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At least 1,369 records · Page 76Linked to original sources
PubMed1979

The effect of raising the scala tympani potassium concentration on the tone-induced cochlear responses of the guinea pig.

Scala tympani (ST) in guinea pig was perfused with modified Ringer's solutions containing 5--50 mM potassium; tone-induced cochlear responses from the basal turn of ST were compared before, during and after perfusions. The compound nerve action potential (N1) and afterpotential (a/p) amplitudes were reduced, especially above 20 mM; the summating potential (SP) was variable, but its onset shape changed consistently with 13--20 mM levels. However, the cochlear microphonic amplitude (CM) remained substantially unchanged even at the 35 mM level. K+ concentration was monitored in ST with ion-sensitive pipettes. Stable levels were reached within 2 min, but N1 responses continued to fall beyond this time. Recovery to normal K+ levels took place spontaneously and the concentration curve which resulted showed a 2-slope characteristic. These experiments question whether elevated potassium concentration in scala tympani depolarizes the hair cells, and if it does, whether the hear cell resting potential is involved in the generation of the CM.

Acoustic Stimulation↗
PubMed2000

Cochlear implantation of auditory neuropathy.

Auditory neuropathy (AN) is a hearing disorder that presents with a grossly abnormal or absent neural response as measured by evoked potentials in the presence of normal outer hair cell function evidenced by present otoacoustic emissions or cochlear microphonics. Rehabilitation for patients with AN is challenging due to abnormal temporal encoding at the auditory nerve leading to severely impaired speech perception. Although patients with AN may demonstrate improvement in thresholds with amplification, temporal encoding dysfunction, and consequently speech perception degradation, is not alleviated by amplification. Another issue is the heterogeneity of the AN population in terms of audiologic and neurologic findings, in addition to uncertain etiology and pathophysiology. For children with prelingual onset of AN, development of auditory and oral communication skills is particularly compromised. All children with hearing loss in the severe-to-profound range who do not benefit from conventional amplification can be considered candidates for a cochlear implant (CI). This paper presents a case study of a child with AN who received a CI. Whereas no synchronous neural response auditory brainstem response could be elicited to acoustic stimuli, an electrically evoked auditory nerve action potential was evident following implantation, suggesting restoration to some degree of neural synchrony. Significant improvement in speech perception was found post-CI. Recommendation to implant all patients with AN would be premature, but these findings suggest that electrical stimulation in some cases of auditory neuropathy can be a viable option.

Cochlea↗
PubMed2003

Modulation of cochlear afferent response by the lateral olivocochlear system: activation via electrical stimulation of the inferior colliculus.

The olivocochlear (OC) efferent innervation of the mammalian inner ear consists of two subdivisions, medial (MOC) and lateral (LOC), with different peripheral terminations on outer hair cells and cochlear afferent terminals, respectively. The cochlear effects of electrically activating MOC efferents are well known, i.e., response suppression effected by reducing outer hair cells' contribution to cochlear amplification. LOC peripheral effects are unknown, because their unmyelinated axons are difficult to electrically stimulate. Here, stimulating electrodes are placed in the inferior colliculus (IC) to indirectly activate the LOC system, while recording cochlear responses bilaterally from anesthetized guinea pigs. Shocks at some IC sites produced novel cochlear effects attributable to activation of the LOC system: long-lasting (5-20 min) enhancement or suppression of cochlear neural responses (compound action potentials and round window noise), without changes in cochlear responses dominated by outer hair cells (otoacoustic emissions and cochlear microphonics). These novel effects also differed from classic MOC effects in their lack of dependence on the level and frequency of the acoustic stimulus. These effects disappeared on sectioning the entire OC bundle, but not after selective lesioning of the MOC tracts or the cochlea's autonomic innervation. We conclude that the LOC pathway comprises two functional subdivisions, capable of inducing slow increases or decreases in response magnitudes in the auditory nerve. Such a system may be useful in maintaining accurate binaural comparisons necessary for sound localization in the face of slow changes in interaural sensitivity.

