Outer hair cell electromotility and otoacoustic emissions

Chick hair cells do not exhibit voltage-dependent somatic motility

It is generally believed that mechanical amplification by cochlear hair cells is necessary to enhance the sensitivity and frequency selectivity of hearing. In the mammalian ear, the basis of cochlear amplification is believed to be the voltage-dependent electromotility of outer hair cells (OHCs). The avian basilar papilla contains tall and short hair cells, with the former being comparable to inner hair cells, and the latter comparable to OHCs, based on their innervation patterns. In this study, we sought evidence for somatic electromotility by direct measurements of voltage-dependent length changes in both tall and short hair cells at nanometre resolution. Microchamber and whole-cell voltage-clamp techniques were used. Motility was measured with a photodiode-based measurement system. Non-linear capacitance, an electrical signature of somatic motility, was also measured to complement motility measurement. Significantly, chick hair cells did not exhibit somatic motility nor express non-linear capacitance. The lack of somatic motility suggests that in avian hair cells the active process resides elsewhere, most likely in the hair cell stereocilia.

Physiopathological significance of distortion-product otoacoustic emissions at 2f1-f2 produced by high- versus low-level stimuli

Distortion product otoacoustic emissions emitted by the cochlea at 2f1-f2 in response to pairs of pure tones at f1 and f2 (DPOAE) form a class of otoacoustic emissions and as such, are viewed as a reliable tool for screening outer hair cell (OHC) dysfunctions on a pass/fail basis. However, the persistence of residual DPOAEs from impaired cochleae at high stimulus levels has suggested that above 60-70 dB SPL, instead of reflecting "active" cochlear motion, DPOAEs might represent another "passive" modality: they would thus become unsuitable for analyzing cochlear function. The present work reports the consequences on high- vs low-level DPOAEs of three types of cochlear impairments involving OHCs: progressive OHC degeneration of genetic origin in CD1 mice, complete cochlear ischemia in gerbils, and furosemide injection vs ischemia-reperfusion in gerbils. An alternative to the "active-passive" model was used wherein regardless of stimulus level, cubic DPOAEs are produced by N (probably OHC-borne) nonlinear elements driven by input I and modulated by a function F3 of their operating point o; thus, DPOAE proportional to NI3F3(o). When OHCs degenerated, thereby implying a decrease of N, DPOAE levels also decreased regardless of the stimulus level up to 80 dB SPL, in line with the previous formula but at variance with the prediction of the active-passive concept. Instead of affecting N, the other two experiments impaired the efficiency of the cochlear feedback loop as a result of its electrical drive being decreased by strial dysfunction. As it is well accepted that the impaired basilar-membrane motion, although greatly reduced at low levels, tends to catch up with a normal one at higher levels, it was assumed the same was true with I so that DPOAE levels had to be, and indeed were little affected at high levels while plummeting at low levels, without any need for invoking two modalities for DPOAE generation. Finally, comparisons of furosemide vs ischemia effects revealed additional influences on DPOAEs, possibly accounted for by function F3(o). These results lead to the proposal that although high-level DPOAEs are expected to be poor audiometric indicators, they seem well adapted to assessing the functional integrity of nonlinear elements in OHCs, i.e., presumably their mechanoelectrical transduction channels.

Use of ABR threshold and OAEs in detection of noise induced hearing loss

OBJECTIVE

To determine which measure is the most sensitive to noise induced hearing loss (NIHL): auditory nerve brainstem response (ABR), distortion product otoacoustic emission (DPOAE) or transient evoked otoacoustic emission (TEOAE), and how to assess possible changes in these responses.

SUBJECTS & METHODS

Four groups of rats were exposed to various durations of 113 dB SPL broadband noise: 5 or 10 minutes (temporary changes in cochlear function), and 3 or 4 hours (permanent changes). Group means and data from individual animals were compared before and after exposure.

RESULTS

Mean group DPOAE amplitude reduction showed no clear advantage over mean ABR threshold elevation in detection of temporary and permanent NIHL. Data from individual rats, however, indicated a clinical advantage for DPOAEs in detecting slight temporary, but not permanent, changes. TEOAEs were more sensitive in detecting changes in individual rats than as a group measure.

