Does electrical stimulation of the crossed olivo-cochlear bundle produce movement of the organ of Corti?

Slow oscillatory changes of DPOAE magnitude and phase after exposure to intense low-frequency sounds

Sensitive sound detection within the mammalian cochlea is performed by hair cells surrounded by cochlear fluids. Maintenance of cochlear fluid homeostasis and tight regulation of intracellular conditions in hair cells are crucial for the auditory transduction process but can be impaired by intense sound stimulation. After a short, intense low-frequency sound, the cochlea shows the previously described "bounce phenomenon," which manifests itself as slow oscillatory changes of hearing thresholds and otoacoustic emissions. In this study, distortion product otoacoustic emissions (DPOAEs) were recorded after Mongolian gerbils were exposed to intense low-frequency sounds (200 Hz, 100 dB SPL) with different exposure times up to 1 h. After all sound exposure durations, a certain percentage of recordings (up to 80% after 1.5-min-long exposure) showed oscillatory DPOAE changes, similar to the bounce phenomenon in humans. Changes were quite uniform with respect to size and time course, and they were independent from sound exposure duration. Changes showed states of hypo- and hyperactivity with either state preceding the other. The direction of changes was suggested to depend on the static position of the cochlear operating point. As assessed with DPOAEs, no indication for a permanent damage after several or long exposure times was detected. We propose that sensitivity changes occur due to alterations of the mechanoelectrical transduction process of outer hair cells. Those alterations could be induced by different challenged homeostatic processes with slow electromotility of outer hair cells being the most plausible source of the bounce phenomenon. NEW & NOTEWORTHY Low-frequency, high-intensity sound can cause slowly cycling activity changes in the mammalian cochlea. We examined the effect of low-frequency sound duration on the degree of these alterations. We found that cochlear changes showed a stereotypical biphasic pattern independent of sound exposure duration, but the probability that significant changes occurred decreased with increasing sound duration. Despite exposure durations of up to 1 h, no permanent or transient impairments of the cochlea were detected.

Integrating the active process of hair cells with cochlear function

Uniquely among human senses, hearing is not simply a passive response to stimulation. Our auditory system is instead enhanced by an active process in cochlear hair cells that amplifies acoustic signals several hundred-fold, sharpens frequency selectivity and broadens the ear's dynamic range. Active motility of the mechanoreceptive hair bundles underlies the active process in amphibians and some reptiles; in mammals, this mechanism operates in conjunction with prestin-based somatic motility. Both individual hair bundles and the cochlea as a whole operate near a dynamical instability, the Hopf bifurcation, which accounts for the cardinal features of the active process.

The physics of hearing: fluid mechanics and the active process of the inner ear

Most sounds of interest consist of complex, time-dependent admixtures of tones of diverse frequencies and variable amplitudes. To detect and process these signals, the ear employs a highly nonlinear, adaptive, real-time spectral analyzer: the cochlea. Sound excites vibration of the eardrum and the three miniscule bones of the middle ear, the last of which acts as a piston to initiate oscillatory pressure changes within the liquid-filled chambers of the cochlea. The basilar membrane, an elastic band spiraling along the cochlea between two of these chambers, responds to these pressures by conducting a largely independent traveling wave for each frequency component of the input. Because the basilar membrane is graded in mass and stiffness along its length, however, each traveling wave grows in magnitude and decreases in wavelength until it peaks at a specific, frequency-dependent position: low frequencies propagate to the cochlear apex, whereas high frequencies culminate at the base. The oscillations of the basilar membrane deflect hair bundles, the mechanically sensitive organelles of the ear's sensory receptors, the hair cells. As mechanically sensitive ion channels open and close, each hair cell responds with an electrical signal that is chemically transmitted to an afferent nerve fiber and thence into the brain. In addition to transducing mechanical inputs, hair cells amplify them by two means. Channel gating endows a hair bundle with negative stiffness, an instability that interacts with the motor protein myosin-1c to produce a mechanical amplifier and oscillator. Acting through the piezoelectric membrane protein prestin, electrical responses also cause outer hair cells to elongate and shorten, thus pumping energy into the basilar membrane's movements. The two forms of motility constitute an active process that amplifies mechanical inputs, sharpens frequency discrimination, and confers a compressive nonlinearity on responsiveness. These features arise because the active process operates near a Hopf bifurcation, the generic properties of which explain several key features of hearing. Moreover, when the gain of the active process rises sufficiently in ultraquiet circumstances, the system traverses the bifurcation and even a normal ear actually emits sound. The remarkable properties of hearing thus stem from the propagation of traveling waves on a nonlinear and excitable medium.

Using the cochlear microphonic as a tool to evaluate cochlear function in mouse models of hearing

The cochlear microphonic (CM) can be a useful analytical tool, but many investigators may not be fully familiar with its unique properties to interpret it accurately in mouse models of hearing. The purpose of this report is to develop a model for generation of the CM in wild-type (WT) and prestin knockout mice. Data and modeling results indicate that in the majority of cases, the CM is a passive response, and in the absence of outer hair cell (OHC) damage, mice lacking amplification are expected to generate WT levels of CM for inputs less than approximately 30 kHz. Hence, this cochlear potential is not a useful metric to estimate changes in amplifier gain. This modeling analysis may explain much of the paradoxical data in the literature. For example, various manipulations, including the application of salicylate and activation of the crossed olivocochlear bundle, reduce the compound action potential but increase or do not change the CM. Based on this current evaluation, CM measurements are consistent with early descriptions where this AC cochlear potential is dominated by basal OHCs, when recorded at the round window.

