Distortion Product Otoacoustic Emissions Recordings Research Articles

Optimizing distortion product otoacoustic emission recordings in normal-hearing ears by adopting cochlear place-specific stimuli.

Distortion product otoacoustic emissions (DPOAEs) provide a window into active cochlear processes and have become a popular clinical and research tool. DPOAEs are commonly recorded using stimulus with fixed presentation levels and frequency ratio irrespective of the test frequency. However, this is inconsistent with the changing mechanical properties of the cochlear partition from the base to the apex that lend specific frequency-dependent spatial properties to the cochlear traveling wave. Therefore, the frequency and level characteristics between the stimulus tones should also need to be adjusted as a function of frequency to maintain optimal interaction between them. The goal of this investigation was to establish a frequency-specific measurement protocol guided by local cochlear mechanics. A broad stimulus parameter space extending up to 20 kHz was explored in a group of normal-hearing individuals. The stimulus frequency ratio yielding the largest 2f1-f2 DPOAE level changed as a function of frequency and stimulus level. Specifically, for a constant stimulus level, the frequency ratio producing the largest DPOAE level decreased with increasing frequency. Similarly, at a given f2 frequency, the stimulus frequency ratio producing the largest DPOAE level became wider as stimulus level increased. These results confirm and strengthen our current understanding of DPOAE generation in the normally functioning cochlea and expand our understanding to previously unexamined higher frequencies. These data support the use of frequency- and level-specific stimulus frequency ratios to maximize DPOAE generation.

The effect of increase in blood glucose level on hearing loss

ObjectivePrevious studies have shown that hearing function is also vulnerable to the effects of diabetes mellitus which can be shown by brainstem auditory evoked potential and distortion product otoacoustic emission recordings. This study aimed to investigate the changes of brainstem auditory evoked potential and distortion product otoacoustic emission in hyperglycemia and whether there is a relationship between reactive oxygen substances production and hearing deterioration in the rat model. Methods25 streptozotocin induced diabetic rats were divided into three groups: control, high blood glucose, and diabetes mellitus. Brainstem auditory evoked potential and distortion product otoacoustic emission were recorded, and thiobarbituric acid reactive substances levels were measured in the brainstem tissue. ResultsAt 8 kHz, the latencies of I, II, III, IV, and V brainstem auditory evoked potential waves in high blood glucose and diabetes mellitus groups were elongated, at 16 kHz, only these wave latencies of the diabetes mellitus group were prolonged compared with the control group. A significant decrease was also found in distortion product otoacoustic emission amplitudes at 4, 6, 8, and 10 kHz in the high blood glucose and diabetes mellitus groups compared to the control group. There was a significant increase in thiobarbituric acid reactive substances values due to the increase in blood glucose levels in the high blood glucose and diabetes mellitus groups compared to the control group. ConclusionThese results suggested that high blood glucose levels may cause hearing impairment not only in the diabetic state but also in the period of hyperglycemia before the onset of manifest diabetes mellitus and reactive oxygen substances may play an important role in the pathophysiology of diabetes mellitus. We suggest that regulating high glucose levels even before the onset of manifest diabetes mellitus may prevent hazardous effects on hearing function. Level of evidenceLevel 3.

Characterizing the Access of Cholinergic Antagonists to Efferent Synapses in the Inner Ear

Stimulation of cholinergic efferent neurons innervating the inner ear has profound, well-characterized effects on vestibular and auditory physiology, after activating distinct ACh receptors (AChRs) on afferents and hair cells in peripheral endorgans. Efferent-mediated fast and slow excitation of vestibular afferents are mediated by α4β2*-containing nicotinic AChRs (nAChRs) and muscarinic AChRs (mAChRs), respectively. On the auditory side, efferent-mediated suppression of distortion product otoacoustic emissions (DPOAEs) is mediated by α9α10nAChRs. Previous characterization of these synaptic mechanisms utilized cholinergic drugs, that when systemically administered, also reach the CNS, which may limit their utility in probing efferent function without also considering central effects. Use of peripherally-acting cholinergic drugs with local application strategies may be useful, but this approach has remained relatively unexplored. Using multiple administration routes, we performed a combination of vestibular afferent and DPOAE recordings during efferent stimulation in mouse and turtle to determine whether charged mAChR or α9α10nAChR antagonists, with little CNS entry, can still engage efferent synaptic targets in the inner ear. The charged mAChR antagonists glycopyrrolate and methscopolamine blocked efferent-mediated slow excitation of mouse vestibular afferents following intraperitoneal, middle ear, or direct perilymphatic administration. Both mAChR antagonists were effective when delivered to the middle ear, contralateral to the side of afferent recordings, suggesting they gain vascular access after first entering the perilymphatic compartment. In contrast, charged α9α10nAChR antagonists blocked efferent-mediated suppression of DPOAEs only upon direct perilymphatic application, but failed to reach efferent synapses when systemically administered. These data show that efferent mechanisms are viable targets for further characterizing drug access in the inner ear.

