Stimulus Frequency Otoacoustic Emission Delays and Generating Mechanisms in Guinea Pigs,Chinchillas, and Simulations

According to coherent reflection theory (CRT), stimulus frequency otoacoustic emissions (SFOAEs) arise from cochlear irregularities coherently reflecting energy from basilar membrane motion within the traveling-wave peak. This reflected energy arrives in the ear canal predominantly with a single delay at each frequency. However, data from humans and animals indicate that (1) SFOAEs can have multiple delay components, (2) low-frequency SFOAE delays are too short to be accounted for by CRT, and (3) “SFOAEs” obtained with a 2nd (“suppressor”) tone ≥2 octaves above the probe tone have been interpreted as arising from the area basal to the region of cochlear amplification. To explore these issues, we collected SFOAEs by the suppression method in guinea pigs and time-frequency analyzed these data, simulated SFOAEs, and published chinchilla SFOAEs. Time-frequency analysis revealed that most frequencies showed only one SFOAE delay component while other frequencies had multiple components including some with short delays. We found no systematic patterns in the occurrence of multiple delay components. Using a cochlear model that had significant basilar membrane motion only in the peak region of the traveling wave, simulated SFOAEs had single and multiple delay components similar to the animal SFOAEs. This result indicates that multiple components (including ones with short delays) can originate from cochlear mechanical irregularities in the SFOAE peak region and are not necessarily indicative of SFOAE sources in regions ≥2 octaves basal of the SFOAE peak region. We conclude that SFOAEs obtained with suppressors close to the probe frequency provide information primarily about the mechanical response in the region that receives amplification, and we attribute the too-short SFOAE delays at low frequencies to distortion-source SFOAEs and coherent reflection from multiple cochlear motions. Our findings suggest that CRT needs revision to include reflections from multiple motions in the cochlear apex.     

Two-tone distortion at different longitudinal locations on the basilar membrane

When listening to two tones at frequency f1 and f2 (f2>f1), one can hear pitches not only at f1 and f2 but also at distortion frequencies f2-f1, (n+1)f1-nf2, and (n+1)f2-nf1 (n=1,2,3...). Such two-tone distortion products (DPs) also can be measured in the ear canal using a sensitive microphone. These ear-generated sounds are called otoacoustic emissions (OAEs). In spite of the common applications of OAEs, the mechanisms by which these emissions travel out of the cochlea remain unclear. In a recent study, the basilar membrane (BM) vibration at 2f1-f2 was measured as a function of the longitudinal location, using a scanning laser interferometer. The data indicated a forward traveling wave and no measurable backward wave. However, this study had a relatively high noise floor and high stimulus intensity. In the current study, the noise floor of the BM measurement was significantly decreased by using reflective beads on the BM, and the vibration was measured at relatively low intensities at more than one longitudinal location. The results show that the DP phase at a basal location leads the phase at an apical location. The data indicate that the emission travels along the BM from base to apex as a forward traveling wave, and no backward traveling wave was detected under the current experimental conditions.     

Backward Propagation of Otoacoustic Emissions

IntroductionsSounds impinging the eardrum are transmitted viamiddle ear ossicles to the oval window. Stapes vibra-tion creates pressure difference between the scala tym-pani and the scala vestibuli. This pressure differencecauses a movement of cochlear partition and adjacentcochlear fluids. Basilar membrane vibrations result indeflection of hair cell stereocilia, which gate ion chan-nels on their tips. This mechanical-to-electrical trans-duction process converts mechanical vibrations intoelec…     

Confirmation of G. von Békésy's theory of paradoxical wave propagation along the cochlear partition by means of bone-conducted auditory brainstem responses

In order to investigate the propagation time of the traveling wave in the cochlea after bone-conduction stimulation of the inner ear, bone-conducted auditory brainstem responses (ABRs) were recorded in 6 normally hearing subjects after masking the basal cochlear region using high-pass filtered noise. As in air-conducted ABRs, Jewett V wave latency is delayed corresponding to the propagation time of the traveling wave front traversing the desynchronized hair cell region. These results support the theory of paradoxical wave propagation proposed by von Békésy in 1952, who postulated that wave motion always starts from the stiffest part of the basilar membrane, independent of the location of the vibrating force. In addition, we also found a latency delay of the Jewett V wave of bone-conducted ABRs in 8 patients with high-frequency hearing loss which corresponded to the severity of their hearing impairment.     

