Otoacoustic emissions measured with a physically open recording system

Compensating for ear-canal acoustics when measuring otoacoustic emissions

Otoacoustic emissions (OAEs) provide an acoustic fingerprint of the inner ear, and changes in this fingerprint may indicate changes in cochlear function arising from efferent modulation, aging, noise trauma, and/or exposure to harmful agents. However, the reproducibility and diagnostic power of OAE measurements is compromised by the variable acoustics of the ear canal, in particular, by multiple reflections and the emergence of standing waves at relevant frequencies. Even when stimulus levels are controlled using methods that circumvent standing-wave problems (e.g., forward-pressure-level calibration), distortion-product otoacoustic emission (DPOAE) levels vary with probe location by 10-15 dB near half-wave resonant frequencies. The method presented here estimates the initial outgoing OAE pressure wave at the eardrum from measurements of the conventional OAE, allowing one to separate the emitted OAE from the many reflections trapped in the ear canal. The emitted pressure level (EPL) represents the OAE level that would be recorded were the ear canal replaced by an infinite tube with no reflections. When DPOAEs are expressed using EPL, their variation with probe location decreases to the test-retest repeatability of measurements obtained at similar probe positions. EPL provides a powerful way to reduce the variability of OAE measurements and improve their ability to detect cochlear changes.

Reverse transmission along the ossicular chain in gerbil

In a healthy cochlea stimulated with two tones f (1) and f (2), combination tones are generated by the cochlea's active process and its associated nonlinearity. These distortion tones travel "in reverse" through the middle ear. They can be detected with a sensitive microphone in the ear canal (EC) and are known as distortion product otoacoustic emissions. Comparisons of ossicular velocity and EC pressure responses at distortion product frequencies allowed us to evaluate the middle ear transmission in the reverse direction along the ossicular chain. In the current study, the gerbil ear was stimulated with two equal-intensity tones with fixed f (2)/f (1) ratio of 1.05 or 1.25. The middle ear ossicles were accessed through an opening of the pars flaccida, and their motion was measured in the direction in line with the stapes piston-like motion using a laser interferometer. When referencing the ossicular motion to EC pressure, an additional amplitude loss was found in reverse transmission compared to the gain in forward transmission, similar to previous findings relating intracochlear and EC pressure. In contrast, sound transmission along the ossicular chain was quite similar in forward and reverse directions. The difference in middle ear transmission in forward and reverse directions is most likely due to the different load impedances-the cochlea in forward transmission and the EC in reverse transmission.

Forward and reverse transfer functions of the middle ear based on pressure and velocity DPOAEs with implications for differential hearing diagnosis

Recently it was shown that distortion product otoacoustic emissions (DPOAEs) can be measured as vibration of the human tympanic membrane in vivo, and proposed to use these vibration DPOAEs to support a differential diagnosis of middle-ear and cochlear pathologies. Here, we investigate how the reverse transfer function (r-TF), defined as the ratio of DPOAE-velocity of the umbo to DPOAE-pressure in the ear canal, can be used to diagnose the state of the middle ear. Anaesthetized guinea pigs served as the experimental animal. Sound was delivered free-field and the vibration of the umbo measured with a laser Doppler vibrometer (LDV). Sound pressure was measured 2-3 mm from the tympanic membrane with a probe-tube microphone. The forward transfer function (f-TF) of umbo velocity relative to ear-canal pressure was obtained by stimulating with multi-tone pressure. The r-TF was assembled from DPOAE components generated in response to acoustic stimulation with two stimulus tones of frequencies f(1) and f(2); f(2)/f(1) was constant at 1.2. The r-TF was plotted as function of DPOAE frequencies; they ranged from 1.7 kHz to 23 kHz. The r-TF showed a characteristic shape with an anti-resonance around 8 kHz as its most salient feature. The data were interpreted with the aid of a middle-ear transmission-line model taken from the literature for the cat and adapted to the guinea pig. Parameters were estimated with a three-step fitting algorithm. Importantly, the r-TF is governed by only half of the 15 independent, free parameters of the model. The parameters estimated from the r-TF were used to estimate the other half of the parameters from the f-TF. The use of r-TF data - in addition to f-TF data - allowed robust estimates of the middle-ear parameters to be obtained. The results highlight the potential of using vibration DPOAEs for ascertaining the functionality of the middle ear and, therefore, for supporting a differential diagnosis of middle-ear and cochlear pathologies.

