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Intra-organ of Corti displacement measurements made via optical coherence tomography have provided significant information about cochlear micromechanics in recent years. However, several limitations inherent to this modality have complicated interpretation of these measurements. For one, optical coherence tomography measures the one-dimensional projection onto the optical axis of a three-dimensional motion. Also, the optical axis may make a substantial angle with the basilar membrane normal, meaning that structures along the optic axis, measured in a single measurement, may lie in different tonotopic cross-sections. We have developed a method that accounts for both of these complications, to reconstruct the two-dimensional longitudinal-transverse components of displacements of structures within the organ of Corti. This is performed by taking data at multiple longitudinal positions at two viewing angles, without any a priori knowledge of the measurement locations or viewing angles. We present a sample data set in which we have applied this program to reconstruct the transverse-longitudinal motion in the outer hair cell region in the base of the sensitive gerbil cochlea. The results reinforce the importance of accounting for viewing angle when analyzing and reporting vibration results. [ABSTRACT FROM AUTHOR]

Two-tone suppression (TTS) is the nonlinear phenomenon in which cochlear responses in the region most sensitive to a probe tone are reduced if a second tone (suppressor) of different frequency is simultaneously presented. TTS occurs due to saturation of mechanoelectrical transducers (MET) in the outer hair cells. The nonlinear dependence of MET channel conductance is the main cause of nonlinearity in cochlear amplification. Although the cochlear amplifier primarily acts locally, it influences distant fluid coupled parts of the basilar membrane (BM). The question therefore arises, how the fluid coupling contributes to the TTS. To answer this question, TTS was analysed using a 2D nonlinear cochlear model in two cases: for a suppressor of higher frequency and for a suppressor of lower frequency than the probe tone. It is shown that the BM-BM hydrodynamical coupling redistributed the excess of the power generated by the OHC electromechanical feedback force. It is also shown that the short-range part of the BM-BM coupling plays an important role in power balance along the BM. Moreover, it turned out from the model simulations that the suppression by a low-frequency suppressor causes a basal shift of the amplitude maximum and affects the phase of the BM stationary response. In contrast, the suppression by a high-frequency suppressor does not significantly affect the BM response phase; it flattens the amplitude of the BM stationary response, which leads to an apical shift of the amplitude maximum. [ABSTRACT FROM AUTHOR]

Distortion product otoacoustic emissions (DPOAEs) exhibit features at low frequencies that are distinct from high-frequency ones in that they are less ratio-dependent: their characteristics (amplitude and phase) vary little with the frequency separation of the primary tones. The underlying mechanism may arise from the unique apical cochlear mechanics that are poorly understood. Intracochlear observations provide evidence for the generation mechanism of DPOAEs. In the current study, two-tone induced distortion products (DPs) were recorded from the apically located organ of Corti complex [OCC includes the organ of Corti, the basilar membrane, and the tectorial membrane] of alive gerbil cochleae using optical coherence tomography (OCT) together with simultaneously measured ear canal DPOAEs. Within the OCC, DPs were robust and recorded from all key structures. The ratio-dependency of DPs appeared to link to the characteristics of the primary tones that were broadly tuned and exhibited nonlinearity in a broad region. The upper sideband, 2f2− f1 DP was found ratio independent due to the invariant overlapping region of the primary tones. The lower sideband, 2f1 − f2 DP differed, and varied with the f2/f1 ratio. It included both local and nonlocal (i.e. traveling) DP components. Thus, their correlation to related DPOAEs was not straightforward. Our observation provides insights into the generation of low-frequency DPOAEs and related cochlear mechanics, which helps to improve the precision of DPOAEs in detecting auditory sensory damage in the clinic. [ABSTRACT FROM AUTHOR]

