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PubMed1982

Detection threshold microstructure and its effect on temporal integration data.

Auditory detection thresholds were measured at several preselected frequencies using an adaptive 2IFC procedure. Results confirm the existence of shifts in detection threshold ranging from 2 to 14 dB with quite small changes in signal frequency. There does not appear to be a uniform pattern associated with the microstructure of the detection threshold curve. An additional experiment was performed to determine the effect of signal duration on detection threshold microstructure. Results indicate that the temporal integration function is considerably steeper for more sensitive frequencies (3.7 dB/doubling of duration), than for less sensitive frequencies (1.7 dB/doubling). This probably is not due to differences in processing as much as it is to the effect of the energy spread associated with decreasing signal duration.

Acoustic Stimulation↗
PubMed1981

Psychophysical profiles differentiate drugs of abuse.

Behavioral reaction times and both auditory and visual absolute thresholds were determined in baboons before and after the administration of stimulant and depressant drugs of abuse. Pentobarbital and diazepam increased reaction times and elevated the visual threshold in a dose-dependent manner. Diazepam also raised the auditory threshold though pentobarbital did not. Chlordiazepoxide affected neither sensory thresholds nor reaction times over the same dose range; d-methylamphetamine elevated the visual threshold, but did not elevate the auditory threshold or increase reaction times.

Amobarbital↗
PubMed1984

Correlates of tone-on-tone masked thresholds in the chinchilla auditory nerve.

In an attempt to determine neural correlates of tone-on-tone masking, discharge patterns of chinchilla auditory-nerve fibers were obtained in response to a set of two-tone stimuli for which behavioral masking had been previously measured (Long, G.L. and Miller, J.D. (1981): Hearing Res. 4, 279-285). The lowest masked thresholds in a sample of fibers were quantitatively similar to the chinchilla's behavioral masked thresholds. In addition, the neural data were in qualitative agreement with other previously-described characteristics of tone-on-tone masking, such as the contribution of cochlear distortion products and the upward spread of masking. It thus appears that the limitations imposed by peripheral frequency analysis determine the tone-on-tone masking pattern.

Animals↗
PubMed1995

Comparison of tympanic membrane-recorded electrocochleography and the auditory brainstem response in threshold determination.

The purpose of this study was to evaluate the usefulness of electrocochleography (ECoG) recorded with a tympanic membrane electrode as an adjunctive measure to auditory brainstem response (ABR) in frequency-specific threshold estimation. In a group of 10 normally hearing and 10 sensorineural hearing-impaired subjects, ABR and ECoG were simultaneously recorded in response to tone-burst stimuli centered at 500, 1000, 2000, and 4000 Hz. At each frequency, stimulus intensity was reduced in 12-dB decrements from an initial level of 110 dB SPL until no replicable response could be discerned. Electrocochleography and ABR thresholds were determined at each frequency, and correlation to behavioral audiometric threshold was determined. Input/output functions were also computed. At 2000 and 4000 Hz, both ABR and ECoG thresholds correlated with behavioral audiometric threshold. At 1000 Hz, ABR threshold correlated with behavioral audiometric threshold; ECoG did not. At 500 Hz, neither ABR nor ECoG threshold correlated with behavioral audiometric threshold. Input/output functions were steeper for ECoG than for ABR at all frequencies tested in the normally hearing group.

Auditory Threshold↗
PubMed2002

Gap detection threshold in the rat before and after auditory cortex ablation.

Gap detection threshold (GDT) was measured in adult female pigmented rats (strain Long-Evans) by an operant conditioning technique with food reinforcement, before and after bilateral ablation of the auditory cortex. GDT was dependent on the frequency spectrum and intensity of the continuously present noise in which the gaps were embedded. The mean values of GDT for gaps embedded in white noise or low-frequency noise (upper cutoff frequency 3 kHz) at 70 dB sound pressure level (SPL) were 1.57+/-0.07 ms and 2.9+/-0.34 ms, respectively. Decreasing noise intensity from 80 dB SPL to 20 dB SPL produced a significant increase in GDT. The increase in GDT was relatively small in the range of 80-50 dB SPL for white noise and in the range of 80-60 dB for low-frequency noise. The minimal intensity level of the noise that enabled GDT measurement was 20 dB SPL for white noise and 30 dB SPL for low-frequency noise. Mean GDT values at these intensities were 10.6+/-3.9 ms and 31.3+/-4.2 ms, respectively. Bilateral ablation of the primary auditory cortex (complete destruction of the Te1 and partial destruction of the Te2 and Te3 areas) resulted in an increase in GDT values. The fifth day after surgery, the rats were able to detect gaps in the noise. The values of GDT observed at this time were 4.2+/-1.1 ms for white noise and 7.4+/-3.1 ms for low-frequency noise at 70 dB SPL. During the first month after cortical ablation, recovery of GDT was observed. However, 1 month after cortical ablation GDT still remained slightly higher than in controls (1.8+/-0.18 for white noise, 3.22+/-0.15 for low-frequency noise, P<0.05). A decrease in GDT values during the subsequent months was not observed.