Spontaneous otoacoustic emissions measured using an open ear-canal recording technique

Spontaneous otoacoustic emissions (SOAEs) and synchronized spontaneous otoacoustic emissions (SSOAEs) were recorded using both the standard closed-canal method of recording and a novel open-canal method which involved suspending the probe at the entrance to the ear canal with no occluding tip. In both conditions, a probe tube microphone was inserted down the ear canal to measure the acoustic pressure near the tympanic membrane. Open- and closed-canal recordings were obtained in twelve otologically normal ears, all of which exhibited SSOAEs, and 6 of which exhibited SOAEs. The results were analysed to identify any differences in response to frequency and amplitude. The different recording conditions appeared to have no significant effect on SOAE or SSOAE frequency, suggesting little effect on the SOAE generator within the cochlea. Below about 2 kHz, the amplitude for both types of emission was less for the open-canal recording when compared to the closed-canal recordings. Above 2 kHz, SSOAE amplitudes were greater in the open- than the closed-canal condition. Model stimulations of the ear canal and middle-ear acoustics are presented which were in qualitative agreement with the results shown for the effects on emission amplitudes.

Reconciling the origin of the transient evoked ototacoustic emission in humans

A pervasive theme in the literature for the transient evoked otoacoustic emission (TEOAE) measured from the human ear canal has been one of the emission arising solely (or largely) from a single, place-fixed mechanism. Here TEOAEs are reported measured in the absence of significant stimulus contamination at stimulus onset, providing for the identification of a TEOAE response beginning within the time window that is typically removed by windowing. Contrary to previous studies, it was found that in humans, as has previously been found in guinea pig, the TEOAE appears to arise from two generation mechanisms, the relative contributions of these two mechanisms being time and stimulus-level dependent. The method of windowing the earliest part of the ear canal measurement to remove stimulus artifact removes part of the TEOAE i.e., much of the component arising from a nonlinear generation mechanism. This reconciliation of TEOAE origin is consistent with all OAEs in mammals arising in a stimulus-level dependent manner from two mechanisms of generation, one linear, one nonlinear, as suggested by Shera and Guinan [J. Acoust. Soc. Am. 105, 782-798 (1999)].

In search of basal distortion product generators

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

Middle ear forward and reverse transmission in gerbil

The middle ear transmits environmental sound to the inner ear. It also transmits acoustic energy sourced within the inner ear out to the ear canal, where it can be detected with a sensitive microphone as an otoacoustic emission. Otoacoustic emissions are an important noninvasive measure of the condition of sensory hair cells and to use them most effectively one must know how they are shaped by the middle ear. In this contribution, forward and reverse transmissions through the middle ear were studied by simultaneously measuring intracochlear pressure in scala vestibuli near the stapes and ear canal pressure. Measurements were made in gerbil, in vivo, with acoustic two-tone stimuli. The forward transmission pressure gain was about 20-25 dB, with a phase-frequency relationship that could be fit by a straight line, and was thus characteristic of a delay, over a wide frequency range. The forward delay was about 32 micros. The reverse transmission pressure loss was on average about 35 dB, and the phase-frequency relationship was again delaylike with a delay of about 38 mus. Therefore to a first approximation the middle ear operates similarly in the forward and reverse directions. The observation that the amount of pressure reduction in reverse transmission was greater than the amount of pressure gain in forward transmission suggests that complex motions of the tympanic membrane and ossicles affect reverse more than forward transmission.