When elicited by two stimulus tones (at frequencies f1 and f2, f2 > f1), the amplitudes of specific distortion-product otoacoustic emission (DPOAE) components exhibit a characteristic bandpass shape as the ratio between f2 and f1 is varied. This bandpass shape has been attributed to various mechanisms including intracochlear resonance, suppression, and wave interference, and has been proposed to be related to cochlear frequency tuning. While human studies suggest modest correlations between psychophysical tuning and the tuning of DPOAE amplitude vs. f2/f1 ratio functions, a relationship between the latter and the tuning of cochlear mechanical responses has yet to be established. This was addressed here through direct comparisons of DPOAEs and cochlear vibrations in wild-type CBA/CaJ mice. DPOAEs were elicited using a fixed-f2, swept-f1 paradigm, and optical coherence tomography was used to measure displacements from cochlear locations with characteristic frequencies near f2. The tuning sharpness of 2f1−f2 DPOAE ratio functions was found to be remarkably similar to that of basilar membrane and/or tectorial membrane responses to single tones, with the tuning sharpness of all responses increasing similarly with decreasing stimulus level. This relationship was observed for f2 frequencies ranging from ∼8 to 22 kHz. Intracochlear distortion products did not exhibit a bandpass shape as the f2/f1 ratio was varied, indicating that interference between distortion products traveling to the stapes may be responsible for the tuning of the DPOAE ratio function. While these findings suggest that DPOAE ratio functions could be used to noninvasively infer cochlear tuning, it remains to be determined whether this relationship holds in other species and for lower frequency regions. [ABSTRACT FROM AUTHOR]

The inner ear response to a tone can be diminished in the presence of another tone through the process of suppression. Typically, the strongest suppression is expected when both tones, the probe and the suppressor, are of similar frequency and fall within the region of active amplification on the basilar membrane (BM). However, suppression can occur in otoacoustic emission (OAE) responses even when the suppressor tone is more than an octave above the evoking probe stimulus—a result that does not mimic BM behavior. The recent development of new imaging techniques (Optical Coherence Tomography, OCT) has brought an unexpected twist to these results: Whereas two-tone suppression is limited to the tonotopic peak region when measured at the BM, the same does not hold true when vibrations are measured in the organ of Corti (OoC). For instance, vibrations at the top of OoC are actively amplified and suppressed over a broad region corresponding to the tail of the traveling wave. The purpose of this study is to compare suppression patterns as measured in ear-canal pressure and organ of Corti vibrations. We measured cochlear vibrations and ear-canal pressure in mice in response to tones swept in frequency at narrow (1.1) and wide (2.1) suppressor-to-probe ratios. The OAE response (residual) is extracted by vector subtraction of ear-canal responses to probe-alone vs. probe swept together with the suppressor. Preliminary data reveal that OAE residuals can be detected for both narrow (1.1) and wide (2.1) ratios in mice. The OAE residuals obtained with wide ratios grow linearly with the stimulus. In contrast, the narrow-ratio OAE residuals tend to have more compressive growth. When residual responses are extracted from the vibrations of the organ of Corti, a similar pattern emerges: the wide-ratio residuals tend to have more linear growth compared to the narrow-ratio residuals. These patterns suggest that wide-ratio OAE residuals may correlate more closely with vibrations within the organ-of-Corti vibrations than with the BM. [ABSTRACT FROM AUTHOR]

Our exquisite sense of hearing involves micromechanical inner-ear processes that precede sound transduction by hair cells. These mechanics are however not well understood, especially in the apical regions of the cochlea that are essential for hearing the relatively low frequencies important for the intelligibility of speech. Recent observations in the high-frequency region of the cochlea's sensory organ of Corti (ooC) revealed unanticipated and complex motions, but low-frequency responses are not known. Here, we measured sound-evoked motions from the outer hair cell (OHC) region and the lateral compartment over an extended region in the apex of gerbil cochleae, and compared their phases to the basilar membrane (BM) response. By combining data from multiple tonotopic recording locations, low-frequency traveling waves that propagated along the BM with frequency dependent wavelengths between 1–3 mm were observed. In addition, we found a nonzero OHC−BM phase difference which systematically depended on the angle between the OCT optical beam and the BM along its longitudinal axis. This result establishes that the OHC vibrations were predominantly in the longitudinal direction, orthogonal to the up-down motion of the BM. Such motion is unavoidable in the presence of traveling, surface (BM) waves, but has such a significant magnitude that it masks the detection of OHC vibrations within the cross-sectional plane. With this, interpreting relative ooC motions in terms of the cochlear micro-mechanisms underlying our ear's remarkable sensitivity and frequency selectivity is tentative because their directions are not a priori known and vibrations along the third cochlear dimension must be considered as an important contributor to the recorded response. [ABSTRACT FROM AUTHOR]

The cochlear tonotopic map determines where along the basilar membrane traveling waves of different frequencies peak. Endolymphatic hydrops has been hypothesized to shift the tonotopic map by altering the stiffness of the cochlear partition, especially in the apex. In this exploratory study performed in a handful of normal and hydropic ears, we report preliminary measurements of interaural differences assayed using behavioral pitch-matching supplemented by measurements of reflection otoacoustic-emission phase-gradient delays. [ABSTRACT FROM AUTHOR]