Contralateral Acoustic Stimulation Modulates Low-Frequency Biasing of DPOAE: Efferent Influence on Cochlear Amplifier Operating State?

The mammalian efferent medial olivocochlear system modulates active amplification of low-level sounds in the cochlea. Changes of the cochlear amplifier can be monitored by distortion product otoacoustic emissions (DPOAEs). The quadratic distortion product f2–f1 is known to be sensitive to changes in the operating point of the amplifier transfer function. We investigated the effect of contralateral acoustic stimulation (CAS), known to elicit efferent activity, on DPOAEs in the gerbil. During CAS, a significant increase of the f2–f1 level occurred already at low contralateral noise levels (20 dB SPL), whereas 2f1–f2 was much less affected. The effect strength depended on the CAS level and as shown in experiments with pure tones on the frequency of the contralateral stimulus. In a second approach, we biased the position of the cochlear partition and thus the cochlear amplifier operating point periodically by a ipsilateral low-frequency tone, which resulted in a phase-related amplitude modulation of f2–f1. This modulation pattern was changed considerably during contralateral noise stimulation, in dependence on the noise level. The experimental results were in good agreement with a simple model of distortion product generation and suggest that the olivocochlear efferents might change the operating state of cochlear amplification.

Estimating the operating point of the cochlear transducer using low-frequency biased distortion products

Distortion products in the cochlear microphonic (CM) and in the ear canal in the form of distortion product otoacoustic emissions (DPOAEs) are generated by nonlinear transduction in the cochlea and are related to the resting position of the organ of Corti (OC). A 4.8 Hz acoustic bias tone was used to displace the OC, while the relative amplitude and phase of distortion products evoked by a single tone [most often 500 Hz, 90 dB SPL (sound pressure level)] or two simultaneously presented tones (most often 4 kHz and 4.8 kHz, 80 dB SPL) were monitored. Electrical responses recorded from the round window, scala tympani and scala media of the basal turn, and acoustic emissions in the ear canal were simultaneously measured and compared during the bias. Bias-induced changes in the distortion products were similar to those predicted from computer models of a saturating transducer with a first-order Boltzmann distribution. Our results suggest that biased DPOAEs can be used to non-invasively estimate the OC displacement, producing a measurement equivalent to the transducer operating point obtained via Boltzmann analysis of the basal turn CM. Low-frequency biased DPOAEs might provide a diagnostic tool to objectively diagnose abnormal displacements of the OC, as might occur with endolymphatic hydrops.

ATP-gamma-S shifts the operating point of outer hair cell transduction towards scala tympani

ATP receptor agonists and antagonists alter cochlear mechanics as measured by changes in distortion product otoacoustic emissions (DPOAE). Some of the effects on DPOAEs are consistent with the hypothesis that ATP affects mechano-electrical transduction and the operating point of the outer hair cells (OHCs). This hypothesis was tested by monitoring the effect of ATP-gamma-S on the operating point of the OHCs. Guinea pigs anesthetized with urethane and with sectioned middle ear muscles were used. The cochlear microphonic (CM) was recorded differentially (scala vestibuli referenced to scala tympani) across the basal turn before and after perfusion (20 min) of the perilymph compartment with artificial perilymph (AP) and ATP-gamma-S dissolved in AP. The operating point was derived from the cochlear microphonics (CM) recorded in response low frequency (200 Hz) tones at high level (106, 112 and 118 dB SPL). The analysis procedure used a Boltzmann function to simulate the CM waveform and the Boltzmann parameters were adjusted to best-fit the calculated waveform to the CM. Compared to the initial perfusion with AP, ATP-gamma-S (333 microM) enhanced peak clipping of the positive peak of the CM (that occurs during organ of Corti displacements towards scala tympani), which was in keeping with ATP-induced displacement of the transducer towards scala tympani. CM waveform analysis quantified the degree of displacement and showed that the changes were consistent with the stimulus being centered on a different region of the transducer curve. The change of operating point meant that the stimulus was applied to a region of the transducer curve where there was greater saturation of the output on excursions towards scala tympani and less saturation towards scala vestibuli. A significant degree of recovery of the operating point was observed after washing with AP. Dose response curves generated by perfusing ATP-gamma-S (333 microM) in a cumulative manner yielded an EC(50) of 19.8 microM. The ATP antagonist PPADS (0.1 mM) failed to block the effect of ATP-gamma-S on operating point, suggesting the response was due to activation of metabotropic and not ionotropic ATP receptors. Multiple perfusions of AP had no significant effect (118 and 112 dB) or moved the operating point slightly (106 dB) in the direction opposite of ATP-gamma-S. Results are consistent with an ATP-gamma-S induced transducer change comparable to a static movement of the organ of Corti or reticular lamina towards scala tympani.

