Search PubMed⌕ Search

Biomedical subjects

D C Mountain

Publications and source records attributed to D C Mountain.

At least 19 recordsLinked to original sources
PubMed2001

Longitudinal coupling in the basilar membrane.

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.

Acoustic Stimulation↗
PubMed2001

Sensing scenes with silicon.

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.

Animals↗
PubMed2000

Effects of acoustic trauma on acoustic enhancement of electrically evoked otoacoustic emissions.

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.

Animals↗
PubMed1999

Multiple modes of inner hair cell stimulation.

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.

Acoustic Stimulation↗
PubMed1998

Nonlinear characteristics of electrically evoked otoacoustic emissions.

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.

Acoustic Stimulation↗
PubMed1998

Measurements of the stiffness map challenge a basic tenet of cochlear theories.

The cochlear frequency map is believed to depend on the progressive decrease in partition stiffness from base to apex. Measurements on cochleae from human cadavers by von Békésy (1960) suggested that the elasticity of the partition increases by a factor of 100 from the stapes to the helicotrema. However, conventional models require a factor of nearly 10,000 to support the frequency range of normal hearing if entirely determined by partition stiffness. To test this assumption, we measured point stiffness along the width and length of the partition in the gerbil cochlea. Two major findings result from this study: (1) contrary to von Békésy's results, both cellular and extracellular elements of the sensory epithelium exhibit stiffness gradients; and (2) the stiffness changes by only a factor of 100 over the whole cochlea. Our results imply that present ideas regarding partition vibration need to be significantly revised.

Anatomy, Artistic↗
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↗