While direct measurements of the basilar membrane (BM) and auditory nerve fiber tuning are possible in lab animals [1] but no psychoacoustic measurements (with rare exceptions [2]), only otoacoustic emission and psychophysical tuning curve measurements [2,3,4] are possible in humans. In an attempt to partly address this missing link, the three-dimensional physiology-based nonlinear mechanical-electrical-acoustic (MEA) model of the human cochlea is developed in this study to enable prediction of both otoacoustic emissions as well as BM and organ of Corti vibration tuning. The nonlinear MEA model of the human cochlea adapts our prior linear MEA model of the guinea pig cochlea [5,6] to humans and includes the nonlinearity due to mechano-electrical transduction in hair bundles. The transmission line human middle ear model, inspired by O'Connor and Puria [7], is integrated with the nonlinear MEA model. The passive BM displacement predicted by the model is compared with the in vivo physiological measurements for the BM displacement in human cadavers [8], and the predicted active BM tuning curves are compared with human psychophysical tuning curves (PTC) from Moore (1978) [9]. The distortion product otoacoustic emissions (DPOAE), predicted by the model at equal stimulus levels and at different stimulus frequencies, have a maximum at around f2/f1=1.2, agreeing with measured data in the literature [10,11]. [ABSTRACT FROM AUTHOR]

The role of cochlear outer hair cells (OHCs) in mammal hearing is compromised by the 'RC time constant problem'. The issue arises because if the cells are to operate at high acoustic frequencies conventional voltage driven 'electromotility' is low pass filtered by the cell membrane. By synthesising a description of the OHC as a piezoelectric actuator and its position between resonant basilar and tectorial membranes it is shown that sharp tuning can arise at high frequency if prestin/SLC26A5 is dynamically tensioned. The model predicts an imaginary (dissipative) component to the OHC capacitance as well as two distinct frequency tuning curve (ftc) shapes: a near symmetrical ftc at low CF, and a low frequency 'tail' on the more sharply tuned high CF ftcs [ABSTRACT FROM AUTHOR]

Psychophysical models often begin with a bank of filters and assume that the energy in the filter outputs is the basis for coding complex sounds. The filter bandwidths are estimated based on masked detection thresholds and are referred to as critical bands. While this conceptual model structure can explain many psychophysical phenomena, it also fails for many tasks. We propose a different conceptual model: a bank of peripheral filters, each followed by a saturating nonlinearity that represents transduction by inner hair cells (IHCs). The metric of interest is the amplitude of low-frequency fluctuations in the hair-cell responses. Low-frequency fluctuations in the IHC voltage drive fluctuations in the auditory-nerve (AN) responses, referred to here as "neural fluctuations" (NF). NFs are a temporal response feature distinct from phase-locking to temporal fine structure, and they influence AN responses across a wide range of frequencies. The saturating nonlinearity that follows each peripheral filter results in neural fluctuations that differ markedly from both the envelope of the stimulus and the envelope of a bandpass-filtered stimulus representing the basilar membrane response. A key difference between neural fluctuations and stimulus-related envelopes occurs when IHCs tuned near a spectral peak are "captured" (or dominated) by the peak in the stimulus spectrum (Deng and Geisler, 1987; Zilany and Bruce, 2007). A peripheral channel that is captured has relatively small fluctuations in the response, whereas channels that are not captured can have strongly fluctuating responses, due to beating between multiple stimulus components. The profile of neural fluctuation amplitudes along the characteristic-frequency (CF) axis encodes key features of the stimulus spectrum. For example, in the case of masking paradigms, a local decrease in the neural-fluctuation amplitude profile encodes the presence and frequency of the target tone. An understanding of the role of the saturating IHC transduction nonlinearity in the proposed model is facilitated by a computational model for the periphery that includes the interaction between the saturating nonlinearity and compressive cochlear gain. We will demonstrate that neural-fluctuation profiles in response to psychophysical masking paradigms can predict performance in several tasks for which the classical critical-band model fails (as well as those for which it succeeds.) Our goal is to determine whether a single model can predict results for several masked-detection tasks, including band-widening paradigms, with or without roving-levels, and co-modulation masking release. The profile of neural fluctuations along the tonotopic axis is of interest because most midbrain neurons are excited or suppressed over a range of amplitude-modulation frequencies (Kim et al., 2020). The fluctuation profiles that are set up in the periphery are thus transformed into rate profiles at the level of the auditory midbrain. Rate profiles across populations of model midbrain neurons were the basis for the psychophysical predictions described below. Finally, the degree of contrast in the neural-fluctuation profiles is reduced by sensorineural hearing loss, suggesting that fluctuation profiles may provide insight into deficits associated with hearing loss. [ABSTRACT FROM AUTHOR]

