Latent cochlear damage in personal stereo users: a study based on click-evoked otoacoustic emissions.

OBJECTIVE: To assess the effects of use of personal stereo systems (PS) on hearing by means of the objective measure of transient-evoked otoacoustic emissions. PARTICIPANTS AND SETTING: People aged between 10 and 59 years who had otoacoustic emissions recorded by the National Acoustic Laboratories between 1989 and 1997 were eligible for inclusion. Recordings from participants with hereditary disorders or any form of aural disease (e.g., otitis media, otosclerosis, fluctuant hearing loss, Meniere's syndrome, or exposure to ototoxic substances) were excluded. METHODS: Transient-evoked otoacoustic emission (TEOAE) records were obtained with a standard 260 repetitions of an 80 dB train of clicks used for recording outer hair cell activity. The measure of otoacoustic emission strength was the Otodynamics ILO88 variable Waverepro%. For each participant, all the key factors relating to their hearing history were assessed from patient referral information or from demographic information obtained in writing at the time of recording either in the form of a detailed questionnaire or verbal assessment. Otoacoustic emission data were analysed according to age, industrial noise exposure and personal stereo use. RESULTS: Usable otoacoustic emission records were obtained from 1724 people (1066 males and 658 females). Otoacoustic emission strength declined with age, and was significantly lower in males than females, lower in people exposed to industrial noise than those not exposed, and significantly lower in users of personal stereo systems than non-users. People with both kinds of noise exposure had values which were significantly lower again, indicating an additive effect. CONCLUSIONS: As only 39 people with PS exposure admitted any hearing problems, decline in otoacoustic emission strength forewarns premature hearing loss in personal stereo users.

Evoked otoacoustic emissions and auditory selective attention.

The auditory system has an extensive peripheral efferent innervation. The question addressed in this paper is whether the olivocochlear bundle (OCB) efferent system innervating the outer hair cells (OHC) of the cochlea plays a role in selective attention. As evoked otoacoustic emissions (EOAE) provide a measure of the active micromechanical properties of OHCs, they can be used to assess the role of the efferent system in attention. Six experiments using tone-pip EOAEs are reported. In each experiment, EOAEs generated by 1 or 2 kHz tone pips when they were attended were compared with EOAEs to the same stimuli when they were unattended. In three experiments (1-4), a non-linear stimulus difference method was used to record a pure cochlear component of EOAEs. In Exps. 1-5, 1 and 2 kHz tone pips were delivered to the same ear and the difficulty of the subjects' task was manipulated in order to produce a more focussed attentional state or contralateral noise was presented to determine whether attention effects are dependent upon having an already activated efferent system. In Exp.6, the 1 and 2 kHz stimuli were delivered to opposite ears. A total of 70 subjects participated in the six experiments. There were no effects of attention on EOAEs in any of the experiments in the direction of previously reported effects. The results of these first six experiments employing simple attention switches between fixed auditory objects do not support active cochlear involvement in selective attention.

A threshold decrease for electrically stimulated motor responses of isolated aging outer hair cells from the pigmented guinea pig.

When outer hair cells are isolated from guinea pig cochleas and are placed in normal Hank's medium, they exhibit aging as a slow tonic reduction in length and increase in diameter. During this time the lateral subsurface cisternae become progressively vesiculated and the optical density of the border seen under phase-contrast microscopy decreases. A study of 65 outer hair cells was carried out using video imaging of this process. The base of each cell bonded to the Petri dish and the motility of the cuticular plate was recorded in two ways. To quantify the slow contraction of each preparation, the dimensions of the cell were measured from video replay. Displacements of the cuticular plate in response to an alternating electric field in line with the cell axis were also monitored using a video tracking technique. The amplitude of a 1 Hz stimulus required to cause a visually detectable motor response above baseline noise decreased as the cell degraded. Typically, fresh cylindrical cells exhibiting high optical contrast showed relatively small movements for field strengths up to 2 kVm-1. However, as the cell depolarized, the rigidity initially decreased and the cell could respond to field strengths down to 0.1 kVm-1 before cell death ultimately occurred. Such a threshold phenomenon in the isolated OHC has not been demonstrated directly until now. This result explains the variability of electromotility in aging in vitro preparations from the cochlea.

