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PubMed1993

Combined effects of solvents on the rat's auditory system: styrene and trichloroethylene.

Because exposures to toxic agents typically involve more than one substance, it is necessary to know if combined exposures pose different risks than those to single agents. Many solvents have been implicated in central nervous disorders and some of them are known to produce hearing loss, probably mediated by damage to cochlear hair cells. Hearing loss was studied by recording the brainstem auditory evoked response (BAER) in male Long Evans rats exposed 8 h/day for 5 days to mixtures of styrene (STY) and trichloroethylene (TCE). Dose groups included air or solvent pairs (STY/TCE) in the following concentrations (ppm): (0:3000), (250:2250), (500:1500), (750:750) and (1000:0). Decreased BAER amplitude, indicative of hearing loss, was correlated with blood levels of total solvent. The effects were as predicted by a linear dose-addition model, indicating neither synergistic nor antagonistic interactions at the concentrations studied.

Acoustic Stimulation↗

PubMed1981

Short-term auditory deprivation: effect on brainstem electrical response.

Recent research has associated long-standing conductive hearing losses and changes in brainstem neural responses. Investigations on human subjects to date have used behavioral modes in evaluating response changes for both short-term (induced) and long-term conductive losses. The present study measured brainstem electrical responses (BSERs) from 30 normally hearing adults divided into unilateral-induced conductive-loss groups of 10, 20 and 30 h. This auditory deprivation produced a significant decrease in the wave-I latency (auditory nerve response) across all groups. It is concluded that short-term auditory deprivation can result in VIII nerve fiber hyper-excitability when initially stimulated after the deprivation period. The occurrence of temporary changes under very short-term deprivation raises the possibility that alterations to brainstem neural responses may also be measurable electrophysiologically in individuals with long-term conductive losses, such as children with otitis media.

Adult↗

PubMed1982

Changes in the organization of actin filaments in the stereocilia of noise-damaged lizard cochleae.

Alligator lizards exposed to 105 dB broadband noise for 24 h showed a 33 dB loss in hearing which was almost completely recovered 11 days after removal from the noise. Two lesions were found in the actin filament organization which could affect the rigidity of the stereocilia and thus could account for this hearing loss. These lesions preferentially affect the tallest stereocilia. The more common one is a depolymerization of the actin filaments at the base of the stereocilium where it makes contact with the cuticular plate. This results in a displacement and detachment of the stereocilium from its rootlet, thereby affecting the orientation of the tallest stereocilium. The other lesion involves a loss in crossbridges between adjacent actin filaments in the stereocilium. We demonstrate that such a loss will dramatically affect the rigidity of the stereocilia.

Actins↗

PubMed1984

Single-neuron labeling and chronic cochlear pathology. II. Stereocilia damage and alterations of spontaneous discharge rates.

The spontaneous discharge rates (SRs) sampled from auditory-nerve fibers in cases of chronic cochlear pathology are often abnormally low [17]. The application of intracellular labeling techniques to noise-exposed ears makes it possible to accurately correlate fiber populations showing SR abnormalities with the cochlear locations from which these responses originate. The correlations reveal that a decrease in the mean rates of spontaneous discharge is typically associated with selective loss of the tallest row of stereocilia from the inner hair cells. In cochlear regions where virtually all of the tall stereocilia are missing from the inner hair cells, the maximum rates of spontaneous discharge are less than 1/3 normal values. We suggest that the loss of tall stereocilia causes the decrease in SR because much of the resting current in the inner hair cell normally flows through the stereocilia membrane. Thus, the loss of that membrane leads to a hyperpolarization of the inner hair cell which, in turn, decreases the spontaneous release of vesicles at the synapse. An interpretation is also suggested for the "compression" of the SR distribution commonly seen among high-frequency neurons in normal animals [9].

Animals↗

PubMed1992

Suprathreshold comparisons of derived and enhanced compound action potentials.