Dysfunction of the outer hair cells (OHCs) is thought to lead to hearing loss. However, direct diagnosis of the OHC dysfunction is difficult, and the activity of the OHC is indirectly estimated by measuring distortion product otoacoustic emissions (DPOAEs). In this study, a finite-element model of the normal cochlea considering the activity of the OHC and hearing impairment models in which the activity of the OHC was partially absent were constructed. A pure tone or a complex tone was applied to the models, and the relationship between DPOAE level and non-active portion on the basilar membrane was investigated. As a result, it was speculated that the dysfunction of the OHC at the basal side of the position of the characteristic frequency for the input frequency affects pure tone hearing and DPOAE levels. [ABSTRACT FROM AUTHOR]

Stimulus-frequency otoacoustic emissions (SFOAEs) are evoked by a single tone presented into the ear. The emissions are generated by reflection of a forward traveling wave (TW) on localized irregularities in the impedance along the basilar membrane (BM). The strongest wavelets are reflected from the place where the TW reaches its largest amplitude. Since the irregularities are localized in fixed places along the basilar membrane, the latency of the SFOAE phase can be used to estimate delay of cochlear filters. We used an iterative approach of Shera et al. [JASA (2005) 118:287-313] to obtain an analytical solution for SFOAEs in a nonlinear two-dimensional cochlear model. The solution allowed for decomposition of a reflection component and a component due to perturbation of the nonlinear force. The nonlinear force which in the smooth cochlea may reflect the forward TW backwards [Talmadge et al., JASA (2000) 108, 2911-2931], is perturbed due to irregularities. The perturbed nonlinear force in the model generates an SFOAE component with latency comparable with the latency of the component due to reflection. This means that although the component is evoked due to nonlinear force, it has a comparable (long) latency to the SFOAE component due to reflection. The amplitude of this component due to perturbation of the nonlinear force grows as the amplitude of irregularities increases. In addition, the component amplitude also grows with the tone intensity. The growth is determined by the amplitude of the nonlinear force. It is almost cubic at the lowest intensities and quickly saturates at levels above about 40 dB SPL (the intensity is determined by the position of the compressive nonlinearity in the input/output function of the basilar membrane displacement). An interesting result is that the nonlinear component partly cancels the reflection component because its phase is shifted by approximately 0.5 cycles, independent of stimulus intensity and frequency. As the level increases, the component due to perturbation reaches an amplitude comparable to the component due to coherent reflection, especially in the portions of SFOAEs with the longest latencies. The destructive interference between the components due to perturbation of the nonlinear force and coherent reflection emphasizes shorter latency wavelets in the overall SFOAEs as stimulus intensity increases and contributes to the saturation of SFOAE amplitude reported, for example, in human experimental data of Abdala and Kalluri [JASA (2017) 142, 812-824]. [ABSTRACT FROM AUTHOR]

