intensity (L3). Points in f3 and L3 which caused a criterion reduction in primary emission amplitude (at 2 f1-f2) were used to construct emission suppression tuning curves (STCs). Only for primary frequencies, f2, at 8 kHz and above were the emission STCs found to be simple, with shapes similar to neural frequency-tuning curves. At lower primary frequencies, particularly for relatively low primary frequency ratios (low f2/f1), three-tone responses were very complex. This complex response usually included a region of anomalous suppression in which very low suppression levels (L3) could result in significant decreases in the primary emission amplitude, often exceeding 12 dB. Regions of such anomalous suppression were typically observed under the following conditions: (1) for all f2 frequencies from 0.5 to 4 kHz; (2) for f3 frequencies between 1.4 and 8 kHz; (3) i.e., for f3 frequencies 1-3 octaves above the primary frequency, f2; (4) at L3 levels often 10 dB lower or more than the usual "best frequency" threshold, i.e., even lower than the relative minimum threshold found near the primary stimulus frequencies; (5) exhibiting sharp amplitude decreases often accompanied by emission phase shifts of about 180 deg; (6) present in both cubic emissions (2 f1-f2 and 2 f2-f1); (7) to be less extreme at larger primary stimulus frequency ratios (larger f2/f1); and (8) less extreme at larger intensity ratios (larger L1/L2). Because of the anomalous behavior at f2 frequencies below 8 kHz, "simple" emission STCs were typically only obtainable, if at all, near the extreme boundaries of the parameter space giving measurable emission amplitudes.

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.

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.

Methods for early identification of noise-induced hearing loss

An ideal test for identifying shifts in cochlear function would be highly repeatable and sensitive to minor damage. Three types of otoacoustic emission (OAE) test and pure-tone audiometry were evaluated for this purpose. They were compared in terms of test-retest repeatability within subjects and sensitivity to differences between subjects. The OAE measures were transiently evoked either conventionally (TEOAE) or using maximum length sequences (TEOAE-MLS), or continuously evoked as distortion products (DPOAEs). Several stimulus conditions were evaluated for each type. Thirty eight subjects with normal hearing or mild hearing losses were tested on all measures. Test-retest repeatability was rescaled according to the sensitivity of each measure to differences in hearing threshold level, thus allowing a direct comparison across methods. The most repeatable method thus defined was TEOAE-MLS which gave a rescaled standard deviation of 1.8 dB on replication. This was followed by TEOAE and DPOAE which gave rescaled standard deviations of 2.9 and 3.1 dB, respectively. All were more reliable than pure-tone audiometry which had a standard deviation of 4.9 dB. It is concluded that the various OAE measures have the potential to distinguish small changes in cochlear function from measurement uncertainty, and hence show promise for monitoring cochlear function in ears exposed to noise or other hazards.

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

High-resolution measurements of distortion product otoacoustic emissions (DPOAEs) from three different experimental paradigms are shown to be in agreement with the implications of a realistic "two-source" cochlear model of DPOAE fine structure. The measurements of DPOAE amplitude and phase imply an interference phenomenon involving one source in the region of strong nonlinear interaction of the primary waves (the strong "overlap" or generation region), and the other source region around the DPOAE tonotopic place. The component from the DPOAE place can be larger than the one from the generator region. These findings are supported by the analysis of the onset and offset of the DPOAE when the higher-frequency primary is pulsed on and off. The two-source hypothesis was further tested by adding a third tone closer in frequency to the DPOAE which modifies the amplitude of the component from the DPOAE place and leaves the one from the generator region unchanged. The results agree well with the model prediction that the variation with frequency, and implied latency, of the phase of the DPOAE tonotopic-place component are greater than the corresponding quantities for the component from the generation region.

Onset of basilar membrane non-linearity reflected in cubic distortion tone input-output functions

The basilar membrane (BM) input output (I/O) function is a non-linear compressive function over much of its operating range. A low level non-compressive region with a break-point or compression threshold between 20 and 40 dB SPL has been identified. To date, no similar compression threshold in cubic distortion tone otoacoustic emission (CDT) data, which would illustrate the dependence of the CDT on BM growth, has been demonstrated. A Taylor series expansion of the outer hair cell gating function yields an amplitude term for 2f1-f2 of p.A1(2).A2, where A1 and A2 are the displacement amplitudes of the BM for two pure tone input stimuli of levels L1 and L2, p a constant. By selectively varying either L1 or L2 with f2/f1 appropriately chosen to reduce suppression effects, the CDT I/O function can be examined for deviation from the power law. In particular, if the amplitude of the CDT were dependent on BM displacement amplitude, then it should be possible by an appropriate choice of parameters to measure compression threshold. We have examined CDT I/O functions for an f2 of 8 kHz in the guinea pig and found them to be consistent with the expected power law. With L1 held constant, L2 varied and f2/f1 = 1.6, a low level region with a slope of one and a compressive region with a slope of 0.14-0.27 corresponding to the analogous regions of the BM I/O function was identified, with a break-point or compression threshold of 22-33 dB SPL.

