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At least 847 records · Page 47Linked to original sources
PubMed1987

Directional properties of the auditory periphery in the guinea pig.

The directional sensitivity of the outer ear of the guinea pig was determined by recording changes in the amplitude of the cochlear microphonic to frequencies between 1 and 20 kHz as the location of the sound source was changed throughout 360 degrees of horizontal auditory space. The directional responses to frequencies below 3 kHz were almost omnidirectional. The directional responses for frequencies between 3 and 12 kHz were progressively more directional toward the anterior midline. The responses for frequencies above 12 kHz were highly directional along the ipsilateral interaural axis. In contrast, the directional responses to all frequencies in animals whose pinnae had been removed were orientated along the ipsilateral interaural axis. The observations suggest that the orientation and strength of the directional response of the auditory periphery in the guinea pig are dependent on frequency and that this dependence is attributable, at least in part, to the acoustic properties of the pinna. The observations also indicate that there is a substantial change in the interaural intensity difference at various frequencies and in the spectral transfer function of the ear according to the location of the sound source in the ipsilateral hemifield. The observation that these changes are asymmetrical about the interaural axis for a substantial part of the auditory range of the animal is consistent with the hypothesis that the frequency dependent directionality of the auditory periphery provides a spectral cue for the localization of broad band sounds in the free field.

Animals↗
PubMed1987

Development of 2f1-f2 otoacoustic emissions in the rat.

2f1-f2 otoacoustic emissions have been recorded from the rat cochlea during its development. Acoustic responses were recorded at 3, 5 and 7 kHz using a fixed value of the f2/f1 ratio (= 1.17). The first 2f1-f2 acoustic responses were obtained at 12 days after birth for 2f1-f2 = 7 and 5 kHz, and 2 days later for 2f1-f2 = 3 kHz. Adult-like patterns of the acoustic responses were achieved by day 18 for 2f1-f2 = 3 kHz, by day 20 for 2f1-f2 = 5 kHz and by day 28 for 2f1-f2 = 7 kHz. These results are discussed in relation to the available anatomical and functional data on the cochlear development of the rat. The delayed appearance of the 3 kHz acoustic responses might be related to the basal-to-apical gradient of morphological cochlear maturation. The fact that the 2f1-f2 otoacoustic emissions reached adult characteristics from the low to high frequencies is consistent with the development of the tuning properties of the basilar membrane. The long development of the 2f1-f2 acoustic responses at 7 kHz suggests that the organ of Corti undergoes subtle changes well after the end of its apparent maturation.

Aging↗
PubMed1988

Investigations into the nature of the association between threshold microstructure and otoacoustic emissions.

Three studies are described which investigate the nature of the association between threshold microstructure and otoacoustic emissions. In the first study, threshold dips (similar in shape to those seen in threshold microstructure) are produced by introducing a low-level masker. Threshold microstructure is not abolished when tonal probes are replaced by narrowband-noise probes, while dips induced by external tonal maskers are eliminated. These findings rule out a simple interpretation of the microstructure dips as an instance of masking by otoacoustic emissions. In the second study, ear-canal measurements of the interactions of external tones with spontaneous emissions indicate that, although beating is often detected near threshold maxima, stimuli close to threshold minima are perceived as tonal because the emission is frequency locked by the external tone. The last study shows that reduction of the levels of otoacoustic emissions by aspirin consumption is associated with an initial reduction of thresholds in regions of threshold microstructure, with the greatest reduction occurring at threshold maxima. This suggests that threshold maxima may be due, at least in part, to interference or masking by the nearby otoacoustic emissions. A simple analog (driven Van der Pol oscillator) of an external tone interacting with a spontaneous emission is used to interpret ear-canal pressure waveforms and associated psychophysical percepts (including threshold detection), for tones close in frequency to emissions.

Aspirin↗
PubMed1988

Effect of elevated potassium concentration in the perilymph on the nonlinearity of cochlear microphonics in the guinea-pig cochlea.

Various characteristics of cochlear microphonic responses (CM) were measured in the guinea-pig after perfusing scala tympani with KCl solutions. In addition to a decrease of CM magnitude, the nonlinearity and the symmetry of waveform showed typical modifications suggesting, in addition to a depolarization of hair cells, some mechanical alterations.

Animals↗
PubMed1988

Acute effects of noradrenalin related vasoactive agents on the ototoxicity of aspirin: an experimental study in the guinea pig.

Aspirin is known to be ototoxic when administered at high doses. Its mode of action is unknown but an alteration of the vascular function has been suspected. To further document this hypothesis, acute effects of some vasoactive agents on the ototoxicity of aspirin were tested in experiments on the guinea pig using sensori-neural electrophysiological responses and morphometry of the vessels of the stria and the spiral lamina. Electrophysiological measures showed no modification of sensory responses but neural responses revealed clear changes after administration of noradrenalin related agents, limited modifications after a drug acting partly as a serotonin antagonist, and no change after a dopaminergic agent. Morphometric studies showed no modification of the strial but some effect on the spiral vessels. The results are compatible with the hypothesis of a vascular involvement in the ototoxicity of aspirin and they point toward an interaction with the noradrenergic sympathetic cochlear system in the spiral lamina.

