A cochlear model using feedback from motile outer hair cells

Cochlear motion across the reticular lamina implies that it is not a stiff plate

Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40-47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters' cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.

Probing hair cell's mechano-transduction using two-tone suppression measurements

When two sound tones are delivered to the cochlea simultaneously, they interact with each other in a suppressive way, a phenomenon referred to as two-tone suppression (2TS). This nonlinear response is ascribed to the saturation of the outer hair cell’s mechano-transduction. Thus, 2TS can be used as a non-invasive probe to investigate the fundamental properties of cochlear mechano-transduction. We developed a nonlinear cochlear model in the time domain to interpret 2TS data. The multi-scale model incorporates cochlear fluid dynamics, organ of Corti (OoC) mechanics and outer hair cell electrophysiology. The model simulations of 2TS show that the threshold amplitudes and rates of low-side suppression are dependent on mechano-transduction properties. By comparing model responses to existing 2TS measurement data, we estimate intrinsic characteristics of mechano-transduction such as sensitivity and adaptation. For mechano-transduction sensitivity at the basal location (characteristic frequency of 17 kHz) at 0.06 nm⁻¹, the simulation results agree with 2TS measurements of basilar membrane responses. This estimate is an order of magnitude higher than the values observed in experiments on isolated outer hair cells. The model also demonstrates how the outer hair cell’s adaptation alters the temporal pattern of 2TS by modulating mechano-electrical gain and phase.

Modelling cochlear mechanics

The cochlea plays a crucial role in mammal hearing. The basic function of the cochlea is to map sounds of different frequencies onto corresponding characteristic positions on the basilar membrane (BM). Sounds enter the fluid-filled cochlea and cause deflection of the BM due to pressure differences between the cochlear fluid chambers. These deflections travel along the cochlea, increasing in amplitude, until a frequency-dependent characteristic position and then decay away rapidly. The hair cells can detect these deflections and encode them as neural signals. Modelling the mechanics of the cochlea is of help in interpreting experimental observations and also can provide predictions of the results of experiments that cannot currently be performed due to technical limitations. This paper focuses on reviewing the numerical modelling of the mechanical and electrical processes in the cochlea, which include fluid coupling, micromechanics, the cochlear amplifier, nonlinearity, and electrical coupling.

Physiologically inspired signal preprocessing for auditory prostheses: Insights from the electro-motility of the OHC

We designed a non-linear functional model of the outer hair cell (OHC) functioning in the filtering system of the cochlea and then isolated from it two second-order structures, one employing the mechanism of the somatic motility and the other the hair bundle motion of the OHC. The investigation of these circuits showed that the main mechanism increasing the sensitivity and frequency selectivity of the filtering system is the somatic motility. The mechanism of the active hair bundle motion appeared less suitable for realization of the band-pass filtering structures due to the dependence of the sensitivity, natural frequency and selectivity on the signal intensity. We combined three second-order filtering structures employing the mechanism of the somatic motility and the lateral inhibition to form a parallel-type filtering channel of the sixth order with the frequency characteristics of the Butterworth-type and Gaussian-type. The investigation of these channels showed that the Gaussian-type channel has the advantage over the Butterworth-type channel. It is more suitable for realization of a filter bank with common lateral circuits and has less distorted frequency characteristic in the nonlinear mode.

Comment on "Mutual suppression in the 6 kHz region of sensitive chinchilla cochleae" [J. Acoust. Soc. Am. 121, 2805-2818 (2007)]

Rhode [J. Acoust. Soc. Am. 121, 2805-2818 (2007)] acknowledges that two-tone neural rate responses for low-side suppression differ from those measured in basilar membrane mechanics, making one question whether this aspect of suppression has a mechanical correlate. It is suggested here that signal coding between mechanical and neural processing stages may be responsible for the fact that the total rate response (but not the basilar membrane response) for low-frequency suppressors is smaller than that for the probe-alone condition. For example, the velocity dependence of inner hair cell (IHC) transduction, membrane/synaptic filtering and the sensitivity difference between ac and dc components of the IHC receptor potential all serve to reduce excitability for low-side suppressors at the single-unit level. Hence, basilar membrane mechanics may well be the source of low-side suppression measured in the auditory nerve.