Delay dependence for the origin of the nonlinear derived transient evoked otoacoustic emission

In the guinea pig it has been shown that the nonlinear derived transient evoked otoacoustic emission (TEOAEnl) is comprised of significant amounts of intermodulation distortion energy. It is expected that intermodulation distortion arising from a nonlinear distortion mechanism will contribute to the overall TEOAE in a stimulus-level-dependent manner, being greatest when basilar-membrane vibration in response to a click stimulus is greatest; with decay of vibration of the basilar membrane subsequent to stimulation by a click, nonlinear interaction along the cochlear partition should reduce and so provide for a linear mechanism to dominate TEOAEnl generation, i.e., the contributions of each of these mechanisms should be delay dependent. To examine this delay dependence, TEOAEnl evoked by acoustic clicks of varying bandwidth were time-domain windowed using a recursive exponential filter in an attempt to separate two components with amplitude and phase properties consistent with different mechanisms of OAE generation. It was found that the part of the TEOAEnl occurring first in time can have a relatively constant amplitude and shallow phase slope, consistent with a nonlinear distortion mechanism. The latter part of the TEOAEnl has an amplitude microstructure and a phase response more consistent with a place-fixed mechanism.

Cochlear delays measured with amplitude-modulated tone-burst-evoked OAEs

Delay times in the mammalian cochlea, whether from measurement of basilar membrane (BM) vibration or otoacoustic emissions (OAEs) have, to date, been largely based on phase-gradient estimates from steady-state responses. Here we report cochlear delays measured directly in the time domain from OAEs evoked by amplitude-modulated tone-burst (AMTB) stimuli. Measurement using OAEs provides a non-invasive estimate of cochlear delay but is confounded by the complexity of generation of such OAEs. At low to moderate stimulus levels, and provided that the stimulus frequency range does not include a region of the cochlea where there is a large change in effective reflectance, AMTB stimuli evoke an OAE with an envelope shape that is similar to the stimulus and allow a direct calculation of cochlear group delay. Such delays are commensurate with BM estimates of delay, estimates of cochlear delay inferred from neural recordings, and previous OAE measures of delay in the guinea pig. However, a nonlinear distortion mechanism, variation in effective reflectance, and intermodulation distortion products generated by the nonlinear interaction in the cochlea of the carrier and sidebands of the AMTB stimulus, may all contribute to OAEs arising with envelope shapes that are not a scaled representation of the stimulus, confounding the estimation of cochlear group delay.

The origin of SFOAE microstructure in the guinea pig

Human stimulus-frequency otoacoustic emissions (SFOAEs) evoked by low-level stimuli have previously been shown to have properties consistent with such emissions arising from a linear place-fixed reflection mechanism with SFOAE microstructure thought to be due to a variation in the effective reflectance with position along the cochlea [Zweig and Shera, J. Acoust. Soc. Am. 98 (1995) 2018-2047]. Here we report SFOAEs in the guinea pig obtained using a nonlinear extraction paradigm from the ear-canal recording that show amplitude and phase microstructure akin to that seen in human SFOAEs. Inverse Fourier analysis of the SFOAE spectrum indicates that SFOAEs in the guinea pig are a stimulus level-dependent mix of OAEs arising from linear-reflection and nonlinear-distortion mechanisms. Although the SFOAEs are dominated by OAE generated by a linear-reflection mechanism at low and moderate stimulus levels, nonlinear distortion can dominate some part of, or all of, the emission spectrum at high levels. Amplitude and phase microstructure in the guinea pig SFOAE is evidently a construct of (i). the complex addition of nonlinear-distortion and linear-reflection components; (ii). variation in the effective reflectance with position along the cochlea; and perhaps (iii). the complex addition of multiple intra-cochlear reflections.

