Basilar Membrane Motion Research Articles

A computational algorithm for computing nonlinear auditory frequency selectivity.

Computational algorithms that mimic the response of the basilar membrane must be capable of reproducing a range of complex features that are characteristic of the animal observations. These include complex input output functions that are nonlinear near the site's best frequency, but linear elsewhere. This nonlinearity is critical when using the output of the algorithm as the input to models of inner hair cell function and subsequent auditory-nerve models of low- and high-spontaneous rate fibers. We present an algorithm that uses two processing units operating in parallel: one linear and the other compressively nonlinear. The output from the algorithm is the sum of the outputs of the linear and nonlinear processing units. Input to the algorithm is stapes motion and output represents basilar membrane motion. The algorithm is evaluated against published chinchilla and guinea pig observations of basilar membrane and Reissner's membrane motion made using laser velocimetry. The algorithm simulates both quantitatively and qualitatively, differences in input/output functions among three different sites along the cochlear partition. It also simulates quantitatively and qualitatively a range of phenomena including isovelocity functions, phase response, two-tone suppression, impulse response, and distortion products. The algorithm is potentially suitable for development as a bank of filters, for use in more comprehensive models of the peripheral auditory system.

331 基底板振動と聴神経インパルスの関係

331 基底板振動と聴神経インパルスの関係

F-0934 原子間力顕微鏡を用いた外有毛細胞のヤング率の計測(G02-4 細胞)(G02 バイオエンジニアリング部門一般講演)

It is established that the isolated outer hair cells (OHCs) can elongate and contract in response to an electrical stimulation, and it is considered that the force produced by the OHC, which accompanies its motility, affects the basilar membrane motion. As a result, the force production of the OHC presumably leads to the fine tuning of the mammalian cochlea. As the force production of the OHC is known to be related to the mechanical properties of the OHC, it is important to understand its mechanical properties. In this study, in order to understand the local Young's modulus of the OHC, the difference in the Young's modulus along the cell axis is measured with an atomic force microscope (AFM). Then, the relationship between the Young's modulus of the OHC and its length is discussed.

Improvement of an IIR asymmetric compensation gammachirp filter.

An IIR implementation of the gammachirp filter has been proposed to simulate basilar membrane motion efficiently (Irino and Unoki, 1999). A reasonable filter response was provided by a combination of a gammatone filter and an IIR asymmetric compensation (AC) filter. It was noted, probably however, that the rms error was high when the absolute values of the parameters are large, because the coefficients of the IIR-AC filter were selected heuristically. In this report, we show that this is due to the sign inversion of the phase of poles and zeros in the conventional model. We propose a new definition of the IIR-AC filter and we describe a method of systematic determining the optimum coefficients and number of cascade for the second-order filter. This results in a reduction of the error to about 1/3 that produced by the conventional model.

Modulated otoacoustic emissions reveal features of the human cochlear impedance at low frequencies

Basilar membrane (BM) displacement by loud low-frequency (LF) tones periodically alters the level of distortion-product otoacoustic emissions (DPOAEs) as a function of the phase of the modulating LF tone. For three human subjects the level of a modulating LF tone (15–320 Hz) was adjusted to maintain a constant DPOAE-modulation depth, indicating a constant periodic LF BM displacement. The resulting modulation LF-tone levels match the iso-loudness contours (ISO/R226) except for an antiresonance centered at about 55 Hz. Here, an increase in level of approximately 5 dB above the iso-loudness contour is required to maintain constant periodic LF BM displacement. The antiresonance separates two distinct regions of the cochlear impedance: a slope of 12 dB/octave at frequencies below 55 Hz suggests a mass-controlled impedance resulting from perilymph flow through the helicotrema; a slope of 6 dB/octave above 55 Hz suggests the existence of a traveling wave with resistive impedance. Psychophysical threshold experiments did not show the antiresonance. Antiresonances have been observed previously in animal experiments using cochlear microphonics or perilymph pressure measurements to estimate BM displacement. Modulated DPOAEs are a noninvasive method to investigate the BM movement at low frequencies.