Contextual modulation of cochlear hearing desensitization depends on the type of loud sound trauma

In ears in which cochlear efferent pathways were cut and with testing done under anaesthetic conditions that preclude middle ear muscle activity (so as to examine the "intrinsic" effects of loud sound on the cochlea without any confounding effect of efferent pathways to the auditory periphery), atraumatic background white noise (WN) increases cochlear hearing loss (temporary threshold shifts, TTSs) induced by a traumatic pure tone but reduces TTSs caused by traumatic 5-kHz wide narrow band (NB) sound. The short-duration moderately intense traumata used in these studies most likely cause TTSs by affecting cochlear mechanics and these WN modulatory effects, exerted directly on the cochlea's intrinsic susceptibility to TTSs, are not predicted by any current description of cochlear mechanics. Here it is demonstrated that background WN reduces trauma-induced TTSs with even a relatively small increase in trauma bandwidth beyond that of a pure tone, discounting the alternative that contextual modulatory effects transition systematically along a continuum as trauma bandwidth increases from a pure tone to a broader bandwidth (albeit 2 kHz-wide NB) trauma. These results have implications for cochlear mechanics as the TTSs due to the traumatic sound of this study are most likely due to changes in cochlear mechanics but are not easily explained by what is currently known of cochlear mechanics.

Contextual modulation of olivocochlear pathway effects on loud sound-induced cochlear hearing desensitization

This study shows that the cochlear hearing losses [temporary threshold shifts (TTSs)] induced by traumatic sound and the effect of olivocochlear (OC) pathways to the cochlea on these hearing losses depend on the context of the sound. Background atraumatic white noise (WN) has been shown to 1) exacerbate loud-pure-tone-induced TTSs, and 2) promote the modulation of TTSs by the uncrossed OC (UOC) pathways additional to the action on TTSs, elicited by binaural loud tones themselves, by the crossed OC (COC) pathway. Here the same atraumatic WN reduced TTSs caused by loud narrow band sound. It also reduced TTS modulation by OC pathways. The UOC no longer exerted any effects on TTSs, and COC effects were significantly reduced in two discrete frequency bands: low frequencies within the narrow band ("within-band" frequencies) and high frequencies outside the band ("high-side" frequencies). COC effects were unchanged at high frequencies within the band. Despite these reductions in OC effects, because the WN itself reduced TTSs, the total effect of OC pathways and background WN now produced larger TTS reductions, especially at higher frequencies. Thus the modulatory effects of the OC pathways on TTSs depend on how background WN modulates cochlear state. It is postulated that the WN background and the OC pathways both modulate TTSs by acting on the outer hair cells, in a way that promotes the reduction of TTSs caused by the narrow band sound trauma. This joint promotion of a protective end-effect on TTSs to narrow band sound trauma contrasts against the effects seen with pure tone trauma where the same background WN exacerbated TTSs at high-side frequencies.

The influence of transducer operating point on distortion generation in the cochlea

Distortion generated by the cochlea can provide a valuable indicator of its functional state. In the present study, the dependence of distortion on the operating point of the cochlear transducer and its relevance to endolymph volume disturbances has been investigated. Calculations have suggested that as the operating point moves away from zero, second harmonic distortion would increase. Cochlear microphonic waveforms were analyzed to derive the cochlear transducer operating point and to quantify harmonic distortions. Changes in operating point and distortion were measured during endolymph manipulations that included 200-Hz tone exposures at 115-dB SPL, injections of artificial endolymph into scala media at 80, 200, or 400 nl/min, and treatment with furosemide given intravenously or locally into the cochlea. Results were compared with other functional changes that included action potential thresholds at 2.8 or 8 kHz, summating potential, endocochlear potential, and the 2 f1-f2 and f2-f1 acoustic emissions. The results demonstrated that volume disturbances caused changes in the operating point that resulted in predictable changes in distortion. Understanding the factors influencing operating point is important in the interpretation of distortion measurements and may lead to tests that can detect abnormal endolymph volume states.

Modifications of a single saturating non-linearity account for post-onset changes in 2f1-f2 distortion product otoacoustic emission

2f1-f2 distortion product otoacoustic emissions (DPOAEs) were recorded from guinea pigs. DPOAEs showed complex time dependence at the onset of stimulation. The DPOAE, measured during the first 500 ms, can either decrease or increase at the onset depending on both the frequencies and levels of the primary tones. These changes are closely associated with amplitude minima (notches) of the DPOAE I/O functions. These notches are characteristic of DPOAE growth functions measured from guinea pigs for primary tones of 50-60-dB sound-pressure level (SPL). Apparent changes in the DPOAE amplitude occur because the notch shifts to higher levels of the primaries during the onset of stimulation. This shift of the notch to higher levels increases for lower f2/f1 ratios but does not exceed about 2 dB. DPOAE amplitude increases for a constant level of the primaries if the onset emission is situated at the low-level, falling slope of the notch. If the onset DPOAE is located on the high-level, rising slope of the notch, then the upward shift of the notch causes the emission either to decrease monotonically, or to decrease initially and then increase. By establishing that the 2f1-f2 onset changes reflect a shift in the growth-function notch, it is possible to predict the temporal behavior of DPOAEs in the two-dimensional space of the amplitude of the primaries and for their different frequency ratios.