Establishing auditory steady-state response thresholds to narrow band CE-chirps in full-term neonates

Narrow band CE-chirps^® were developed to provide a better synchronization of neural response due to the compensation of the traveling wave delay in the basilar membrane. These stimuli combined with a detection method that includes higher response harmonics on the auditory steady-state response (ASSR) recording was studied in this research.     

How does the inner ear generate distortion product otoacoustic emissions?. Results from a realistic model of the human cochlea

There is general agreement that distortion product (DP) otoacoustic emissions elicited by stimuli up to 80-90 dB SPL originate from the saturating nonlinearity of the cochlear amplifier at the basilar membrane site, S, where the responses to the two primary tones overlap. There are, however, different interpretations of how the inner ear transmits the effects of this process to the stapes. The supporters of transmission line models assert that the phenomenon depends upon two main mechanisms: (1) the generation of forward and backward traveling waves (TWs) by DP oscillations at S; (2) the backward propagation of wave components reflected by 'micromechanical impedance perturbations' at the sites where the DP TWs peak. However, quantitative predictions based on this view are still lacking. In contrast, here we show, using a nonlinear hydrodynamic model, that the emissions are propagated almost instantaneously through the fluid.     

Reflection- and Distortion-Source Otoacoustic Emissions: Evidence for Increased Irregularity in the Human Cochlea During Aging

Previous research on distortion product otoacoustic emission (DPOAE) components has hinted at possible differences in the effect of aging on the two basic types of OAEs: those generated by a reflection mechanism in the cochlea and those created by nonlinear distortion (Abdala and Dhar in J Assoc Res Otolaryngol 13:403–421, 2012). This initial work led to the hypothesis that micromechanical irregularity (“roughness”) increases in the aging cochlea, perhaps as the result of natural tissue degradation. Increased roughness would boost the backscattering of traveling waves (i.e., reflection emissions) while minimally impacting DPOAEs. To study the relational effect of aging on both types of emissions and address our hypothesis of its origin, we measured reflection- and distortion-type OAEs in 77 human subjects aged 18–76 years. The stimulus-frequency OAE (SFOAE), a reflection emission, and the distortion component of the DPOAE, a nonlinear distortion emission, were recorded at multiple stimulus levels across a four-octave range in all ears. Although the levels of both OAE types decreased with age, the rate of decline in OAE level was consistently greater for DPOAEs than for SFOAEs; that is, SFOAEs are relatively preserved with advancing age. Multiple regression analyses and other controls indicate that aging per se, and not hearing loss, drives this effect. Furthermore, SFOAE generation was simulated using computational modeling to explore the origin of this result. Increasing the amount of mechanical irregularity with age produced an enhancement of SFOAE levels, providing support for the hypothesis that increased intra-cochlear roughness during aging may preserve SFOAE levels. The characteristic aging effect—relatively preserved reflection-emission levels combined with more markedly reduced distortion-emission levels—indicates that SFOAE magnitudes in elderly individuals depend on more than simply the gain of the cochlear amplifier. This relative pattern of OAE decline with age may provide a diagnostic marker for aging-related changes in the cochlea.     

Effects of Low-Frequency Biasing on Otoacoustic and Neural Measures Suggest that Stimulus-Frequency Otoacoustic Emissions Originate Near the Peak Region of the Traveling Wave