A Targeted Deletion in α-Tectorin Reveals that the Tectorial Membrane Is Required for the Gain and Timing of Cochlear Feedback

α-tectorin is an extracellular matrix molecule of the inner ear. Mice homozygous for a targeted deletion in α-tectorin have tectorial membranes that are detached from the cochlear epithelium and lack all noncollagenous matrix, but the architecture of the organ of Corti is otherwise normal. The basilar membranes of wild-type and α-tectorin mutant mice are tuned, but the α-tectorin mutants are 35 dB less sensitive. Basilar membrane responses of wild-type mice exhibit a second resonance, indicating that the tectorial membrane provides an inertial mass against which outer hair cells can exert forces. Cochlear microphonics recorded in α-tectorin mutants differ in both phase and symmetry relative to those of wild-type mice. Thus, the tectorial membrane ensures that outer hair cells can effectively respond to basilar membrane motion and that feedback is delivered with the appropriate gain and timing required for amplification.

Medial efferent effects on auditory-nerve responses to tail-frequency tones II: alteration of phase.

It is often assumed that at frequencies in the tuning-curve tail there is a passive, constant coupling of basilar-membrane motion to inner hair cell (IHC) stereocilia. This paper shows changes in the phase of auditory-nerve-fiber (ANF) responses to tail-frequency tones and calls into question whether basilar-membrane-to-IHC coupling is constant. In cat ANFs with characteristic frequencies > or = 10 kHz, efferent effects on the phase of ANF responses to tail-frequency tones were measured. Efferent stimulation caused substantial changes in ANF phase (deltaphi) (range -80 degrees to +60 degrees, average -15 degrees, a phase lag) with the largest changes at sound levels near threshold and 3-4 octaves below characteristic frequency (CF). At these tail frequencies, efferent stimulation had much less effect on the phase of the cochlear microphonic (CM) than on ANF phase. Thus, since CM is synchronous with basilar-membrane motion for low-frequency stimuli in the cochlear base, the efferent-induced change in ANF phase is unlikely to be due entirely to a change in basilar-membrane phase. At tail frequencies, ANF phase changed with sound level (often by 90 degrees-180 degrees) and the deltaphi from a fiber was positively correlated with the slope of its phase-versus-sound-level function at the same frequency, as if deltaphi were caused by a 2-4 dB increase in sound level. This correlation suggests that the processes that produce the change in ANF phase with sound level at tail frequencies are also involved in producing deltaphi. It is hypothesized that both efferent stimulation and increases in sound level produce similar phase changes because they both produce a similar mix of cochlear vibrational modes.

A kinship with unusual auditory perception

Craig, while investigating vibrotactile sensation for blind readers, encountered several individuals with extraordinary perceptual ability [Science 196, 450 (1977)]. We report a two-generation kinship (three polyglots) with extraordinary ability to classify certain sets of synthetic acoustic stimuli. With sets of three or four formant /ge/-/ye/ which controls typically identify as containing two or three sounds (/e/, /ge/, /ye/), the mother and son could distinguish as many as six classes of sounds; the father could distinguish five. The additional classes involved stimuli with brief transitions; one class encompassed control transition durations. Each individual could also classify these same stimulus sets using two or three classes; these results were homogeneous (p<0.0001) with the results from a population of 45 age/gender-matched controls. These results require fine-grained differences in rate or duration of basilar membrane movements be represented in auditory cortex.

High calcium permeability and calcium block of the α9 nicotinic acetylcholine receptor

At the synapse between olivocochlear efferent fibers and outer hair cells (OHCs) of the cochlea, a non-classical ionotropic cholinergic receptor allows Ca 2+ entry into the hair cell, thus activating a Ca 2+-sensitive K + current which hyperpolarizes the cell’s membrane. In the mammalian ear, this leads to a reduction in basilar membrane motion, altering auditory nerve fiber activity and reducing the dynamic range of hearing. The α9 nicotinic acetylcholine receptor (nAChR) subunit mediates synaptic transmission between cholinergic olivocochlear fibers and OHCs. Given that Ca 2+ is a key player at this inhibitory synapse, we evaluated the permeability to Ca 2+ of the recombinant α9 receptor expressed in Xenopus laevis oocytes and the modulation of its activity by extracellular Ca 2+. Our results show that the α9 receptor is highly permeable to Ca 2+ and that this cation potently blocks monovalent currents through this channel (IC 50=100 μM, at −70 mV) in a voltage-dependent manner. At a Ca 2+ concentration similar to that found in the perilymph bathing the base of the OHCs, approximately 90% of the Na + current through the α9 receptor is blocked, suggesting that one of the main functions of this channel could be to provide a pathway for Ca 2+ influx.