CONCLUSIONS

TEOAE and DPOAE monitoring may improve detection of NIHL, though it should be used in conjunction with audiometric threshold monitoring.

Effect of click intensity on click-evoked otoacoustic emission waveforms: implications for the origin of emissions

The rather shallow growth of click-evoked otoacoustic emissions (CEOAE) with click intensity, namely <1 dB/dB, distinguishes genuine CEOAEs from stimulus artifacts, thereby providing the rationale for the popular 'derived nonlinear recording' method. However, other CEOAE nonlinearities regarding phase or envelope dependence on stimulus intensity have been barely acknowledged so far. The present work used CEOAEs from 20 normal ears recorded in response to 50-86 dB peak equivalent SPL clicks. The phases of CEOAE spectral components varied considerably with click intensity (sometimes more than 120 degrees ), mostly in a monotonic manner and in such a way that in the majority of ears, phase lagged with increasing intensity. When present, synchronized spontaneous otoacoustic emissions exhibited the same behavior. In a few instances, conspicuous frequency shifts of CEOAE spectral peaks were seen. In contrast to CEOAE phases, envelopes were almost intensity-invariant. This behavior contrasts with that of basilar membrane motion at the place tuned to the stimulus frequency, as consistently disclosed by several recent publications, i.e., no phase shift and large envelope shift with stimulus intensity. It is thought that the phase invariance of basilar membrane motion implies that whatever they do, outer hair cells cannot alter the resonance frequency of the cochlear partition. If one elaborates along this line of reasoning, the large phase shift of CEOAEs with click intensity implies that CEOAEs at frequency f cannot come from the place tuned to f and that instead, they may be intermodulation distortion products produced by nonlinear interactions between spectral components of the click stimulus over a significant length of the basilar membrane.

DPOAE level shifts and ABR threshold shifts compared to detailed analysis of histopathological damage from noise

A detailed comparison of 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) level shifts (LS) and auditory brainstem response (ABR) threshold shifts with noise-induced histopathology was conducted in chinchillas. DPOAE levels (i.e., L(1) and L(2)) at f(1) and f(2), respectively, ranged from 55-75 dB sound pressure level (SPL), with f(2)/f(1)=1.23, 6 points/octave, f(2)=0.41-20 kHz, and ABR thresholds at 0.5-20 kHz, 2 points/octave, were determined pre-exposure. The exposure was a 108 dB SPL octave band of noise centered at 4 kHz (1-1.75 h, n=6) or 80-86 dB SPL (24 h, n=5). DPOAE LSs (magnitude pre- minus post-exposure) and ABR threshold shifts (TS) were determined at 0 days and up to 28 days post-exposure. The cochleae were fixed, embedded in plastic and dissected into flat preparations. The length of the organ of Corti (OC) was measured; missing inner (IHC) and outer (OHC) hair cells counted; stereocilia damage rated; and regions of OC and nerve-fiber loss determined. Cytocochleograms were made showing functional loss and structural damage with the LS and TS overlaid. Some unexpected results were obtained. First, the best correlation of LS with histopathology required plotting the DPOAE data at f(1) with respect to the chinchilla-place map. The best correlation of TS was with IHC and nerve-fiber loss. Second, wide regions of up to 10% scattered OHC loss in the apical half of the OC showed little or no LS. Third, with the 108 dB SPL noise, there was 20-40 dB of recovery for DPOAEs at mid-high frequencies (3-10 kHz) in eight of 12 cochleae where there was 70-100% OHC loss in the basal half of the OC. The largest recovery at mid-high frequencies occurred in regions where the OC was entirely missing. Fourth, with the 80-86 dB SPL noise, there was no LS at small focal lesions (100% loss of OHCs over 0.4 mm) when the frequency place of either f(1) or f(2) was within the lesion but not both. There was no correlation of LS with OHC stereocilia loss, fusion or disarray. These results suggest that, after noise exposure, DPOAEs at mid-high frequencies can originate from or be augmented by generators located at someplace other than the frequency place of f(2), possibly the basal 20% of the OC when this region is intact. Also, noise-induced DPOAE LSs seemed to reflect differing mechanisms for temporary and permanent hearing loss.