Transitory endolymph leakage induced hearing loss and tinnitus: depolarization, biphasic shortening and loss of electromotility of outer hair cells.

There are types of deafness and tinnitus in which ruptures or massive changes in the ionic permeability of the membranes lining the endolymphatic space [e.g., of the reticular lamina (RL)] are believed to allow potassium-rich endolymph to deluge the low [K+] perilymphatic fluid (e.g., in the small spaces of Nuel). This would result in a K+ intoxication of sensory and neural structures. Acute attacks of Ménière's disease have been suggested to be an important example for this event. The present study investigated the effects of transiently elevated [K+] due to the addition of artificial endolymph to the basolateral cell surface of outer hair cells (OHC) in replicating endolymph-induced K+ intoxication of the perilymph in the small spaces of Nuel. The influence of K+ intoxication of the basolateral OHC cell surface on the transduction was then examined. Intoxication resulted in an inhibition of the physiological repolarizing K+ efflux from hair cells. This induced unwanted depolarizations of the hair cells, interfering with mechanoelectrical transduction. A pathological longitudinal OHC shortening was also found, with subsequent compression of the organ of Corti possibly influencing the micromechanics of the mechanically active OHC. Both micromechanical and electrophysiological alterations are proposed to contribute to endolymph leakage induced attacks of deafness and possibly also to tinnitus. Moreover, repeated or long-lasting K+ intoxications of OHC resulted in a chronic and complete loss of OHC motility. This is suggested to be a pathophysiological basis in some patients with chronic hearing loss resulting from Ménière's syndrome.

Functional role of the olivo-cochlear bundle: a motor unit control system in the mammalian cochlea.

A fiber optic lever is applied to the measurement of the motion of the basilar membrane motion in guinea pigs. In response to intense tones from either ear, the motion includes a substantial summating shift in the mean position in addition to a travelling wave originally described by von Békésy. His stroboscopic technique and most techniques used since have been concentrated upon measuring vibrations of the basilar membrane synchronous with the stimulus and have been insensitive to variations in the baseline position such as a summating component of motion analogous to the extracellular summating potential. In addition to the role of the outer hair cells in providing normal hearing sensitivity, they evidently play a role in regulating the mean position of the basilar membrane. For a fixed frequency, the polarity of the mean position varies systematically with sound level and place and summates with time since onset. Since these cells are the target cells for the olivocochlear bundle, homeostasis in the cochlea would appear to be linked efferent function and involve cochlear mechanics. The negative damping hypothesis asserts that hair cell activity is necessary for low thresholds. The results presented here demonstrate that OHC activity exists independent of neural thresholds. The discussion develops the concept that threshold losses are due to a mismatch of opposing tonic forces which normally maintain the mean position of the basilar membrane. Structure is examined in relation to function and the group of outer hair cells innervated by a single medial efferent neuron is identified as a motor unit. Implications of central control of individual motor units include peripheral involvement in selective attention tasks.

The application of a capacitive probe technique for direct observation of electromechanical processes in the guinea pig cochlea.

The demonstration of evoked mechanical responses of the outer hair cells in the mammalian cochlea by indirect measurements introduces a new range of problems into direct mechanical measurements. Direct and indirect measurements indicate that the frequency spectra of evoked electromechanical responses may extend well into the range of audio frequencies, revealing a need to develop terminology and protocols for distinguishing evoked mechanical responses from the traditional traveling wave when both are apparently superimposed on the motion of the basilar membrane in the normally functioning cochlea. Details are presented of a frequency-modulation capacitive probe technique for measurement of vibrating structures of the guinea pig ear. Considerations include the design of the transducer, calibration, sensitivity, linearity, and sources of noise, as well as the influence of the technique upon the animal preparation, and in particular the issues associated with draining scala tympani for the measurement. Relative advantages and disadvantages of the technique are compared with salient features of other techniques currently available. In view of the apparent complexity of cochlear mechanics some preliminary experiments are required to elucidate some of the key questions about reverse-transduction processes in general. A "simple" first experiment is to test existence of any rectifying or motile response.