Over the last half century, the intense activity upon the mechanics of mammalian hearing falls into two broad categories: 1) Studies by sensory physiologists (e.g., Brownell, Flock, Russell) interested in the holistic active mechanical properties of the outer hair cells (OHC) and 2) studies by theoreticians and practitioners who have been attracted into the field seeking to explain the central question of audition, i.e. What accounts for the extreme sensitivity and vulnerability of mammalian hearing? How can the critical mechanism which goes missing not be best described as amplification, if, when sensitivity is lost, it can be compensated by use of an external amplifier? The historic starting point has been the Bekesy two-chamber model of the cochlea. The general, the Fourier transform of basilar membrane (BM) vibrations would normally have a zero-frequency (or dc-) component. However, because of the historic basis of the standard model, any such component has been systematically disqualified from consideration. This presenter was the first to publish that a dc-component of BM motion not only does exist, but it covaries with the vibrational component in ways which mimicked the polarity variation of the summating potential (LePage, JASA, 1987). The later published data (full method details, LePage, Hear. Res., 1989) did not have the full support of all collaborators beyond the development of the fiber-optic sensor. However, using the finished device, it was shown, to the real-time disbelief of B.M. Johnstone, that the BM displacement displayed properties very similar to the contractile components of the OHC both in temporal dependence and amplitudes equalling fractions of the length of the OHC. These fiber-optic data have been extracted from many paper-chart records, digitised, then reprocessed with MATLAB(TM) to generate a video to bring to new life what was originally only rapid large excursions of the pen on the single channel chart recorder. These BM dc-shifts are indeed highly tuned. Their tuning varies as expected with the manipulations originally described. Moreover, the velocity of these excursions is of the same order with the estimates of velocity of BM vibrations from dozens later investigations which have uniformly assumed the unseen motion was exclusively a vibration at the stimulus frequency. This presentation therefore raises deep questions about whether the two-chamber model has remained a dominant influence on the progressively-refined design of BM measurements – as well as the widespread rejection of data from many studies whose results made no sense in conventional terms of a fragile and "mysterious" cochlear amplifier (CA). Could it be that the expected super fragile tuning mechanism was the oft-employed explanation, when an equally plausible explanation could be a robust mechanism based on the OHC, modulated by an unknown but critical tiny BM baseline bias? [ABSTRACT FROM AUTHOR]

Stimulus frequency otoacoustic emissions (SFOAEs), which are sounds emitted by the cochlea at the frequency of the stimulus, have been used as a noninvasive measure of cochlear function. The gradient of the SFOAE phases characterizes the latency of emission and is associated with the frequency selectivity and sharpness of tuning of the mammalian cochlea. However, whether the phase-gradient delay of SFOAE can be used as a indicator of cochlear tuning and sensitivity reliably when the proper-ties of the cochlea change remains unclear. The objective of this study is to address this question by varying cochlear model activity, tectorial membrane (TM) properties and organ of Corti (OoC) micromechanical properties to change cochlear tuning. In this work, a three-dimensional gerbil cochlear model that couples mechanical, electrical and acoustic domains with cochlear roughness has been used. The roughness is implemented on outer hair cells (OHCs) force acting on the basilar membrane (BM). Parameters that control the activity levels, TM longitudinal coupling and OoC impedance are varied. The results show that changes in sharpness of tuning due to reduction in cochlear activity and TM longitudinal coupling can be detected by using SFOAE phase-gradient delay. However, changes in cochlear tuning due to changes in OoC impedance are not necessarily reflected by corresponding changes in SFOAE phase-gradient delay. [ABSTRACT FROM AUTHOR]

A detailed model of the physical processes occurring within the organ of Corti can be compared with experimental data and can give an indication of the mechanisms of its active behaviour. On the other hand, a model that involves wave propagation in the cochlea can give more insight into its coupled response. In principle many kinds of waves can propagate in the cochlea, but it is a single travelling wave that is generally assumed to mainly determines its coupled response, which can be characterized by the frequency variation of its complex wavenumber. In the case of a cochlear model using a locally reacting basilar membrane (BM) and 1D fluid coupling, this wavenumber can be calculated explicitly. When longitudinal coupling, due to the mechano-electrical structure of the Organ of Corti or other forms of fluid coupling are introduced however, additional wave types are possible, and it is not so straightforward to calculate the wavenumber of the main travelling wave. This paper is the second of a series, in which a method is presented of deriving the wavenumber distribution associated to different wave types, based on an elemental model of the cochlea. This allows an investigation of the effect of different forms of longitudinal coupling on the wave motion. In general, the main travelling wave dominates the BM response, and when the model is active, the imaginary part of its wavenumber is positive in a frequency region just before the characteristic frequency. The extent of this active region depends on the form of longitudinal coupling assumed in the organ of Corti. [ABSTRACT FROM AUTHOR]