Interaction between adenosine triphosphate and mechanically induced modulation of electrically evoked otoacoustic emissions

It was shown previously that electrically evoked otoacoustic emissions (EEOAEs) can be amplitude modulated by low-frequency bias tones and enhanced by application of adenosine triphosphate (ATP) to scala media. These effects were attributed, respectively, to the mechano-electrical transduction (MET) channels and ATP-gated ion channels on outer hair cell (OHC) stereocilia, two conductance pathways that appear to be functionally independent and additive in their effects on ionic current through the OHC. In the experiments described here, the separate influences of ATP and MET channel bias on EEOAEs did not combine linearly. Modulated EEOAEs increased in amplitude, but lost modulation at the phase and frequency of the bias tone (except at very high sound levels) after application of ATP to scala media, even though spectral components at the modulation sideband frequencies were still present. Some sidebands underwent phase shifts after ATP. In EEOAEs modulated by tones at lower sound levels, substitution of the original phase values restored modulation to the waveform, which then resembled a linear summation of the separate effects of ATP and low-frequency bias. While the physiological meaning of this procedure is not clear, the result raises the possibility that a secondary effect of ATP on one or more nonlinear stages in the transduction process, which may have caused the phase shifts, obscured linear summation at lower sound levels. In addition, "acoustic enhancement" of the EEOAE may have introduced nonlinear interaction at higher levels of the bias tones.

Effects of 4-aminopyridine on electrically evoked cochlear emissions and mechano-transduction in guinea pig outer hair cells

Stimulation of the cochlea with alternating current produces sound in the ear canal. These electrically evoked oto-acoustic emissions (EEOAEs) are attributed to electro-motility of outer hair cells (OHCs). Earlier work suggested EEOAEs were sensitive to the open probability of OHC mechano-electrical transduction (MET) channels. They were attenuated by 4-aminopyridine (4-AP) and amplitude-modulated by low frequency sound, consistent with current gaining access to a motility source via the MET conductance. However, inconsistencies in the behaviour as well as physical considerations argued against this simple interpretation. In this study the behaviour of EEOAEs in the presence of 4-AP in scala media was examined along with OHC transfer functions derived from low frequency cochlear microphonic (CM) waveforms. Both the level and the modulation of the EEOAEs were reduced by 4-AP, but disproportionately more so than the 4-AP-induced loss of CM. In addition, the modulation as well as the level of the EEOAEs recovered more rapidly than the CM. Both these results indicated that 4-AP modified the process of EEOAE generation independently of its effect on the gross receptor current through the MET conductance. Changes in the derived OHC transfer functions, specifically shifts in the estimated operating bias of the MET channels, indicated the effects of 4-AP applied to the endolymphatic surface of OHCs were complex. It is suggested that both direct and indirect consequences of a 4-AP blockade may have contributed. 4-AP was ineffective when applied to scala tympani.

Unilateral hearing losses alter loud sound-induced temporary threshold shifts and efferent effects in the normal-hearing ear

In animals with bilaterally normal hearing, olivocochlear pathways can protect the cochlea from the temporary shifts in hearing sensitivity (temporary threshold shifts; TTSs) caused by short-duration intense loud sounds. The crossed olivocochlear pathway provides protection during binaural loud sound, and uncrossed pathways protect when monaural or binaural loud sounds occur in noise backgrounds. Here I demonstrate that when there is a chronic unilateral hearing loss, effects of loud sounds, and efferent effects on loud sound, in the normal-hearing ear differ markedly from normal. Three categories of test animals with unilateral hearing loss were tested for effects at the normal-hearing ear. In all categories a monaural loud tone to the normal-hearing ear produced lower-than-normal TTSs, apparently because of a tonic re-setting of that ear's susceptibility to loud sound. Second, in the two test categories in which the hearing-loss ear was only partly damaged, binaural loud sound exacerbated TTSs in the normal-hearing ear because it caused threshold shifts that were a combination of "pure" TTSs and uncrossed efferent suppression of cochlear sensitivity. (In normal cats, this binaural tone results in crossed olivocochlear protection that reduces TTS.) Binaural loud sound did not produce such uncrossed efferent effects in the test category in which the nontest ear had suffered total hearing loss, suggesting that this uncrossed efferent effect required binaural input to the CNS. It is noteworthy that, in the absence of this uncrossed efferent suppression, the pure loud sound-alone induced TTSs after binaural exposure were low. Thus in the absence of any efferent effect, the normal-hearing cochlea had a reduced susceptibility to loud tone-induced damage. Finally, the results suggest that, with respect to cochlear actions at high sound levels, uncrossed and crossed efferent pathways may exert different effects at the one type of receptor cell.

Centrifugal pathways protect hearing sensitivity at the cochlea in noisy environments that exacerbate the damage induced by loud sound

Loud sounds damage the cochlea, the auditory receptor organ, reducing hearing sensitivity. Previous studies demonstrate that the centrifugal olivocochlear pathways can moderately reduce these temporary threshold shifts (TTSs), protecting the cochlea. This effect involves only the olivocochlear pathway component known as the crossed medial olivocochlear system pathway, originating from the contralateral brainstem and terminating on outer hair cells in the cochlea. Here I demonstrate that even moderate noise backgrounds can significantly exacerbate the cochlear TTSs induced by loud tones, but this is prevented because in such conditions there is additional activation of uncrossed olivocochlear pathways, enhancing protection of cochlear hearing sensitivity. Activation of the uncrossed pathways differs from that of the crossed pathway in that it is achieved only in noise backgrounds but can then be obtained under monaural conditions of loud tone and background noise. In contrast, activation of the crossed pathway is achieved only by binaural loud tones and is not further enhanced by background noise. Thus, conjoint activation of both crossed and uncrossed efferent pathways can occur in noise backgrounds to powerfully protect the cochlea under conditions similar to those encountered naturally by humans.