Acoustic Stimulation↗
PubMed2002

Potentiation of noise-induced hearing loss by low concentrations of hydrogen cyanide in rats.

Noise-induced hearing loss is the most prevalent occupational injury in the United States despite the adoption of clear permissible exposure limits and protocols for hearing conservation. This study identifies low-level chemical asphyxiant exposure as a risk factor capable of potentiating noise-induced hearing loss. Rats were exposed to 10, 30, and 50 ppm hydrogen cyanide (HCN) alone for 3.5 h (n = 28) or in combination with 2 h octave band noise exposure (100 dB(lin); n = 28). Additional groups received noise exposure alone (n = 16) and no treatment other than placement in an inhalation chamber with clean air and quiet (n = 16). Pure tone compound action potential (CAP) thresholds were determined 4 weeks following the exposure in order to assess pure tone auditory sensitivity and permanent threshold impairment. Cochleae from an additional 13 subjects were processed for light microscopy to permit assessment of hair cell loss. The results demonstrate that the noise exposure alone impaired CAP threshold by about 10 dB, averaged between 12-40 kHz, and produced a 5% loss of outer hair cells at the base of the cochlea, but no inner hair cell loss. The combined exposure to noise and HCN caused a cyanide dose-dependent CAP threshold impairment that exceeds the noise exposure alone. This effect reached statistical significance at a HCN level of 30 ppm. Combined exposure also produced more outer hair cell loss than noise alone. HCN alone did not cause significant hearing loss or hair cell loss. A risk assessment analysis was conducted for the auditory threshold data using benchmark dose software published by the U. S. EPA (BMDS version 1.3). A continuous model showed that the data could be described by a linear function. For a benchmark response corresponding to a 5 dB increase in auditory threshold above the effect of noise alone, the lower bound on the 95% confidence interval for the benchmark dose was 9 ppm. The benchmark dose that impaired auditory threshold 10% above the effect of noise alone had a lower bound of 2 ppm. The lower bound to the HCN dose that produced a 1 SD elevation in noise-induced hearing loss was 16 ppm. These exposure levels provide a range of concentrations below to slightly above the short-term exposure limit for HCN. However, if these levels are adjusted for an 8 h time-weighted average (TWA), the resulting levels are below the permissible exposure level (PEL) for HCN.

Animals↗
PubMed1991

Frequency-specific auditory brainstem responses relationship to behavioural thresholds in cochlear-impaired adults.

Auditory brainstem response (ABR) thresholds to ipsilaterally masked tone pip stimuli were obtained from three groups of hearing-impaired subjects. Using high-pass (for 500-Hz tone pips) and notched noise (for 1-, 2- and 4-kHz tone pips), ABR thresholds in subjects with low-frequency, high-frequency or flat cochlear hearing losses were compared to conventional pure-tone audiometric thresholds. A strong positive relationship was found between ABR and behavioural threshold elevation. Absolute ABR thresholds at 500 Hz were significantly higher than those at other frequencies. The results of this study indicate that frequency-specific ABR testing can provide an approximation of both degree and configuration of cochlear hearing losses in adults. Further refinements of testing and judging procedures are needed however to reduce the variation evident in our results and thus achieve the accuracy required for most clinical applications.

Adult↗
PubMed1998

Threshold-duration functions of chinchilla auditory nerve fibers.

The purpose of the present study was to measure the change in threshold as a function of stimulus duration in single auditory nerve fibers. Thresholds were measured at each neuron's characteristic frequency (CF) for eight stimulus durations ranging from 8 to 1024 ms. Using an adaptive, two-interval, forced-choice threshold-tracking procedure with a 2-down, 1-up rule, thresholds were estimated based on a decision criterion of one spike or greater difference between tone and no-tone intervals. The results showed that mean thresholds decreased with increasing stimulus duration by approximately 14.6 dB over the range of durations tested. Analysis of group and individual data showed that thresholds decreased by approximately 6-7 dB per decade of duration. The slope of threshold improvement decreased systematically with increasing CF, consistent with previous physiological and psychophysical data.