Recent optical measurements of vibrations of the organ of Corti contrast with those of the basilar membrane in their larger amplitude and much wider bandwidth of active gain and nonlinearity [6-9, 15, 16, 19, 20]. A report by Ren and He (19) demonstrates a considerably richer spectrum of intermodulation distortion in the motion of the reticular lamina than in the basilar membrane of the Mongolian gerbil cochlea. Data from this species from my lab also show a broad distortion spectrum much more similar to the reticular lamina than the basilar membrane in the Ren and He data. This result, along with other evidence of broad-band nonlinearity in ear canal acoustics strongly suggests the need to reevaluate otoacoustic emission (OAE) phenomena. In particular, these experimental data do not support reverse slow basilar membrane traveling waves [23] as the dominant mode of conducting OAE signals to the ear canal, but they are instead conducted primarily by fluid pressure in scala vestibuli, at least for this species in the basal region of the cochlea. Several important unresolved issues in OAEs may be resolved by simultaneous measurements of vibrations in the organ of Corti and basilar membrane, together with comparable measurements in ear canal acoustics. [ABSTRACT FROM AUTHOR]

The synchronized spontaneous (SS) otoacoustic emission (OAE) refers to the long-lasting oscillation evoked by clicks after transient-evoked OAEs (TEOAEs) attenuate. This research aims to study the SSOAE spectro-temporal dynamics by comparing human data and simulation. The simulation of SSOAE is based on a computer model of cochlear mechanics. In the model, the roughness of the basilar membrane (BM) can be adjusted and its effects on SSOAE patterns are observed. It is noted that the change in roughness level (5%, 10%, 15%, and 20%) affects the likelihood distribution between single-frequency, two-frequency, and a band-of–frequencies components. There is a general trend that band-of-frequencies occur more as the roughness level increases. The simulation result shows that, unlike the roughness level of 5% and 10%, roughness levels of 15% and 20% can generate SOAE oscillation below 2 kHz. Signal patterns of simulation are similar to what is observed in human SSOAE data, and thus support the standing wave theory of SOAE. [ABSTRACT FROM AUTHOR]

The effect of different forms of longitudinal coupling on the active response of the cochlea are analysed using an elemental approach, based on a previous finite element model of the guinea pig cochlea that has three mechanical degrees of freedom. The overall basilar membrane (BM) admittance can be readily calculated using the elemental method, and since this is the only aspect of the organ of Corti dynamics that couples into the fluid, it is a useful indicator of its overall behaviour. As has been shown in previous studies, mechanical longitudinal coupling in the TM, together with 3D fluid coupling, are most important in obtaining a coupled BM frequency response that is both tall and broad, as observed in experimental data. An intuitive and efficient method is used for representing the longitudinal coupling in the fluid, by decomposing the 3D fluid coupling in the wavenumber domain into a local near-field mass loading of the organ of Corti and the analytically simpler 1D fluid coupling, representing long range fluid-structure coupling. The 3D fluid coupling is also necessary to obtain a phase variation that is consistent with experimental measurements. The model of the organ of Corti can be represented as a mechanical three degree of freedom system, and when it is locally reacting all the mechanical elements, and hence the BM admittance, are independent of the wavenumber. When longitudinal coupling is introduced in the TM, the stiffness and damping associated with the TM shear motion then depends on the wavenumber. Similarly, when the near field fluid coupling is associated with the BM mass in the mechanical model, this too is now a function of wavenumber. The model can then be used to calculate a wavenumber-dependent BM admittance, which can be combined with the simple analytical equation for 1D fluid coupling to give the dispersion equation for the waves in the coupled cochlea. [ABSTRACT FROM AUTHOR]

Based on a plethora of basilar membrane vibration measurements, it had been presumed that cochlear nonlinearity due to outer hair cell (OHC) forces is limited to the peak region of a given stimulus. However, recent experiments showing two-tone suppression of the reticular lamina (RL) indicate that OHCs provide electromotile feedback over a much broader basal region. In this work, a computational model of the cochlea containing organ of Corti (OoC) structures, a fluid domain coupled to the basilar membrane (BM), and an electrical model to represent OHC activity is calibrated to experimental data for the mechanical and electrical responses to a pure tone and then used to make two-tone suppression predictions. These predictions are in line with recent experimental measurements, capturing extended nonlinearity in the RL response compared to that of the BM. By comparing the OHC force of pure tone and two-tone model predictions, we examine the root cause of broader nonlinearity seen in the response of OoC structures at the top end of the OHCs. [ABSTRACT FROM AUTHOR]