Generation of DPOAEs in the guinea pig

In humans, distortion product otoacoustic emissions (DPOAEs) at frequencies lower than the f(2) stimulus frequency are a composite of two separate sources, these two sources involving two distinctly different mechanisms for their production: non-linear distortion and linear coherent reflection [Talmadge et al., J. Acoust. Soc. Am. 104 (1998) 1517-1543; Talmadge et al., J. Acoust. Soc. Am. 105 (1999) 275-292; Shera and Guinan, J. Acoust. Soc. Am. 105 (1999) 332-348; Kalluri and Shera, J. Acoust. Soc. Am. 109 (2001) 662-637]. In rodents, DPOAEs are larger, consistent with broader filters; however the evidence for two separate mechanisms of DPOAE production as seen in humans is limited. In this study, we report DPOAE amplitude and phase fine structure from the guinea pig with f(2)/f(1) held constant at 1.2 and f(2) swept over a range of frequencies. Inverse Fast Fourier Transform analysis and time-domain windowing were used to separate the two components. Both the 2f(1)-f(2) DPOAE and the 2f(2)-f(1) DPOAE were examined. It was found that, commensurate with human data, the guinea pig DPOAE is a composite of two components arising from different mechanisms. It would appear that the 2f(1)-f(2) emission measured in the ear canal is usually dominated by non-linear distortion, at least for a stimulus frequency ratio of 1.2. The 2f(2)-f(1) DPOAE exhibits amplitude fine structure that, for the animals examined, is predominantly due to the variation in amplitude of the place-fixed component. Cochlear delay times appear consistent with a linear coherent reflection mechanism from the distortion product place for both the 2f(1)-f(2) and 2f(2)-f(1) place-fixed components.

Ototoxic effects of cisplatin in a Sprague-Dawley rat animal model as revealed by ABR and transiently evoked otoacoustic emission measurements

The ototoxic effects of cisplatin in a Sprague-Dawley rat model were evaluated by recordings of auditory brainstem responses (ABR) and transiently evoked otoacoustic emissions (TEOAEs). The ABR responses were evoked from alternating clicks and 8, 10, 12, 16, 20 and 30 kHz tone pips in a range from 40 to 100 dB SPL range. The TEOAEs were recorded with a non-linear protocol, and were evoked by a 63.5 dB SPL click stimulus. Twenty five male Sprague-Dawley rats were used in the study, 20 animals were treated with cisplatin (16 mg/kg, body weight) and five animals served as controls. The data showed that 72 h after the cisplatin administration, the TEOAE and ABR variables were significantly altered. The relationship between the ABR and TEOAE variables was shown to be non-linear. The most significant relationships were observed between the TEOAE correlation and the ABR threshold values at 10, 12, and 16 kHz.

Evaluation of anesthesia effects in a rat animal model using otoacoustic emission protocols

Anesthesia effects on otoacoustic emission (OAE) recordings were evaluated in a group of 72 Sprague-Dawley rats (mean weight 225+/-20 gr). Two anesthesia dosages (high and normal) and two anesthetic protocols (ketamine-xylazine, ketamine-xylazine-atropine) were tested. Transient evoked OAE (TEOAE) and distortion product OAE (DPOAE) responses were recorded in 10 min intervals, for a total period of 60 min. Analyses of the data with repeated measure models indicated the following: (1) The animals receiving a high dose of anesthesia (cumulative dose 66.6 mg of ketamine and 13.2 mg of xylazine/kg of body weight) presented significant alterations of the TEOAE response level and the signal to noise ratio at 3.0 kHz; (2) the animals receiving a normal dose of ketamine-xylazine anesthesia (cumulative dose 50 mg of ketamine and 10 mg of xylazine/kg of body weight) presented TEOAE and DPOAE responses invariant in terms of time; (3) significant differences were observed in the DPOAE responses from animals anesthetized with ketamine-xylazine and ketamine-xylazine-atropine. The data support the hypothesis that the ketamine anesthesia OAE suppressing mechanism is related to middle-ear mechanics.

Effects on cochlear responses of activation of descending pathways from the inferior colliculus