Animals↗
PubMed1989

Evoked otoacoustic emissions as cochlear Bragg reflections.

The reflection of cochlear waves is mathematically described for various assumptions about the dependence of the echo-generating mechanism on place and frequency, without requiring knowledge of the physical details of the mechanism. Any 'wave-related' reflections, caused by intrinsic features of the resonance or active-feedback mechanism, cannot explain long group delays in an approximately shift-similar basilar membrane (BM). Only 'BM related' scattering due to inhomogenities is a possible explanation then. Reflection at a fixed point of the BM could cause only narrow-band evoked otoacoustic emissions (EOAEs). Long group delays in a wider frequency band can be obtained by assuming Bragg reflection at an approximately periodic inhomogenity of BM parameters, possibly due to spatial variations of active undamping. This explains the long delays, about inversely proportional to frequency, the decreasing instantaneous frequency, the often found multi-packet or modulated structure of echoes (vanishing at higher SPL), the spectra of simultaneous and delayed EOAEs and their relation to the threshold of hearing. These phenomena do not necessarily require multiple reflection due to middle/inner-ear mismatch.

Auditory Threshold↗
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↗
PubMed1990

Comparison of low- and high-side two-tone suppression in inner hair cell and organ of Corti responses.

Two-tone interactions were measured from inner hair cells and from the organ of Corti fluid space in the third turn of the guinea pig cochlea. At relatively low stimulus levels, a low-side suppressor caused frequency response functions to become broader. Phase changes exhibited a lag/lead transition around the characteristic frequency in harmony with the change in magnitude. These patterns are similar to those previously documented for a high-side suppressor (Cheatham and Dallos 1989) and suggest that suppression is not simply an attenuation phenomenon since level reductions for single-tone inputs produce response patterns which are mirror images of those obtained for the two-tone conditions. In contrast to the low-level results, data measured at moderately high stimulus levels indicate that the magnitude changes produced by both low- and high-side suppressors are qualitatively similar to changes generated by reducing the input sound level. In other words, ac frequency response functions become narrower, partially reversing the broadening of these functions which occurs as sound level increases. Companion phase measures, however, demonstrate that low- and high-side suppressors, in spite of producing similar changes in filter shape, do not produce similar changes in response phase. In fact, neither of the two-tone conditions produce response patterns similar to the one associated with reducing the input sound level.

Acoustic Stimulation↗
PubMed1991

Development of otoacoustic emissions in gerbil: evidence for micromechanical changes underlying development of the place code.

The development of the acoustic distortion product (ADP) 2f1-f2 was studied in gerbils, beginning 12 days after birth (P12). ADPs were measured as a function of stimulus frequency region (1.0 to 13.0 kHz) and level (10 to 80 dB SPL). There was an orderly progression in the appearance and maturation of the emissions, with responses to high-frequency stimuli (f2 = 13.0 kHz) appearing first, at P13-14. Responses to mid and high frequencies (f2 = 3.9 to 13.0 kHz) matured earlier than responses to lower frequencies. Responses to low-frequency stimuli (f2 = 1.3 KHz) did not appear until P18-19 and were not mature until after one month of age. The first emissions to develop in a given frequency region had elevated thresholds, were reduced in amplitude, and displayed monotonic input-output functions. As the auditory system matured, emission growth functions became non-monotonic displaying saturation, but initially retained a reduced dynamic range. Data from the developing gerbil suggest that initially its cochlear mechanics are passive and that active elements associated with normal outer hair cell function mature first in the basal turn and last near the apex. Furthermore, the development of active nonlinear elements underlying ADP generation is consistent with the development of frequency selectivity and developmental shifts in the place code which have been demonstrated in the gerbil.

Acoustic Stimulation↗
PubMed1991

Guinea pigs show post-natal stability in frequency mapping at the basal turn.

Others have shown that in animals born with immature cochleae the basal turn undergoes changes in function during the early days of postnatal life. We examined whether similar changes could be detected in the guinea pig, an animal born with a functionally mature cochlea. Our results indicate that no changes occur in the function of the basal turn of the guinea pig cochlea immediately after birth.

Acoustic Stimulation↗
PubMed1991

Cochlear microphonic enhancement in two tone interactions.

Two tone interaction functions of the cochlear microphonic (CM) were obtained from pigmented guinea pigs. First (basal) cochlear turn recording locations show optimally enhanced levels of CM when the interfering tone (F2) was positioned about 4 kHz above probe tones (F1) of 12 kHz and 20 kHz. Maximum enhancement occurred for equal level tones. No enhancement was seen for a probe tone of 4 kHz. When basal turn cochlear sensitivity was compromised, CM enhancement caused by the interfering tone was altered and only CM reduction was then seen. The CM reduction was the typical characteristic described by many earlier studies. Guinea pigs with various changes in cochlear sensitivity were studied, providing evidence in support of earlier reports that CM interference (both reductions and enhancements) depends on far field vector summation of the outputs of hair cells from a restricted area of the basilar membrane. No CM enhancement was seen in micropipette recordings from within the organ of Corti.

