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PubMed1980

The effect of varying stimulus polarity (rarefaction Vs. condensation) on early auditory evoked potentials (EAEPs).

A patient with a vascular brain stem lesion was investigated with EAEP. The results suggest that rare-faction and condensation stimuli produce different responses. The alternating mode is a compromise and helps to suppress stimulus-related artifacts and cochlear microphonics. Wave form alterations and latency shifts must be recognized and taken into account in neurological applications of this test.

Adult↗
PubMed1979

Effect of kanamycin sulfate on the endocochlear dc potential of guinea pigs.

The magnitudes of the endocochlear dc potential (EP) during adequate ventilation and during five minutes of anoxia were recorded in control guinea pigs and guinea pigs administered various dosages of kanamycin sulfate. The magnitudes of the EP after five minutes of anoxia were -44.6 +/- 5.9 mV in the controls and +26.6 +/- 9.9 mV in the guinea pigs that received kanamycin for seven or ten days. This change occurred after five or six days of administration of kanamycin. However, there was no significant change in the magnitude ot the EP with adequate ventilation with kanamycin intoxication. There is a significant correlation between the magnitude of the negative EP and the maximum output of the cochlear microphonics (r = -.770, P less than .001). These results suggest that the EP may not be the mathematical summation of the positive electrogenic potential and the negative diffusion potential. The mechanism for generating the negative EP during anoxia may have some relationship to hair cell integrity.

Animals↗
PubMed1981

The second filter is real, but how does it work?

The following speculative suggestions are offered informally concerning the "second filter" in the cochlea. Increased sensitivity is an advantage as important as increased sharpness of tuning. These two features and the "cochlea echo" strongly suggest a resonant system with high Q, which would also explain the prolonged latency near threshold of the action potential of the auditory nerve. The viscous damping of a mechanical resonator in the organ of Corti might be overcome by positive feedback. The seond filter is mechanically fragile and is vulnerable to anoxia. The source of the energy for positive feedback might be the cochlear microphonic of the outer hair cells. A possible transducer is hyaluronic acid located between the cilia and known to have piezoelectric properties.

Action Potentials↗
PubMed1990

Nephrotoxic and ototoxic agents.

It is well established that many drugs, such as the aminoglycoside antibiotics and the chemotherapeutic drug cisplatin, are capable of inducing both nephrotoxicity and ototoxicity. The factors that selectively predispose the kidney and inner ear to the toxic effects of these agents as well as the mechanism by which damage is produced are not well defined. The two organs differ greatly in their exposure to these toxic agents. The kidney has an abundant vascular supply and tends to selectively concentrate a number of drugs within the renal cortex or medulla, often to toxic levels. The vascular supply of the inner ear is not as extensive. In addition, the stria vascularis of the cochlea may act as a functional regulator of drug entry into inner ear fluids. The absorption of drugs into perilymph and endolymph is poorly understood. Selective accumulation theories of drug accumulation in the inner ear must be questioned because of the results of recent pharmacokinetic studies, which give contrary data. Drug-induced ototoxicity and nephrotoxicity can be explained on a cellular level. Studies using radiolabeled gentamicin suggest that binding mechanisms of the drug to the plasma membrane of the outer hair cells of the cochlea and vestibular apparatus and to the brush border receptors of the renal proximal convoluted tubules are similar. This suggests the same receptor sites for aminoglycosides occur in otic and renal organs. Calcium channels are implicated because of the reversibility of aminoglycoside-induced changes in the cochlear microphonic by calcium and other divalent cations. Calcium channel blockers, such as verapamil, reduce the nephrotoxicity of a number of drugs that are also ototoxic. Studies are needed to assess potential prevention of ototoxicity by use of these same calcium channel blocking agents. Aminoglycosides concentrate within the lysosomes of renal proximal tubular cells. Possibly, they also may concentrate in lysosomes within the cells of cochlear and vestibular structures. Nephrotoxic heavy metals concentrate within proximal tubular cells and, some, such as lead or bismuth, specifically concentrate within intracytoplasmic or intranuclear inclusion bodies. Studies are necessary to determine if the same metals accumulate within the cochlear and vestibular cells, inclusion bodies, or both. These questions and others must be answered before it can be determined why many nephrotoxic drugs and agents are also ototoxic.

Adolescent↗
PubMed1986

The frequency-following response to continuous tones in humans.