The inferior colliculus (IC) has been shown anatomically to make direct descending connections with medial olivocochlear (MOC) neurones in the auditory brainstem. The MOC neurones project to the outer hair cells in the cochlea and inhibit cochlear neural output. This study investigated the effect of IC stimulation on cochlear output in both guinea pigs and rats in order to determine the functional significance of the IC-to-olivocochlear system projection. Stimulation of the central nucleus and the external cortex of the IC in paralysed guinea pigs, both contra- and ipsilaterally to the test cochlea, resulted in a small increase of the cochlear microphonic amplitude and a small decrease of the compound action potential (CAP) amplitude, the latter equivalent to a 3-6 dB change in acoustic input. Effects on the CAP were maximal in the frequency range 6-10 kHz. These effects were consistent with partial activation of the MOC system. In unparalysed rats, stimulation of the inferior colliculus evoked a large, prolonged suppression ranging from 5-12 dB in the amplitude of distortion product otoacoustic emissions (2f(1)-f(2); DPOAE), as reported previously by Scates et al. (1999). However, this suppression was decreased to only 0-3 dB when the animals were paralysed, suggesting that the larger suppression in the unparalysed state was the consequence of either a general masking effect caused by animal movement, or activation of middle ear muscles by the inferior colliculus stimulation. The results indicate a small but significant excitatory effect of the inferior colliculus on the medial olivocochlear system under conditions of anaesthesia and paralysis.

Changes to low-frequency components of the TEOAE following acoustic trauma to the base of the cochlea

Several studies have shown that acoustic trauma to the base of the cochlea can result in loss of transient-evoked otoacoustic emission (TEOAE) energy at frequencies much lower than those affected in the audiogram. We have extended these studies to show that the low-frequency emission energy was substantially affected if the transient stimulus included frequencies within the range affected by the trauma, otherwise the change observed was small. In keeping with the suggestion that TEOAEs are predominantly comprised of intermodulation distortion energy (Yates and Withnell, Hear. Res. 136 (1999) 49-64), trauma to the basal region of the cochlea was found to affect emission energy across a broad frequency range in response to a wide-band acoustic stimulus. Further, group delay measurements demonstrated that the dominant contribution to the TEOAE originated from the basal region of the cochlea.

The role of intermodulation distortion in transient-evoked otoacoustic emissions

Transient-evoked otoacoustic emissions (TEOAEs) are low-intensity sounds recorded in the external ear canal immediately following stimulation by a transient stimulus, typically a click. While the details of their production is unknown, there is evidence to suggest that the amplitude of each component frequency reflects the physiological condition of the corresponding region of the cochlea. Certain observations are at variance with this assumption, however, suggesting that pathology at a basal site within the cochlea might affect the production of emissions at frequencies which are not characteristic for that site. We have recorded click-evoked emissions in guinea pigs using high-pass clicks and found emissions at frequencies which are not present in the stimulus and which could not, therefore, have originated from the characteristic place for those emission frequencies. These new frequencies are, by definition, intermodulation distortion frequencies and must have been generated from combinations of frequencies in the stimulus by non-linear processes within the cochlea. Further processing of the emissions by Kemp's technique of non-linear recovery showed that the magnitude of emissions at frequencies within the stimulus frequency pass-band was approximately the same as that of frequencies not present in the stimulus. We propose that, in guinea pigs at least, most of the click-evoked emission energy is generated as intermodulation distortion, produced by non-linear intermodulation between various frequency components of the stimulus. If this result is confirmed in humans, many of the anomalies in the literature may be resolved.

Enhancement of the transient-evoked otoacoustic emission produced by the addition of a pure tone in the guinea pig

This study examined the transient-evoked otoacoustic emission obtained in response to a click stimulus presented in combination with a pure tone in the guinea pig. Low-pass filtered click waveforms were digitally generated using a sin(t)/t function windowed over 3 ms with an elevated cosine envelope. Transient-evoked otoacoustic emissions were obtained using the nonlinear derived response technique. Phase locked pure tones of various frequencies at approximately 70 dB SPL were electrically mixed with electrical clicks, with the pure tone present only for the three lower level stimuli in the train of four stimuli. Enhancement in the amplitude of the response spectrum at frequencies which corresponded to regions of the basilar membrane apical to the tone was observed with the addition of the tone. This finding is inconsistent with the transient-evoked otoacoustic emission being the result of independent generators. It suggests that intermodulation distortion energy may contribute to the transient-evoked otoacoustic emission, the enhancement in the emission response spectrum at frequencies below the pure tone being a result of a complex interaction on the basilar membrane of intermodulation distortion products.