The Derived cochlear nerve compound action potential (CAP) and the Enhanced CAP are both measures which demonstrate the ability of a forward masker to increase the amplitude of the CAP produced by a probe stimulus. Enhancement occurs whenever the amplitude of a masked CAP is larger than that of a nonmasked CAP, whereas the derived CAP is produced by the subtraction of the entire masked CAP waveform from that of the nonmasked waveform. Therefore, a derived CAP is created whenever the masker produces a difference of amplitude, latency, and/or waveform shape. The present experiments compare these two measures by observing the effects of 13 kHz maskers varying from levels of -10 to +70 db SPL on CAPs produced by 50 or 60 dB SPL, 13 kHz probe stimuli. Enhancement is characterized by a nonmonotonic increase of CAP amplitude (and sometimes a decrease of latency) as a function of increasing levels of the forward masker, whereas this pattern seldom occurs with the derived CAP. Enhancement is typically seen with forward making, but seldom seen with simultaneous masking, whereas the derived CAP is very similar under these two types of masking.

Acoustic Stimulation↗

PubMed1995

A model of frequency tuning in the basilar papilla of the Tokay gecko, Gekko gecko.

This paper uses the quantitative details of the anatomy of the auditory papilla in the Tokay gecko Gekko gecko (as described in the companion paper) to make a quantitative model predicting the tonotopic organization of two of the three papillar areas. Assuming that hair-cell bundle stiffness is similar to that of other species, a model of resonance frequencies for the apical areas of the papilla was constructed, taking into account factors such as the number of hair cells per resonant unit, their bundle dimensions, the volume of the tectorial mass, etc. The model predicts that the apical pre- and postaxial areas, although anatomically adjacent, respond to different frequency ranges, a phenomenon not yet reported from any vertebrate. The model predicts that together, these areas respond best to frequencies between 1.1 and 5.3 kHz, close to the range found physiologically [Eatock et al. (1981) J. Comp. Physiol. 142, 203-218] (0.8 to 5 kHz) for the high-frequency range for this species. Only physiological experiments tracing responses to specific papillar nerve fibres can confirm or refute these interesting predictions of the model. The model also indicates that, compared to free-standing hair-cell bundles, the semi-isolated tectorial structures called sallets not only lower the range of characteristic frequencies but also increase the frequency selectivity of the attached hair cells.

Acoustic Stimulation↗

PubMed1995

Quantitative anatomical basis for a model of micromechanical frequency tuning in the Tokay gecko, Gekko gecko.

The basilar papilla of the Tokay gecko was studied with standard light- and scanning electron microscopy methods. Several parameters thought to be of particular importance for the mechanical response properties of the system were quantitatively measured, separately for the three different hair-cell areas that are typical for this lizard family. In the basal third, papillar structure was very uniform. The apical two-thirds are subdivided into two hair-cell areas running parallel to each other along the papilla and covered by very different types of tectorial material. Both of those areas showed prominent gradients in hair-cell bundle morphology, i.e., in the height of the stereovillar bundles and the number of stereovilli per bundle, as well as in hair cell density and the size of their respective tectorial covering. Based on the direction of the observed anatomical gradients, a 'reverse' tonotopic organization is suggested, with the highest frequencies represented at the apical end.

Acoustic Stimulation↗

PubMed1994

Phase weighting: a method to improve objective detection of steady-state evoked potentials.

Objective response detection statistics such as magnitude-squared coherence (MSC) reflect the degree to which subaverage phases are clustered or dispersed, but not their agreement with an expected, or target, phase. Using signal-plus-noise simulations and human 40-Hz auditory evoked potentials, we tested MSC performance with and without phase weighting, in which MSC values were multiplied by weights related to the phase error between measured phase and target phase. Phase weighting improved MSC performance for both simulated and 40-Hz auditory evoked potential data. However, the improvement was greater for the simulations, probably because target phase was precisely known.

Acoustic Stimulation↗

PubMed1995

OHC response recruitment and its correlation with loudness recruitment.

After proper noise exposure, Hensen's cells, which have been shown to follow closely the response characteristics of the outer hair cells, suffered a loss of sensitivity at low and moderate SPLs. The lower the stimulus level, the greater was the loss. When the low-SPL loss did not exceed about 40 dB, input-output functions showed an increased rate of amplitude growth, so that the post-exposure response caught up with its pre-exposure counterpart between 60 and 90 dB SPL, depending on the severity of the loss. These results, together with preceding clinical observations, led us to the conclusion that loudness recruitment occurs at least in part at the hair cell level and is basically a local event as opposed to a pathological spread of excitation. The response recruitment we have discovered appears to result from a decreased effect of the active feedback when the passive cochlear mechanisms are intact. Evidence for these relationships is presented and an explanation is offered for previous experimental successes and failures in observing a steepening of rate-intensity functions in auditory nerve fibers after noise exposures or administration of ototoxic drugs.