Medial efferent effects on auditory-nerve responses to tail-frequency tones. I. Rate reduction

One way medial efferents are thought to inhibit responses of auditory-nerve fibers (ANFs) is by reducing the gain of the cochlear amplifier thereby reducing motion of the basilar membrane. If this is the only mechanism of medial efferent inhibition, then medial efferents would not be expected to inhibit responses where the cochlear amplifier has little effect, i.e., at sound frequencies in the tails of tuning curves. Inhibition at tail frequencies was tested for by obtaining randomized rate-level functions from cat ANFs with high characteristic frequencies (CF > or = 5 kHz), stimulated with tones two or more octaves below CF. It was found that electrical stimulation of medial efferents can indeed inhibit ANF responses to tail-frequency tones. The amplitude of efferent inhibition depended on both sound level (largest near to threshold) and frequency (largest two to three octaves below CF). On average, inhibition of high-CF ANFs responding to 1 kHz tones was around 5 dB. Although an efferent reduction of basilar-membrane motion cannot be ruled out as the mechanism producing the inhibition of ANF responses to tail frequency tones, it seems more likely that efferents produce this effect by changing the micromechanics of the cochlear partition.

The radial pattern of basilar membrane motion evoked by electric stimulation of the cochlea

Electric current applied to the cochlea can evoke in situ electromotile responses of the organ of Corti. These nonsound-generated responses can give insight into the mechanics of the organ as the putative forces produced by outer hair cells (OHC) must couple to the modes of vibration of the basilar membrane (BM). In this study, platinum-iridium wire electrodes were positioned into the scala vestibuli and scala tympani of the first cochlear turn in the guinea pig. Current (1.5 ms rectangular-shaped pulses) was applied to these electrodes at levels to 500 μA peak. A laser Doppler velocimeter was used to record the velocity or displacement of the basilar membrane at the tonotopic 18 kHz place via an opening into the scala tympani of the first cochlear turn. Beads were positioned across the width of the BM so that the velocity or displacement of the BM could be studied in the radial direction. It was found that the current pulses evoked linear displacements of up to 2 nm for current levels of 500 μA (higher levels were damaging to the organ of Corti). The pattern of motion across the width of the BM was such that maximum displacement and velocity was located near the first row of OHCs and the position of the outer pillar cell footplate. The BM motion was biphasic in that the zona arcuata moved in the opposite direction to that of the zona pectinata. The results of this study demonstrate that the level of force produced by OHCs is effective in moving the BM and that the distribution of force within the organ of Corti leads to a multimodal motion pattern of the BM for this experimentally artificial means of evoking OHC motion.

Basilar membrane motion in relation to two-tone suppression

It is proposed that two-tone suppression of rate responses in auditory-nerve fibres by a low-side suppressor cannot be explained in terms of basilar membrane motion. In a model, the amplitude of the mechanical response, either to the tone at characteristic frequency (CF), or to the CF tone combined with a second, lower frequency tone (a suppressor), is taken as the effective stimulus to inner hair cells (IHC), the voltage response of which is considered responsible for excitatory drive to auditory-nerve fibres. Many empirical mechanical and physiological effects are simulated accurately by the model, particularly phenomena observed in two-tone experiments using low-side suppressor tones, that authors have described as two-tone suppression. It is argued in this paper, however, that such phenomena strictly do not constitute suppression in the cochlear response and provide no explanation for rate suppression in nerve fibres. According to the model presented here and consistent with experimental data, suppression of the spike response to a CF tone in an auditory-nerve fibre by a low-side suppressor cannot be explained in terms of the mechanics of the BM. Conclusions by others that experiments support a mechanical explanation for low-side rate suppression are shown to be questionable. It is concluded that low-side suppression of neural responses is explicable only in terms of a non-mechanical factor derived from the response to the low frequency tone, that depresses responsiveness in fibres at the CF location. Adherence to the model of low-side neural rate suppression depending on reduced net mechanical response of the BM is contrary to experimental evidence; furthermore it overlooks a profound influence additional to synaptic drive, that is implied in the shaping of responses in auditory-nerve fibres.