Cochlear microphonics and otoacoustic emissions in chronically de-efferented chinchilla

The effects of eliminating the olivocochlear bundle (OCB) on cochlear electromechanical properties were examined by measuring cochlear microphonics (CM) and distortion product otoacoustic emissions (DPOAEs) in chronically de-efferented chinchillas. The OCB fibers to the right ears were successfully sectioned in six out of 15 adult chinchillas via a posterior paraflocular fossa approach. At the end of the experiment, these ears were histologically verified as being deprived of both lateral and medial OCB fibers. The opposite (left) ears from the animals served as controls. Following de-efferentation, changes of the inter-modulation distortion components (2f(1)-f(2), f(2)-f(1), 3f(1)-2f(2), 3f(2)-2f(1)) varied, depending on the frequencies and levels of the stimuli. DPOAE amplitudes to low-level stimuli were within the 95% confidence intervals around mean DPOAE amplitudes of the control ears at all the frequencies (1-8 kHz). At high stimulus levels, DPOAE amplitudes increased by 5-20 dB at 1 and 2 kHz while remaining in the normal range at 4 and 8 kHz. In contrast, the CM input/output functions to stimuli from 1 to 8 kHz were significantly reduced by approximately 40-50% at all input levels. The results suggest that the OCB may play a role in modulating electrical properties of the outer hair cells and in reducing the magnitude of cochlear distortion to high-level stimuli.

Monitoring noise susceptibility: sensitivity of otoacoustic emissions and subjective audiometry

The capacity of different audiological methods to detect a high noise susceptibility was examined in 20 normally hearing and 26 especially noise-susceptible subjects. The latter were selected from 422 soldiers in field studies: they had shown a temporary threshold shift (TTS) in pure tone audiometry (PTA) after regular training with firearms. In laboratory experiments, the TTS-positive soldiers were re-examined using greatly reduced sound intensities, which caused no TTS in a control subject group. Before and after acoustic stimulation, different subjective (PTA, high frequency audiometry (HFA), upper limit of hearing (ULH)) and objective (transiently evoked otoacoustic emissions (TEOAE), distortion products (DPOAE)) audiological tests were performed. After exposure to low impact noise in the laboratory, in both PTA and HFA, a TTS was observed in 11.5% (N = 3) of the noise-susceptible group (compared to 0% in the control group). In the TTS-positive group, deterioration of the ULH occurred in 28% (N = 7) (compared to 15% (N = 3) in the control group). An ULH improvement occurred in only one subject (3.8%) (compared to 25% (N = 5) in the control group). Significant alterations of click-evoked OAE-amplitudes were found in 26.9% (N = 7) of the selected groups, whereas stable emissions were observed in all but one subject (5%) of the control group. However, DPOAE alterations were seen in 19.2% (N = 5) of the TTS-positive soldiers but also in 25% (N = 5) of the control group. These results suggest that TEOAE provides a more sensitive and more objective method of detecting a subtle noise-induced disturbance of cochlear function than do PTA or DPOAE.

Boltzmann analysis of CM waveforms using virtual instrument software

We describe a modification to our technique for the rapid analysis of low-frequency cochlear microphonic (CM) waveforms in the basal turn of the guinea pig cochlea (Patuzzi and Moleirinho, 1998). The transfer curve relating instantaneous sound pressure in the ear canal to instantaneous receptor current through the outer hair cells (OHCs) is determined from the distorted microphonic waveform generated in the extracellular fluid near the hair cells, assuming a first-order Boltzmann activation curve. Previously, the analysis was done in real time using custom-built electronic circuitry. Here, the same task is performed numerically using virtual instrument software (National Instruments LabVIEW 4.1) running on a personal computer. The assumed theoretical function describing the CM waveform is Vcm = Voff + Vsat/[1 + exp[(Eo+Z.Po.sin(2pi f + phi(tot)))/kT]], where the six parameters are (i) a DC offset voltage (Voff); (ii) the frequency of the sinusoidal stimulus (f); (iii) the phase of the sinusoidal stimulus (phi(tot)); (iv) the maximal amplitude of the distorted microphonic signal (Vsat); (v) the sensitivity of the transduction process (Z); and (vi) the operating point on the sigmoidal transfer curve (Eo). The software obtains the least-squares fit to the CM waveforms by continuously deriving the six parameters at a speed of about one determination per second. The independent fitting of the frequency and phase allows the data to be analysed off-line from data previously recorded to tape (i.e. the frequency and phase of the microphonic response need not be known accurately beforehand). We present here an outline of the software we have used, and give an example of the changes which can be monitored using the technique (transient asphyxia). The method's advantages and limitations have been discussed in our previous paper. The virtual instrument described here is available from the authors on request.