Acoustic Stimulation↗
PubMed1987

Auditory-nerve single-neuron thresholds to electrical stimulation from scala tympani electrodes.

Single auditory-nerve neuron thresholds were studied in sensory-deafened squirrel monkeys to determine the effects of electrical stimulus shape and frequency on single-neuron thresholds. Frequency was separated into its components, pulse width and pulse rate, which were analyzed separately. Square and sinusoidal pulse shapes were compared. There were no or questionably significant threshold differences in charge per phase between sinusoidal and square pulses of the same pulse width. There was a small (less than 0.5 dB) but significant threshold advantage for 200 microseconds/phase pulses delivered at low pulse rates (156 pps) compared to higher pulse rates (625 pps and 2500 pps). Pulse width was demonstrated to be the prime determinant of single-neuron threshold, resulting in strength-duration curves similar to other mammalian myelinated neurons, but with longer chronaxies. The most efficient electrical stimulus pulse width to use for cochlear implant stimulation was determined to be 100 microseconds/phase. This pulse width delivers the lowest charge/phase at threshold. The single-neuron strength-duration curves were compared to strength-duration curves of a computer model based on the specific anatomy of auditory-nerve neurons. The membrane capacitance and resulting chronaxie of the model can be varied by altering the length of the unmyelinated termination of the neuron, representing the unmyelinated portion of the neuron between the habenula perforata and the hair cell. This unmyelinated segment of the auditory-nerve neuron may be subject to aminoglycoside damage. Simulating a 10 micron unmyelinated termination for this model neuron produces a strength-duration curve that closely fits the single-neuron data obtained from aminoglycoside deafened animals. Both the model and the single-neuron strength-duration curves differ significantly from behavioral threshold data obtained from monkeys and humans with cochlear implants. This discrepancy can best be explained by the involvement of higher level neurologic processes in the behavioral responses. These findings suggest that the basic principles of neural membrane function must be considered in developing or analyzing electrical stimulation strategies for cochlear prostheses if the appropriate stimulation of frequency specific populations of auditory-nerve neurons is the objective.

Animals↗
PubMed2006

Additive effects of antiepileptic drugs and pentylenetetrazole on hearing.

The long-term effect of three of the most widely used antiepileptic drugs at relevant doses on the hearing decline that accompanies pentylenetetrazole (PTZ)-induced experimental epilepsy was investigated here, and compared with the effect of vinpocetine (VPC), which is a drug with antiepileptic potential. For this purpose, cortical activity (monitored by the EEG) and auditory sensitivity, as indicated by brainstem auditory evoked potential (BAEP) threshold at 4 and 8 kHz tone frequencies, were determined in guinea pigs daily injected for 28 days with vehicle (control), 20 mg/kg carbamazepine (CBZ), 6 mg/kg phenytoin (PHT), 30 mg/kg valproate (VPA) or 2 mg/kg vinpocetine (VPC) before and after the administration of PTZ at a convulsing dose (100 mg/kg). Results show that all the antiepileptic drugs tested were more or less effective in preventing PTZ-induced seizures. The long-term treatment with VPC decreased the auditory threshold, whereas the long-term treatment with CBZ, PHT or VPA increased the auditory threshold to a similar extent as the convulsing agent, PTZ. The combined effects of the antiepileptic drugs and PTZ on auditory threshold were additive. Therefore, only VPC prevented the increase in the auditory threshold induced by PTZ. It is concluded that the hearing loss produced by the long-term treatment with the most commonly used antiepileptic drugs could be aggravated by the illness. The prevention exerted by VPC on the hearing decline that accompanies experimental epilepsy, along with its capacity to control seizures at low doses in this and other animal models of epilepsy, would make VPC a valid candidate for the treatment of epilepsy.

Acoustic Stimulation↗
PubMed2001

Spiral ligament pathology: a major aspect of age-related cochlear degeneration in C57BL/6 mice.

Data from systematic, light microscopic examination of cochlear histopathology in an age-graded series of C57BL/6 mice (1.5-15 months) were compared with threshold elevations (measured by auditory brain stem response) to elucidate the functionally important structural changes underlying age-related hearing loss in this inbred strain. In addition to quantifying the degree and extent of hair cell and neuronal loss, all structures of the cochlear duct were qualitatively evaluated and any degenerative changes were quantified. Hair cell and neuronal loss patterns suggested two degenerative processes. In the basal half of the cochlea, inner and outer hair cell loss proceeded from base to apex with increasing age, and loss of cochlear neurons was consistent with degeneration occurring secondary to inner hair cell loss. In the apical half of the cochlea with advancing age, there was selective loss of outer hair cells which increased from the middle to the extreme apex. A similar gradient of ganglion cell loss was noted, characterized by widespread somatic aggregation and demyelination. In addition to these changes in hair cells and their innervation, there was widespread degeneration of fibrocytes in the spiral ligament, especially among the type IV cell class. The cell loss in the ligament preceded the loss of hair cells and/or neurons in both space and time suggesting that fibrocyte pathology may be a primary cause of the hearing loss and ultimate sensory cell degeneration in this mouse strain.