Medial olivocochlear bundle activation and perceived auditory intensity in humans

In order to test the hypothesis of a role of cochlear efferent activity in intensity perception in humans, loudness functions, loudness integration, and loudness summation were measured in the absence and in the presence of contralateral noise in normal-hearing subjects. Additionally, relationships with the effect of the noise on evoked otoacoustic emissions (EOAEs) were tested, and comparisons with vestibular neurotomy patients were performed. Overall, the results failed to demonstrate significant effects of contralateral noise stimulation on loudness functions and loudness integration. Furthermore, no significant differences were found in vestibular neurotomy patients. A significant effect of contralateral noise on loudness summation was noted, but was not related to the effect on otoacoustic emissions. The present results fail to support the notion that efferent influences onto the cochlear compression have a significant perceptual effect.

Effect of acoustic tumor extension into the internal auditory canal on distortion-product otoacoustic emissions

We studied the effects on distortion-product otoacoustic emissions (DPOAEs) of internal auditory canal (IAC) extension of acoustic neuromas (ANs) with the hypothesis that cochlear patterns of DPOAEs would be more commonly observed when the IAC was completely filled with tumor because of direct tumor involvement of either the inner ear or its blood supply. In a retrospective analysis of 86 patients with surgically proven ANs, DPOAEs were classified as having cochlear or noncochlear patterns on the basis of comparisons with the behavioral pure tone thresholds. The results of behavioral audiometry and DPOAEs were compared with the extension of the tumor into the IAC, which was categorized as full or partial. Of the 86 patients, 58 had tumors with full IAC extension, and 28 had tumors with partial IAC involvement. Cochlear patterns of DPOAEs were found in 55.2% of the tumors in the full IAC group and in 71.4% of those in the partial IAC group (not statistically different). It was concluded that the extent of IAC involvement by ANs was not significantly related to the negative effects of the tumor on cochlear function as represented by DPOAEs.

Ototoxic effects of cisplatin in a Sprague-Dawley rat animal model as revealed by ABR and transiently evoked otoacoustic emission measurements

The ototoxic effects of cisplatin in a Sprague-Dawley rat model were evaluated by recordings of auditory brainstem responses (ABR) and transiently evoked otoacoustic emissions (TEOAEs). The ABR responses were evoked from alternating clicks and 8, 10, 12, 16, 20 and 30 kHz tone pips in a range from 40 to 100 dB SPL range. The TEOAEs were recorded with a non-linear protocol, and were evoked by a 63.5 dB SPL click stimulus. Twenty five male Sprague-Dawley rats were used in the study, 20 animals were treated with cisplatin (16 mg/kg, body weight) and five animals served as controls. The data showed that 72 h after the cisplatin administration, the TEOAE and ABR variables were significantly altered. The relationship between the ABR and TEOAE variables was shown to be non-linear. The most significant relationships were observed between the TEOAE correlation and the ABR threshold values at 10, 12, and 16 kHz.

Audiologic assessment of children with suspected hearing loss

In assessing children with suspected hearing loss, audiologists are faced with the challenge of attempting to describe auditory function in a population that presents with a wide range developmental levels and abilities. Fortunately, there are a number of behavioral and physiologic tools available for the evaluation of auditory sensitivity and system integrity in children of all ages and developmental abilities. As powerful as many of these tools may be, however, it is important that the diagnosis of hearing loss be based on the outcome of independent measures of auditory function, which serve to cross-check and complement one another. Failure to do so is likely to result in inaccurate or incomplete delineation of auditory function.

Evaluation of anesthesia effects in a rat animal model using otoacoustic emission protocols

Anesthesia effects on otoacoustic emission (OAE) recordings were evaluated in a group of 72 Sprague-Dawley rats (mean weight 225+/-20 gr). Two anesthesia dosages (high and normal) and two anesthetic protocols (ketamine-xylazine, ketamine-xylazine-atropine) were tested. Transient evoked OAE (TEOAE) and distortion product OAE (DPOAE) responses were recorded in 10 min intervals, for a total period of 60 min. Analyses of the data with repeated measure models indicated the following: (1) The animals receiving a high dose of anesthesia (cumulative dose 66.6 mg of ketamine and 13.2 mg of xylazine/kg of body weight) presented significant alterations of the TEOAE response level and the signal to noise ratio at 3.0 kHz; (2) the animals receiving a normal dose of ketamine-xylazine anesthesia (cumulative dose 50 mg of ketamine and 10 mg of xylazine/kg of body weight) presented TEOAE and DPOAE responses invariant in terms of time; (3) significant differences were observed in the DPOAE responses from animals anesthetized with ketamine-xylazine and ketamine-xylazine-atropine. The data support the hypothesis that the ketamine anesthesia OAE suppressing mechanism is related to middle-ear mechanics.