Analysis of Force Generation Mechanism of Outer Hair Cell Using Elastic Shell Theory.

Recent experiments have shown that the isolated outer hair cell (OHC) can elongate and contract in response to electrical stimulation. From the OHC's position relative to the basilar membrane, it is deduced that the OHC length change directly affects the basilar membrane motion. Thus, this electromotile property of the OHC may play a key role in cochlear tuning. In order to evaluate the OHC's ability to influence the basilar membrane motion and tuning, a few experiments and numerical analyses have already been reported. However, the OHC motility within the range of the human ear (20 Hz∼20 kHz) and the force generated by the OHC are unknown. In this study, an attempt is made to analyze the OHC motility by using the shell theory. The OHC is modelled as a cylindrical composite shell and the function of the motor protein is taken into consideration. Substituting the estimated mechanical properties of the plasma membrane and cortical lattice to the shell model, the force generated by the OHC is obtained numerically. Then these mechanical properties are determined by comparing the numerically obtained OHC force with the experimental data of the previous report. Also the dynamic equations of the shell model are derived, and the dynamic behavior of the OHC is examined. The results indicate that the axial and circumferential stiffnesses of the cortical lattice are 1.3×10-2N/m and 2.0×10-2N/m, respectively, and that the numerically obtained dynamic behavior of the OHC is consistent with the experimental data.

The hearing organ: active sound amplifier and highly sensitive measuring system

The present paper provides a brief review on topical issues of auditory physiology. Recent data on transduction mechanism and adaptation in hair cells as well as on the possible role of outer hair cells in amplifying basilar membrane motion are presented. Strategies of present physiological research in dealing with sensorineural deafness are discussed.

A computational model of early stages of dolphin hearing

A computational model of the early stages of dolphin hearing was developed to model dolphin echolocation performance. The first stage consists of a bank of 49 gammatone filters with center frequencies from about 2.5 to 147 kHz. This stage computes the basilar membrane motion that results from the echo time series used as input. The second stage of the model computes a neural activation pattern that rectifies and log transforms the basilar membrane motion and computes lateral inhibition in both frequency and time. The third stage integrates and aligns the neural activation pattern from multiple echoes, a process that has been demonstrated empirically in dolphin echolocation. The result of this processing is a stable auditory ‘‘image’’ or time-frequency map of the signal. The parameters of this model are tuned for the anatomy of the dolphin cochlea. The model has been applied to echo returns from a synthesized dolphin signal reflecting from cylinders in clutter. The cylinders were made of different materials and had an outer diameter of 3.8 cm and a length of 10.2 cm.

Impediment of basilar membrane motion reduces overload protection but not threshold sensitivity: evidence from clinical and experimental hydrops

The mechanical function of the basilar membrane (BM) in the mammalian cochlea has been newly debated after the discovery of frequency selectivity of single hair cells. Decisive information on this matter can be expected from hydropic ears, since hydrops presumably alters cochlear mechanics by (1) impeding BM motion and (2) uncoupling outer hair cell (OHC) stereocilia from the tectorial membrane (TM). Therefore hearing in Menière's disease (MD) was examined analysing data on epidemiology of MD types and audiogram types, 2f thresholds, over-recruitment, loudness intolerance, and otoacoustic emissions. Further, hearing in experimental hydrops (XH) was examined analysing data on: morphological changes on TM and OHCs in relation to hydrops duration; morphological and electrophysiological changes upon acoustic overstimulation. The results were unequivocal on two points: (1) co-occurrence of hydrops and normal hearing thresholds can appear both in MD and XH, (2) co-occurrence of non-hearing-loss hydrops and loudness hypersensitivity is typical both in MD and XH. The conclusion is that BM motion apparently is no necessary element in the chain of cochlear sound transmission but obviously is an auxiliary element for overload protection through resonant absorption. The results are further indicative of audiometric methods for an early detection of incipient MD.

A review of active and passive basilar membrane cochlear mechanics.