Isoflurane increases amplitude and incidence of evoked and spontaneous otoacoustic emissions

The volatile anesthetic isoflurane was tested for its effect on cochlear function by means of measuring distortion product otoacoustic emissions (DPOAE) and spontaneous otoacoustic emissions (SOAE) in the mustached bat (Pteronotus parnellii parnellii). Averaged growth functions of DPOAE and spontaneous otoacoustic emissions were assessed and compared between the control group (no isoflurane application) and the isoflurane group (application of isoflurane at vaporizer settings sof about 1.5-2%). Isoflurane significantly increases the DPOAE amplitude, e.g. at a primary tone level l2 of 40 dB SPL by 10.7 dB. Additionally, the incidence of SOAEs was highly increased during application of isoflurane. The sound-evoked efferent effect on the generation of otoacoustic emissions was significantly reduced in the isoflurane group. We suggest that isoflurane might affect the postsynaptic action of acetylcholine (ACh) released by the efferent terminals of outer hair cells (OHCs). This could lead to the observed decrease of efferent suppression and to a disinhibition of cochlear amplification.

Integration of outer hair cell activity in a one-dimensional cochlear model

Recently, significant progress has been made in understanding the contribution of the mammalian cochlear outer hair cells (OHCs) to normal auditory signal processing. In the present paper an outer hair cell model is incorporated in a complete, time-domain, one-dimensional cochlear model. The two models control each other through cochlear partition movement and pressure. An OHC gain (gamma) is defined to indicate the outer hair cell contribution at each location along the cochlear partition. Its value ranges from 0 to 1: gamma=0 represents a cochlea with no active OHCs, gamma=1 represents a nonrealistic cochlea that becomes unstable at resonance frequencies, and gamma=0.5 represents an ideal cochlea. The model simulations reveal typical normal and abnormal excitation patterns according to the value of gamma. The model output is used to estimate normal and hearing-impairment audiograms. High frequency loss is predicted by the model, when the OHC gain is relatively small at the basal part of the cochlear partition. The model predicts phonal trauma audiograms, when the OHC gain is random along the cochlear partition. A maximum threshold shift of about 60 dB is obtained at 4 kHz.

A non-linear circuit for simulating OHC of the cochlea

In the present paper, referring to known characteristics of the outer hair cells functioning in the cochlea of the inner ear, a functional model of the outer hair cells is constructed. It consists of a linear feed-forward circuit and a non-linear positive feedback circuit. The feed-forward circuit reflects the contribution of local basilar and tectorial membrane areas and passive outer hair cells' physical parameters to the forming of low-selectivity resonance characteristics. The non-linear positive feedback circuit reflects the non-linear outer hair cell signal transduction processes and the active role of efferents from the medial superior olive in altering circuit sensitivity and selectivity. Referring to an analytical description of the circuit model and computer simulation results, an explanation is given over the biological meaning of the outer hair cells' non-linearities in signal transduction processes and the role of the non-linearities in achieving the following: signal compression, the dependency of circuit sensitivity and frequency selectivity upon the input signal amplitude, the compatibility of high-frequency selectivity and short transient response of the biological filtering circuits.

New tunes from Corti's organ: the outer hair cell boogie rules

The amplification of acoustic stimuli is a feature of hair cells that evolved early on in vertebrates. Though standard stereocilia mechanisms to promote such amplification may persist in the mammal, an additional mechanism evolved to enhance high frequency sensation. Only in mammals, a special cell type, the outer hair cell, arose that possesses a remarkably fast somatic mechanical response, which probably endows the passive cochlea with a boost in sensitivity by a factor of 100 (40dB), at least. Experiments conducted over the past few years have shed light on many aspects of outer hair cell electromotility, including the molecular identification of the motor, the effects of a knockout, and underlying mechanisms of action. A review of this remarkable progress is attempted.

How can the cochlear amplifier be realized by the outer hair cells which have nothing to push against?

A theoretical consideration is given on how the constituent cells of the cochlear partition can amplify its motion and increase its momentum without resorting to external forces, and it leads to a micromechanical model that explains the role of the cells in the active amplification. The triangle composed of the outer hair cell, the phalanx of Deiters cell and the reticular lamina forms a mechanical unit that stores up and releases strain. When outer hair cells contract in a region along the cochlear partition, strain accumulates in the triangle causing deformation of the region that pushes down the basilar membrane, and hence it appears as a transverse pressure that drives the basilar membrane. The momentum of the region increases at the cost of the momentum of neighboring regions, and the total momentum of the cochlear partition is not altered by the internal forces generated by the outer hair cells. The model can produce a frequency-response curve that compares favorably with experimental data.