Excess plasma membrane and effects of ionic amphipaths on mechanics of outer hair cell lateral wall

The interaction between the outer hair cell (OHC) lateral wall plasma membrane and the underlying cortical lattice was examined by a morphometric analysis of cell images during cell deformation. Vesiculation of the plasma membrane was produced by micropipette aspiration in control cells and cells exposed to ionic amphipaths that alter membrane mechanics. An increase of total cell and vesicle surface area suggests that the plasma membrane possesses a membrane reservoir. Chlorpromazine (CPZ) decreased the pressure required for vesiculation, whereas salicylate (Sal) had no effect. The time required for vesiculation was decreased by CPZ, indicating that CPZ decreases the energy barrier required for vesiculation. An increase in total volume is observed during micropipette aspiration. A deformation-induced increase in hydraulic conductivity is also seen in response to micropipette-applied fluid jet deformation of the lateral wall. Application of CPZ and/or Sal decreased this strain-induced hydraulic conductivity. The impact of ionic amphipaths on OHC plasma membrane and lateral wall mechanics may contribute to their effects on OHC electromotility and hearing.

Magnitudes and phases of human distortion-product otoacoustic emissions at 2f(1)-f(2) against f(2)/f(1): effects of an audiometric notch

The influence of the frequency ratio f(2)/f(1) of two pure-tone stimuli on the distortion-product otoacoustic emissions (DPOAEs) at 2f(1)-f(2) was assessed in 14 hearing-impaired human ears exhibiting a narrow audiometric notch at 4 kHz, whereas 11 normally hearing ears served as controls. A fixed-f(2) paradigm was used, with f(2) values varying from 2 to 8 kHz in 1-kHz steps. The intensities of the two stimuli were either 60 or 70 dB SPL. The magnitudes and phases of DPOAEs were extracted so that the bandpass filter-like profile of DPOAE magnitude against f(2)/f(1) (f(2)/f(1) magnitude function) could be characterized by the presence and position of its maximum, and DPOAE group delays were derived from the phase gradient of the DPOAE when f(1) varied. The main difference between normal and impaired ears occurred at 4 kHz in that, in contrast with normal ears, nine impaired ears out of 14 did not present any peak in their f(2)/f(1) magnitude function, while the remaining five ears only differed from normal ones by a slightly shifted maximum toward larger f(2)/f(1)s. Group delays were significantly shortened in all impaired ears, with a tendency to be shorter in the subset of ears with flat DPOAE magnitude profiles with f(2)/f(1). No clear effect of notch depth was observed, and with the chosen stimulus characteristics, no abnormal DPOAE result was observed whenever f(2) fell outside the audiometric notch. It is concluded that DPOAE group delays apparently provide useful objective clues of cochlear dysfunction, more sensitive than DPOAE magnitudes inasmuch as many of them remained near the normal range. Although a majority of f(2)/f(1) magnitude functions were clearly flattened when f(2) was impaired, this feature was less systematic.

Differential ototoxicities induced by lead acetate and tetraethyl lead

Lead poisoning disrupts many biological structures and functions, including those of the auditory system. This study examined the ototoxic effects of lead acetate (LA) and tetraethyl lead (TEL) of equal lead content on cochlear function and the ability of alpha-phenyl-tert-butyl-nitrone (PBN) to attenuate such effects. Baseline 1.0 microV cochlear microphonic (CM) and compound action potential (CAP) responses were recorded and animals administered either PBN (100 mg/kg, i.p.) or an equal volume of 0.9% saline, followed by an i.p. injection of LA (50 mg/kg) in an ethanol vehicle, TEL (42.7 mg/kg) in a corn oil vehicle, corn oil or ethanol vehicle alone. Two hours after administration, post-exposure CM and CAP responses were recorded. CAP threshold shifts in the saline-LA group were elevated by 5-10 dB at mid to high frequencies relative to controls (20-24 kHz, P<0.05). Mean CAP threshold shifts in the saline-TEL were significantly greater than those of both control groups at all tested frequencies except 2 kHz (P<0.001). However, threshold shifts in the group receiving PBN prior to TEL were significantly smaller than shifts in the group receiving saline prior to TEL (P<0.01). These data suggest that TEL is more ototoxic than is LA and that free radicals partially mediate TEL-induced CAP disruption.