The Goldman-Hodgkin-Katz equation and graphical 'load-line' analysis of ionic flow through outer hair cells

While the 'membrane potential' of a cell which has a homogeneous membrane and surrounding environment, and which is not pumping ions electrogenically (passing no net current through its membranes), can be estimated from the Goldman voltage equation, this equation is inappropriate for other cells. In the mammalian cochlea such problematic cells include the cells of stria vascularis and the sensory hair cells of the organ of Corti. Not only is the Goldman voltage equation inappropriate, but in asymmetric cells the concept of a single 'membrane potential' is misleading: a different transmembrane voltage is required to define the electrical state of each section of the cell's heterogeneous membrane. This paper presents a graphical 'load-line analysis' of currents through one such asymmetric cell, the outer hair cells of the organ of Corti. The approach is extremely useful in discussing the effects of various cochlear manipulations on the electrical potential within hair cells, even without a detailed knowledge of their membrane conductance. The paper discusses how modified Goldman-Hodgkin-Katz equations can be used to describe stretch-activated channels, voltage-controlled channels, ligand-mediated channels, and how the combination of these channels and the extracellular ionic concentrations should affect the hair cell's resting intracellular potential and resting transcellular current, its receptor current and receptor potential, and the extracellular microphonic potential around these cells. Two other issues discussed are the role of voltage-controlled channels in genetically determining membrane potential, and the insensitivity of hair cells to changes of extracellular potassium concentration under some conditions.

A four-state kinetic model of the temporary threshold shift after loud sound based on inactivation of hair cell transduction channels

A model of the temporary threshold shift (TTS) following loud sound is presented based on inactivation of the mechano-electrical transduction (MET) channels at the apex of the outer hair cells (OHCs). This inactivation is assumed to reduce temporarily the OHC receptor current with a consequent drop in the mechanical sensitivity of the organ of Corti. With acoustic over-stimulation some of the hair cells' MET channels are assumed to adopt one of three closed and non-transducing conformations or 'TTS states'. The sound-induced inactivation is assumed to occur because the sound makes the TTS states more energetically favourable when compared with the transducing states, and the distribution between these states is assumed to depend on the relative energies of the states and the time allowed for migration between them. By lumping the fast transducing states (one open and two closed) into a single transducing 'pseudo-state', the kinetics of the inactivation and re-activation processes (corresponding to the onset and recovery of TTS) can be described by a four-state kinetic model. The model allows an elegant description of the onset and recovery of TTS time-course in a human subject under a variety of continuous exposure conditions, and some features of intermittent exposure as well. The model also suggests that recovery of TTS may be accelerated by an intermittent tone during the recovery period which may explain some variability TTS in the literature. Other implications of the model are also discussed.

Automatic monitoring of mechano-electrical transduction in the guinea pig cochlea

We have estimated the transfer curve relating instantaneous sound pressure in the ear canal to instantaneous receptor current through the outer hair cells (OHCs) in the basal turn of the guinea pig cochlea using the cochlear microphonic (CM) elicited by continuous 200 Hz tones. The transfer curve is well approximated by a Boltzmann activation curve which has been automatically analysed using a custom-built electronic circuit which continuously derives the three parameters defining the curve with a time resolution of seconds. This technique offers a convenient method of monitoring changes in OHC mechano-electrical transduction due to cochlear disturbances, and allows the investigation of cochlear homeostasis over the course of hours. We present here details of the technique, evidence that the recordings are minimally contaminated by neural responses, and normative data on the changes in the parameters with sound level. As the level of the 200 Hz tone increases, the equivalent operating point on the transfer curve migrates in a way consistent with a movement of the organ of Corti towards scala tympani or a contraction of the outer hair cells. Surprisingly, the effective slope of the curve which represents the mechanical sensitivity of the transduction process decreases over an 8 to 1 range as the level of the 200 Hz tone is increased. The effect of this variation is that the amplitude of the equivalent mechanical displacement input to the mechano-electrical transduction process appears to increase by a mere 2 to 1 while the sound level increases by a factor of 20 to 1. These changes are not neurally mediated, since they also occur in the presence of tetrodotoxin and the blocker of afferent neurotransmission, kainate.

Enhancement of electrically evoked oto-acoustic emissions associated with low-frequency stimulus bias of the basilar membrane towards scala vestibuli

Electrically evoked oto-acoustic emissions (EEOAEs) are sounds present in the ear canal when ac current is passed into the cochlea. EEOAEs are attributed to the activation of fast electromotile responses in outer hair cells (OHCs). An interesting property of EEOAEs is the phenomenon of "acoustic enhancement," where the emission amplitude is increased by moderate-level sound [D. C. Mountain and A. E. Hubbard, Hear. Res. 42, 195-202 (1989)]. In this report a form of enhancement is described which occurs with displacements of the basilar membrane toward scala vestibuli, during amplitude modulation of the EEOAE waveform by low-frequency tones. This "SV-bias enhancement" possibly consists of two components: (i) a low-level component induced by sound at levels which produce nonlinear growth of the cochlear microphonic and which may be equivalent to the "acoustic enhancement" described previously, and (ii) a high-level component which occurs at sound levels well above those which cause saturation of the cochlear microphonic. The low-level component could be explained by either an increased access of the extrinsically applied current to a membrane-based source of OHC motility, perhaps coupled with a reduction in negative feedback, or an increase in electromotile output during scala vestibuli displacements, but the origin of the high-level component is obscure.