Intensity-invariance of fine time structure in basilar-membrane click responses: implications for cochlear mechanics

Basilar-membrane and auditory-nerve responses to impulsive acoustic stimuli, whether measured directly in response to clicks or obtained indirectly using cross- or reverse-correlation and/or Fourier analysis, manifest a striking symmetry: near-invariance with stimulus intensity of the fine time structure of the response over almost the entire dynamic range of hearing. This paper explores the origin and implications of this symmetry for cochlear mechanics. Intensity-invariance is investigated by applying the EQ-NL theorem [de Boer, Aud. Neurosci. 3, 377-388 (1997)] to define a family of linear cochlear models in which the strength of the active force generators is controlled by a real-valued, intensity-dependent parameter, gamma (with 0 < or = gamma < or = 1). The invariance of fine time structure is conjectured to imply that as gamma is varied the poles of the admittance of the cochlear partition remain within relatively narrow bands of the complex plane oriented perpendicular to the real frequency axis. Physically, the conjecture implies that the local resonant frequencies of the cochlear partition are nearly independent of intensity. Cochlear-model responses, computed by extending the model obtained by solution of the inverse problem in squirrel monkey at low sound levels [Zweig, J. Acoust. Soc. Am. 89, 1229-1254 (1991)] with three different forms of the intensity dependence of the partition admittance, support the conjecture. Intensity-invariance of cochlear resonant frequencies is shown to be consistent with the well-known "half-octave shift," describing the shift with intensity in the peak (or best) frequency of the basilar-membrane frequency response. Shifts in best frequency do not arise locally, via changes in the underlying resonant frequencies of the partition, but globally through the intensity dependence of the driving pressure. Near-invariance of fine time structure places strong constraints on the mechanical effects of force generation by outer hair cells. In particular, the symmetry requires that the feedback forces generated by outer hair cells (OHCs) not significantly affect the natural resonant frequencies of the cochlear partition. These results contradict many, if not most, cochlear models, in which OHC forces produce significant changes in the reactance and resonant frequencies of the partition.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

Three-dimensional numerical modeling for global cochlear dynamics

A hybrid analytical-numerical model using Galerkin approximation to variational equations has been developed for predicting global cochlear responses. The formulation provides a flexible framework capable of incorporating morphologically based mechanical models of the cochlear partition and realistic geometry. The framework is applied for a simplified model with an emphasis on application of hybrid methods for three-dimensional modeling. The resulting formulation is modular, where matrices representing fluid and cochlear partition are constructed independently. Computational cost is reduced using two methods, a modal-finite-element method and a boundary element-finite-element method. The first uses a cross-mode expansion of fluid pressure (2.5D model) and the second uses a waveguide Green's-function-based boundary element method (BEM). A novel wave number approach to the boundary element formulation for interior problem results in efficient computation of the finite-element matrix. For the two methods a convergence study is undertaken using a simplified passive structural model of cochlear partition. It is shown that basilar membrane velocity close to best place is influenced by fluid and structural discretization. Cochlear duct pressure fields are also shown demonstrating the 3D nature of pressure near best place.

A model of cochlear micromechanics

A cochlear model is presented which has two degrees of freedom, the motion of the basilar membrane and that of the tectorial membrane (TM), in every cross section of the organ of Corti. It assumes that the reticular lamina is virtually rigid, so that the apical portion of the outer hair cells is firmly anchored to it, and also that the TM is directly driven by the BM through the marginal net of the TM, which anchor to the phalanges of the outermost row of Deiters' cells or Hensen's cells, or both. It is shown that the model can produce more than 40 dB of cochlear gain in the frequency-domain simulation and 30 dB in time-domain simulation. Transiently evoked otoacoustic emissions and cochlear microphonics are simulated in the time domain.