Click-evoked otoacoustic emissions recorded from untreated congenital hypothyroid newborns

Thyroid hormone plays an important role in hearing development. Both a genetic or non-genetic hypothyroidism is often associated with congenital hearing loss. The exact incidence of hearing impairment in untreated congenital hypothyroid (CH) patients is unknown. This paper will present the results of measuring of the transient-evoked otoacoustic emissions (TEOAE) in a population of 29 newborns, who tested positive on a screening test for hypothyroidism (CH group) and in 68 well babies (control group) randomly chosen from all the newborns, classified as PASS, included in the Hearing Screening Program of the San Raffaele Hospital in Milan. TEOAE were recorded in all newborns within 1 month after birth and before beginning L-thyroxine treatment with conventional commercial instrumentation. Both temporal and time-frequency analyses of the emitted responses were conducted by means of a wavelet transform. The comparison of the characteristics of the temporal and frequency content of the responses of the two groups (CH and control) showed no statistically significant difference. No correlation was found between outer hair cell dysfunction and hypothyroidism.

Membrane tether formation from outer hair cells with optical tweezers

Optical tweezers were used to characterize the mechanical properties of the outer hair cell (OHC) plasma membrane by pulling tethers with 4.5-microm polystyrene beads. Tether formation force and tether force were measured in static and dynamic conditions. A greater force was required for tether formations from OHC lateral wall (499 +/- 152 pN) than from OHC basal end (142 +/- 49 pN). The difference in the force required to pull tethers is consistent with an extensive cytoskeletal framework associated with the lateral wall known as the cortical lattice. The apparent plasma membrane stiffness, estimated under the static conditions by measuring tether force at different tether length, was 3.71 pN/microm for OHC lateral wall and 4.57 pN/microm for OHC basal end. The effective membrane viscosity was measured by pulling tethers at different rates while continuously recording the tether force, and estimated in the range of 2.39 to 5.25 pN x s/microm. The viscous force most likely results from the viscous interactions between plasma membrane lipids and the OHC cortical lattice and/or integral membrane proteins. The information these studies provide on the mechanical properties of the OHC lateral wall is important for understanding the mechanism of OHC electromotility.

Micro- and nanomechanics of the cochlear outer hair cell

Outer hair cell electromotility is crucial for the amplification, sharp frequency selectivity, and nonlinearities of the mammalian cochlea. Current modeling efforts based on morphological, physiological, and biophysical observations reveal transmembrane potential gradients and membrane tension as key independent variables controlling the passive and active mechanics of the cell. The cell's mechanics has been modeled on the microscale using a continuum approach formulated in terms of effective (cellular level) mechanical and electric properties. Another modeling approach is nanostructural and is based on the molecular organization of the cell's membranes and cytoskeleton. It considers interactions between the components of the composite cell wall and the molecular elements within each of its components. The methods and techniques utilized to increase our understanding of the central role outer hair cell mechanics plays in hearing are also relevant to broader research questions in cell mechanics, cell motility, and cell transduction.

The effect of noise bandwidth on the contralateral suppression of transient evoked otoacoustic emissions

The purpose of this study was to determine whether the bandwidth or loudness of a contralateral stimulus is the most important factor in evoking suppression of transient evoked otoacoustic emissions (TEOAEs). TEOAEs were measured in both ears of 10 women in quiet and in the presence of one of three contralateral noise bands; narrow band (NB), wide band (WB) and equalized (EQ), all centered at 2000 Hz. The NB (100 Hz bandwidth) and WB (2200 Hz bandwidth) noises were presented at 60 dB SPL. The SPL of the EQ (100 Hz bandwidth) noise was adjusted such that it was equal in loudness to the WB noise as determined using a psychoacoustic procedure. Only the WB noise was associated with a significant reduction of TEOAE levels. It is believed that this effect occurred because the WB noise has greater effective energy representation across frequency on the basilar membrane as it may receive more gain from the action of the cochlear amplifier. Results of the present study indicate that noise bandwidth is the most important factor in the contralateral suppression of TEOAEs.

