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D C Mountain

Publications and source records attributed to D C Mountain.

At least 19 recordsLinked to original sources
PubMed1995

Acoustic enhancement of electrically evoked otoacoustic emissions reflects basilar membrane tuning: a model.

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.

Acoustic Stimulation↗
PubMed1995

Electrically evoked basilar membrane motion.

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.

Animals↗
PubMed1994

Effects of electrical biasing on electrically-evoked otoacoustic emissions.

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.

Acoustic Stimulation↗
PubMed1994

A piezoelectric model of outer hair cell function.

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.

Animals↗
PubMed1994

Mapping the cochlear partition's stiffness to its cellular architecture.

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.

Acoustic Stimulation↗
PubMed1994

Transfer characteristic of the inner hair cell synapse: steady-state analysis.

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.

Acoustic Stimulation↗
PubMed1994

Electrically evoked otoacoustic emissions from the apical turns of the gerbil cochlea.

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.

Acoustic Stimulation↗
PubMed1993

Acoustic enhancement of electrically-evoked otoacoustic emissions reflects basilar membrane tuning: experiment results.

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.

Acoustic Stimulation↗
PubMed1993

The envelope following response (EFR) in the Mongolian gerbil to sinusoidally amplitude-modulated signals in the presence of simultaneously gated pure tones.

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.

Acoustic Stimulation↗
PubMed1992

The envelope following response: scalp potentials elicited in the Mongolian gerbil using sinusoidally AM acoustic signals.

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.

Acoustic Stimulation↗
PubMed1991

In vivo measurement of basilar membrane stiffness.

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.

Animals↗
PubMed1990

Haircell forward and reverse transduction: differential suppression and enhancement.

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.

Animals↗
PubMed1989

Low-frequency responses of inner hair cells: evidence for a mechanical origin of peak splitting.

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.

Acoustic Stimulation↗
PubMed1989

Measurement of low-frequency receptor potentials in inner hair cells: a theoretical analysis.

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.

Acoustic Stimulation↗
PubMed1989

Rapid force production in the cochlea.

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.

Animals↗
PubMed1985

Model of d.c. potentials in the cochlea: effects of voltage-dependent cilia stiffness.

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.

Animals↗
PubMed1983

Alternating current delivered into the scala media alters sound pressure at the eardrum.

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.

Acoustic Stimulation↗