Acoustic Stimulation↗

PubMed1995

Distortion-product otoacoustic emissions and their anaesthesia sensitivity in the European starling and the chicken.

The aim of the present experimental series was to provide further information on the distortion-product otoacoustic emissions (DP) of birds and contribute to a general understanding of DP generation. Basic characteristics of the DP 2f1-f2 and 2f2-f1 were measured in the ear canal of both awake and anaesthetized European Starlings and chickens. The effect of a third suppressive tone and the behaviour of the DP under anaesthesia were also studied. In general, the DP characteristics of both bird species resembled those of lizards and mammals, but first appeared at somewhat higher primary-tone levels. The best frequencies of third tones suppressing 2f1-f2 lay near the first primary tone (f1), but for 2f2-f1, the situation was more complex. Facilitation via a third tone was also seen for both DP, often at levels below those eliciting suppression. The DP 2f1-f2 disappeared completely at the onset of deep anaesthesia and recovered to its original magnitude when the anaesthesia was lightened, sometimes with a considerable delay. The compound action potential (CAP) was somewhat more sensitive to anaesthesia than the DP. Control experiments showed that the anaesthesia effect was not a result of hypoxia. Avian DP at low and intermediate sound levels are thus physiologically-sensitive manifestations of normal hair-cell function that are, in contrast to mammals, also anaesthesia-sensitive.

Acoustic Stimulation↗

PubMed2006

Hearing assessment in pre-school children with speech delay.

OBJECTIVE: The purpose of this study was to detect any underlying hearing loss among the healthy pre-school children with speech delay. METHODS: 76 children, aged from 1 to 5 years, underwent a thorough audiological examination consisting of tympanometry, free field testing, otoacoustic emission recordings and auditory brainstem responses (ABRs). If hearing was normal, then they were evaluated by a child neurologist-psychiatrist. RESULTS: According to our findings, the children were classified into 3 groups; those with normal hearing levels (group I, 52 children, 68.4%), sensorineural hearing loss (group II, 22 children, 28.9%) and conductive hearing loss (group III, 2 children, 2.6%). In group I, speech delay was attributed to pervasive developmental disorder (PDD), which represents high-functioning autistic children (37 cases). Other causes were specific language impairment (SLI)-expressive (3 cases), bilingualism (2 cases), and unknown etiology (10 cases). More than half (59%) of the children diagnosed with PDD evidenced significant language impairment limited to more than two words. Children with SLI-expressive and bilingualism used a maximum of two words. In group II, 13 children suffered from profound hearing loss in both ears, 3 from severe, 3 had profound hearing loss in one ear and severe in the other, 2 from moderate, and 1 had moderate in one ear and severe in the other. No child had mild sensorineural hearing loss. The children with profound hearing loss in at least one ear had total language impairment using no word at all (10 cases), or a maximum of two words (6 cases). When hearing loss was moderate to severe, then the speech vocabulary was confined to several words (more than two words-6 cases). Only two children suffering from conductive hearing loss both presented with complete lack of speech. CONCLUSION: A great number of healthy pre-school children with speech delay were found to have normal hearing. In this case, the otolaryngologist should be aware of the possible underlying clinical entities, especially of psychiatric nature. The children with profound sensorineural hearing loss exhibited more severe speech delay than those with moderate to severe. Regardless of etiology, the early identification and intervention contribute to positive outcome in this critical period of childhood for language development.

Acoustic Impedance Tests↗

PubMed2005

Effects of age and spectral shaping on perception and neural representation of stop consonant stimuli.