Reliability of Contralateral Suppression in Evoked Distortion Product Otoacoustic Emissions.

Introduction Distortion product otoacoustic emissions (DPOAE) and their suppression may be considered useful in monitoring cochlear function and the efferent auditory pathway inhibitory effect. Nonetheless, the establishment of reliable parameters of response variations is of great importance. Objectives To verify the replicability of test and retest in the research of the inhibitory effect of the efferent pathway using contralateral suppressing stimulus during DPOAE recording for clinical applicability. Methods Cross-sectional study with 48 volunteers, aged 18 to 30 years, with normal audiometric thresholds. The procedures included were audiometric and immittance measures to overrule any conductive or sensorineural conditions and DPOAE recordings without and with contralateral suppression with a 60 dBHL white noise. Distortion product otoacoustic emissions amplitudes were analyzed and compared in both conditions with Wilcoxon test, and the Spearman correlation test was used to assess test-retest reliability. Results The comparative analysis showed differences between amplitudes in test and retest conditions only in 1,500 Hz for DPOAE measures with all other tested frequencies showing no differences, and no difference was observed in all recorded frequencies in the test and retest comparison for DPOAE suppression. The degree of correlation between test and retest of DPOAE amplitude was good at 6,000 Hz and strong (r > 0.880) at the other frequencies. For DPOAE with suppression, all frequencies presented strong correlation between test and retest: 1,500 Hz (r = 0.880), 2,000 Hz (r = 0.882), 3,000 Hz (r = 0.940), and 6,000 Hz (r = 0.957). Conclusions The study found good replicability in contralateral suppression of DPOAE with potential clinical applicability, and we recommend conducting the test from 2000Hz to higher frequencies for more reliable results.

ABO Blood Group and Cochlear Status: Otoacoustic Emission Markers.

There are an increasing number of research studies examining the effects of ABO blood group on susceptibility to disease. However, little is known regarding the potential relationship between blood group and hearing. Higher risk of noise-induced hearing loss was linked to blood group O in several occupational health studies. Based on this finding, a recent study of cochlear status was conducted with normal-hearing female participants representing equal numbers of the four blood groups in the ABO blood group system. ABO blood group was associated with cochlear characteristics, including the prevalence of spontaneous otoacoustic emissions (SOAEs) and the amplitudes of transient-evoked otoacoustic emissions (TEOAEs) and distortion-product otoacoustic emissions (DPOAEs). Females with blood group O showed significantly lower amplitudes of DPOAEs at some frequencies and lower prevalence of SOAEs compared with participants with blood group B. There was a general trend of reduced TEOAE and DPOAE amplitudes in blood group O individuals compared with participants with non-O blood groups. Following from this finding, and based on known sex differences in otoacoustic emission characteristics, the present study examined the possible effects of blood group on otoacoustic emission status in males. Sixty clinically normal-hearing males aged between 18 and 26 years, with equal numbers of participants in each of the ABO blood groups, were recruited by purposive sampling. SOAE, DPOAE, and linear and nonlinear TEOAE recordings were collected from all participants, as well as tympanometric data related to external and middle ear characteristics. The male blood group O participants exhibited significantly lower SOAE prevalence and reduced amplitudes of DPOAEs on average, and in the midfrequency range, than participants with blood group B, and lower nonlinear and linear TEOAE amplitudes at a number of frequencies when compared with participants with blood groups A and B. A consistent trend of lower TEOAE and DPOAE response amplitudes was observed in participants with blood group O. No significant difference was noted among blood groups for outer or middle ear characteristics. These results were consistent with previous findings of reduced otoacoustic emission responses in female blood group O individuals. Results support the hypothesis that blood group O individuals may be at increased risk of cochlear damage from noise exposure. Further investigation on the potential link between ABO blood group and auditory status, including potentially differential effects of noise exposure on cochlear function, is needed. The possible effects of ABO blood group on other aspects of audition, such as hearing sensitivity, speech understanding, and auditory processing, should be evaluated.