Auditory-nerve fiber responses to clicks in guinea pigs with a damaged cochlea

This paper describes auditory-nerve single-fiber responses to clicks in noise-damaged cochleas. Poststimulus time histograms (PSTHs) were recorded for various click intensities and for the two click polarities. The PSTHs found in fibers with elevated thresholds are discussed in relation to the frequency threshold curves (FTCs) measured in these fibers. Five types of abnormal FTCs are distinguished. Type I is elevated as a whole, type II has an elevated (and often broadened) tip and a tail at normal level, type III has low thresholds in the tail (often hypersensitive), type IV represents a flat tuning, and type V has no tip but shows a clear appearance of the tail (often hypersensitive). The click PSTHs of abnormal fibers were compared to normal PSTHs at equal sound-pressure levels, and various abnormal trends were found corresponding to the type of FTC. PSTHs for type I have longer dominant-peak latencies and smaller amplitudes; PSTHs for type II were normal well above the fiber's threshold; PSTHs for type III revealed remarkable patterns with multiple peaks, part of them with a latency strongly varying with polarity; PSTHs for type IV showed narrow peaks and steep amplitude/intensity curves; PSTHs for type V showed a multiple peaked pattern and large amplitudes and steep amplitude/intensity curves to rarefaction polarity. The various features in the click responses were in most cases consistent with the type of FTC. The results can be used to explain deviations in whole-nerve recordings in abnormal cochleas.

Patch clamped responses from outer hair cells in the intact adult organ of Corti

Outer hair cells (OHCs) from the mammalian cochlea act as both sensory cells and motor cells. We report here whole-cell tight seal recordings of OHC activity in their natural embedding tissue, the intact organ of Corti, using a temporal bone preparation. The mean cell resting potential, -76 +/- 4 mV (n = 19) and input conductance (10 +/- 3 nS at -70 mV) of third turn hair cells were significantly lower than have been found in isolated cells. Two main K+ currents in the cell were identified. One current, activated positive to -100 mV, was reduced by 5 mM BaCl2. The other current, activated above -40 mV, was reduced by 100 microM 4-aminopyridine (4-AP) and by 30 mM tetraethylammonium (TEA). Both of these currents have been also identified in recordings reported from isolated cells. On stepping to different membrane potentials, cells imaged in the organ of Corti changed length by an amount large enough to cause visible distortions in neighbouring cells. By quantifying such distortions we estimate that the forces generated by OHCs can account for the enhanced response to sound required by the cochlear amplifier.

Modeling the active process of the cochlea: phase relations, amplification, and spontaneous oscillation

The high sensitivity and sharp frequency selectivity of acoustical signal transduction in the cochlea suggest that an active process pumps energy into the basilar membrane's oscillations. This function is generally attributed to outer hair cells, but its exact mechanism remains uncertain. Several classical models of amplification represent the load upon the basilar membrane as a single mass. Such models encounter a fundamental difficulty, however: the phase difference between basilar-membrane movement and the force generated by outer hair cells inhibits, rather than amplifies, the modeled basilar-membrane oscillations. For this reason, modelers must introduce artificially either negative impedance or an appropriate phase shift, neither of which is justified by physical analysis of the system. We consider here a physical model based upon the recent demonstration that the basilar membrane and reticular lamina can move independently, albeit with elastic coupling through outer hair cells. The mechanical model comprises two resonant masses, representing the basilar membrane and the reticular lamina, coupled through an intermediate spring, the outer hair cells. The spring's set point changes in response to displacement of the reticular lamina, which causes deflection of the hair bundles, variation of outer hair cell length and, hence, force production. Depending upon the frequency of the acoustical input, the basilar membrane and reticular lamina can oscillate either in phase or in counterphase. In the latter instance, the force produced by hair cells leads basilar-membrane oscillation, energy is pumped into basilar-membrane movement, and an external input can be strongly amplified. The model is also capable of producing spontaneous oscillation. In agreement with experimental observations, the model describes mechanical relaxation of the basilar membrane after electrical stimulation causes outer hair cells to change their length.