In 2013 we published an experimental study (Versteegh and Van der Heijden, doi.org/10.1007/s10162-013-0393-0) that introduced a traveling-wave perspective of cochlear nonlinearity, the so-called spatial buildup of compression and suppression. We recorded basilar membrane (BM) motion in the base of the gerbil cochlea, and obtained experimental evidence for the spatial buildup by independently varying the frequencies of the suppressor and the probe. The study concluded that the nonlinear response at any point in the cochlea can only be properly understood in terms of a spatial buildup of compression. Traveling waves start out as linear responses to the stapes drive and gradually accumulate compressive nonlinearity after entering their peak region, ∼½ octave short of their best place. Likewise, the nonlinear effects of intensity on vibration phase can only be understood in terms of local changes in propagation velocity of the wave. Once viewed through this lens, a large body of published data on the effects of compression and suppression, often featuring complex patterns and seemingly contradicting trends, was shown to fit in a unifying framework. Despite the emphasis on spatial buildup, the data underling the 2013 study were obtained from single BM locations, due to the need for reflective microbeads with laser Doppler vibrometry. The spatial aspect was only explored by proxy, i.e., by varying the stimulus frequency and exploiting the tonotopy of the cochlear response ("scaling"). Optical coherence tomography (OCT) vibrometry has liberated us from this limitation, and in the current work we actually traced the spatial buildup of compression. In the BM and OHC region of the 13-24-kHz region of the gerbil cochlea, we recorded responses to equal-amplitude tone pairs centered at 15 kHz, spaced by 20 Hz. The resulting beating pattern affords a dynamic way of obtaining I/O curves. Spectral analysis of the responses yielded an accurate quantitative description of the compressive growth. We observed that the amount of compression (the difference in gain between the peak and dip episodes of the beat) increased gradually and systematically along the course of the slowly propagating wave, which confirms the spatial buildup claimed in our 2013 study. With increasing instantaneous intensity, the spatial rate of compression increased systematically over the nonlinear region. Overall, the conclusions of the 2013 study were confirmed, emphasizing the tight relation between cochlear compression and the wave character of cochlear vibrations. [ABSTRACT FROM AUTHOR]

The goal of this presentation is two-fold: The primary goal is to discuss my present understanding of cochlear function. A secondary goal is to review my earlier (1970-2021) cochlear modeling work, along with the roles of four close friends: Egbert De Boer, Steve Neely, Paul Fahey and George Zweig. To understanding of how the cochlea works, one needs an understanding of the experimental data on: 1) cochlear function (both basilar (BM) and tectorial membranes (TM)), 2) tympanic membrane, 3) middle ear (ME), 4) inner and outer hair cells (IHC, OHC), 5) auditory nerve (AN), and 6) cochlear amplifier (CA). My views on these topics have been greatly sharpened by looking back and unifying this complex puzzle. A great deal of progress has been made in the last 50 years. Conclusions: My recent review of neural tuning curve data from 1985, using nonlinear (NL) distortion product generation, has revealed a deeper understanding of cochlear function. The most important, and surprising result, is that the cochlea is much more linear in its filtering properties than I previously assumed. When the suppressor frequency fs is at least 1/2 octave lower than the characteristic ("best") frequency (fcf), it is best known as "low-side" suppression. There is no "low-side" suppression for suppressors below 65 [dB-SPL] Fahey and Allen [1]. For suppressors above 65 [dB-SPL], suppression is engaged, with a slope between 1-2 [dB/dB]. Since the excitation threshold is also 65 [dB-SPL], we conclude that the neural threshold of excitation to both the inner and outer hair cells have nearly the same threshold. That is the suppression threshold of the OHC are nearly equal to, the IHC threshold. This raises the interesting question: If the IHC and OHC 65 [dB] thresholds are the same in the tails of the tuning curves, how can the CA function at threshold levels? Furthermore this is a highly unexpected result because low-side suppression, as measured on the basilar membrane, has a 20-30 [dB] higher threshold [2, 3]. Is the OHC action restricted to the neighborhood of the neuron's best frequency? This would require that the neural low-side suppression and loudness recruitment (the reduced loudness of low-intensity sounds in the hearing-impaired ear) are closely related (i.e., are the same phenomena). The ramifications of this observation seem significant as they must impact our fundamental understanding of hearing and thus hearing loss [4], (p. 332, Allen90) [5]. In summary: Two-tone suppression acts like an automatic gain control, elevating the loudness threshold, with little audible distortion. We then discuss the properties of the CA, functionally measuring the CA gain. The URL for cited manuscripts: https://auditorymodels.org/index.php?n=Main.Publications; https://www.mechanicsofhearing.org/ [ABSTRACT FROM AUTHOR]