A Comparison of Distortion Product Otoacoustic Emission Properties in Ménière's Disease Patients and Normal-Hearing Participants.

Postmortem examination of temporal bones of Ménière's disease patients consistently show dilated endolymphatic spaces of the inner ear, for which the term endolymphatic hydrops has been coined. During the past decade, magnetic resonance imaging techniques for the inner ear appeared, advancing the diagnosis of Ménière's disease. They require, however, a field-strength of at least 3 T, are costly and not universally available. Alternative, noninvasive, cost-effective tests with high sensitivity and specifity for endolymphatic hydrops are desirable. In this study, we test the suitability of distortion product otoacoustic emissions (DPOAEs) for endolymphatic hydrops detection. Previous measurements of the commonly recorded cubic DPOAEs mainly register cochlear hearing loss and are not specific for Ménière's disease. Simultaneous recordings of cubic and quadratic DPOAEs might be more suitable to detect endolymphatic hydrops, because both DPOAE orders react differently to changes of the cochlear operating point as they might occur in Ménière's disease patients. Cubic and quadratic DPOAEs were recorded in normal-hearing participants (N = 45) and in the affected and unaffected ears of patients with a diagnosis of definite Ménière's disease (N = 32). First, to assess the integrity of DPOAE-generating mechanisms, cubic DPOAE-grams were obtained with primary tone frequencies f2 between 1 and 8 kHz with primary tone levels l1 = 60 dB SPL and l2 = 50 dB SPL, and a fixed primary tone frequency ratio of 1.22. Then, cubic and quadratic DPOAEs were simultaneously recorded with primary tone levels l1 = l2 = 65 dB SPL and at primary tone frequencies f2 = 4 and 5 kHz, where f1 was successively varied such that the ratio f2/f1 ranged between 1.1 and 1.6 in 0.04 steps while quadratic and cubic DPOAE levels were extracted from the same recording. Cubic DPOAEs were significantly reduced in the affected ears of Ménière's disease patients, and slightly reduced in the unaffected ears of Ménière's disease patients, relative to the ears of normal-hearing participants. In contrast, no significant changes could be seen in quadratic DPOAEs across the ears of normal-hearing participants and Ménière's disease patients. We could identify a relatively good preservation of quadratic DPOAE levels in relation to a reduction of cubic DPOAE levels as a potential noninvasive diagnostic approach in the early stage of suspected Ménière's disease. Future studies validating the differential diagnostic power of this parameter in control groups with nonhydropic forms of hearing loss are warranted.

Distortion product otoacoustic emissions in geriatric dogs

Recordings of distortion product otoacoustic emissions (DPOAE) were taken from 28 geriatric dogs aged 12.2 ± 2.2 years and 15 control dogs aged 5.9 ± 3.0 years (mean ± standard deviation) to demonstrate frequency-specific changes in cochlear responses. Recordings were performed for primary frequencies of 2–12 kHz in 2 kHz increments. Brainstem auditory evoked response (BAER) recordings were also made from geriatric dogs for comparison with DPOAE responses. Significant decreases in DPOAE response amplitudes were observed at frequencies of 6–12 kHz in geriatric dogs compared to control dogs, reflecting loss of cochlear outer hair cells along the length of the cochlea. Significant decreases in response amplitudes were not seen at frequencies of 2 or 4 kHz. Decreases in BAER response amplitudes subjectively paralleled the depressed DPOAE amplitudes. No significant linear regression relationships were found for DPOAE response amplitude vs. age despite the progressive nature of age-related hearing loss. The reductions in response at all frequencies starting at the age where dogs are considered geriatric indicate that age-related hearing loss begins earlier in the life span. DPOAE recordings provide a means to assess cochlear function across different portions of the auditory spectrum for assessing hearing loss associated with aging, and potentially for losses from other causes of decreased auditory function.

Interaural attention modulates outer hair cell function.