Efficacy of topotecan treatment on an experimental model of transient evoked otoacoustic emissions

OBJECTIVE

The aim of this study was to investigate the effects of topotecan (Hycamtin), a topoisomerase I inhibiting anticancer agent, on Transient Evoked Otoacoustic Emissions (TEOAEs) of the rabbits. We planned to investigate whether this test might provide a method for monitoring early ototoxic influence of drug administration to the cochlea.

METHODS

The study was conveyed in two groups each consisting of five rabbits with a total of ten ears. Rabbits in group I received i.v. topotecan (0.5 mg/kg once daily) for 3 days. Rabbits in group II received i.v. topotecan (0.25 mg/kg once daily) for 3 days. Cochlear function was serially monitored using transient evoked otoacoustic emissions before administration (BA) and on the 4th and 15th days after administration of topotecan. TEOAEs were analysed in terms of mean stimulus, stability and emission amplitude at 1.0-4.0 kHz.

RESULTS

For group I and II, intergroup and intragroup differences were not statistically significant in the mean stimulus, stability and emission amplitudes at 1.0-4.0 kHz.

CONCLUSIONS

We evaluated the potential role of TEOAEs in early identification of cochlear dysfunction induced by topotecan. It was concluded that topotecan did not have ototoxic effects on the cochlea in the early period of administration. TEOAEs may be useful to monitor the cochlear function and to detect the late stage of ototoxicity especially in the presence of potentially toxic factors for the prevention of permanent damage.

Electrically evoked otoacoustic emissions from the chicken ear

The outer hair cell electromotile response is believed to underlie the sharp tuning and exquisite sensitivity of the mammalian inner ear, and contribute to the production of electrically evoked otoacoustic emissions (EEOAEs) and sound-evoked otoacoustic emissions (OAEs). Avian ears are also sharply tuned, extremely sensitive and generate spontaneous and sound-evoked OAEs, but avian hair cells do not exhibit somatic electromotility. However, stereocilia bundle movements have been observed in avian and amphibian hair cells suggesting that EEOAEs might arise from electrically evoked bundle movements. Here, we demonstrate for the first time that AC current applied to the round window of the chicken evokes EEOAE of up to 18 dB SPL. The EEOAE produces a bandpass response with maximum amplitude in the 1000-3000 Hz range; the response drops off rapidly above 4000 Hz and below 500 Hz. The impulse response to current pulses is characterized by a large peak sometimes followed by a damped oscillation with a frequency around 2000 Hz. EEOAEs decreased significantly after anoxia and paraformaldehyde damage of the cochlea. Kanamycin-induced hair cell loss also caused a significant reduction in EEOAE and distortion product OAE; these emissions showed only a small recovery at long recovery times, when most hair cells should have regenerated. These results suggest that the EEOAE has a biological origin in the cochlea, which could presumably involve electrically evoked stereocilia bundle movements.

Protection of outer hair cells from reperfusion injury by an iron chelator and a nitric oxide synthase inhibitor in the guinea pig cochlea

To examine whether an active process of the cochlea was injured by ischemia-reperfusion, time courses of distortion-product otoacoustic emissions (DPOAEs) were examined before, during and after 30 min cochlear ischemia using albino guinea pigs. DPOAEs decreased to the minimum level when the animals were subjected to ischemia. When the cochlea was recirculated, DPOAEs initially recovered with time until 20 min after the onset of reperfusion. However, thereafter the amplitude of DPOAEs gradually decreased toward the noise level. Administration of deferoxamine (an iron chelator) or N-nitro-L-arginine (a nitric oxide synthase inhibitor) ameliorated this decrease of DPOAEs during reperfusion and significantly increased the DPOAE amplitudes 60 min after the onset of reperfusion as compared with those in non-treated animals. These results suggest that cochlear reperfusion as well as ischemia injured the active process of the cochlea and that free radicals and nitric oxide play important roles in this injury.