Shapes of cat auditory nerve fiber tuning curves

Tuning curves of auditory nerve fibers in normal-hearing cats were fitted by a computational model comprising four processes. One process accounts for sensitivity in tuning curve tails and consists of an approximation to bandpass filtering by extracochlear structures. The second and third processes describe passive and active components of basilar membrane (BM) mechanics, respectively. The former consists of a lowpass filter function, which provides baseline threshold sensitivity and filtering above characteristic frequency (CF), and the latter consists of a Gaussian that accounts for sharp tuning and high sensitivity around CF. A fourth process, modeled as a high-pass filter, was needed in many fits to account for breaks and plateaus in threshold sensitivity at frequencies above CF. The latter three processes operated on cochlear spatial coordinates rather than stimulus frequency. The four-process description closely accounted for shapes of most tuning curves. Tuning curve tails possessed minima at 40-80 dB SPL, and minima increased with fiber CF. High-frequency cutoffs of tail filters tended to increase with CF, but low-frequency cutoffs were generally constant across CF. Functions describing tails varied from ear to ear but behaved in a similar manner for fibers from a single ear. Passive components of BM resonances possessed baselines with sensitivities that decreased with CF and cutoff slopes that increased with CF. The magnitude of the active component increased smoothly with CF over an 80 + dB range, and its spatial extent was essentially constant at 1.5 mm or 6% of cochlear length regardless of gain magnitude, fiber CF, or threshold sensitivity. Tuning curves from fibers with high and medium spontaneous rates (SRs) and similar CFs had nearly identical shapes, with the sole difference being essentially constant differences in sensitivity across the entire excitatory frequency range. Tuning curve shapes from fibers with low SRs were more variable. These could either resemble those obtained from similarly-tuned fibers with higher SRs, or they could exhibit lower tip-to-tail ratios and reduced active component magnitudes. The latter were typically associated with low maximum discharge rates.

Distortion product otoacoustic emissions in an active nonlinear model of the cochlea

An active nonlinear model of the cochlea in the form of a transmission line was presented, in which the active feedback system by outer hair cells (OHCs) was expressed as a series of low-pass filters on the basilar membrane (BM) which were transducing basilar membrane displacement to feedback force. The model could produce distortion product oto-acoustic emissions (DPOAEs) explicitly as well as sharp tuning curves of BM, and it was possible to discuss the cause of DPOAEs in terms of the active feedback. It was inferred that the nonlinearity of the cochlea which causes DPOAEs may be related to a saturating property of the feedback system by OHCs.

Responses of peripheral auditory neurons to two-tone stimuli during development: II. Factors related to neural responsiveness

In an accompanying paper (Fitzakerley et al., 1994), it was demonstrated that there is a significant developmental correlation between the appearance of tuning and two-tone suppression. However, it was also found that some sharply tuned neurons meeting minimal adult standards did not exhibit suppression. Therefore, in order to investigate other factors that may be related to the demonstration of suppression in peripheral auditory neurons, the relationship of two-tone suppression with various parameters related to neural responsiveness was studied in perinatal kittens. A positive correlation was made between the observance of suppression and driven discharge rate, characteristic frequency (CF), and threshold, all properties which change significantly during the final stages of cochlear differentiation. In older animals, suppression was also observed in higher percentages of neurons having low spontaneous rates (< 1 spk/s). Suppression was evoked less often by test tones placed below CF than above CF in neurons recorded from younger animals, and was generally produced by a narrower range of test tone intensities than those recorded from adults. As a result, the conventional description of suppression as observed in peripheral auditory neurons in mature animals must be extended to include these factors when responses of immature neurons are considered.

Responses of peripheral auditory neurons to two-tone stimuli during development: I. Correlation with frequency selectivity

The responses of peripheral auditory neurons to two-tone stimuli were used to inferentially examine the nature of cochlear processing during development. Rate suppression was not seen in the youngest animals, and was first observed at 77 gestational days, in units exhibiting adultlike frequency selectivity. Suppression was highly correlated with the degree of tuning, and neurons were segregated into three classes based on these responses. Broadly tuned neurons (type IB) with low characteristic frequencies (CFs) did not exhibit suppression, and were observed early in postnatal life. Sharply tuned, but still immature neurons (type IS) exhibited suppression, but to a lesser degree than mature neurons (type M). One interpretation of these results is that basilar membrane mechanics are linear during the final stages of cochlear development, indicating that the immature signal transduction process is fundamentally different from that of adults.