Mounting evidence suggests that auditory attention tasks may modulate the sensitivity of the cochlea by way of the corticofugal and the medial olivocochlear (MOC) efferent pathways. Here, we studied the extent to which a separate efferent tract, the 'uncrossed' MOC, which functionally connects the two ears, mediates inter-aural selective attention. We compared distortion product otoacoustic emissions (DPOAEs) in one ear with binaurally presented primaries, using an intermodal target detection task in which participants were instructed to report the occurrence of brief target events (visual changes, tones). Three tasks were compared under identical physical stimulation: (i) report brief tones in the ear in which DPOAE responses were recorded; (ii) report brief tones presented to the contralateral, non-recorded ear; and (iii) report brief phase shifts of a visual grating at fixation. Effects of attention were observed as parallel shifts in overall DPOAE contour level, with DPOAEs relatively higher in overall level when subjects ignored the auditory stimuli and attended to the visual stimulus, compared with both of the auditory-attending conditions. Importantly, DPOAE levels were statistically lowest when attention was directed to the ipsilateral ear in which the DPOAE recordings were made. These data corroborate notions that top-down mechanisms, via the corticofugal and medial efferent pathways, mediate cochlear responses during intermodal attention. New findings show attending to one ear can significantly alter the physiological response of the contralateral, unattended ear, probably through the uncrossed-medial olivocochlear efferent fibers connecting the two ears.

Analysis of subtle auditory dysfunctions in young normal-hearing subjects affected by Williams syndrome

ObjectiveTo assess if young subjects affected by Williams syndrome (WS) with normal middle ear functionality and normal hearing thresholds might have subtle auditory dysfunctions that could be detected by using clinically available measurements. MethodsOtoscopy, acoustic reflexes, tympanometry, pure-tone audiometry, and distortion product otoacoustic emissions (DPOAEs) were measured in a group of 13 WS subjects and in 13 age-matched, typically developing control subjects. Participants were required to have normal otoscopy, A-type tympanogram, normal acoustic reflex thresholds, and pure-tone thresholds ≤15dB HL at 0.5, 1, and 2kHz bilaterally. To limit the possible influence of middle ear status on DPOAE recordings, we analyzed only data from ears with pure-tone thresholds ≤15dB HL across all octave frequencies in the range 0.25–8kHz, middle ear pressure (MEP)>−50daPa, static compliance (SC) in the range 0.3–1.2cm3, and ear canal volume (ECV) in the range 0.2–2ml, and we performed analysis of covariance to remove the possible effects of middle ear variables on DPOAEs. ResultsNo differences in mean hearing thresholds, SC, ECV, and gradient were observed between the two groups, whereas significantly lower MEP values were found in WS subjects as well as significantly decreased DPOAEs up to 3.2kHz after adjusting for differences in middle ear status. ConclusionsResults revealed that WS subjects with normal hearing thresholds (≤15dB HL) and normal middle ear functionality (MEP>−50daPa, SC in the range 0.3–1.2cm3, ECV in the range 0.2–2ml) might have subtle auditory dysfunctions that can be detected by using clinically available methods. Overall, this study points out the importance of using otoacoustic emissions as a complement to routine audiological examinations in individuals with WS to detect, before the onset of hearing loss, possible subtle auditory dysfunctions so that patients can be early identified, better monitored, and promptly treated.

Extraction of otoacoustic distortion product sources using pulse basis functions

Distortion product otoacoustic emissions (DPOAEs) acquired in normal-hearing subjects show considerable variation in amplitude with varying frequency. This is known as DPOAE fine structure. It is widely accepted that fine structure results from wave interference from two DPOAE sources, a non-linear generation component and a coherent reflection component. Here a method is presented that decomposes short-pulse DPOAE recordings into pulse basis functions and enables the quantification of both source components in the time domain, independent of their relative phase and at low cost of measurement time. Input-output functions utilizing the extracted primary-source component are analyzed.