Previous studies of the frequency-following response (FFR) in man suggest that it has multiple sources. Identification of these sources has been complicated by the use of tone bursts to evoke FFRs and the lack of precise methods to calculate their amplitude and latency. Tone bursts produce transient responses which confound measurements of the FFR. The use of continuous tones avoids this problem and the Fast Fourier Transform can be used to assess accurately and efficiently the presence, amplitude and phase angle of the FFR. In this study we systematically examined the frequency and intensity range over which FFRs to continuous tones could be evoked using FFRs to tone bursts for comparison. We then analyzed FFRs to continuous tones to determine the sources of this potential. FFRs to both stimuli have similar thresholds (65-90 dB SPL) and can be evoked by the same range of frequencies. Neurogenic FFRs in man occur only below 1000 Hz. The source for this potential has a latency of 8.2 +/- 0.1 ms (mean +/- SD) and is consistent with a midbrain source. At higher frequencies FFRs have a latency of less than 1 ms and are most likely cochlear microphonic. The small variation in the latency of the neurogenic FFR suggests this as a possible tool for assessing neurological disorders.

Adolescent↗
PubMed2006

Speech processing in vocoder-centric cochlear implants.

The principles of the most recent cochlear implant processors are similar to that of the channel vocoder, originally used for transmitting speech over telephone lines with much less bandwidth than that required for transmitting the unprocessed speech signal. An overview of the various vocoder-centric processing strategies proposed for cochlear implants since the late 1990s is provided including the strategies used in different commercially available implant processors. Special emphasis is placed on reviewing the strategies designed to enhance pitch information for potentially better music perception. The various noise suppression strategies proposed over the years based on multi-microphone and single-microphone inputs are also described.

Acoustic Stimulation↗
PubMed1976

Cochlear microphonics and SDH activity in the hair cells under anoxia.

Cochlear microphonics and succinic dehydrogenase (SDH) activity of the hair cells during anoxia were studied in guinea pigs. After injection of the glucose-glucose oxidase mixture into the cerebellomedullary cistern, the deprivation of the oxygen tension in CSF and perilymph was measured in different time courses. In CSF, oxygen tension was reduced to zero within 60 min while in perilymph the maximal reduction was about a half the original level. The cochlear microphonics responded well to the oxygen tension deprivation in the perilymph. SDH activity, particularly in the outer hair cells was quite parallel to the oxygen tension in perilymph and also to the behavior of the cochlear microphonics. The present results indicate a close relationship between the electrophysiological and histochemical findings and that cochlear microphonics represent the function of the outer hair cells only.

Action Potentials↗
PubMed1970

Auditory frequency-following response: neural or artifact?

An electrical response which reproduces the waveform and frequency of the sound stimulus can be recorded from the central neural pathway for audition. Controversy has existed for some years over whether this frequency-following response (FFR) is neural or an artifact such as remote pickup of the cochlear microphonic or cross talk in the recording system. Two experiments resolve this issue by demonstrating that the frequency-following response depends upon functionally intact neural pathways. The frequency-following response, as well as auditory evoked potentials, is abolished by section of the eighth nerve; it is reversibly abolished by cooling of the cochlear nucleus.

Cochlear Nerve↗
PubMedDate not supplied

The prognostic value of electrocochleography in severely hearing-impaired infants.

This paper presents a longitudinal evaluation of electrocochleographic assessment in severely hearing-impaired infants. Electrophysiological data were obtained by transtympanic electrocochleography to tone-burst stimuli at octave frequencies of 500 to 8000 Hz at the age of 0-6 years in a group of 126 subjects. The results are compared with auditory thresholds determined at school age in the same children by means of pure-tone audiometry. Cochlear microphonics could be recorded in virtually all ears, although the majority of subjects had hearing losses of 90 dB and more. Compound action potentials (CAPs) showed waveforms varying from normal to a wide range of abnormalities. Audiometric thresholds correlated generally well with the compound action potential (CAP) thresholds obtained in infancy. The error in the predicted audiometric thresholds is between 15 and 20 dB, as compared with 11 dB reported for more moderate hearing losses. It is shown that, in spite of the high stimulus levels used, substantial frequency-specific threshold information is retained. Occasional large discrepancies in thresholds were often associated with markedly abnormal response waveforms. Among the many cases in which no ABR could be elicited, 68 per cent produced detectable electrocochleographic responses in the 1000-4000 Hz range. It is concluded that electrocochleography is a valuable method for the assessment of residual hearing in infants suspected of having a severe hearing impairment.

Audiometry, Evoked Response↗
PubMed1990

Fine structure of the intracochlear potential field. I. The silent current.