OBJECTIVE: To determine if (1) aging affects neural representation of a dynamic spectral speech cue and (2) spectrally-shaped gain applied to the cue reduces any aging effects. METHODS: Psychometric functions and cortical evoked responses were compared in young and older listeners with normal hearing. Stimuli were consonant-vowels along a /bdg/ place-of-articulation continuum in an unshaped and shaped condition. Shaped stimuli enhanced audibility of the F2 formant transition relative to the rest of the stimulus. RESULTS: Compared with younger listeners, older listeners had larger /d/ categories, longer P2 latencies, and larger N1 amplitudes to unshaped stimuli. To shaped stimuli, older listeners had /d/ categories and P2 latencies more similar to those measured from younger listeners, while N1 amplitudes were larger. CONCLUSIONS: Aging significantly affects the processing of dynamic spectral information. For some measures, differences due to aging were minimized with spectrally-shaped stimuli. SIGNIFICANCE: Aging reduces neural responsiveness to dynamic spectral cues. If the cue is enhanced, neural responsiveness is increased and perceptual measures are more like those from the younger listeners for some stimuli. This suggests that aging may decrease responsiveness of intact neurons as opposed to destroying neurons and/or distorting spectral coding.

Acoustic Stimulation↗

PubMed2006

Abnormal brainstem auditory evoked responses in mitochondrial neurogastrointestinal encephalomyopathy (MNGIE): evidence of delayed central conduction time.

OBJECTIVE: To assess the usefulness of brain auditory evoked potentials (BAEPs) in the study of asymptomatic white matter alterations in brain MRI observed in mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients. METHODS: The authors studied the neurophysiological characteristics of the BAEPs in four genetically confirmed MNGIE patients who presented varying degrees of leukoencephalopathy in brain MRI. RESULTS: Prolonged I-III and I-V interpeak latencies were the most common abnormalities found, with a correlation between the extent of brain MRI lesions and BAEPs. CONCLUSIONS: The findings suggest a delayed central conduction time along the brainstem. BAEPs may be useful in the neurophysiological evaluation of central white matter lesions in MNGIE. Similar neurophysiological findings have been reported in other myelin disorders in the central nervous system. SIGNIFICANCE: The BAEPs abnormalities identified should be interpreted as an indirect sign of CNS involvement in MNGIE patients and provide comprehensive and integrated information concerning brainstem dysfunction. Further studies are necessary in order to identify whether there is a correlation between BAEPs and the clinical progression of the disease.

Adult↗

PubMed2004

Quantitative analysis of apoptotic and necrotic outer hair cells after exposure to different levels of continuous noise.

We have reported that by 2 days after noise exposure the size of cochlear lesion was expanding by outer hair cells (OHCs) dying either by apoptosis or necrosis. The current study was designed to compare the prevalence of the two cell death pathways as a function of time after exposure to noises of different levels. Chinchillas were exposed to a narrow band noise at either 104 or 108 dB SPL for 1 h. At three time points (1, 4 and 30 days) after the noise exposure, the numbers of missing, apoptotic and necrotic OHCs in the cochleas were identified and documented with a combination of TUNEL, caspase-3 and propidium iodide labeling. The subjects exposed to the 108-dB noise showed significantly more apoptotic OHCs than necrotic OHCs in the cochleas examined at days 1 and 4 after the noise exposure. By day 30, apoptotic and necrotic pathologies continued, although in small quantity, with no significant difference in quantity between two types of cell death. The subjects exposed to the 104-dB noise showed a significant difference in the numbers of apoptotic and necrotic OHCs at day 1 after the noise exposure, whereas the difference became statistically insignificant at day 4 and day 30 after the noise exposure. The results of the study indicate that the early expansion of cochlear lesion is attributed primarily to apoptosis, whereas the later stage of lesion expansion is likely the result of an equal contribution from apoptosis and necrosis.

Animals↗

PubMed2003

A model of two-stage newborn hearing screening with automated auditory brainstem response.