Distortion-product otoacoustic emissions and cochlear microphonics: relationships in patients with and without endolymphatic hydrops

OBJECTIVES

Because endolymphatic hydrops causes cochlear malfunction, and both otoacoustic emissions and cochlear microphonics measure specific cochlear activities, some insight into the pathology of Meniere's disease might be gained by using these two test modalities. Specifically, the involvement of cochlear outer hair cells in patients with endolymphatic hydrops may be detected. Furthermore, it is hoped that these two tests might help determine which regions of the cochlea are affected by endolymphatic hydrops, as well as where along the auditory pathway abnormalities are present.

STUDY DESIGN

Data were gathered prospectively on patients presenting to a private, tertiary referral otology/neurotology practice.

METHODS

From February 1999 to April 2000, clinical information was collected on patients presenting with vertigo, hearing loss (HL), sudden HL, fluctuant HL, aural fullness, and/or tinnitus. Data included demographics, diagnosis, pure-tone and speech audiometry, tympanometry, summating potential, action potential, cochlear microphonic, and distortion-product otoacoustic emissions. Descriptive statistics were calculated, and relationships between distortion-product otoacoustic emissions and cochlear microphonics in patients with hydrops (defined as summating potential to action potential ratio > or =0.40) and without hydrops were analyzed.

RESULTS

Distortion-product otoacoustic emissions were present more often and had larger amplitudes at the lower frequencies. No differences were found in the presence of distortion-product otoacoustic emissions across the frequencies for the two groups, but larger mean amplitudes were found for hydropic ears at 7966 Hz. As hearing levels worsened, both hydropic and nonhydropic ears were less likely to have emissions present; however, 18% of hydropic ears had emissions unexpectedly present when the pure-tone thresholds were > or =50 dB. The cochlear microphonic from the hydrops group tended to be smaller, but this was not statistically significant. Analysis of variance showed a small negative correlation between summating potential to action potential ratio and level of emission at 1968 Hz in hydropic ears; otherwise, there was no relationship between the ratio and emissions. The only statistically significant finding when analyzing the relationship between cochlear microphonic and otoacoustic emission was a small positive correlation between level of microphonic and level of emission at 1406 Hz in hydropic ears. No significant relationships were found between hearing thresholds and emissions or microphonics.

CONCLUSIONS

Even though both distortion-product otoacoustic emissions (DPOAEs) and cochlear microphonics (CMs) measure specific cochlear activities, they were not found to be useful for differentiating patients with hydrops from those without. In some patients, however, unexpected distortion-product otoacoustic emissions were present. This may represent localizing information about which regions of the cochlea are being affected by hydrops in these patients. Also, a small positive correlation between the CM and the DPOAE at 1406 Hz was detected in the hydrops group, which may represent the effects of endolymphatic hydrops on the outer hair cell. Future investigations involving hydropic patients with unexpected DPOAEs and studies looking for more DPOAE and CM correlations at frequencies surrounding 1406 Hz are being planned.

Evidence of peripheral auditory activity modulation by the auditory cortex in humans

At the auditory periphery, the medial olivocochlear system is assumed to be involved in complex sound processing and may be influenced by feedback from higher auditory nuclei. Indeed, the descending auditory pathway includes fibers coming from the auditory cortex that are anatomically well positioned to influence the superior olivary complex, and thus the medial efferent system. The aim of the present study was to verify the hypothesis of an implied influence of the auditory cortex on the peripheral auditory system. In three rare cases of patients presenting with intractable temporal lobe epilepsy, Heschl's gyrus (i.e. the temporal superior gyrus) was surgically removed in the right hemisphere in two patients and in the left hemisphere in a third patient, in order to minimize epilepsy attacks, as preoperative stereoencephalography had shown the epileptic focus or tumor to be situated in those locations. In all three cases, several weeks after the operation the medial olivocochlear system was clearly less functional on both sides, but especially on the side contralateral to the resection. In healthy controls, no such pattern was obtained. In four other epileptic patients, who were operated unilaterally at the anterior temporal pole, amygdala and hippocampus with the temporal gyrus partially spared, efferent suppression grew stronger in the ear ipsilateral to surgery. These results revealed that, in humans, the primary and secondary auditory cortex play a role in modulating auditory periphery activity through direct or indirect efferent fibers. In accordance with previous findings, this descending influence may improve the auditory afferent message by adapting the hearing function according to cortical analysis of the ascending input.