Stimulus-frequency otoacoustic emissions (SFOAEs) have been used to study a variety of topics in cochlear mechanics, although a current topic of debate is where in the cochlea these emissions are generated. One hypothesis is that SFOAE generation is predominately near the peak region of the traveling wave. An opposing hypothesis is that SFOAE generation near the peak region is deemphasized compared to generation in the tail region of the traveling wave. A comparison was made between the effect of low-frequency biasing on both SFOAEs and a physiologic measure that arises from the peak region of the traveling wave—the compound action potential (CAP). SFOAE biasing was measured as the amplitude of spectral sidebands from varying bias tone levels. CAP biasing was measured as the suppression of CAP amplitude from varying bias tone levels. Measures of biasing effects were made throughout the cochlea. Results from cats show that the level of bias tone needed for maximum SFOAE sidebands and for 50% CAP reduction increased as probe frequency increased. Results from guinea pigs show an irregular bias effect as a function of probe frequency. In both species, there was a strong and positive relationship between the bias level needed for maximum SFOAE sidebands and for 50% CAP suppression. This relationship is consistent with the hypothesis that the majority of SFOAE is generated near the peak region of the traveling wave.     

Effects of changes in stimulus level on phases of distortion product otoacoustic emissions

Effect of changes in stimulus levels of both lower (f(1)) and higher (f(2)) stimulus tones on phases of 2f(1)-f(2) component of the distortion product otoacoustic emission (DPOAE) was examined in five normal hearing adults. The f(2) was fixed at 4004 Hz in all of the measurements, and the stimulus frequency ratio (f(2)/f(1)) was varied from 1.15 to 1.3. Change of the level of lower stimulus tone (L(1)) and the level of higher stimulus tone (L(2)) showed different effects on the DPOAE phases. The phase lags increased with increasing L(1), when f(2)/f(1) was above 1.22, whereas the phase gains increased with increasing L(1), when f(2)/f(1) was below 1.22. On the other hand, the difference in L(2) minimally affected DPOAE phase at most f(1)s. The previous studies about basilar membrane vibration revealed that phase lags increase with increasing stimulus level, when the stimulus frequency is below the best frequency, while phase gains increase with increasing stimulus level, when the stimulus frequency is above the best frequency, and the effect of phase change in stimulus level diminished, when the stimulus frequency was far above the best frequency. Based on the comparison between the results of the present study and the previous findings of others concerning basilar membrane vibration, the DPOAE generation site is assumed to be located at apical of the peak of the f(2) traveling wave.     

Exploration of stimulus-frequency otoacoustic emission suppression tuning in hearing-impaired listeners

Objective: Otoacoustic emissions (OAEs) can provide useful measures of tuning of auditory filters. We previously established that stimulus-frequency (SF) OAE suppression tuning curves (STCs) reflect major features of behavioral tuning (psychophysical tuning curves, PTCs) in normally-hearing listeners. Here, we aim to evaluate whether SFOAE STCs reflect changes in PTC tuning in cases of abnormal hearing. Design: PTCs and SFOAE STCs were obtained at 1 kHz and/or 4 kHz probe frequencies. For exploratory purposes, we collected SFOAEs measured across a wide frequency range and contrasted them to commonly measured distortion product (DP) OAEs. Study sample: Thirteen listeners with varying degrees of sensorineural hearing loss. Results: Except for a few listeners with the most hearing loss, the listeners had normal/nearly normal PTCs. However, attempts to record SFOAE STCs in hearing-impaired listeners were challenging and sometimes unsuccessful due to the high level of noise at the SFOAE frequency, which is not a factor for DPOAEs. In cases of successful measurements of SFOAE STCs there was a large variability in agreement between SFOAE STC and PTC tuning. Conclusions: These results indicate that SFOAE STCs cannot substitute for PTCs in cases of abnormal hearing, at least with the paradigm adopted in this study.     

Effects of a perilymphatic fistula on the passive vibration response of the basilar membrane

In this study, a three-dimensional finite-element model of the passive human cochlea was created. Dynamic behavior of the basilar membrane caused by the vibration of the stapes footplate was analyzed considering a fluid-structure interaction with the cochlear fluid. Next, the effects of a perilymphatic fistula (PLF) on the vibration of the cochlea were examined by making a small hole on the wall of the cochlea model. Even if a PLF existed in the scala vestibuli, a traveling wave was generated on the basilar membrane. When a PLF existed at the basal end of the cochlea, the shape of the traveling wave envelope showed no remarkable change, but the maximum amplitude became smaller at the entire frequency range from 0.5 to 5kHz and decreased with decreasing frequency. In contrast, when a PLF existed at the second turn of the cochlea, the traveling wave envelope showed a notch at the position of the PLF and the maximum amplitude also became smaller. This model assists in elucidating the mechanisms of hearing loss due to a PLF from the view of dynamics.     