Development of the base of the cochlea: place code shift in the gerbil

Distortion product otoacoustic emission measurements were made at 1/12 octave intervals before and after the injection of furosemide in gerbils aged 15 days after birth to adult, in order to obtain estimates of cochlear amplifier gain as a function of stimulus frequency. The frequency at which the gains went sharply to zero, defined as the 'base cutoff frequency', increased from about 20 kHz to over 50 kHz during development. This increase provides further confirmation of the hypothesis that the place code changes during development in the basal part of the cochlea. If the measured base cutoff frequency is identified with the characteristic frequency at the basal end of the cochlea, as defined by electrophysiological measures, then these emission data can be used to generate a frequency-place map as a function of age. The derived place code shift is consistent with published electrophysiological measures, and can be used to extend these measures. Near the base cutoff frequency, the observed cochlear amplifier gain typically dropped sharply from a relative maximum to zero, over a distance of about a half octave. Specifically, this distance appeared to exhibit a curvilinear variation with age, reaching a maximum of 3/4 of an octave at 19-21 days. After transforming from frequency to place using the map derived from emissions, however, the distance between the extreme base and the place associated with the peak gain decreased monotonically from about 1.2 mm at age 15-19 days to 0.6 mm at maturity. This distance is assumed to be characteristic of the length of the active amplification zone for the cochlear amplifier in the base region. Over the same time period, there was approximately a doubling of the rate of amplification with distance from the base, so that the cochlear amplifier gain at the peak actually changed very little from 15 days to adult.

Effects of loop diuretics on the suppression tuning of distortion-product otoacoustic emissions in rabbits

The suppression tuning of distortion-product otoacoustic emissions (DPOAEs) is commonly assumed to measure frequency selectivity, because the dominant features of suppression-tuning curves (STCs) are similar to the principal properties of the neural-tuning curves (NTCs) of single auditory-nerve fibers. In the present study, several common loop diuretics were used to affect the DPOAE-generation process to determine if reversible ototoxicity could adversely modify the characteristics of STCs, in a manner similar to that shown previously for NTCs. Contour plots of DPOAE level in the presence of a series of variable-level suppressor tones were obtained before and after administering diuretic drugs that reversibly reduced or eliminated DPOAEs. Primary-tone pairs were centered at 2.8 or 4 kHz, with L1 = L2, or L2 < L1. From the resulting plots, STC parameters including tip frequency, threshold at the tip frequency, and Q10 dB measures of tuning were extracted for four suppression criteria of 3, 6, 9, and 12 dB. In the pre-drug nonototoxic state, suppression tuning depended on both primary-tone level (L1, L2), and the relative levels of the primaries (L1-L2), with tuning being sharper for lower- than for higher-level equilevel primaries, and sharpest for offset-level primary tones. Following drug injection, the expected decrease in sharpness of tuning evidenced by changes in Q10 dB as well as the dramatically elevated tip thresholds normally seen for NTCs under similar conditions, were not observed. Overall, Q10 dB increased or decreased more or less randomly, with a slight tendency for STCs to become sharper than prior to drug dosing, for the two highest suppression criteria. The STC-tip frequencies demonstrated significant decreases following diuretic administration that were weakly correlated with the associated decreases in DPOAE amplitude. The most consistent changes in response to the drug-induced reduction in DPOAE level were increases in the STC-tip thresholds. However, these changes were relatively small and rarely exceeded 10 dB. In the absence of notable changes in overall STC shape, a major finding was a change in the effectiveness of suppression following ototoxic insult. However, when the amount of suppression was expressed as a percentage of the DPOAE remaining, the effects of diuretic dosing were often almost completely obscured. Overall, the results demonstrated that when the generation of DPOAEs was interfered with by the introduction of a suppressor tone to produce STCs that resemble NTCs, STCs behaved quite differently following reversible cochlear insult than their previously documented neural counterparts. These findings imply that STCs do not assess the frequency-selective aspects of the cochlear amplification process in a manner similar to NTCs.

Interpretation of distortion product otoacoustic emission measurements. II. Estimating tuning characteristics using three stimulus tones

The simple model introduced in Part I [J. Acoust. Soc. Am. 102, 413-429 (1997)] is used to simulate the response of the cochlea to three stimulus tones. The focus is on "emission suppression tuning curves" constructed using a third tone to suppress the cubic distortion tone emission (CDT, 2f1-f2) generated by two primary tones at frequencies f1 and f2 (intensities L1 and L2). A criterion decrease (here, 5 dB) of the CDT emission amplitude defines the 2f1-f2 emission suppression tuning curve. Applying traditional tuning curve measures to emission suppression tuning curves appears ineffective in determining the underlying cochlear amplifier characteristics. However, it is shown that there are three characteristics of emission suppression tuning curves which are particularly useful: (1) the "f2 threshold" which is the level of the third tone, L3, required for the criterion CDT amplitude decrease, under the condition that the third tone frequency, f3, is approximately equal to f2; (2) the "shoulder threshold" similarly defined for f3 << f2; and (3) the "tuning width," w40. The tuning width is defined to be the distance (in octaves) from the frequency f2 to the upper f3 frequency for which there is a criterion CDT decrease, in this case using the L3 level which is 40 dB above the f2 threshold. Model calculations appropriate to gerbils show that these measures are most accurately related to the underlying cochlear amplifier characteristics for parameters where the primary stimulus amplitudes satisfy L1/L2 > 20 dB and for which L1 is 25 dB or more below the sharp "notch" seen in the two tone input-output function. In this parameter region, the cochlear amplifier characteristics are related to measured quantities by the relationships wr approximately equal to w40 and Ga approximately equal to TE + w40gp. Here, Ga is the gain (dB) of the cochlear amplifier, defined as the total increase in cochlear response over the passive response, wr is the distance (octaves) over which the active cochlear response rises to a maximum, and gp is the passive increase (dB/octave) of the traveling wave along the basilar membrane. The measured quantities are TE, the difference (dB) between the shoulder threshold and the f2 threshold, and the tuning width, w40 (octaves), defined above. Model predictions are confirmed by measurements in adult gerbils.