Demonstrating True DPOAE Modulation by the Efferent Pathway

Objective: 1) To isolate the distortion product otoacoustic emission (DPOAE) components using an ipsilateral and individualized suppressor tone. 2) To demonstrate the modulation of DPOAEs by the medial olivocochlear efferent pathway once component interference has been eliminated. Method: DPOAE (2f1-f2 ) recordings using a frequency-sweep paradigm with ipsilateral suppressor tones and CAS were made on 8 normal-hearing subjects in a sound-treated audiology booth from January to February 2012. Estimates of DPOAE level and phase were obtained using fast Fourier transform and least-squares-fit techniques. Results: Fine structure depth was significantly reduced with the use of an ideal ipsilateral suppressor tone, demonstrating the effectiveness of the suppressor tone at separating DPOAE components. In addition, once the DPOAE components were separated to remove phase interference, MOC activation resulted in consistent suppression of DPOAE level as compared to nonseparated DPOAE level which showed enhancement as well as suppression. These results help to support the theory of DPOAE component interference leading to enhancements often seen with efferent activation and to demonstrate a more consistent way to view DPOAE modulation by the efferent pathway. Conclusion: This study demonstrates that DPOAE fine structure enhancement seen with activation of the MOC pathway is likely due to DPOAE component interference as opposed to true DPOAE modulation. In addition, the results show that DPOAE component isolation can be achieved by an ipsilateral suppressor tone, however it must be individualized.

Cognitive task demands modulate the sensitivity of the human cochlea.

Recent studies lead to the conclusion that focused attention, through the activity of corticofugal and medial olivocochlear (MOC) efferent pathways, modulates activity at the most peripheral aspects of the auditory system within the cochlea. In two experiments, we investigated the effects of different intermodal attention manipulations on the response of outer hair cells (OHCs), and the control exerted by the MOC efferent system. The effect of the MOCs on OHC activity was characterized by measuring the amplitude and rapid adaptation time course of distortion product otoacoustic emissions (DPOAEs). In the first, DPOAE recordings were compared while participants were reading a book and counting the occurrence of the letter “a” (auditory-ignoring) and while counting either short- or long-duration eliciting tones (auditory-attending). In the second, DPOAEs were recorded while subjects watched muted movies with subtitles (auditory-ignoring/visual distraction) and were compared with DPOAEs recorded while subjects counted the same tones (auditory-attending) as in Experiment 1. In both Experiments 1 and 2, the absolute level of the averaged DPOAEs recorded during the auditory-ignoring condition was statistically higher than that recorded in the auditory-attending condition. Efferent-induced rapid adaptation was evident in all DPOAE contours, under all attention conditions, suggesting that two medial efferent processes act independently to determine rapid adaptation, which is unaffected by attention, and the overall DPOAE level, which is significantly affected by changes in the focus of attention.

A New Method to Analyze Distortion Product Otoacoustic Emissions (DPOAEs) in the High-Frequency Range Up to 18 kHz Using Windowed Periodograms

Distortion product otoacoustic emissions (DPOAEs) are widely used as an objective examination procedure to determine cochlear function. In a clinical routine setting, the amplitude of the DPOAE signal at 2f1 .. f2 is applied as an indicator for a potential hearing loss up to 8 kHz. Due to their poor signal to noise ratio, meatal nodes from standing waves and calibration issues, high-frequency DPOAEs > 8 kHz have hardly been addressed in experimental and clinical audiology so far. Here, we present a new method of measuring DPOAE signal levels based on optimal maximum likelihood estimation with windowed power spectral density estimation of stochastic signals and filtering theory. Analysis of simulated data showed that the proposed method effectively reduces the disturbing noise floor compared to conventional averaging techniques. Robust DPOAE signals were measured in 20 ears from 10 normally hearing young adults (21 to 27 years) from 0:5 to 18 kHz. Repeated DPOAE recordings in one individual yielded a good to very good testretest reliability of the proposed method. These observations are discussed in the context of DPOAE signal processing and possible clinical applications of high-frequency DPOAE measurements.

Assessing repeatability and reproducibility using hierarchical modeling: a case-study of distortion product otoacoustic emissions

We present how the repeatability and reproducibility of a measurement device can be estimated from a suitably defined hierarchical linear model. The methodology is illustrated using a collection of eight data sets which consist of the distortion product otoacoustic emission recordings collected from both ears of ten young Sprague-Dawley rats at different frequencies under eight different recording conditions. We formulate a model which extends the commonly used one-way random effects model (5) to account for an experimental setup that is more elaborated than the ones traditionally used in interlaboratory experiments. The fitted model is easily interpretable and furnishes as a by-product the frequencies at which the highest response level is achieved under the eight recording conditions. These values together with the repeatability and reproducibility limits of the protocols are crucial in contributing to the enhancement of the research capabilities on the possible causes of hearing impairment.