Our purpose was to evaluate a two-stage newborn hearing screening program using automated auditory brainstem response (AABR) before discharge and to describe our follow-up program. This study used 4085 infants born in the Seirei-Hamamatsu and Mikatahara General Hospitals during a 2-year period. The initial screening test was performed 2 or 3 days after birth at an intensity of 35 dBnHL. For the infants who were referred from this test, the re-screening test was performed 5 or 6 days after birth. Diagnostic work-up with auditory brainstem response (ABR), otoacoustic emissions (OAE), and a conditioned orientation reflex audiometry (COR) test were performed by the age of 3-6 months. The referral rate was 1.20% (49/4085 infants) in the first test and 0.71% (29/4085 infants) in the two-stage screening. The two-stage screening procedure was able to reduce the false-positive rate from 0.83 to 0.34%. The incidence of bilateral and unilateral congenital hearing loss diagnosed by ABR was 8/4085 (0.20%) infants and 7/4085 (0.17%) infants, respectively. One infant with congenital cytomegalovirus infection, who passed the two-stage AABR tests, was diagnosed with hearing loss 1 month after birth, using ABR. The two-stage measurement of AABR is effective and time efficient due to significant decreases in the referral rate and the false-positive rate.

Audiometry↗

PubMed1998

Auditory screening in high-risk pre-term and full-term neonates using transient evoked otoacoustic emissions and brainstem auditory evoked potentials.

The present report concerns a 3 year, 8 month hearing screening in 1531 high-risk neonates by means of two successive transient evoked otoacoustic emission (TEOAE) recordings followed, in cases of suspected hearing loss, by brainstem auditory evoked potential (BAEP) recording and otolaryngology (ORL) consultation. After TEOAE 1 and 2 and BAEP testing, 1361 infants (88.9%) were declared normal, and 170 (11.1%) suspected of hearing loss. Of these 170, 58 showed bilateral and 26 unilateral impairment. Definite hearing loss on ORL consultation was diagnosed in 14 infants (0.9% of the screened population as a whole); 22 are still being followed, while 86 (5.6%) failed to consult for diagnosis. The mean age on diagnosis of definite hearing loss was 9.9 +/- 4.9 (range 4-20) months. Several auditory function risk factors proved more frequent in deaf than in normal children. Our results show that early hearing loss screening in at-risk neonates needs to be pursued.

Auditory Threshold↗

PubMed2000

Prognostic factors for hearing preservation in vestibular schwannoma surgery.

OBJECTIVE: To determine whether prognostic indicators for hearing preservation could be identified in patients with vestibular schwannoma undergoing middle fossa craniotomy resection. STUDY DESIGN: Prospective case review. SETTING: Private practice tertiary referral center. PATIENTS: 333 patients with serviceable hearing and vestibular schwannoma resected by middle fossa craniotomy from 1992 to 1998. MAIN OUTCOME MEASURES: Potential prognostic indicators, including tumor size and nerve of origin, preoperative pure-tone average, speech discrimination, distortion product otoacoustic emission testing, age, auditory brainstem response (ABR), and electronystagmography. RESULTS: Postoperative hearing near preoperative levels was attained in 167 patients (50%), with an American Academy of Otolaryngology-Head and Neck Surgery Class A hearing result in 33% and a Class B result in 26%. Comparison of potential prognostic indicators between groups with hearing preserved and the group with no measurable hearing revealed significant differences in preoperative hearing, ABR, and tumor origin data. Better preoperative hearing, shorter intraaural wave V latency, shorter absolute wave V latency, and superior vestibular nerve origin were associated with higher rates of hearing preservation. CONCLUSIONS: Preoperative hearing status, ABR, and intraoperative tumor origin data were shown to be of value as prognostic indicators.

Adolescent↗

PubMed1980

Electrophysiologic studies of the auditory cortex in the awake monkey.

An overview of some recent developments in the study of central auditory processes is presented. We describe findings from single cell physiologic studies that contribute to our understanding of central auditory function, the development of behavioral techniques permitting precise evaluation of hearing in animals, and the power and potential of integrative neurophysiologic-behavioiral investigations in defining and analyzing central neural mechanisms that underlie normal perception and imperception. The influence of changes in intermodality attention on evoked activity of 25 cells of the auditory cortex is described and compared to effects of other attentional changes. A small but consistent increase in excitatory cell evoked discharge rate and a reduction in initial latency of response are shown to be correlated with a shift from a visual to an auditory task. In general, behavioral states ranging from sleep-waking to attention to a specific acoustic cue are shown to influence responsiveness of the auditory system and variability of response. Procedures are described for effective control of attentional factors and reduction of electrophysiologic variability. The contributions of these data and procedures to studies of the encoding of complex auditory signals and studies of neural mechanisms underlying such disorders as noise induced hearing loss are discussed.

Animals↗