Acoustic Stimulation↗
PubMedDate not supplied

[Effect of microphone potential feedback on formation of adjusted curves of the hearing periphery].

The model of the formation of the exiting factor on the neuron endings of the cochlear nerve taking into consideration the feedback mechanisms described in recent experimental investigations is proposed. It is shown that the functioning of the feedback in the model give the possibility to explain the principle of action of the mechanism of enhancement of mechanical duct tuning which produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibers. At the same time the utilisation in the model the mechanism of the lateral inhibition provide it the property similar to the effect of inhibition of the neurone response to a given tone signal when another tone signal arises in a nearby frequency region.

Animals↗
PubMed1999

[Auditory neuropathy].

OBJECTIVE: To study the clinical feature and audiological and electrophysiological characteristics of auditory neuropathy. METHODS: The history, audiometry, stapedial reflex, electrocochleogram, auditory brainstem response, promontory stimulation tests, otoacoustic emissions (OAE) and its contralateral supression tests were studied in 17 cases of auditory neuropathy. RESULTS: The patients were predominently female teenagers with a history of gradual onset of bilateral hearing loss, pure tone auditometry revealed symmetric ascending audiograms with moderate hearing loss at frequencies below 1 kHz, but relatively normal hearing at 2 kHz and above. Speech reception thresholds were between 40 to 85 dB HL, discrimination score was from 29% to 68%. Stapedial reflex, auditory brainstem response and compound action potentials were absent or threshold elevated disproportionately to the pure tone threshold. All of the patients had normal cochlear microphonics and robust spontaneously and transiently evoked otoacoustic emissions (SOAE and TEOAE), the TEOAE showed no contralateral suppression effects as seen in normal subjects. CONCLUSION: The diagnosis of the disease depends on the discrepancies between subjective and objective hearing and pre-neural and neural responses. The crossed olivocochlear bundle function testing in terms of contralateral acoustic stimulation induced otoacoustic emission reduction is specific for differential diagnosis. There is a need for more awareness of the disease among otolaryngologists and neurologists.

Acoustic Impedance Tests↗
PubMedDate not supplied

Auditory and visual dysfunction following lead exposure.

The effects of lead exposure on cognitive function have been intensively studied during the past decade, but relatively little effort has been made to understand the impact on sensory function. Subtle impairments of visual and/or auditory processing, however, could have profound effects on learning. The objectives of this paper are to review what is known about the effects of lead exposure on visual and auditory function and to identify related research needs. In particular, the effects of lead exposure on sensory function in children, which have not been studied adequately, will be discussed. Evidence from human and animal studies reveal that lead exposure impairs auditory function. The cochlear nerve and more central structures appear to be preferentially sensitive in both developing and mature humans and experimental animals. Elevations in hearing thresholds and increased latencies of brainstem auditory evoked potential have been reported at low-moderate levels of lead exposure. Higher doses of lead increase the threshold of the auditory nerve action potential, produce segmental demyelination and axonal degeneration of the cochlear nerve, but appear to have no effect on cochlear microphonics or structure. Lead exposure affects both the retina and visual cortex of the developing and mature visual system. Low to moderate level developmental lead exposure produces selective rod deficits which can be detected with electrophysiological and behavioral techniques. At slightly higher levels of lead exposure the visual cortex is affected. A wide range of functional and neurochemical effects on retinal function occurring at blood lead levels below 20 micrograms/dl, the current level of concern, have been observed in rats. Structural, biophysical and photochemical similarities of rods in rats, monkeys and humans argue the relevance of this data for pediatric lead screening. To date, however, rod-mediated visual functions have not been examined in lead-exposed children. Undetected sensory deficits of these kinds may have profound impact on the motor and mental development of children as well as on the quality of life of affected adults. There is clearly a need for more extensive sensory testing in children and workers to screen for lead-induced health effects and in animal models to clarify the mechanisms of lead neurotoxicity.

Auditory Perceptual Disorders↗
PubMed2000

Hearing function in a hyperbaric environment.

