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PubMed2002

Interaction between adenosine triphosphate and mechanically induced modulation of electrically evoked otoacoustic emissions.

It was shown previously that electrically evoked otoacoustic emissions (EEOAEs) can be amplitude modulated by low-frequency bias tones and enhanced by application of adenosine triphosphate (ATP) to scala media. These effects were attributed, respectively, to the mechano-electrical transduction (MET) channels and ATP-gated ion channels on outer hair cell (OHC) stereocilia, two conductance pathways that appear to be functionally independent and additive in their effects on ionic current through the OHC. In the experiments described here, the separate influences of ATP and MET channel bias on EEOAEs did not combine linearly. Modulated EEOAEs increased in amplitude, but lost modulation at the phase and frequency of the bias tone (except at very high sound levels) after application of ATP to scala media, even though spectral components at the modulation sideband frequencies were still present. Some sidebands underwent phase shifts after ATP. In EEOAEs modulated by tones at lower sound levels, substitution of the original phase values restored modulation to the waveform, which then resembled a linear summation of the separate effects of ATP and low-frequency bias. While the physiological meaning of this procedure is not clear, the result raises the possibility that a secondary effect of ATP on one or more nonlinear stages in the transduction process, which may have caused the phase shifts, obscured linear summation at lower sound levels. In addition, "acoustic enhancement" of the EEOAE may have introduced nonlinear interaction at higher levels of the bias tones.

Adenosine Triphosphate↗
PubMed2002

Auditory brainstem responses in adult budgerigars (Melopsittacus undulatus).

The auditory brainstem response (ABR) was recorded in adult budgerigars (Melopsittacus undulatus) in response to clicks and tones. The typical budgerigar ABR waveform showed two prominent peaks occurring within 4 ms of the stimulus onset. As sound-pressure levels increased, ABR peak latency decreased, and peak amplitude increased for all waves while interwave interval remained relatively constant. While ABR thresholds were about 30 dB higher than behavioral thresholds, the shape of the budgerigar audiogram derived from the ABR closely paralleled that of the behavioral audiogram. Based on the ABR, budgerigars hear best between 1000 and 5700 Hz with best sensitivity at 2860 Hz-the frequency corresponding to the peak frequency in budgerigar vocalizations. The latency of ABR peaks increased and amplitude decreased with increasing repetition rate. This rate-dependent latency increase is greater for wave 2 as indicated by the latency increase in the interwave interval. Generally, changes in the ABR to stimulation intensity, frequency, and repetition rate are comparable to what has been found in other vertebrates.

Acoustic Stimulation↗
PubMed2002

Characterizing cochlear mechano-electric transduction with a nonlinear system identification technique: the influence of the middle ear.

Previously a third-order polynomial equation characterizing mechano-electric transduction was obtained from a nonlinear system identification procedure applied to an ear canal acoustic signal and cochlear microphonic (CM/AC). In this paper, we examine the influence of the linearity and frequency response of the intervening middle ear on the nonlinearity, frequency response, and coherence of the third-order polynomial model of mechano-electric transduction (MET). Ear canal sound pressure (AC), cochlear microphonics (CM), and stapes velocity (SV) were simultaneously recorded from Mongolian gerbils. Linear and nonlinear transfer and coherence functions relating stapes velocity to the acoustic signal (SV/AC), CM to the acoustic signal (CM/AC), and CM to the stapes velocity (CM/SV) were computed. The results showed that SV/AC was linear while CM/AC and CM/SV were not, indicating that the nonlinearity of CM/AC was not due to nonlinearity of the middle ear. The frequency response of the linear term of CM/AC was similar to that of ST/AC but differed from that of CM/SV while the cubic term of CM/AC was similar to that of CM/SV. This indicates that the frequency dependence of CM/AC was due to both the middle ear and frequency dependence of the inner ear. Finally the fit of the polynomial model of MET without the middle ear (CM/SV) did not improve from the fit including the middle ear (CM/AC). A cochlear model of the CM indicated that the lack of improvement was due to the limitations of a third-order polynomial equation characterizing the hair cell transducer function.

Animals↗
PubMed2003

Objective estimates of cochlear tuning by otoacoustic emission analysis.

A new method is presented for estimating cochlear tuning starting from measurements of either the transient evoked otoacoustic emission latency or the spontaneous otoacoustic emission minimal spacing. This method could be useful in obtaining indirect information about the tuning curve, particularly for subjects that, like neonates, cannot be studied with psycho-acoustical techniques. Theoretical models of the acoustic transmission along the cochlea based on the transmission line formalism predict a relation between the otoacoustic emission latency and the frequency. This relation depends on the tuning curve, i.e., the frequency dependence of the quality factor of the cochlear resonances. On the other hand, models for the generation of spontaneous emissions based on the concept of coherent scattering from cochlear random inhomogeneities imply an independent relation between the tuning curve and the minimal frequency spacing between spontaneous emissions. In this study, experimental measurements of the otoacoustic emission latency and of the minimal spacing between spontaneous emissions are presented. Theoretical relations are derived, which connect these two measured quantities and the tuning curve. The typically longer latency of neonates implies a higher degree of tuning at high levels of stimulation.

Adolescent↗
PubMed2005

Modeling high-frequency electromotility of cochlear outer hair cell in microchamber experiment.

Cochlear outer hair cells (OHC) are critically important for the amplification and sharp frequency selectivity of the mammalian ear. The microchamber experiment has been an effective tool to analyze the OHC high-frequency performance. In this study, the OHC electrical stimulation in the microchamber is simulated. The model takes into account the inertial and viscous properties of fluids inside and outside the cell as well as the viscoelastic and piezoelectric properties of the cell composite membrane (wall). The closed ends of the cylindrical cell were considered as oscillatory rigid plates. The final solution was obtained in terms of Fourier series, and it was checked against the available results of the microchamber experiment. The conditions of the interaction between the cell and pipette was analyzed, and it was found that the amount of slip along the contact surface has a significant effect on the cell electromotile response. The cell's length changes were computed as a function of frequency, and their dependence on the viscosities of both fluids and the cell wall was analyzed. The distribution of the viscous losses inside the fluids was also estimated. The proposed approach can help in a better understanding of the high-frequency OHC electromotility under experimental and physiological conditions.

Cell Movement↗
PubMed1979

The spectral content of the cochlear microphonic measured in scala media of the guinea pig cochlea.

Cochlear microphonic (CM) in response to low-frequency tonal stimuli, measured as a function of sound-pressure level (SPL) in scala media of the guinea pig cochlea, was averaged and Fourier analyzed. The slope of the amplitude of the Nth CM harmonic versus sound intensity in log-log coordinates was approximately N (1 less than or equal to N less than or equal to 5) in the first three cochlear turns, but notable variations on such a slope rule were found to apply to CM in turn I. CM harmonic phases plotted versus SPL were found to group into two distinctive categories expressly delimited by whether the order of the harmonic was even or odd. Some difference between CM recorded between scala media and scala tympani and recorded between scala media and the animal's neck could be attributed to neural contamination. We also found CM in its saturation region to have a hysteretic relation to the input sound pressure. At high sound levels, large, physiologically produced acoustic harmonics existed at the animal's eardrum. Our data support an asymmetrical, saturating, single-valued nonlinearity as a model for CM generation at low sound levels. At higher sound levels a different, more complex, hysteretic nonlinearity seems mandatory.

Acoustic Stimulation↗
PubMed1979

Comments on the effects of overstimulation on microphonic sensitivity.

Utilizing the differential recording technique, microphonic isopotential measurements were carried out before and after overstimulation. Recordings were made in the first and third turns of guinea pig cochleas. The exposure stimuli were pure tones of frequencies near the best frequency of the recording site. The results were suggestive of differences in the effects of overstimulation in different turns of the cochlea, at least as they are manifested electrophysiologically.

Acoustic Stimulation↗
PubMed1980

Sound-induced resistance changes in the inner ear.

A new technique for measuring sound-induced resistance changes (CR) in scala media in response to pure-tone stimuli by injecting alternating current into guinea-pig cochleas was reported recently [C.D. Geisler et al., J. Acoust. Soc. Am. 61, 1557-1566 (1977)]. Detailed measurements with this technique indicate that while the CR behaves approximately as does the cochlear microphonic (CM) there can be very significant differences between the two variables under certain experimental conditions. Computer analysis of simultaneously recorded CR voltage components and CM indicates that the CR harmonics, in both amplitude and phase, behaved differently with sound intensity and with asphyxia than did the CM harmonies (A.E. Hubbard et al., J. Acoust. Soc. Am 66, 431-445 (1979)]. Direct current injection and stimulation of the crossed olivocochlear bundle (COCB) indicate further differences between CM and CR (D.C. Mountain, Ph.D. thesis, University of Wisconsin-Madison, 1978). Positive dc caused a relative augmentation of CM that grew with sound intensity, and a relative reduction in CR magnitude that decreased with intensity. Negative dc caused effects of similar magnitude but opposite sign. COCB stimulation caused enhancement of both CM and CR. Present models cannot account quantitatively for these results.

Acoustic Stimulation↗
PubMed1980

Sound pressures in the basal turn of the cat cochlea.

Techniques were developed for measuring sound pressure in the cochlea with calibrated, liquid-filled, piezoelectric probe microphones. Sound pressures were measured in scala vestibuli and scala tympani in the basal turn in 25 cats for tones from 20--10 000 Hz. Control experiments indicated that intracochlear pressures were essentially uninfluenced by the measuring technique, and were conducted to the cochlea via the ossicular chain. Intracochlear pressures are linearly related to pressure at the tympanic membrane for tone levels at least as high as 105 dB SPL, and are relatively independent of depth of probe insertion in the scalae. The transfer ratio of sound pressure in scala vestibuli to that at the tympanic membrane increases in magnitude over the frequency range 50--1000 Hz to reach a maximum value of 15--30 dB, and decreases at higher frequencies, thus demonstrating that the middle ear provides a frequency-dependent pressure gain. At frequencies below 40 Hz, the pressures in scala vestibuli and scala tympani are approximately equal and are both determined by the round-window membrane compliance. At frequencies above 100 Hz, the round-window membrane impedance is small compared to the acoustic input impedance of the cochlea, and the pressure in scala vestibuli considerably exceeds that in scala tympani; consequently, the pressure difference across the cochlear partition is approximately equal to the pressure in scala vestibuli.

Animals↗
PubMed1981

Identification of local and propagating distortion products from cochlear microphonic responses.

Careful measurements show that sound pressures of 40 dB re 20 mu N/m2 are sufficient to reveal two varieties of well-behaved, nonlinear distortion products in the cochlear microphonic (CM). The first variety appears in the primary-tone area and is designated as a local CM distortion product. The second type exists apical to the primary-tone area and is identical to the mechanically propagated distortion seen in the phase-locked responses of primary auditory nerve fibers. The existence of the propagating distortion product forces the conclusion that there must be a mechanical contribution to the local "CM" distortion product as well. The intrusion of nonlinear mechanical responses at such low levels (less than 40 dB SPL) indicates that the effective mechanical input to the hair cells may be nonlinear over most of the audible range. An important but unanswered question is the range over which the transducer characteristic of CM could be effectively linear, for that would determine whether CM could be used to probe mechanical nonlinear effects in the primary tone area.

Acoustic Stimulation↗
PubMed1982

Correspondence principle in cochlear mechanics.

When only long waves play the most important part in the cochlea, the response can be described by a most simplified model, the one-dimensional model. When short waves are to be included, a more complex model is needed. The response then depends on the dimensionality of the model and is much harder to obtain. This applies especially to the region in the neighborhood of the point where the basilar membrane shows resonance. Both two- and three-dimensional models have been studied to assess the effects of short and long waves. The relative importance of the part played by short waves depends on the damping constant (or loss factor )delta associated with the resonance of the basilar membrane (BM). For very small delta a three-dimensional model is really necessary, it cannot be replaced by a model of lower dimensionality. When delta is small, but not too small, the three-dimensional model can be made equivalent to a two-dimensional one, provided the latter is modified ina specific manner. This paper shows why this is so and which conditions have to be met. The two-dimensional model must undergo two modifications to effect this equivalence. The first modification ensures that the model has the same long-wave behavior. In the second place, a specific additional mass ("added mass") reactance should be added to Z(kappa). An expression for the limiting value of delta, above which this correspondence is valid, is given in the paper. A second, larger, limit is presented as well: when delta is above this limit, the responses of both the three-dimensional and the two-dimensional model are equivalent to that of an appropriately chosen one-dimensional model. In this case too, long-wave behavior must be matched and an "added mass" reactance must be included in Z(kappa). This holds true for the entire cochlea including the region of resonance. For both types of transition the amount of "added mass" is given.

Basilar Membrane↗
PubMed1982

Effects of low-frequency biasing on auditory-nerve activity.

Intensity functions of single fibers in the auditory nerve of gerbils were obtained to tone bursts alone and tone bursts superimposed on a low-frequency biasing signal. The bias was a triangular sound-pressure waveform with a period of 100 ms and was usually presented at 90 dB SPL. Cochlear microphonics (CM) recorded in scala media of the basal and second turns of the cochlea were trapezoidal in shape in response to the triangular waveform of sound pressure. Assuming the CM waveform reflects basilar-membrane displacement at very low frequencies, it is concluded that the input impedance of the gerbil cochlea is substantially resistive at frequencies as low as 10 HZ. Intensity functions of single fibers with characteristic frequencies (CFs) below about 12 kHZ exhibited enhanced thresholds to CF tones associated with basilar-membrane displacement toward scala tympani; conversely, thresholds to CF tones were suppressed during displacement toward scala vestibuli. Tone-alone thresholds were usually between the two biased conditions. The bias had little effect on responses of tones placed below or above CF and on the activity of fibers with CFs greater than about 12 kHZ. These physiological results are compatible with corresponding psychophysical measures obtained from human observers.

Animals↗
PubMed1982

A cochlear nonlinear transmission-line model compatible with combination tone psychophysics.

Human psychophysical measurements of the cubic combination tone (2f1-f2) have shown that at low and moderate stimulus levels its phase decreases at 6 degrees-12 degrees per dB increase in stimulus level. This finding contrasts with physiological measurements in anaesthetized animals where the CT phase is insensitive to stimulus level. We have characterized quantitatively the difference in cochlear nonlinear response between humans and animals in terms of a cochlear nonlinear transmission line model having different nonlinear elements for human and animal. Following Hall [J. Acoust. Soc. Am. 56, 1818-1828 (1974)], a nonlinearity was introduced in the resistance of the cochlear partition (model A) for describing the animal cochlea. To model the human cochlea, we found that adding a nonlinear stiffness to the nonlinear mechanical loading of the basilar membrane gave the correct phase-amplitude dependence (model B). Simulation was used to solve the nonlinear models in the time domain. For high amplitude stimuli, both models predict similar results, mainly saturation in the response. The significant differences between the models occur at low and moderate stimulus intensities. According to model B the site of the resonant frequency along the basilar membrane depends on the stimulus level, while it is independent of stimulus level according to model A. As a result of the shift in the resonant site location in model B, the phase response profile is shifted as well, so that the phase response at the original resonant site depends on stimulus level. The psychophysical data on CT cancellation were predicted by model B, while physiological data on CT cancellation are predicted by model A.

Animals↗
PubMed1983

Cochlear microphonic responses to acoustic clicks in guinea pig and their relation with microphonic responses to pure tones.

Cochlear microphonic (CM) responses to acoustic transient stimuli were studied at the three more basal turns of the cochlea in the guinea pig. The responses to rarefaction and condensation pressure pulses of less than 100-mus duration were recorded using the differential electrode technique. In some animals the CM response to pure tones was recorded at the same position at which the transient response was obtained. The transient responses recorded at the three turns of the cochlea displayed a damped oscillation at a frequency consistent with the values of cutoff frequency already known for the electrode positions. Some of the responses were significantly less damped than click responses previously reported. There was a good correlation between the cutoff frequency in the frequency response curve and the frequency of oscillation in the transient response for recordings obtained at the same position in the cochlea. A nonlinear effect was observed for changes in stimulus intensity. There was a less than proportional decrease in amplitude of the initial part of the damped oscillation for a decrease of the stimulus intensity, while the late part of the response behaved almost linearly. This nonlinearity observed in the CM transient response could not be explained by a nonlinear characteristic of the sort reported in the basilar membrane of the squirrel monkey by Robles et al. [J. Acoust. Soc. Am. 59, 926-939 (1976)]; rather it seems to be a saturation nonlinearity similar to the one known for sinusoidal stimulation.

Acoustic Stimulation↗
PubMed1983

Magnitude and phase-frequency response to single tones in the auditory nerve.

In this paper we describe magnitude and phase measurements obtained from primary single unit recordings in the cat auditory nerve. Levels range from threshold to 100 dB SPL, with frequencies from 0.1-30.0 kHz. The upper limit on the phase measurements was limited by the loss of neural phase locking at 4-5 kHz. For each unit, the frequency tuning curve (FTC) was measured by the method of Kiang and Moxon [M.C. Liberman, J. Acoust. Soc. Am. 63, 442-445 (1978)] to establish the threshold frequency response of the unit. Data from several selected animals, organized by characteristic frequency (CF), are presented showing phase response, group delay, frequency tuning, and tuning slope for each CF range. The major emphasis in this paper is on the "linear" aspects of the data as characterized by the filter properties of the single unit response, however a number of nonlinear (level-dependent) effects are described. Data are presented showing the phase response normalized by the cochlear microphonic (CM) recorded at the round window membrane. This normalization simplifies the phase data since it produces a constant phase slope with respect to frequency (constant group delay) for high CF units (f CF greater than 1 kHz) for frequencies more than one octave below their characteristic frequencies. A model of CM, as measured at the round window (RW), is presented and compared to experimental CM measurements. The CM model gives a reasonable fit to the experimental data above 500 Hz. Our interpretation of the CM normalization is that it removes driver and middle ear effects. In the model we assume that the CM is generated by the displacement of the basilar membrane near the round window recording site.

Animals↗
PubMed1983

Chinchilla auditory-nerve responses to low-frequency tones.

Single unit activity was recorded in the auditory nerves of chinchillas. Period histograms were constructed for responses to tones with frequencies 30-1000 Hz. For low-frequency tones at near-threshold levels, peak period histogram phases for low- and medium-best-frequency (BF) neurons (less than or equal to kHz) ranged from synchronous with condensation at the eardrum to 90 degrees leading it. At near-threshold (but high absolute) levels, high-BF (greater than or equal to 8 kHz) neurons responded in phase with rarefaction. At even higher levels, period histograms for responses of high-BF neurons tended to become bimodal, with one of the modes lagging rarefaction by 90 degrees. Using cochlear microphonics as an indicator of basilar membrane (BM) displacement, at threshold levels, response phase of low- and medium-BF neurons fall within a range between displacement and velocity of the BM toward scala vestibuli. High-BF neurons respond, at threshold (but high) intensities, in phase with BM displacement toward scala tympani. The rates of growth of frequency sensitivity in responses of low-BF (+ 18 dB/oct) and high-BF (+ 12 dB/oct) neurons are consistent with preferred response phases corresponding to BM SV velocity and ST displacement, respectively. At supra-threshold levels high-BF neurons may fire preferentially to both scala tympani displacement and scala vestibuli velocity. These results support the notion that, for high-intensity, low-frequency stimuli, OHC hyperpolarization can induce excitation of the dendrites innervating IHCs.

Animals↗
PubMed1985

Specification of the acoustical input to the ear at high frequencies.

The sound fields that arise in the auditory canals of cats have been examined both experimentally and theoretically. Of particular interest was the spatial variation of sound pressure near the eardrum, where reference probes are typically located. Using a computer controlled data acquisition system, sound pressure was measured between 100 Hz and 33 kHz for constant driver input at 14 different locations in the ear canal of a cat, and the standing wave patterns formed. The shape of the patterns could be predicted quite well above 12 kHz using a theory that requires specification of only the geometry of the ear canal. This theory, an extension of the one-dimensional horn equation, applies to three-dimensional, rigid-walled tubes that have both variable cross section and curvature along their lengths. Large variations of sound pressure along the ear canal and over the surface of the eardrum are found above about 10 kHz. As a consequence it is not possible to define the acoustical input to the ear from sound pressure level measured at any single location. Even in comparative experiments, in which only the constancy of the acoustical input is important, any uncertainty in reference probe location would lead to an uncertainty in sound pressure level when different sets of measurements are compared. This error, calculated for various probe locations and frequencies, is especially large when the probe is near a minimum of the sound field. Spatial variations in pressure can also introduce anomalous features into the measured frequency response of other auditory quantities when eardrum sound pressure is used as a reference. This is illustrated with measurements of the round window cochlear microphonic.

Animals↗