Frequency-dependent self-induced bias of the basilar membrane and its potential for controlling sensitivity and tuning in the mammalian cochlea.

A displacement-sensitive capacitive probe technique was used in the first turn of guinea pig cochleas to examine whether the motion of the basilar membrane includes a displacement component analogous to the dc receptor potentials of the hair cells. Such a "dc" component apparently exists. At a given location on the basilar membrane, its direction toward scala vestibuli (SV) or scala tympani (ST) varies systematically with frequency of the acoustic stimulus. Furthermore, it appears to consist of two parts: a small asymmetric offset response to each gated tone burst plus a progressive shift of the basilar membrane from its previous position. The mean position shift is cumulative, increasing with successive tone bursts. The amplitude of the immediate offset response, when plotted as a function of frequency, appears to exhibit a trimodal pattern. This displacement offset is toward SV at the characteristic frequency (CF) of the location of the probe, while at frequencies either above or below the CF the offset is relatively larger, and toward ST. The mechanical motion of the basilar membrane therefore appears to contain the basis for lateral suppression. The cumulative mean position shift, however, appears to peak toward ST at the apical end of the traveling wave envelope and appears to be associated with a resonance, not of the basilar membrane motion directly, but coupled to it. The summating potential, measured concurrently at the round window, shows a more broadly tuned peak just above the CF of the position of the probe. This seems to correspond to the peak at the CF of the mechanical bias. As the preparation deteriorates, the best frequency of the vibratory displacement response decreases to about a half-octave below the original CF. There is a corresponding decrease in the frequency of the peaks of the trimodal pattern of the asymmetric responses to tone bursts. The trimodal pattern also broadens. In previous experiments the basilar membrane has been forced to move in response to a low-frequency biasing tone. The sensitivity to high-frequency stimuli varies in phase with the biasing tone. The amplitudes of slow movement in these earlier experiments and in the present experiments are of the same order of magnitude. This suggests strongly that the cumulative shift toward ST to a high-frequency acoustic stimulus constitutes a substantial controlling bias on the sensitivity of the cochlea in that same high-frequency region. Its effect will be to reduce the slope of neural rate-level functions on the high-frequency side of CF.

A spatial template for the shape of tuning curves in the mammalian cochlea.

The shape of the tuning curve of primary auditory neurons of four mammals is characterized using a simple exponential model. The regression analysis formalizes a distinction between the characteristic frequency of a neuron and its "nominal" characteristic frequency in cases of temporary threshold loss in high-frequency neurons. Second, the model offers a stronger quality test for sharpness of tuning than the Q10dB since it takes into account the threshold of the neuron at its characteristic frequency and its "characteristic place" of origin along the cochlear partition. Third, the model reveals that the low-frequency side of the tip segment of the tuning curve is bounded by a constraint or template which is most simply expressed in spatial terms. The template describes the basal-side boundary of an "excitatory region" whose length along the cochlear partition is proportional to the square root of the sound pressure. Tuning curve variability arises because biological dependencies influence the basic template. A "spatial-filter" hypothesis is developed and its generality is discussed, particularly in regard to the case of the acoustic "fovea" of the horseshoe bat. Finally, the possibility is discussed that the template possesses a simple physiological correlate in the form of a spatially localized region marked by a "dc" shift of the mean position of the basilar membrane which sets the sensitivity of the tuning mechanism [E.L. LePage, J. Acoust. Soc. Am. 82, 139-154 (1987)].