Two hydrodynamic effects are introduced in the standard transmission-line formalism, the focusing of the pressure and fluid velocity fields near the basilar membrane and the viscous damping at the fluid-basilar membrane interface, which significantly affect the cochlear response in the short-wave region. In this region, in which the wavelength is shorter than the cochlear duct height, only a layer of fluid of order of the wavelength is effectively involved in the traveling wave. This has been interpreted [8] as a reduced fluid contribution to the system inertia in the peak region, which is a viewpoint common to the 3-D FEM solutions. In this paper we propose an alternative approach, from a slightly different physical viewpoint. Invoking the fluid flux conservation along the traveling wave propagation direction, we can derive a rigorous propagation equation for the pressure integrated along the vertical axis. Consequently, the relation between the average pressure and the local pressure [4] at the fluid-BM interface can be written. The local pressure is amplified by a factor dependent on the local wavenumber with respect to the average pressure, a phenomenon we refer to as "fluid focusing", which plays a relevant role in the BM total amplification gain. This interpretation of the hydrodynamic boost to the pressure provides a physical justification to the strategy [10] of fitting the BM admittance with a polynomial containing both a conjugated pole and a zero. In the short-wave region, the sharp gradients of the velocity field yield a second important effect, a damping force on the BM motion, proportional to the local wavenumber, which stabilizes active models and shifts the peak of the response towards the base, with respect to the resonant place. This way, the peaked BM response is not that of a proper resonance, corresponding to a sharp maximum of the admittance, but rather a focusing-driven growth toward the resonant place, which is "aborted" before reaching it by the sharply increasing viscous losses. The large values of the wavenumber that ensure strong focusing are ultimately fueled, against viscosity, by the nonlinear OHC mechanism, hence the otherwise puzzling observation of a wide nonlinear gain dynamics with almost level-independent admittance. [ABSTRACT FROM AUTHOR]

One-dimensional measurements of sound-evoked motion were made using optical coherence tomography (OCT) in the high frequency, basal turn of the cochlea in deeply anesthetized guinea-pigs. Wideband acoustic stimuli were used to evoke nanometer-scale vibrations of the basilar membrane (BM) and organ of Corti (OoC). Both macro- and micro-mechanical aspects of the responses were analysed, where macro-mechanical refers to 'motions observed on the BM', and micro-mechanical refers to 'motions of OoC and related structures, relative to those observed on the BM'. Both macro- and micro-mechanical motions exhibited compressive nonlinearity across wide dynamic ranges (typically from ∼30 to >80 dB SPL near a site's characteristic frequency or CF). Only the macro-mechanical motion was sharply tuned, however – the micro-mechanical motion, which included a transverse 'squeezing' type of OoC deformation, was low-pass filtered (from well below CF), and not sharply tuned in its own right. The micro-mechanical motion was also delayed, accumulating ∼0.5 cycles of phase-shift with respect to the macro-mechanics between 0 Hz and CF. Putative cell-length changes were also derived in the Deiters' (DC) and outer hair cell (OHC) regions: Both DC and OHC length changes exceeded macro-mechanical movements by factors of ∼2-4 at low frequencies and moderate sound levels, but decreased in relative magnitude with increasing frequency and/or sound level. At low frequencies, OHCs became longer and shorter, and DCs became shorter and longer, when the BM moved towards the scala tympani and scala vestibuli, respectively. These phase relationships reversed at frequencies closer to CF, in keeping with the ∼0.5 cycles of phase-accumulation in the micro-mechanics. These observations are discussed with respect to previous findings, including a recent theoretical study that considered frequency-dependent changes in the mode-shape of a wave propagating along a simple physical waveguide [1]. [ABSTRACT FROM AUTHOR]

Using optical coherence tomography (OCT) vibrometry we studied wave propagation by recording at multiple longitudinal positions in the gerbil and guinea pig cochlea. Our aims are to (1) fully portray the unusual dispersive character of the wave; (2) derive from these properties the underlying physics of the wave and the way it is regulated by physiological processes; (3) challenge cochlear modelers to report and analyze wave propagation in their models. In the current study we report two unexpected features of the spatial profiles of sound-evoked vibrations of the basilar membrane. The profiles of low-frequency components show very little systematic phase accumulation, whereas the slow waves near the best frequency often exhibit abrupt, place-fixed transitions in propagation velocity. [ABSTRACT FROM AUTHOR]