Acoustic Stimulation↗
PubMed1991

Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons.

In the anesthetized cat, some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIs) in these responses. The ISIs were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length, were very regular. Except at threshold levels of stimulation, mean ISIs were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIs suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.

Acoustic Stimulation↗
PubMed1992

Hypothetical roles of middle ear muscles in the guinea-pig.

The attenuation of incoming sounds induced by acoustic reflex triggering was evaluated from the cochlear microphonic response to test tones in 45 awake guinea-pigs. Although control electromyographic measurements proved that the stapedius muscle was contracting, neither impedance changes nor attenuation induced by contralateral reflex-eliciting sounds were detectable in 30 cases out of 45. For ipsi- and bilateral stimulations, an attenuation was detectable for 7 guinea-pigs out of 10. In the guinea-pigs for which a reflex-induced change was found on CM, the mean attenuation was weak i.e. of the order of 2 dB at 20 dB above reflex threshold. These results were quite different from those obtained during control experiments in the rabbit for which CM attenuation was much larger. However, large attenuations associated with middle ear muscle contractions were found in the guinea-pig in other circumstances, i.e. during self-vocalization or when spontaneous muscle contractions occurred during anaesthesia. It is concluded that middle ear muscles can have several different functions, and that even when it exists, attenuation of loud sounds might not be their primary role.

Acoustic Impedance Tests↗
PubMed1992

Intracochlear salicylate reduces low-intensity acoustic and cochlear microphonic distortion products.

Salicylate is well-known to produce reversible hearing loss and tinnitus. The site and mechanism of salicylate's ototoxic actions, however, remain unresolved. Recent experiments demonstrating primarily low-intensity effects on cochlear afferent outflow and effects on otoacoustic emissions (OAEs) suggest that salicylate acts to compromise active, energy-enhancing processes within the cochlea (i.e., the active process). We tested this hypothesis by examining the effect of salicylate on distortion product emissions. Distortion product responses to two-tone stimulation were monitored in the guinea pig before, during, and after intracochlear administration of increasing concentrations of salicylate (0.6-5 mM). These responses were recorded as acoustic signals in the ear canal spectrum (ADP), and as present in the cochlear microphonic (CM) recorded from a wire in basal turn scala vestibuli (CMDP). We also recorded the CM response to a single tone. Cochlear perfusion of salicylate resulted in a dose-responsive reduction in ADPs that was greater for low intensities of stimulation. CMDPs also demonstrated a concentration-dependent reduction at low intensities, but were increased slightly, though not significantly, by salicylate when elicited by high intensity primaries. CM was essentially unchanged by intracochlear salicylate. These results are consistent with an action of salicylate that involves the outer hair cells (OHCs) and are in harmony with the hypothesis that salicylate may selectively compromise the active process.

Animals↗
PubMed1995

Laser-feedback measurements of turtle basilar membrane motion using direct reflection.

In mammalian hearing, the frequency-dependent spatial pattern of movement in the basilar membrane/organ of Corti complex forms the basis of frequency discrimination. This is not necessarily the case in lower vertebrates; the turtle, for example, has an electrical resonance mechanism in its auditory receptor cells that varies in best frequency from cell to cell. But how much, if any, of the frequency separation by the turtle is done mechanically by the basilar membrane complex? Attempts to find an investigative approach that avoided placing objects on the basilar membrane led to the rediscovery of laser-feedback interferometry. Laser-feedback interferometric investigations of the vibrational amplitude and phase of the turtle basilar membrane in response to imposed nanometer displacements of the eardrum reveal that the membrane reflects the broadly-tuned middle-ear filter characteristics. Phase-angle measurements of the basilar membrane as a function of frequency, and the best frequency of the obtained amplitude tuning curves, did not vary as a function of position within each specimen. Input-output functions of the basilar membrane were generally linear. The middle ear demonstrates a negative gain of 2-6 while the central region of the basilar membrane has a positive gain of 4-18 dependent on location and biological variability.

Acoustic Stimulation↗
PubMed1996

The effects of continuous versus interrupted noise exposures on distortion product otoacoustic emissions in guinea pigs.

Distortion product otoacoustic emissions (DPOAE) were measured serially in guinea pigs before and following 4-h exposures to a half-octave band of noise centered at 6 kHz. Stimulus parameters used to elicit the DPOAE were f2/f1 = 1.26 and L2 = L1-10. The 80 dB SPL exposures resulted in attenuation of emissions, which was maximal at the frequency one-half octave above the exposure when referenced to the f2 stimulus, and which recovered back to baseline after 2 days. The 90 dB SPL exposures resulted in a permanent deficit in emissions elicited by high-frequency stimuli, as measured after 8 days of recovery. A statistically significant difference was also found between animals exposed continuously for 4 h versus animals given two 2-h exposures separated by a 1-h break. Measures of f2-f1 and 3f1-2f2 indicated that they were more sensitive than 2f1-f2 to alterations in cochlear function after noise exposure.

Acoustic Stimulation↗