Navy divers' hearing function was assessed as part of three saturation deep dives to 1,000 feet of sea water (fsw) to determine explanations for threshold shifts observed under hyperbaric conditions. Across the three deep dives, different aspects of the ear were evaluated, including air- and bone-conduction pure-tone thresholds, real ear probe microphone measurements, auditory evoked potentials, and central auditory processing assessments. Attempts to measure middle ear function and cochlear function (through otoacoustic emissions) were unsuccessful. Baseline measurements were obtained at 0 fsw in air before and after the saturation deep dives. Results showed that some aspects of hearing function remained unchanged with increases in depth. In general, audiometric thresholds at depth were similar to those measured on the surface at 500, 1,000, 2,000, 3,000, and 4,000 Hz. However, hearing sensitivity actually improved at depth at 6,000 and 8,000 Hz. The use of a specially designed sound booth for a pressurized heliox environment yielded significantly lower ambient noise levels and improved the accuracy of threshold measurement. Auditory evoked potential measurements and central auditory processing function were relatively unaffected by changes in depth. Significant changes at depth were seen in ear canal resonance which shifted up in frequency; this finding was attributed to the effect of helium on the hearing mechanism. Because objective measurement of middle ear and inner ear function were not methodologically possible, questions still remain regarding the interpretation of middle and inner ear function at depth. Nonetheless, our overall findings suggest that most aspects of hearing functioning are similar under high atmospheric pressures and in heliox as they are on the surface, with the exception of shifts in ear canal resonance and improvements in audiometric thresholds at high frequencies.

Acoustic Impedance Tests↗
PubMed1995

Effects of acoustic overstimulation on 2f1-f2 distortion product in the cochlear microphonics.

The cochlear microphonics (CM) in response to two-tone stimuli as well as the threshold of compound action potential (CAP) were measured before and after exposure to 4 kHz pure tone at 100 dB SPL for 10 min. Although the loss of CM output at the primary frequencies was limited to around 2 dB, the 2f1-f2 distortion products in the CM (CM-DPs) were markedly reduced immediately after the exposure, especially at low primary levels (i.e. less than 65 dB). The low level CM-DPs recovered gradually near the initial level within 7 days from the exposure. The elevation of CAP threshold closely paralleled with the reduction of CM-DPs in not only the acute phase but also in the recovery phase from the exposure. These results show that the active transduction process in the cochlea was affected by acoustic overstimulation. This impairment of the active transduction was postulated to play an important role in developing the noise induced temporary threshold shift.

Acoustic Stimulation↗
PubMed1987

A model of the generation of the cochlear microphonic with nonlinear hair cell transduction and nonlinear basilar membrane mechanics.

A phenomenological model of the generation of the cochlear microphonic of the guinea pig is presented which incorporates sharp tuning and nonlinear growth of the vibration of the cochlear partition, and nonlinear transduction of this response by a single linear array of hair cells (assumed to be the outer hair cells). The contribution to the microphonic potential from each hair cell generator is also assumed to decay exponentially with distance from the hair cell. Using experimentally obtained estimates for the parameters a good agreement between the theoretical microphonic tuning curves and growth functions was obtained without recourse to complex micromechanical motion of the cochlear partition, electrical tuning of the hair cells or a contribution from the inner hair cells of the cochlea.

Acoustic Stimulation↗
PubMed1996

[Lermoyez syndrome--electrocochleographic studies].

BACKGROUND: Lermoyez's syndrome is usually regarded as a rare variant of Menière's disease with hearing improvement and reduction of tinnitus during vertiginous attacks. Electrocochleographic studies of this syndrome are restricted to one published case up to now. METHODS: In our ENT department we clinically and electrocochleographically examined three patients suffering from Lermoyez's syndrome. These results were compared to those typically found in Menière patients. RESULTS: Two of the three patients did not show an elevated summating potential or increased ratio of summating potential and compound action potential (SP/CAP ratio). There was no hearing improvement following oral glycerol administration, and the SP/CAP ratio did not change during a Lermoyez attack. On the other hand, we observed normal cochlear microphonics in ears affected by Lermoyez's syndrome, indicating good hair cell function. In one ear, the threshold of cochlear microphonics was even better than the pure tone threshold. CONCLUSIONS: Our observations suggest that endolymphatic hydrops may not be always the underlying pathologic correlate of Lermoyez's syndrome. Thus Lermoyez's syndrome should not be simply regarded as a variant of Menière's disease.