Effect of contralateral acoustic stimulation on cochlear tuning measured using stimulus frequency and distortion product OAEs

Abstract

Objective: To study whether a change in cochlear tuning, measured using OAEs, could be detected due to contralateral activation of the efferent system using broadband noise. Design: Cochlear tuning measures based on SFOAE phase gradients and SFOAE-2TS ‘Q’ were used to test this hypothesis. SFOAE magnitude and phase gradient were measured using a pure-tone sweep from 1248 to 2496 Hz at 50 dB SPL. 2TS curves of SFOAE were recorded with a suppressor frequency swept from 1120 to 2080 Hz at 50 dB SPL. DPOAE f2-sweep phase gradient was also obtained to allow comparisons with the literature. All three assays were performed across with- and no-CAS conditions. Study sample: Twenty-two young, normal-hearing adults. Results: CAS did not produce a statistically significant change in the tuning metric in any of the OAE methods used, despite producing significant reductions in the OAE magnitude. Conclusion: It is unknown whether this insensitivity to CAS is due to an insensitivity of these three measures to cochlear mechanical tuning. The results suggest that any changes in tuning induced by CAS that may occur are small and difficult to detect using the OAE measurement paradigms used here.     

Correspondence amongst microstructure patterns observed in otoacoustic emissions and Békésy audiometry.

Similar patterns of microstructure have been reported in normal ears for Békésy threshold recordings and various forms of otoacoustic emissions (OAE). It has been suggested that they have a common origin associated with the amplifying function of the outer hair cell system and wave interactions occurring within cochlear mechanics. Fine-frequency Békésy audiometry was conducted in ten normal ears and its microstructure was compared with that recorded using two OAq techniques: stimulus frequency (SFOAE) and distortion product (DPOAE). All sweeps encompassed the frequency range from 992 to 2000 Hz in 16-Hz steps. The same probe was used for all Békésy and OAE recordings to eliminate transducer effects. SFOAEs were obtained with stimulus intensities of 0, 3, 6 and 9 dB. DPOAEs were obtained for 2F1-F2 with primary levels (L1/L2) of 40/30, 45/35, 50/40 and 55/45 dB. Reliable microstructure was recorded in all ears. Mean values of microstructure peak spacing ranged from 5.6 to 9.3 per cent amongst methods, consistent with published data. Microstructure was similar within each OAE method for different stimulus intensities for each subject. However, comparisons between Békésy and OAEs, or between OAE methods, did not show the strong correspondence that would be expected if there were a simple common origin to the microstructure. There was weak support for the expected correspondence between Békésy and SFOAE, but no support for any correspondence between Békésy and DPOAE. It is concluded that the various forms of microstructure cannot be explained by a simple common origin.     

Audiometric predictions using stimulus-frequency otoacoustic emissions and middle ear measurements