The effect of topical castellani solution on outer hair cell function of rats

To evaluate if Castellani solution, which is used for the treatment of mycotic infections of the ear, has any ototoxic potential. There are some studies about the toxic effect of some of the ingredients of the Castellani solution, but the ototoxic effect is not known. The current study was designed to investigate the ototoxicity experimentally. Prospective controlled animal study. Experiments were performed on 40 male Wistar albino rats. The animals were divided into four groups, each consisting of 10 rats. Under general anesthesia, the right tympanic membranes of the all of the rats were perforated and baseline distortion product otoacoustic emission (DPOAE) measurements were performed from the right ears before the use of solutions. The solutions were applied through the ear canal for 2 weeks using 0.1 mL twice a day. The rats in group I (study group) received Castellani solution. The rats in group II and group III received gentamicin (40 mg/mL) and saline solution respectively. The rats in group IV were followed without use of any medication. After 2 weeks DPOAE recordings were repeated. Baseline DPOAE results were normal in the right ears of all animals tested. Animals treated with either Castellani solution or saline and the rats in the control group showed no significant changes in the DPOAE amplitudes. The gentamicin group showed a significant decrease in the DPOAE values. In the rat, Castellani solution, which is topically applied to the middle ear cavity, does not affect DPOAE amplitudes.

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.

Providing basic hearing tests using remote computing technology

We administered pure tone and otoacoustic emissions testing to subjects in a distant community using remote computing technology. Fifteen men and 15 women ranging in age from 18-30 years were tested. An audiometer was used to measure subject pure tone thresholds. In addition, distortion product otoacoustic emissions (DPOAEs) data were recorded using a portable system. Both systems were interfaced to a PC which was connected to the local area network at Minot State University (MSU). An examiner at Utah State University, 1100 km away, could control both the DPOAE and the audiometer equipment at MSU. Overall, the pure tone means for the face-to-face and telemedicine trials were equivalent at each frequency. Moreover, DPOAE recordings exhibited equivalent results at each frequency for telemedicine and face-to-face trials. These results support the use of remote computing as a telemedicine method for providing pure tone audiometry and DPOAE testing to distant communities.

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

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

Comparison between distortion product otoacoustic emissions and nerve fiber responses from the basilar papilla of the frog

The basilar papilla (BP) is one of the three end organs in the frog inner ear that is sensitive to airborne sound. Its anatomy and physiology are unique among all classes of vertebrates. Essentially, the BP functions as a single auditory filter presumably arising from a mechanically-tuned mechanism. As such, both neural and distortion product otoacoustic emission (DPOAE) tuning may reflect a single mechanical filtering mechanism. Using the Duffing oscillator as a simple model for both neural and DPOAE tuning from the BP, two predictions can be made: [1] the characteristic frequency (CF) of neural tuning and the best frequency (BF) of DPOAE tuning will coincide and [2] the neural tuning curve and DPOAE-audiogram have a similar shape when the neural tuning curve is scaled by a factor of 4 along the y-axis. We recorded both neural tuning curves and DPOAE-audiograms from the BP of the leopard frog. These recordings show good agreement with the model predictions when the stimulus tones are related by relatively small stimulus frequency ratios. For larger stimulus frequency ratios, DPOAE recordings clearly deviate from model predictions. These differences are most likely caused by the oversimplified representation of the frog BP by the model.

Cochlear sensitivity in the lesser spear-nosed bat, Phyllostomus discolor.

Behavioral auditory thresholds of Phyllostomus discolor are characterized by two threshold minima separated by an insensitive region at about 55 kHz (Esser and Daucher 1996). To investigate whether these characteristics are due to cochlear properties, we recorded distortion product otoacoustic emissions (DPOAEs) and calculated relative DPOAE threshold curves, which proved to be a good measure of cochlear sensitivity. Our results indicate that in P. discolor, cochlear sensitivity, as assessed by DPOAE recordings, does not show a threshold maximum at 55 kHz. The DPOAE threshold curves display an absolute minimum at approximately 30 kHz, and from that frequency region, the threshold continuously increases without any pronounced irregularities. The frequency tuning properties of the cochlea, as assessed by DPOAE suppression tuning curves (STCs) reveal broad filter bandwidths with Q10dB values between 3.4 and 10.7. There are no frequency-specific specializations of cochlear tuning. The characteristic pattern of subsequent threshold maxima and minima at high frequencies observed in behavioral studies seems to be shaped by transfer characteristics of the outer ear and/or neuronal processing in the ascending auditory pathway rather than by cochlear mechanics.