Microphonic and DPOAE measurements suggest a micromechanical mechanism for the 'bounce' phenomenon following low-frequency tones

Neural auditory thresholds in the guinea pig can be temporarily improved by up to 6 dB about 2 min after the cessation of an moderately intense low-frequency tone (Kirk and Patuzzi, 1997). We have measured changes in the f2-f1 distortion product otoacoustic emission (DPOAE) and low-frequency microphonic potential in scala tympani before, during and after a low-frequency tone (200 Hz) to determine the cause of this so-called bounce phenomenon. In particular we have analysed the low-frequency microphonic waveform in detail to estimate changes in the maximal receptor current through the outer hair cells (OHCs), the sensitivity of the OHC forward transduction process and the change in OHC operating point on the mechano-electrical transduction transfer curve. Our results indicate that a 200 Hz tone changes the maximal current and sensitivity of the OHCs minimally, but more importantly, it transiently changes the operating point on the OHC transfer curve. In particular, the operating point changes are consistent with a movement of the OHC stereocilia away from the OHC basal body at the peak of the bounce. These changes detected using the microphonic potential are associated with changes in the level of the f2-f1 DPOAE that correlate well with the electrical measurements. We suggest that the shift in operating point is largely responsible for the increase in cochlear sensitivity, and is due to a disruption of the salt balance within the cochlea during the intense low-frequency tone.

Transient changes in cochlear potentials and DPOAEs after low-frequency tones: the 'two-minute bounce' revisited

After exposure to a loud, non-traumatic low-frequency tone, auditory thresholds are elevated. Thresholds recover to normal in a non-monotonic manner, decreasing rapidly at first before increasing again, until they finally decrease monotonically towards normal. Although the transient elevation of thresholds after the initial improvement was originally called a 'bounce' by Hirsh and Ward (1952), Kemp (1986) suggests that the initial rapid recovery is the oddity: under some conditions a low-frequency tone can produce hypersensitivity in otoacoustic emissions, psychophysical thresholds, and perceived loudness (Kemp's 'bounce') without a later elevation of threshold (Hirsh and Ward's 'bounce'). Kemp also suggested that the transient hypersensitivity was caused by changes in the sensitivity of the active process within the cochlea. We have investigated the origin of this transient hypersensitivity (Kemp's bounce) in guinea pigs, recording cochlear potentials (CM, CAP, SP and EP) and otoacoustic emissions (DPOAEs at f2-f1, 2f1-f2, 2f2-2f1 and 3f1-2f2). Our results indicate that the bounce does not require neural activity, but is probably produced by non-neural cochlear mechanisms, possibly a transient decrease in the permeability of the organ of Corti which produces a small but significant change in standing current through outer hair cells. At least part of these changes, which are reduced as the stimulation frequency increases, and absent above 2 kHz, seem due to a small and transient movement of the cochlear partition towards scala tympani, probably due to a transient osmotic imbalance.

The influence of the cochlear efferent system on chronic acoustic trauma

The role of the olivocochlear bundle (OCB) in modulating noise-induced permanent injury to the auditory periphery was studied by completely sectioning the OCB fibers in chinchillas and exposing the animals while awake to a broad-band noise at 105 dB SPL for 6 h. Outer hair cell (OHC) function was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAE) at frequencies from 1.2 to 9.6 kHz and cochlear microphonics (CM) at frequencies from 1 to 8 kHz. As a result of de-efferentation, the CM was decreased but the DPOAEs were unchanged in de-efferented ears as compared with efferented control and sham-operated ears. Following noise exposure, the ears that were de-efferented showed significantly more depression of DPOAE input/output functions and greater decrement of CM amplitude. The differences between de-efferented and efferent-innervated ears were evident across all the frequencies. The cochlear lesions of the OHCs reflected by traditional cytocochleograms, however, were minimal in both efferented and de-efferented ears. The results indicate that cochlear de-efferentation decreases the CM in chinchilla and increases the ear's susceptibility to noise-induced permanent hearing damage. More importantly, de-efferentation increases susceptibility at low frequencies as well as high frequencies.

A theoretical basis for the high-frequency performance of the outer hair cell's receptor potential

The frequency response of the outer hair cell (OHC) was studied theoretically. An electrical model of the OHC was analyzed mathematically, taking into account the effect of its inherent voltage-dependent capacitance. It was found that the variations of the capacitance dependent on the membrane potential could enhance the high-frequency response of the OHC, so that its cutoff frequency could be extended into the audio range. It was found further that the enhancement of the frequency response of the OHC was strongly dependent on its resting potential and on the ratio of the maximum voltage-dependent capacitance to the membrane linear capacitance.

The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system

In the unanesthetized mustached bat, stimulation of the ear with an acoustic transient produces damped oscillations which are evident in the cochlear microphonic potential. In this report we demonstrate how the decay time of these oscillations is affected by broadband noise presented to the contralateral ear (CLN). In the absence of CLN, the mean decay time was 1.94 +/- 0.23 ms, but during the presentation of CLN the decay time consistently decreased. The changes were finely graded, the higher the CLN, the greater the change. The effect could be maintained at a constant level for extended periods of time and this was evident when the CLN exceeded 40 dB SPL. The latency of the reflex for 64 dB noise was about 11 ms and near maximum changes occurred within 15 ms of CLN onset. Sectioning medial efferent nerve fibers in the floor of the fourth ventricle or the administration of a single dose of gentamicin eliminated changes produced by CLN. The prominence of CM responses to damped oscillations and the robust changes in response to CLN make the mustached bat an excellent model for studying the influence of the medial efferent system on cochlear mechanics.