Since David Kemp first proposed the active model in 1979, the role of the outer hair cell (OHC) in basilar membrane (BM) mechanics has been hotly debated. In the ‘‘active’’ model view, the OHC modulates the motion of the BM traveling wave on a cycle-by-cycle basis, leading to a negative BM resistance and a traveling-wave power gain. Nonlinearity is introduced by assuming that the negative BM resistance depends on the signal level. In the ‘‘passive’’ model view, the OHC controls the stiffness of the basilar membrane, leading to a level-dependent (nonlinear) relative impedance between the BM and tectorial membrane and a nonlinear basilar membrane and transduction response. Both models seem to be able to achieve the important nonlinear variations in responses seen in the BM, OHC, IHC, and neural response, but with differing assumptions and degrees of physical reality. It is now clear that the OHC nonlinearly compresses both the dynamic range of basilar membrane motion (Rhode, 1971; Ruggero, 1990) and the neural response (Yates, 1989), extending the otherwise limited dynamic range of the IHC response. This role of the OHC may be quantified using psychoacoustic masking patterns, two-tone suppression, loudness growth and recruitment, and OAEs as objective measures.

Steady-state sinusoidal velocity responses of the basilar membrane in guinea pig.

This report provides a detailed analysis of tone-evoked velocity responses of basilar membrane (BM) motion measured in the basal turn of the guinea pig cochlea. A laser Doppler vibrometer, coupled to a compound microscope, measured the velocity of reflective microbeads placed onto the scala tympani surface of the BM. The velocity responses of the stapes footplate were also determined, allowing the calculation of a BM transfer function. The normal transfer function is compared to that seen with cochlear "insensitivity" and postmortem. This comparison results in measures of the active process contribution to the magnitude and phase of the BM transfer function. It was found that the active process contributed as much as 65 dB of "gain" and 270 degrees of phase lag at the best frequency. Other details about features of input/output velocity functions and the derived mechanical frequency-tuning curve at a criterion of 50 micron/s are analyzed.

Nonlinearity of mechanoelectrical transduction of outer hair cells as the source of nonlinear basilar-membrane motion and loudness recruitment.

The sound-induced travelling wave in the mammalian cochlea is believed to be enhanced and sharpened by a positive-feedback mechanism, causing the passive, linear growth function of the basilar membrane (BM) to become nonlinear. Based on direct measurements of the receptor potential of isolated outer hair cells, it is shown here how nonlinear BM motion might be due predominantly to the nonlinear growth function of the receptor potential. Since intensity coding in the inner ear is supposed to depend on an interaction of nonlinear BM motion with afferent fibres of different synaptic thresholds, intensity coding is expected to be directly dependent on the mechanoelectrical transduction of outer hair cells (OHC). According to the present experimental data and the feedback concept of outer hair cell action, disruption of the mechanoelectrical transduction of OHC leads to both a reduction of gain and linearizing of the response; that is, to both hearing loss and loudness recruitment.

Electromotile hearing: evidence from basilar membrane motion and otoacoustic emissions

Electrical stimulation of the cochlea is known to cause auditory sensations in humans and other animals. It also has been shown to produce emissions of sound from the inner ear. In the current study we investigate the relationship between electrically induced motion of the basilar membrane (BM) and the production of otoacoustic emissions. We test the hypothesis that electrical current-induced movements of the outer hair cell (OHC electromotility) result in intracochlear acoustic pressure which causes traveling waves on the BM. Our results demonstrate that the dominant response of the guinea pig inner ear to electric stimulation, at the round window membrane (RW) or across the cochlear duct, is a mechanical response of the organ of Corti. We observed that electrical stimulation of the cochlea produced traveling wave activity on the BM, measured with a laser Doppler velocimeter. The BM motion was accompanied by sound emitted by the cochlea for frequencies up to at least 25 kHz. Furthermore, bipolar rectangular current stimulation produced steady, bipolar displacements of the BM (to 2 nm), indicating functional elongation or contraction of OHCs occurs depending on the polarity of the current pulse. All of the evoked responses were absent after drug treatments eliminated the OHCs. Our data indicate that OHCs undergo electrically evoked displacements capable of producing high-fidelity, high-frequency acoustic energy. The electrically evoked intracochlear energy results in conventional traveling waves within the cochlea, as well as emissions of sound from the cochlea. These data provide direct support for a mechanism of cochlear sensitivity and tuning involving high-frequency OHC electromotility. Moreover, the data also indicate that any intra- or extracochlear electric current which affects the electric polarization of OHCs could induce BM traveling waves and cause ‘electromotile hearing’. This form of hearing would be one component under the more general definition of the electrophonic effect.