Interrelations between psychoacoustical tuning curves and spontaneous and evoked otoacoustic emissions

With the hypothesis that cochlear active mechanisms are the origin of otoacoustic emissions (OAEs) and of the high frequency selectivity exhibited by the ear, psychoacoustical tuning curves (PTCs), transiently evoked otoacoustic emissions (TEOAEs), and spontaneous otoacoustic emissions (SOAEs) have been examined in 50 normal hearing subjects. Using a clinical simplified method, PTCs were successively assessed at three frequencies--1, 2 and 4 kHz--in each subject. The results showed the existence of significant differences in the quality of tuning (Q10dB) of the PTCs between, first, subjects having SOAEs and subjects having no SOAEs (Student's t-test; p < 0.05; df = 35) and, second, subjects having large TEOAEs and subjects having small TEOAEs (Student's t-test; p < 0.05; df = 14). Nevertheless, these significant differences did not appear for all the frequencies studied: the frequency selective relationship between PTCs and OAEs mainly involved the 2 kHz zone. Such results are discussed according to the specificities of the clinical method used for PTC measurement as well as to the spectral characteristics of OAEs.

Volume regulation in cochlear outer hair cells

Many cells placed in a hypotonic medium initially swell and then rapidly undergo a regulatory volume decrease (RVD) to return towards original volume. Re-exposure to the isotonic solution results in the cells shrinking followed by a regulatory volume increase (RVI). Previous studies have shown that isolated outer hair cells (OHCs) placed in a hypotonic medium swell and maintain this shape until returned to the original medium. We re-examined this apparent lack of cell volume regulation in OHCs. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a Hank's balanced salt solution (HBS). In the cells studied, switching the perfusate to a hypotonic HBS (290-280 mmol/kg) for 15 min resulted in an immediate shortening of the OHCs (i.e., volume increase). In 26% of the cells, this increase was followed by a return to original length during the time the cell was perfused with the hypotonic medium, a RVD. Twelve percent of the cells demonstrating a RVD also displayed a RVI. Omitting collagenase and increasing Ca2+ concentration did not increase the percentage of cells displaying a RVD, while gadolinium (Gd3+, 10 microM) decreased the percentage to zero. This is the first report of isolated OHCs undergoing cell volume regulation.

A realizable cochlear model using feedback from motile outer hair cells

A physically realizable form of a recent cochlear model using feedback forces from motile outer hair cells [Geisler (1991) Hear. Res. 54, 105-117] has been developed. The model was computer-simulated in the frequency domain (necessarily linear). Its responses to pure tones are very realistic in terms of sharpness (Q10s of 3-5) and in terms of tip-to-tail ratios (50-60 dB). These large tips are due to the feedback forces, which act as negative resistances (energy-supplying elements) over restricted spatial ranges. Nyquist-criterion analysis indicates that the model is stable. The spatial patterns of the model's output also bear qualitative resemblances to several other phenomena observed in cochleas, both living and excised.

Two-tone suppression and distortion production on the basilar membrane in the hook region of cat and guinea pig cochleae

Two-tone suppression and two-tone distortion were investigated at the level of the basilar membrane in the hook region of cat and guinea pig cochleae using a displacement-sensitive laser interferometric measurement system. The system allowed measurements to be performed at physiological stimulus levels in the cochlear region tuned to 30-35 kHz in cat and 29 kHz in guinea pig. The amplitude of vibration of the basilar membrane due to a probe tone at the characteristic frequency (CF) was attenuated during the presentation of a simultaneous suppressor tone either above or below CF. The amount of suppression depended on the intensities of both probe and suppressor, and the relationship of the suppressor frequency to the CF. Suppressors at frequencies more than an octave below the CF attenuated the responses to the CF probe at a rate of up to 1 dB/dB, with little variation based on suppressor frequency. As the suppressor frequency was increased above CF the rate of suppression decreased rapidly. The lowest suppressor intensity at which attenuation of the probe response was observed did not vary in direct proportion to the probe intensity. This suppression threshold often varied only a few dB SPL when the probe was varied over a 20 dB SPL range. In a few instances the rate of attenuation was as much as a factor of two greater at the lowest probe intensities than at higher intensities. It is noteworthy that suppression was found when the frequency of the suppressor was either above or below CF in the same preparation. Low frequency suppressor tones suppress basilar membrane motion at the CF when the basilar membrane undergoes displacement toward either scala. The maximum suppression occurs around 100 microseconds after the peak excursions caused by the low frequency biasing tone. Two-tone distortion products were often observed even at stimulus levels below those causing two-tone suppression at the site studied. The cubic difference tone (CDT) was the most prominent of the distortion products. The level of the CDT component varied nonmonotonically with the level of either of the primary tones. Responses at the difference frequency between the two primaries were usually below the noise floor of the recording system. The existence of both two-tone distortion and two-tone suppression was dependent on the presence of a cochlear nonlinearity.