Acute and chronic effects of unilateral elimination of auditory nerve activity on susceptibility to temporary deafness induced by loud sound in the guinea pig

The involvement of crossed cochlear pathways in modulating the deafening effects of loud sound was investigated in the anaesthetized guinea pig. Auditory nerve activity was blocked unilaterally, either by surgical cochlear destruction or intracochlear perfusion of lignocaine, and the effect of a standard loud sound exposure in the untreated ear was then assessed using the compound action potential (CAP) audiogram technique. It was found that both cochlear destruction or lignocaine perfusion reduced the amount of threshold elevation in the untreated ear. The effect of lignocaine perfusion was significantly greater than acute cochlear destruction. In animals allowed to survive for 24 h and one week post-cochlear destruction before loud sound exposure, the protective effect was still present and was significantly greater than immediately post-destruction. This long-term protective effect of contralateral cochlear destruction was blocked by administering strychnine prior to the loud sound exposure. The results of lignocaine perfusion and chronic destruction make it unlikely that protection immediately post-destruction is the result of a transient barrage of primary afferent activity. We conclude that elimination of auditory nerve input can alter the effectiveness of brainstem circuitry responsible for protection (possibly the olivocochlear system). Since acoustic stimulation of the contralateral ear also has acute protective effects thought to be mediated by olivocochlear efferents, the circuitry responsible for protection appears to be subject to a complex balance between excitatory and inhibitory influences.

Effects of intra-cochlear perfusion of salicylates on cochlear microphonic and other auditory responses in the guinea pig

The ototoxic action of salicylate was investigated in the guinea pig by perfusion of both salicylate and bromosalicylate through scala tympani. The results qualitatively confirmed experiments using intravenous administration in cats (Stypulkowski, 1990), showing dose-dependent elevations in compound action potential (CAP) thresholds, increases in cochlear microphonics (CM) and level-dependent reductions in 2f1-f2 acoustic distortion products. The endocochlear potential was not significantly affected and iontophoretic injection of salicylate into scala media had no measurable effect on CAP thresholds, consistent with an action on the basolateral walls of the hair cells. Perfusion with indomethacin produced effects similar to those of the salicylates, but at non-physiological doses. Together with the great effectiveness of 5-bromosalicylate, this suggests that salicylate does not act by inhibiting prostaglandin synthesis. The results are qualitatively consistent with the proposition that salicylates act on the basolateral walls of the outer hair cells. However, the magnitude of the CM increases, particularly at high drug concentrations, and the fact that salicylate reduced, but did not eliminate the effects of olivocochlear efferent stimulation on CM amplitude indicate that a simple explanation for salicylate effects based solely on a conductance increase in the outer hair cell membranes may be inadequate.

Additivity of threshold elevations produced by disruption of outer hair cell function

We present a simple model describing the additivity of hearing loss in the mammalian cochlea produced by disruption of the outer hair cell transduction processes. The validity of this model has been tested experimentally in the guinea-pig by inducing threshold elevations using two simultaneous cochlear manipulations, including acoustic overstimulation, two-tone suppression, low-frequency acoustic biasing of the cochlear partition and electrical stimulation of the medial olivo-cochlear system of efferent fibres. The results of these experiments suggest that the model presented is an adequate description, within the measurement error of our experiments, of the hearing losses produced.

Effects of acute negative middle ear pressure on hearing. New answers to old questions and a review of the literature

The magnitude of hearing loss caused by negative middle ear pressure is a controversial issue. We present a clinical, prospective and controlled study on the effect of negative middle ear pressure on hearing in human ears. Sixty-two ears of patients undergoing uvulopalatopharyngoplasty were examined pre- and postoperatively. The test group consisted of 31 ears in which negative postoperative middle ear pressure ranging between -150 and -400 mmH2O had developed. The control group consisted of 31 ears in which no middle ear pressure change was recorded after the same operation. Bone conduction hearing loss up to 20 dB (with a mean of 11 dB) at 250 Hz, up to 25 dB (with a mean of 13.1 dB) at 500 Hz and up to 10 dB (with a mean of 7.6) at 1,000 Hz was the dominant finding. An additional air-bone gap up to 15 dB (with a mean of 4.3 dB) was found only at 250 Hz. Basic theories about the acoustic response of the ear are discussed and new theories proposed.

Cochlear efferent neurones and protection against acoustic trauma: protection of outer hair cell receptor current and interanimal variability

We have measured the changes in neural and microphonic sensitivity in the basal turn of the guinea-pig cochlea produced by intense acoustic overstimulation (10 kHz, 115 dB SPL for 60 s and 150 s). As reported previously, the drop in neural and microphonic sensitivities observed after overstimulation were highly correlated [Patuzzi et al. (1989) Hear. Res. 39, 189-202]. Presentation of a non-traumatizing pure-tone to the contralateral ear (10 kHz, 80 dB SPL) during acoustic overstimulation reduced the amount of acoustic trauma measured using the neural response or the microphonic response. Transection of the medial olivo-cochlear system of efferent fibres at the floor of the fourth ventricle abolished this protective effect of contralateral sound and dramatically reduced the variability in the data. Since the low-frequency microphonic is a simple measure of the receptor current through the outer hair cells, and this current probably plays a part in enhancing the mechanical sensitivity of the cochlea, the protection of the microphonic we have observed suggests that the efferent system protects neural sensitivity by protecting the mechano-electrical transduction of outer hair cells. The drop in variability after sectioning the efferents also suggests that inter-animal variations in susceptibility to noise trauma may be a consequence of differing tonic activity of the efferents, and/or a variation in the sensitivity of the efferent pathway.