Field potentials were recorded along radial tracks in scala tympani and scala vestibuli of the guinea-pig cochlea. A current density analysis revealed standing current density profiles that were qualitatively similar between animals and between the second and third cochlear turns. Radial standing current densities were greatest at or near the spiral ligament. All the scala vestibuli current density profiles were scaled versions of one another while the scala tympani current density profiles showed more variability. Acoustic stimuli modulated the standing current and there was a cochlear microphonic current density peak in scala tympani near the organ of Corti. The results are summarized with a current-density field line model, the key element of which is a constant current pumped into scale media by the stria vascularis. The standing potential gradients drive current from each perilymphatic chamber into the spiral ligament en route to the lateral surface of the stria vascularis. The strial current is divided between the receptor cell pathway and leakage pathways. The standing current through the leakage pathways is indirectly modulated by acoustic stimulation through the modulation of the endocochlear potential. The reciprocal modulation of current between hair cell and leakage pathways suggests that the stria vascularis maintains a constant current during acoustic stimulation. The cochlear standing current is similar to the retinal dark current in its importance for sensory transduction but the fact that the silent current is generated by the stria vascularis and not the receptor cells provides significant benefits for the detection of mechanical stimuli.

Acoustic Stimulation↗
PubMed1990

Cochlear potentials and auditory evoked potentials in the caiman (Caiman crocodilus (L.)).

Brain-stem auditory evoked potentials (BAEPs) and round window compound action potentials (CAPs) in response to rarefaction and condensation clicks were recorded from anaesthetized and artificially respired caiman. The recorded wave forms were substantially different from the brain-stem and round window potentials recorded in mammals, including man. In particular, wave latencies were much longer than in mammals. Wave amplitudes increased and latencies decreased significantly and reversibly with increases in stimulus intensity and body temperature. The latencies of the first positive wave (P1) in the BAEP and the first negative wave (N1) in the CAP are correlated and co-vary with stimulus level and body temperature. BAEP P1 thus represents the response of the auditory nerve. The cochlear microphonic (CM) latency in caiman is unaffected by stimulus intensity and by cooling of the animal.

Action Potentials↗
PubMed2004

Comparison of acute low-tone sensorineural hearing loss versus Meniere's disease by electrocochleography.

To clarify the pathogenesis of acute low-tone sensorineural hearing loss (ALHL), we retrospectively compared the electrocochleographic findings from 20 patients with ALHL with those from 58 patients with Meniere's disease (MD) classified into 4 groups (MD1 through MD4) according to their pure tone average. The mean summating potential-action potential ratio in the ALHL group was 0.35 +/- 0.13, which was significantly higher than the control ratio but similar to the ratio seen in the MD1 group (pure tone average < 25 dB hearing level). The mean detection threshold of the cochlear microphonics in the ALHL group was 32.0 +/- 9.4 dB normal hearing level, which was again similar to that seen in the MDI group. Moreover, more than 50% of patients with ALHL had normal cochlear microphonics input-output curves. We therefore conclude that the pathogenesis of ALHL arises from an endolymphatic hydrops with little or no impairment of hair cells that resembles early-stage MD.

Action Potentials↗
PubMed1970

Cochlear nerve fiber discharge patterns: relationship to the cochlear microphonic.

Fourier analysis of discharge patterns in response to sinusoidal acoustic stimulation provides a consistent and repeatable measure of response phase and amplitude. The variation of the fundamental and harmonic components of the patterns as stimulus parameters are changed is strikingly similar to that of cochlear microphonics. The results are significantly different for single fibers with different characteristic frequencies; the variations parallel those of microphonics recorded from different cochlear turns.

Acoustics↗
PubMed1990

Influence of acoustic deprivation on recovery of hair cells after acoustic trauma.

The purpose of this study was to investigate how the recovery of the cochlea, after acoustic trauma, might be influenced by acoustic stimulation or deprivation. In anaesthetized adult chinchillas, both ears were simultaneously exposed to a traumatizing acoustic stimulus (2 kHz tone, at 117 dB SPL for 15 min). Probe microphones positioned in both bullae were used to ensure identical exposure to the two ears; this was important because the experiment relies on within-animal controls. Cochlear action potential thresholds across frequency (CAP audiograms) were used to verify the similarity of threshold shifts to the two ears. Immediately following, a unilateral ossiculectomy was performed which resulted in one cochlea being acoustically deprived during the recovery period, whilst the other was not. In groups of animals with recovery periods of 1, 3, 6, and 12 weeks, both the acoustically deprived and the normally stimulated cochleas were examined with scanning electron microscopy. To quantify hair cell damage, we used a damage scale based on stereociliar integrity; for each cochlea, a standard region 5.5-8.5 mm from the apex was studied in detail. We found that after acoustic trauma, hair cell damage to the cochlea which is deprived of sound during the recovery period, is significantly greater compared with that in the normally stimulated, contra-lateral cochlea. Our results suggest that mechanical activation of the inner ear acts to inhibit long-term degenerative processes, or influence repair of partially damaged hair cells.

Action Potentials↗