Action Potentials↗
PubMed1990

Quantification of the relation between electrophysiologic and morphologic changes in experimental endolymphatic hydrops.

The degree of hydrops in guinea pig ears that had undergone unilateral endolymphatic sac and duct obliteration was quantified. For 15 guinea pigs from a prior study, the correlation between the degree of hydrops and several electrophysiologic measures of the functional state of the cochlea was determined. A significant correlation between the degree of hydrops and a reduction in the low-frequency cochlear microphonic evoked by a 29-Hz tone was found. No correlation was found between the degree of hydrops and the magnitude of the summating potential evoked by 8-kHz probes. A weak correlation was found between the degree of hydrops and a reduction in the compound action potential evoked by 2-kHz probes. Possible differences in the way the endolymphatic duct obliteration exerts its effect on the different cochlear potentials are discussed.

Animals↗
PubMed1989

Rapid changes in cochlear nucleus cell size following blockade of auditory nerve electrical activity in gerbils.

Large spherical cells of the mammalian anteroventral cochlear nucleus (AVCN) receive direct excitatory input from auditory nerve axons. Trans-synaptic regulation of neuronal cell size and cell number after cochlear ablation has been previously demonstrated in neonates of several vertebrate species, including the gerbil. Such changes may be related to loss of spontaneous or evoked auditory nerve electrical activity or to loss of activity-independent factors. We have developed a method to chronically, yet reversibly, block auditory nerve electrical activity without violating the integrity of the inner ear. Tetrodotoxin (TTX) was embedded in an ethylene-vinyl acetate copolymer resin (Elvax). A small piece of Elvax containing TTX was placed next to the round window membrane, which allowed TTX to diffuse into the inner ear. As a measure of the effectiveness of manipulation, the onset, duration, and magnitude of the auditory threshold shift were measured by the auditory brainstem response. The sound-evoked response was abolished within 10 minutes of placement of TTX on the round window membrane. The duration of threshold shift was dose-dependent and lasted 24-46 hours. Implants of Elvax without TTX did not produce a significant threshold shift. TTX, which blocks voltage-gated sodium channels, did not abolish the potassium-based cochlear microphonic response. The consequence of blocking afferent electrical activity on gerbil AVCN large spherical cells was examined by measuring their cross-sectional area after each of four manipulations: unilateral auditory nerve action potential blockade with TTX; unilateral surgical cochlear ablation; ipsilateral TTX exposure/contralateral cochlear ablation; and unilateral sham operation (Elvax without TTX).(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed2000

Experimental autoimmune inner ear disease: an electrocochleographic and histophysiologic study.

Systemic immunization with swine inner ear antigens in complete Freund's adjuvant induces functional disturbances in the cochlea. Morphometric data indicate that an endolymphatic hydrops develops within 2 weeks. It diminishes 6 weeks after immunization. A progressive decrease in the compound action potential amplitude is observed from 2 to 6 weeks after immunization. Enhancement of the amplitude of the summating potential is present without a clear overall correlation to the presence of endolymphatic hydrops. The amplitude of the cochlear microphonics shows no significant changes after immunization. Western blot analysis of the sera performed 2 and 6 weeks after immunization shows enhanced reactivity at 68, 50, 45, and 27 kd molecular weights, as compared to controls. The same spectrum of cross-reacting antibodies is believed to be instrumental in immune-mediated sensorineural hearing loss in patients. Apparently, cross-reacting antibodies and released mediators disturb cochlear homeostasis, resulting in the observed changes in the electrophysiological responses. However, these changes are not clearly related to structural changes at the light and electron microscopic levels.

Animals↗