OBJECTIVE: The goals of the study are to determine how well stimulus-frequency otoacoustic emissions (SFOAEs) identify hearing loss, classify hearing loss as mild or moderate-severe, and correlate with pure-tone thresholds in a population of adults with normal middle ear function. Other goals are to determine if middle ear function as assessed by wideband acoustic transfer function (ATF) measurements in the ear canal account for the variability in normal thresholds, and if the inclusion of ATFs improves the ability of SFOAEs to identify hearing loss and predict pure-tone thresholds. DESIGN: The total suppressed SFOAE signal and its corresponding noise were recorded in 85 ears (22 normal ears and 63 ears with sensorineural hearing loss) at octave frequencies from 0.5 to 8 kHz, using a nonlinear residual method. SFOAEs were recorded a second time in three impaired ears to assess repeatability. Ambient-pressure ATFs were obtained in all but one of these 85 ears and were also obtained from an additional 31 normal-hearing subjects in whom SFOAE data were not obtained. Pure-tone air and bone conduction thresholds and 226-Hz tympanograms were obtained on all subjects. Normal tympanometry and the absence of air-bone gaps were used to screen subjects for normal middle ear function. Clinical decision theory was used to assess the performance of SFOAE and ATF predictors in classifying ears as normal or impaired, and linear regression analysis was used to test the ability of SFOAE and ATF variables to predict the air conduction audiogram. RESULTS: The ability of SFOAEs to classify ears as normal or hearing impaired was significant at all test frequencies. The ability of SFOAEs to classify impaired ears as either mild or moderate-severe was significant at test frequencies from 0.5 to 4 kHz. SFOAEs were present in cases of severe hearing loss. SFOAEs were also significantly correlated with air conduction thresholds from 0.5 to 8 kHz. The best performance occurred with the use of the SFOAE signal-to-noise ratio as the predictor, and the overall best performance was at 2 kHz. The SFOAE signal-to-noise measures were repeatable to within 3.5 dB in impaired ears. The ATF measures explained up to 25% of the variance in the normal audiogram; however, ATF measures did not improve SFOAEs predictors of hearing loss except at 4 kHz. CONCLUSIONS: In common with other OAE types, SFOAEs are capable of identifying the presence of hearing loss. In particular, SFOAEs performed better than distortion-product and click-evoked OAEs in predicting auditory status at 0.5 kHz; SFOAE performance was similar to that of other OAE types at higher frequencies except for a slight performance reduction at 4 kHz. Because SFOAEs were detected in ears with mild to severe cases of hearing loss, they may also provide an estimate of the classification of hearing loss. Although SFOAEs were significantly correlated with hearing threshold, they do not appear to have clinical utility in predicting a specific behavioral threshold. Information on middle ear status as assessed by ATF measures offered minimal improvement in SFOAE predictions of auditory status in a population of normal and impaired ears with normal middle ear function. However, ATF variables did explain a significant fraction of the variability in the audiograms of normal ears, suggesting that audiometric thresholds in normal ears are partially constrained by middle ear function as assessed by ATF tests.     

Simultaneous Intracochlear Pressure Measurements from Two Cochlear Locations: Propagation of Distortion Products in Gerbil

Sound energy propagates in the cochlea through a forward-traveling or slow wave supported by the cochlear partition and fluid inertia. Additionally, cochlear models support traveling wave propagation in the reverse direction as the expected mechanism for conveying otoacoustic emissions out of the cochlea. Recently, however, this hypothesis has been questioned, casting doubt on the process by which otoacoustic emissions travel back out through the cochlea. The proposed alternative reverse travel path for emissions is directly through the fluids of the cochlea as a compression pressure in the form of a fast wave. In the present study, a custom-made micro-pressure sensor was used in vivo in the gerbil cochlea to map two-tone-evoked pressure responses at distinct longitudinal and vertical locations in both the scala tympani and scala vestibuli. Analyses of the magnitude and phase of intracochlear pressure responses at the primary tone and distortion product frequencies were used to distinguish between fast and slow waves in both the forward- and reverse-propagation directions. Results demonstrated that distortion products may travel in both forward and reverse directions post-generation and the existence of both traveling and compression waves. The forward-traveling component appeared to duplicate the process of any external tone, tuned to the local characteristic-frequency place, as it increased compressively and nonlinearly with primary-tone levels. A compression wave was evidenced at frequencies above the cutoff of the recording site. In the opposite direction, a reverse-traveling wave played the major role in driving the stapes reversely and contributed to the distortion product otoacoustic emission. The compression wave may also play a role in reverse propagation when distortion products are generated at a region close to the stapes.     