Two-tone suppression by a saturating feedback model of the cochlear partition

A model of a small strip of cochlear partition was computer simulated. The model is composed of two elements, approximations to the transfer functions of an inner hair cell (IHC) and an outer hair cell (OHC), respectively. The IHC element was insensitive to DC stimulation. Input was one or two sinusoids. One sinusoid, at the characteristic frequency (CF), was multiplied by the gain of the 'cochlear amplifier'. A second sinusoid, representing a tone with much lower frequency, was not affected by the amplifier gain. This gain was determined by the OHC transfer function. In one form of the model ('fixed-gain'), this gain was set at a fixed number determined from the furthest point reached on the OHC transfer function. This form of the model produced very realistic single-tone responses as well as showing 'two-tone suppression': that is, the IHC DC response produced by CF stimulation was reduced when the lower-frequency sinusoid, at suitable intensities, was added to the stimulus. When a DC component was added to the two-tone stimulus, the magnitude of this two-tone suppression was enhanced. In the second form of the model ('variable-gain'), the cochlear-amplifier gain varied throughout the stimulus cycle. Its value was re-calculated at each instant, determined by the point on the OHC transfer function current at that particular instant. This form of the model showed two-tone suppression only when a DC component was added to the two-tone stimulus.

Lowering extracellular chloride concentration alters outer hair cell shape

In general, increasing external K+ concentration, as well as exposure to hypotonic medium, induces a shortening of outer hair cells (OHCs) accompanied by an increase in width and volume. One possible mechanism suggested for these changes is a movement of Cl- and/or water across the cell membrane. We therefore examined the role of Cl- in OHC volume maintenance by testing the effect of decreasing extracellular Cl- concentration on OHC length and shape. In addition, the effect of hypotonic medium was examined. OHCs were isolated from guinea pig cochleae, mechanically dissociated and dispersed, and placed in a modified Hanks balanced salt solution (HBS). Exposing the cells to a Cl(-)-free HBS produced an initial shortening, which was rapidly followed by an increase in length. After about 9 min of exposure to Cl(-)-free HBS, the cells appeared to lose all water and collapsed. Upon return to normal HBS, the OHCs returned to their normal shape. We speculate that the collapse of the OHCs may be due to the loss of intracellular Cl-, which, in turn, resulted in the loss of intracellular K+ and water. The results indicate that Cl- contributes greatly to the maintenance of OHC volume. In addition, we confirmed that isolated OHCs swell in hypotonic medium and maintain their swollen state until returned to normal medium. The mechanism for maintenance of the swollen state is unknown.

Two-tone suppression in inner hair cell responses: correlates of rate suppression in the auditory nerve

Inner hair cell (IHC) recordings were made from second turn of the guinea pig cochlea where characteristic frequencies are approximately 4000 Hz. In order to compare IHC responses with rate suppression measured in the auditory nerve, suppressors were introduced that produced little or no response in the hair cell. The effects of a variable-frequency suppressor on a constant-frequency probe, placed near characteristic frequency, were also investigated since this paradigm is commonly used in single unit experiments. Resulting magnitude changes were measured in the fundamental component of the ac receptor potential and/or in the total dc produced in the region of temporal overlap between the two stimulus inputs. This latter component is especially important when considering how changes in IHC responses relate to decreases in discharge rate in single auditory nerve fibers. Since the ac receptor potential is filtered by the hair cell's basolateral membrane, the dc component probably controls transmitter release at the characteristic frequency of these second-turn IHCs. Based on results from these and previous experiments, a proposal is advanced to explain the evolution of two-tone suppression in the peripheral auditory system. The paper also discusses the use of excitatory versus non-excitatory suppressors and includes a description of two-tone suppression areas at the mechanical, IHC and single unit levels. The explanation of low-side suppression areas is of special interest since hitherto they have been difficult to model (Kim, 1985).