Aging↗
PubMed1998

Hair cell regeneration after local application of gentamicin at the round window of the cochlea in the pigeon.

Hair cells in the basilar papilla of birds have the capacity to regenerate after injury. Methods commonly used to induce cochlear damage are systemic application of ototoxic substances such as aminoglycoside antibiotics or loud sound. Both methods have disadvantages. The systemic application of antibiotics results in damage restricted to the basal 50% of the papilla and has severe side effects on the kidneys. Loud sound damages only small parts of the papilla and is restricted to the short hair cells. The present study was undertaken to determine the effect of local aminoglycoside application on the physiology and morphology of the avian basilar papilla. Collagen sponges loaded with gentamicin were placed at the round window of the cochlea in adult pigeons. The time course of hearing thresholds was determined from auditory brain stem responses elicited with pure tone bursts within a frequency range of 0.35-5.565 kHz. The condition of the basilar papilla was determined from scanning electron micrographs. Five days after application of the collagen sponges loaded with gentamicin severe hearing loss, except for the lowest frequency tested, was observed. Only at the apical 20% of the basilar papilla hair cells were left intact, all other hair cells were missing or damaged. At all frequencies there was little functional recovery until day 13 after implantation. At frequencies above 1 kHz functional recovery occurred at a rate of up to 4 dB/day until day 21, beyond that day recovery continued at a rate below 1 dB/day until day 48 at the 5.6 kHz. Below 1 kHz recovery occurred up to day 22, the recovery rate was below 2 dB/day. A residual hearing loss of about 15-25 dB remained at all frequencies, except for the lowest frequency tested. At day 20 new hair cells were seen on the basilar papilla. At day 48 the hair cells appeared to have recovered fully, except for the orientation of the hair cell bundles. The advantage of the local application of the aminoglycoside drug over systemic application is that it damages almost all hair cells in the basilar papilla and it has no toxic side effects. The damage is more extensive than with systemic application.

Acoustic Stimulation↗
PubMed2000

Otoprotectant minimizes hearing defects caused by Pseudomonas aeruginosa exotoxin A.

Exotoxin A, produced by Pseudomonas aeruginosa (PaExoA), penetrates from the middle ear in to the cochlea and causes sensorineural hearing loss (SNHL). In this investigation we studied electrophysiological changes in the albino rat following instillation of PaExoA and N(G)-nitro-L-arginine methyl ester (L-NAME), a known inhibitor of nitric oxide synthesis, into the middle ear. Hearing thresholds were measured by auditory brainstem response (ABR) technique. Latency/intensity curves were constructed to distinguish between cochlear and conductive components of hearing loss. PaExoA caused damage to cochleae and SNHL, mainly at high frequencies. This impairment was blocked by (L-NAME). It would appear that nitric oxide may be a significant link in the mechanism of SNHL caused by bacterial toxin. L-NAME acts as an otoprotectant against the deleterious action of PaExoA.

Animals↗
PubMedDate not supplied

Reduction of acoustic reflex threshold in neonates without auditory risk.

UNLABELLED: The auditory sensitization, a tool used in the investigation of acoustic reflex, allows the decrease of acoustic reflex thresholds from a facilitating stimulus. It may be presented before or simultaneously with the elicitor tone. The thresholds after and before the facilitating stimulus are compared and it is expected to see the decreased threshold. From the study of the acoustic reflex it is possible to obtain information about the auditory pathways, such as structures of the brainstem, since the acoustic reflex pathway is related to the auditory nuclei in this site. They are also involved in auditory processing. Thus, alterations of the acoustic reflex could be related to deficits in auditory processing skills. AIM: This study aims at investigating the acoustic reflex sensitization from a high-frequency facilitating tone (6 kHz) in newborns without risk factors to hearing impairment. RESULTS: The acoustic reflex threshold decreased in males and females for all studied frequencies. CONCLUSION: A high-frequency facilitating tone presented simultaneously produced a decrease of the acoustic reflex threshold in newborns without risk factors to hearing impairment.

Acoustic Impedance Tests↗