Development of 2f1-f2 otoacoustic emissions in the rat

2f1-f2 otoacoustic emissions have been recorded from the rat cochlea during its development. Acoustic responses were recorded at 3, 5 and 7 kHz using a fixed value of the f2/f1 ratio (= 1.17). The first 2f1-f2 acoustic responses were obtained at 12 days after birth for 2f1-f2 = 7 and 5 kHz, and 2 days later for 2f1-f2 = 3 kHz. Adult-like patterns of the acoustic responses were achieved by day 18 for 2f1-f2 = 3 kHz, by day 20 for 2f1-f2 = 5 kHz and by day 28 for 2f1-f2 = 7 kHz. These results are discussed in relation to the available anatomical and functional data on the cochlear development of the rat. The delayed appearance of the 3 kHz acoustic responses might be related to the basal-to-apical gradient of morphological cochlear maturation. The fact that the 2f1-f2 otoacoustic emissions reached adult characteristics from the low to high frequencies is consistent with the development of the tuning properties of the basilar membrane. The long development of the 2f1-f2 acoustic responses at 7 kHz suggests that the organ of Corti undergoes subtle changes well after the end of its apparent maturation.     

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.     

Otoacoustic Estimation of Cochlear Tuning: Validation in the Chinchilla

We analyze published auditory-nerve and otoacoustic measurements in chinchilla to test a network of hypothesized relationships between cochlear tuning, cochlear traveling-wave delay, and stimulus-frequency otoacoustic emissions (SFOAEs). We find that the physiological data generally corroborate the network of relationships, including predictions from filter theory and the coherent-reflection model of OAE generation, at locations throughout the cochlea. The results support the use of otoacoustic emissions as noninvasive probes of cochlear tuning. Developing this application, we find that tuning ratios—defined as the ratio of tuning sharpness to SFOAE phase-gradient delay in periods—have a nearly species-invariant form in cat, guinea pig, and chinchilla. Analysis of the tuning ratios identifies a species-dependent parameter that locates a transition between “apical-like” and “basal-like” behavior involving multiple aspects of cochlear physiology. Approximate invariance of the tuning ratio allows determination of cochlear tuning from SFOAE delays. We quantify the procedure and show that otoacoustic estimates of chinchilla cochlear tuning match direct measures obtained from the auditory nerve. By assuming that invariance of the tuning ratio extends to humans, we derive new otoacoustic estimates of human cochlear tuning that remain mutually consistent with independent behavioral measurements obtained using different rationales, methodologies, and analysis procedures. The results confirm that at any given characteristic frequency (CF) human cochlear tuning appears sharper than that in the other animals studied, but varies similarly with CF. We show, however, that the exceptionality of human tuning can be exaggerated by the ways in which species are conventionally compared, which take no account of evident differences between the base and apex of the cochlea. Finally, our estimates of human tuning suggest that the spatial spread of excitation of a pure tone along the human basilar membrane is comparable to that in other common laboratory animals.     

Electrically Evoked Medial Olivocochlear Efferent Effects on Stimulus Frequency Otoacoustic Emissions in Guinea Pigs

Stimulus frequency otoacoustic emissions (SFOAEs) are produced by cochlear irregularities reflecting energy from the peak region of the traveling wave (TW). Activation of medial olivocochlear (MOC) efferents reduces cochlear amplification and otoacoustic emissions (OAEs). In other OAEs, MOC activation can produce enhancements. The extent of MOC enhancements of SFOAEs has not been previously studied. In anesthetized guinea pigs, we electrically stimulated MOC fibers and recorded their effects on SFOAEs. MOC stimulation mostly inhibited SFOAEs but sometimes enhanced them. Some enhancements were not near response dips and therefore cannot be explained by a reduction of wavelet cancelations. MOC stimulation always inhibited auditory-nerve compound action potentials showing that cochlear-amplifier gain was not increased. We propose that some SFOAE enhancements arise because shocks excite only a small number of MOC fibers that inhibit a few scattered outer hair cells thereby changing (perhaps increasing) cochlear irregularities and SFOAE amplitudes. Contralateral sound activation is expected to excite approximately one third of MOC efferents and may also change cochlear irregularities. Some papers suggest that large SFOAE components originate far basal of the TW peak, basal of the region that receives cochlear amplification. Using a time-frequency analysis, we separated SFOAEs into components with different latencies. At all SFOAE latencies, most SFOAE components were inhibited by MOC stimulation, but some were enhanced. The MOC inhibition of short-latency SFOAE components is consistent with these components being produced in the cochlear-amplified region near the TW peak.     

Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity.