Auditory information processing; sensory biophysics; computer simulation; biomedical electronics; biomedical signal processing; environmental engineering.
CURRENT RESEARCH
Dr. Mountain’s research centers around the experimental and theoretical studies of hearing function, including: cochlear biomechanics, otoacoustic emissions, auditory processing of complex sounds, and auditory evoked potentials. Professor Mountain also engages in studies designed to predict the impact of anthropogenic sound sources on marine mammals.
Abstract: Outer hair cell (OHC) somatic motility plays a key role in mammalian cochlear frequency selectivity and hearing sensitivity, but the mechanism of cochlear amplification is not well understood and remains a matter of controversy. We have visualized and quantified the effects of electrically evoked OHC somatic motility within the gerbil organ of Corti using an excised cochlear preparation. We found that OHC motility induces oscillatory motion of the medial olivocochlear fibers where they cross the tunnel of Corti (ToC) in their course to innervate the OHCs. We show that this motion is present at physiologically relevant frequencies and remains at locations distal to the OHC excitation point. We interpret this fiber motion to be the result of oscillatory fluid flow in the ToC. We show, using a simple one-dimensional hydromechanical model of the ToC, that a fluid wave within the tunnel can travel without significant attenuation for distances larger than the wavelength of the cochlear traveling wave at its peak. This ToC fluid wave could interact with the cochlear traveling wave to amplify the motion of the basilar membrane. The ToC wave could also provide longitudinal coupling between adjacent sections of the basilar membrane, and such coupling may be critical for cochlear amplification.
Abstract: The outer hair cell (OHC) of the mammalian inner ear exhibits an unusual form of somatic motility that can follow membrane-potential changes at acoustic frequencies. The cellular forces that produce this motility are believed to amplify the motion of the cochlear partition, thereby playing a key role in increasing hearing sensitivity. To better understand the role of OHC somatic motility in cochlear micromechanics, we developed an excised cochlea preparation to visualize simultaneously the electrically-evoked motion of hundreds of cells within the organ of Corti (OC). The motion was captured using stroboscopic video microscopy and quantified using cross-correlation techniques. The OC motion at approximately 2-6 octaves below the characteristic frequency of the region was complex: OHC, Deiter's cell, and Hensen's cell motion were hundreds of times larger than the tectorial membrane, reticular lamina (RL), and pillar cell motion; the inner rows of OHCs moved antiphasic to the outer row; OHCs pivoted about the RL; and Hensen's cells followed the motion of the outer row of OHCs. Our results suggest that the effective stimulus to the inner hair cell hair bundles results not from a simple OC lever action, as assumed by classical models, but by a complex internal motion coupled to the RL.
Abstract: Anatomical studies suggest that the basilar membrane (BM) supports a radial tension, which is potentially important in cochlear mechanics. Assuming that the tension exists, we have calculated its magnitude from measurements of BM stiffness, longitudinal coupling, and geometry using a BM model. Results for the gerbil cochlea show that the tension decreases from the base to the apex of the cochlea and generates a tensile stress that is comparable in magnitude to the stress generated in other physiological systems. The model calculations are augmented by experiments that investigate the source of BM tension. The experimental results suggest that BM tension is maintained by the spiral ligament.
Abstract: The construction, measurement, and modeling of an artificial cochlea (ACochlea) are presented in this paper. An artificial basilar membrane (ABM) was made by depositing discrete Cu beams on a piezomembrane substrate. Rather than two fluid channels, as in the mammalian cochlea, a single fluid channel was implemented on one side of the ABM, facilitating the use of a laser to detect the ABM vibration on the other side. Measurements were performed on both the ABM and the ACochlea. The measurement results on the ABM show that the longitudinal coupling on the ABM is very strong. Reduced longitudinal coupling was achieved by cutting the membrane between adjacent beams using a laser. The measured results from the ACochlea with a laser-cut ABM demonstrate cochlear-like features, including traveling waves, sharp high-frequency rolloffs, and place-specific frequency selectivity. Companion computational models of the mechanical devices were formulated and implemented using a circuit simulator. Experimental data were compared with simulation results. The simulation results from the computational models of the ABM and the ACochlea are similar to their experimental counterparts.
Abstract: Aminoglycoside treatment induces caspase-dependent apoptotic death in inner ear sensory hair cells. The timing of apoptotic signaling in sensory hair cells following systemic aminoglycoside treatment has not been characterized in vivo. We administered a single subcutaneous injection of the aminoglycoside gentamicin (300 mg/kg) to 12-16-day-old chicks and used immunocytochemical techniques to document the following responses in affected hair cells: T-cell restricted intracellular antigen-related protein (TIAR) translocation from the nucleus to the cytoplasm, cytochrome c release from the mitochondria, caspase-3 activation, nuclear condensation, and an orderly progression of hair cell ejection from the proximal end of the basilar papilla. Hair cells in the proximal tip exhibited TIAR translocation from the nucleus and aggregation into punctate granules in the cytoplasm 12 hours after injection and the response progressed distally. Cytochrome c release from the mitochondria into the cytoplasm and caspase-3 activation were observed in affected hair cells immediately prior to and during ejection. Hair cell ejection occurred between 30 and 54 hours after injection, beginning in the proximal tip and progressing distally. Nuclear condensation accompanied ejection while the loss of: 1) membrane integrity; 2) phalloidin labeling of F-actin; and 3) TO-PRO-1 labeling of nuclear contents occurred within 48 hours following ejection. Our results present a timeline of aminoglycoside-induced inner ear sensory hair cell apoptotic death that includes an 18-hour window between the initial apoptotic response and the later stages of programmed death signaling that accompany ejection and a gradual breakdown of hair cells following ejection.
Abstract: A systematic and detailed study of the longitudinal coupling exhibited by the basilar membrane (BM) was performed in the excised gerbil cochlea. Contrary to the notion that the adjacent regions of the BM are decoupled from each other, the data indicate that: (a) the BM exhibits longitudinal coupling; (b) the length of the coupled region increases from base to apex of the cochlea; and (c) the cells of the organ of Corti (OC) increase the overall coupling exhibited by the BM. Modeling results show that, at a given location, longitudinal coupling increases the effective stiffness of the OC near the characteristic frequency. Therefore, the effect of longitudinal coupling cannot be neglected in the region of the peak of the traveling wave.
Abstract: Scene analysis, the process of converting sensory information from peripheral receptors into a representation of objects in the external world, is central to our human experience of perception. Through our efforts to design systems for object recognition and for robot navigation, we have come to appreciate that a number of common themes apply across the sensory modalities of vision, audition, and olfaction; and many apply across species ranging from invertebrates to mammals. These themes include the need for adaptation in the periphery and trade-offs between selectivity for frequency or molecular structure with resolution in time or space. In addition, neural mechanisms involving coincidence detection are found in many different subsystems that appear to implement cross-correlation or autocorrelation computations.
Abstract: Lobsters are capable of tracking turbulent plumes to their sources faster than can be accomplished by estimating a spatial gradient from time-averaging the concentration signal. We have used RoboLobster, a biomimetic robot lobster to investigate biologically scaled chemotaxis algorithms using two point concentration sampling to track a statistically characterized turbulent plume. Our results identify the range of effectiveness of these algorithms and, with studies of lobster behavior, suggest effective strategies beyond this range. They suggest that a lobster's chemo-orientation strategy entails an unidentified means of dealing with the intermittency of the concentration signal. Extensions of these algorithms likely to improve are discussed.
Abstract: Moderate acoustic trauma results in decreased cochlear sensitivity and frequency selectivity. This decrease is believed to be caused by damage to the cochlear amplifier that is associated with outer hair cells (OHCs) and their nonlinear electromechanical characteristics. A consequence of OHC nonlinearity is the acoustic enhancement effect, in which low-frequency electrically evoked otoacoustic emissions are enhanced by a simultaneous tone. The present study found that acoustic trauma reduced the acoustic enhancement effect and this reduction is correlated with the N1 threshold at the electrode site. This result is consistent with the theory that trauma affects the mechanoelectric transduction process, thus affecting cochlear mechanical nonlinearity. Acoustic trauma also reduced the cochlear microphonic in a way that suggests that the number of functioning tension-gated channels and the stiffness of the gating springs were decreased. In some cases, the electromechanical transduction process was also found to be affected by acoustic trauma.
Abstract: Most current theories of cochlear mechanics assume that the pattern of cochlear partition vibration is simple, similar to that of a bending beam. Recent evidence suggests, however, that the vibration of the organ of Corti can be complex and that multiple vibrational modes may play an important role in cochlear transduction. Inner hair cell (IHC) and auditory nerve responses to pure tones can exhibit large phase shifts and complex response waveforms with increasing stimulus level. In contrast, the comparable basilar membrane (BM) responses are much less complex, exhibiting only small phase shifts and relatively sinusoidal waveforms. To reconcile the differences observed between the published BM data and the IHC data, we have recorded receptor potentials from IHCs and compared these waveform data to the output of two computational models: a traditional linear model where IHC excitation depends only on BM displacement and a new model that assumes that outer hair cell (OHC) force production provides the major mechanical input to the IHC along with two additional mechanical components. Comparisons of the output of the two models with the experimental data show that the new model is capable of reproducing the very complex voltage responses of the IHC recorded in vivo whereas the traditional model performed poorly.
Abstract: To further our knowledge of outer hair cell nonlinearities, we measured the dependence of the electrically-evoked otoacoustic emissions (EEOEs) on current level for a wide range of electrical frequencies. Alternating electrical current was delivered into the scala media of the gerbil cochlea while the EEOE was measured with a probe-tube microphone. While the EEOE scaled linearly with current level for many frequencies and current levels, notable exceptions occurred. For frequencies below 300 Hz and currents above 20-30 microA(peak), the gain (primary EEOE magnitude divided by the current level) increased abruptly. For higher frequencies, the gain often increased slightly with increasing current of up to 30-50 microA(peak), but decreased at even higher current levels. We also investigated the enhancement of the EEOE due to simultaneous acoustic stimulation. The enhancement of the EEOE was relatively insensitive to current level with little change in enhancement for current levels up to 20 microA(peak). For current levels above approximately 40 microA(peak), the enhancement decreased slightly.
Abstract: Electrically induced outer hair cell (OHC) motility, demonstrated by a number of investigators in isolated OHC preparations, has been considered to be a key mechanism in the active process which brings about the excellent sensitivity and frequency selectivity of the mammalian cochlea. In this study, electrical-to-mechanical transduction in the gerbil cochlea was demonstrated in vivo by direct measurement of basilar membrane motion evoked by sinusoidal electrical current injected into the scala media. The characteristic frequency (CF) of the measurement place was approximately 40 kHz as determined by the basilar membrane (BM) responses to acoustic stimulation. The results showed that basilar membrane motion could be evoked by electrical current of frequencies from below 10 Hz to exceeding 40 kHz. The magnitude and phase of the BM velocity response to constant current stimulation, from 100 Hz to 10,000 Hz, were similar to the acoustically driven BM velocity for constant umbo velocity. For frequencies in this range, the BM motion evoked by a current of 50 microA was comparable to the BM motion evoked by a 60 dB SPL acoustic stimulus. The phase of the electrically evoked BM motion indicates that positive current injected into the scala media caused the BM to move toward scala vestibuli for frequencies between 100 and 10 kHz. This result is consistent with the hypothesis that the electrically evoked BM motion is due to electrically evoked OHC length changes.
Abstract: A simple model for the acoustic enhancement of electrically evoked otoacoustic emissions (EEOEs) is presented in this paper. The model is based on the assumption that the enhancement is a result of the local interaction between the electrical current spreading in the scala media and the basilar membrane (BM) response to acoustic input. The analytical, steady-state response of the 1-dimensional linear cable to sinusoidal current injection is derived and is used to predict the current spreading in the cochlea. Acoustic enhancement at an emission generator is modeled as a magnitude change that is a sigmoid function of the local BM motion. The model results are in good agreement with the experimental findings and support our interpretation that the acoustic enhancement of EEOEs reflects BM tuning.
Abstract: Electrically evoked otoacoustic emissions were measured with current delivered to the second and third turns of the gerbil cochlea. The emission magnitude and phase are dependent on the characteristic frequency (CF) of the stimulating microelectrode location. The death of the animal resulted in an initial increase in emission below the CF of the electrode location and a decrease in emission near the CF of the electrode location. The group delay of the electrically evoked emission phase data is twice as large as the acoustically evoked cochlear microphonic (CM) data obtained by Schmiedt and Zwislocki [J. Acoust. Soc. Am. 61, 133-149 (1977)]. This suggests the possibility of two separate propagation modes for the forward and reverse traveling waves.
Abstract: Inner hair cells (IHC) transduce mechanical to electrical energy in the mammalian cochlea producing a receptor potential which is a rectified, filtered representation of the mechanical input to the hair cell. The IHC synapse transfers the information in the receptor potential to the fibers of the auditory nerve (whose cell bodies form the spiral ganglion) where it is encoded as a pattern of action potentials. That transfer was investigated by comparing the steady-state responses in pre- and post-synaptic cells. A nonlinear transfer characteristic describing the synapse was generated by plotting the spiral ganglion cell firing rate as a function of the IHC receptor potential. For each spiral ganglion unit, the operating range maps onto a different portion of the nonlinear inner hair cell operating range, dependent on the neural unit's threshold. Units whose rate-level functions exhibit similar slopes but different thresholds can have dramatically differing sensitivities to changes in the IHC potential. This threshold-dependent mapping supports the concept that information may be distributed amongst nerve fibers according to their threshold.
Abstract: The mechanical properties of the cochlear partition are fundamental to auditory transduction. We measured the point stiffness of the partition, in vivo, at up to 17 radial positions spanning its width, in the basal turn of the gerbil cochlea. We found the linear stiffness at the position that is most likely under the outer pillar cells to be 1.5 times greater than adjacent positions toward the ligament, in the pectinate zone, and five times greater than adjacent positions toward the lamina, in the arcuate zone. This radial variation seems to reflect the cellular geometry of the partition: The pillar cell is positioned as a structural element, and the basilar membrane supports a rich cellular structure in the pectinate zone, whereas it borders a fluid-filled space in the arcuate zone. The radial variation in partition stiffness we find will influence passive cochlear mechanics, and also bears on active cochlear mechanics, since it supports the plausibility of cells as effective force generators. Our results from measurements made in vivo extend the findings of previous measurements made in excised cochleae, in which the cellular contribution to stiffness was less evident.
Abstract: Mammalian outer hair cells (OHC) are believed to increase cochlear sensitivity and frequency selectivity via electromechanical feedback. A simple piezoelectric model of outer hair cell function is presented which integrates existing data from isolated OHC experiments. The model predicts maximum OHC force production to equal 1.25 nN/mV. The model also predicts that the maximum velocity of OHC contraction in situ to be 800 microns/s. These predictions are compared to available experimental data and are found to be in good agreement. The good agreement between the predicted and experimental results suggests that, at the characteristic frequency of a given cochlear location, the OHC receptor current is very efficiently converted into basilar membrane motion.
Abstract: Electrically-evoked otoacoustic emissions were produced using a 10 microA, 750 Hz AC current plus a biasing DC current in the range of +/- 10 microA. Concurrently, a 1643 Hz tonal stimulation was delivered to the eardrum. At low sound levels, negative DC current increased the emission while positive DC current reduced the emission. Such findings are reasonably explained by a negative-feedback model of cochlear function. At high sound levels, negative DC current reduces the emission, while positive current has little effect. These data can be accounted for by voltage-dependent length changes shown to occur in isolated outer hair cells, with the additional requirement that voltage-dependent K+ channels in outer hair cells reduce the effectiveness of positive DC current in changing membrane potential.
Abstract: The envelope following response (EFR) is an auditory-evoked potential recorded from the scalp which is elicited by long duration, amplitude-modulated stimuli. In this paper, the results of a series of experiments exploring the behavior of the EFR elicited with sinusoidally amplitude modulated (SAM) tones in the presence of simultaneously gated, continuous, pure-tone interfering signals of varying intensity are reported. Probe stimuli consisted of SAM tones with carriers ranging in frequency from 800 Hz-4 kHz, modulated at frequencies between 30-150 Hz. Probe signals were presented at intensities between 50 and 75 dB pSPL. Pure-tone interfering signals consisted of frequencies between 100 Hz and 10 kHz and ranged in intensity from -10 to +20 dB re: the probe. In these experiments a maximum reduction in the response to the probe tone, measured at the probe modulation frequency, appeared as a sharp peak within a narrow frequency band above the frequency of the probe carrier and a broader region of reduced response extending to higher frequencies. This reduction in response was asymmetrical, spreading more to high than to low frequencies. With an increase in the intensity of the interfering signal the maximum reduction of the response increased in a saturating, monotonic fashion with a concomitant broadening of the frequency region affected. The obtained interference response pattern may be attributable to both "synchrony capture" (i.e., capture of the EFR of the system by envelope components arising due to the interaction of probe and interfering signals) and "synchrony suppression" (i.e., a reduction in the synchronized response from neurons excited by the probe in the presence of the added interfering tone). It appears that the EFR to SAM stimuli of low to moderate intensity arose primarily from neuronal populations tuned to frequencies at or above the probe fc. The results of the present study suggest that at low intensity levels SAM signals are indeed relatively frequency specific and warrant further study for audiometric applications.
Abstract: Acoustic enhancement of the electrically-evoked otoacoustic emissions (EEOEs) was investigated by systematically varying acoustic frequency and intensity. The results demonstrated that simultaneous acoustic stimulation at frequencies around the characteristic frequency of the electrical current injection place was most effective in enhancing low-frequency EEOEs. Moreover, it was demonstrated that the enhancement was tuned and graded. The enhancement threshold tuning curves (defined as sound pressure level needed to achieve 1 dB of enhancement) resembled basilar membrane tuning at high sound pressure levels. The data suggest that the emissions were generated from a cochlear region near the electrode place, and the magnitude of the enhancement depends on the magnitude of the basilar membrane response to the acoustic stimulus.
Abstract: Scalp potentials which follow the low frequency envelope of a sinusoidally amplitude modulated stimulus waveform were evoked and recorded in anesthetized gerbils. This envelope following response (EFR) is presumably due to the synchronized discharge of populations of neurons in the auditory pathway. The magnitude of the EFR increased and the latency decreased in a near monotonic fashion with increased stimulus intensity and modulation depth. The modulation rate transfer function (MRTF) was determined for modulation frequencies between 10 and 920 Hz imposed on carrier frequencies ranging from 1 to 7 kHz. The MRTF was low pass in character having a corner frequency of 100-120 Hz. Measurements of the group delay, determined from the phase of the response relative to the stimulus phase, indicate that the response is generated in at least three distinct regions within the auditory pathway.
Abstract: Basilar membrane stiffness measurements were made in the base of the gerbil cochlea. Basilar membrane stiffness was determined by contacting the basilar membrane with a stainless steel needle (tip diameter 25 microns) attached to a force transducer, putting the needle/transducer structure through a low-frequency sinusoidal excursion with amplitude 5 or 25 nm, and measuring the restoring force exerted on the needle by the basilar membrane at the applied frequency. Stiffness was calculated as the amplitude of the restoring force divided by the amplitude of the excursion. Stiffness was measured over a 24-microns range of static displacements of the basilar membrane and is presented as stiffness versus static displacement. In cochleas that were not damaged during surgery the stiffness versus displacement characteristic usually had the following features: (1) an initial stiffness plateau with average stiffness 0.6 N/m; (2) a second plateau or level off with average stiffness 9.1 N/m; and (3) an increase in stiffness beyond the second plateau that was consistent with the theoretical stiffness-vs-displacement function of a beam. These features were present both pre- and post-mortem.
Abstract: Cochlear outer haircells are believed to play a significant role in an amplification process which greatly enhances inner ear sensitivity. Haircell forward (mechanical-to-electrical) and reverse (electrical-to-mechanical) transduction may be involved. We have produced decreases in cochlear microphonic and increases in electrically-evoked cochlear emissions using the drug, furosemide. The data indicate forward and reverse transduction are not a simple bi-directional process and suggest that the outer haircells are part of a negative feedback system.
Abstract: Auditory nerve fibres usually respond to a preferred phase of a low frequency sinusoidal stimulus. However, at high sound pressures fibres may either change the preferred phase of response or respond to more than one phase of the stimulus. This complex firing pattern is known as peak splitting. Hypotheses for the origin of peak splitting have ranged from micromechanical models to models incorporating electrical interactions between inner and outer hair cells. In order to determine the origin of peak splitting, the potential across the IHC synaptic membrane has been measured during stimulation with low frequency tones and it is found that the IHC receptor potentials exhibit peak splitting at sound pressures that coincide with the saturation of the outer hair cell receptor potentials. Current injection experiments show that peak splitting is also recorded in the resistance change of the inner hair cell during acoustic stimulation. It is concluded from this evidence that peak splitting is present in the mechanical input to the IHC.
Abstract: The measurement of the low-frequency responses of inner hair cells is complicated by the fact that extracellular cochlear microphonic may be larger than the intracellular inner hair cell potential. A subtraction technique is proposed in which the extracellular and intracellular potentials are recorded sequentially and then subtracted to give an estimate of the true membrane potential. A theoretical analysis is presented which predicts that this calculated inner hair cell membrane potential is a more accurate measure of inner hair cell receptor current than the membrane potential measured with respect to a remote indifferent electrode.
Abstract: Electrical stimulation of the mammalian cochlea causes a mechanical response which produces acoustic signals at the frequency of the electrical current. These electrically-evoked acoustic emissions can be as large as 34 dB SPL. Concurrent acoustic stimuli can enhance the emission response. Comparison of the enhancement effect with the cochlear microphonic (CM) suggests that the emissions originate from the outer hair cells (OHC). Frequency response measurements indicate a rate-limiting time constant for the force-generating process which is less than 35 microseconds.
Abstract: The purpose of this study was to explore possible mechanisms for the generation of the summating potential. Computer simulation was used to model the effects of potential hair cell nonlinearities on extracellular and intracellular d.c. potentials in the cochlea. No one nonlinearity can account for both extracellular and intracellular experimental data. However, a model which includes two nonlinearities (voltage-dependent cilia stiffness and nonlinear transducer channel resistance) produces extracellular and intracellular responses which match experimental data very well.
Abstract: Alternating current delivered into the scala media of the gerbil cochlea modulates the amplitude of a test tone measured near the eardrum. Variations in the electromechanical effect with acoustic stimulus parameters and observed physiological vulnerability suggest that cochlear hair cells are the biophysical origin of the process. Cochlear hair cells have traditionally been thought of as passive receptor cells, but they may play an active role in cochlear micromechanics.
Abstract: The injection of d.c. current into scale media alters both the cochlear microphonic (CM) and the acoustically synchronized changing resistance (CR) measured in scala media. Positive current increases the CM and decreases the CR. The effect on the CM is greatest at high sound pressure level (SPL), whereas the effect on CR is greatest at low SPL. Negative current has a similar but opposite effect on both the CM and the CR. The results suggest that a voltage-dependent nonlinear element exists in cochlear hair cells.
Abstract: Electrical stimulation of the crossed olivo-cochlear bundle (COCB) increases both the cochlear microphonic and the acoustically synchronized changing resistance (CR) and it causes a decrease in the electrical impedance of scala media of the guinea pig. The similarity between the change in CR due to COCB stimulation and the change in CR due to negative d.c. polarization (Mountain, D.C., Hubbard, A.E. and Geisler, C.D. (1980): Hearing Res. 3, 215-229) suggests that the CR is dependent on the hair cell membrane potential measured with respect to scale tympani.
Abstract: 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.
Abstract: Mechanical nonlinearity in the cochlea produces acuoustic distortion products that can be measured in the ear canal. These distortion products can be altered by changes in the endolymphatic potential as well as by stimulation of the crossed olivocochlear bundle, which provides efferent innervation to cochlear hair cells.
Abstract: 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.
Abstract: The harmonic structure of the cochlear microphonic (CM) and that of a sound-elicited signal which we have considered as an (apparent) changing resistance (CR) were simultaneously determined in scala media of the first turn of the guinea pig cochlea. We analyzed our data in the context of the Davis variable resistance hair-cell model (1965), which predicts CM and CR to be proportional to each other. But, plotted as functions of the sound-pressure level, CM and CR were found to have qualitatively similar but quantitatively disproportionate spectra. The preparations with the highest endolymphatic potential showed the least correspondence between the spectra of the two measured quantities. The phase angles of the fundamental components in CM and CR were equal within approximately 10 degrees, but the phase of the even harmonics of the two independent measures commonly differed by approximately 180 degrees at lower SPLs. Although most data were collected using 160-Hz tonal stimulation, tones with frequencies up to 1280 Hz produced qualitatively similar results. The CM and the CR both varied slightly with the level of the alternating current used to probe the CR. Considered on a quantitative basis, consistent with the accuracy of our measurements, any model which reduces to a fixed source, a fixed resistance, and a single linear, time-varying resistance cannot mimic the most significant, commonly found aspects of our CM and CR data. An alternate model incorporating a nonlinear, time-invariant resistance is able to account for some of the data. The output of the model is correctly considered a (time) changing resistance, or apparent changing resistance; but the model demonstrates that similar experimental results are not necessarily evidence for a time-varying resistor as originally proposed by Davis.