Recent advances in the hearing assessment of children

The remarkable specificity and sensitivity of otoacoustic emissions (OAEs) in identifying cochlear dysfunction, and the speed and objectivity with which the test can be conducted has made the OAE procedure the 'standard-of-care' in pediatric audiology assessment. Together with the auditory brainstem responses (ABRs), the OAE procedure not only separates sensory from neural impairment, but also facilitates early audiologic diagnosis and management. This article describes some unique applications of the OAE procedure in the diagnosis, monitoring and treatment of auditory dysfunction.

Distortion product otoacoustic emmissions in Ménière's disease

Distortion product otoacoustic emissions (DPOAEs) are an objective, non-invasive measure for evaluating outer hair cell (OHC) activity. In this study DPOAEs were measured in 70 patients affected by Meniere's disease (MD). In addition 58 out of 70 patients performed both an audiometric threshold evaluation pre- and postglycerol administration (i.e. glycerol test) and DPOAEs pre- and post-osmotic drug assumption. The purpose of this combined form of testing was to explore the effects of glycerol on the active non-linear mechanisms of the cochlea. More than 60% of the ears with MD emitted DPOAEs despite the presence of an average hearing threshold level above 40 dB (HTL). Changes in the DPOAE baseline measures were observed in 32.4% of cases after glycerol administration. Patients were divided in four groups according to the different pattern of DPOAEs shown after the glycerol test. In particular, three-quarters of cases showed a significant increase in DPOAE amplitude, one-quarter of patients, who initially did not express DPOAEs, eventually did after intake of the osmotic agent, while no decreased DPOAEs were observed in these series. The different expressions of DPOAEs should be associated with the evolutive phases of MD. In addiction, elucidation of the relationship between DPOAEs and the stages of MD was an outcome of this test.

Water permeability of cochlear outer hair cells: characterization and relationship to electromotility

The distinguishing feature of the mammalian outer hair cells (OHCs) is to elongate and shorten at acoustic frequencies, when their intracellular potential is changed. This "electromotility" or "electromechanics" depends critically on positive intracellular pressure (turgor), maintained by the inflow of water through yet uncharacterized water pathways. We measured the water volume flow, J(v), across the plasma membrane of isolated guinea pig and rat OHCs after osmotic challenges and estimated the osmotic water permeability coefficient, P(f), to be approximately 10(-2) cm/sec. This value is within the range reported for osmotic flow mediated by the water channel proteins, aquaporins. J(v) was inhibited by HgCl(2), which is known to block aquaporin-mediated water transport. P(f) values that were estimated for OHCs from neonatal rats were of the order of approximately 2 x 10(-3) cm/sec, equivalent to that of membranes lacking water channel proteins. In an immunofluorescence assay we showed that an anti-peptide antibody specific for aquaporins labels the lateral plasma membrane of the OHC in the region in which electromotility is generated. Using patch-clamp recording, we found that water influx into the OHC is regulated by intracellular voltage. We also found that the most pronounced increases of the electromotility-associated charge movement and of the expression of OHC water channels occur between postnatal days 8 and 12, preceding the onset of hearing function in the rat. Our data indicate that electromotility and water transport in OHCs may influence each other structurally and functionally.

Delay and temporal integration in medial olivocochlear bundle activation in humans

OBJECTIVE

Contralateral suppression of the transient-evoked otoacoustic emissions (TEOAEs) provides a means of studying auditory efferent function, but the temporal dynamics of the reflex are not fully understood. The most fundamental parameter is the time-course of activation of contralateral suppression. The stimulus parameters are likely to be important; this may include temporal dynamics of the suppressor itself. This investigation thus was devoted to the further study of 1) delay of contralateral suppression of TEOAEs-effect of delay of the ipsilateral probe-and 2) temporal variation of the suppressor-effect of amplitude modulation of the contralateral noise stimulus.

DESIGN

Measurements were made in three samples of normal-hearing subjects (N(total) = 71), employing well-established methods of TEOAE assessment.

RESULTS

Statistically significant contralateral suppression occurred some 60 msec after onset of the contralateral noise; thereafter, the effect was essentially constant (i.e., to >180 msec). The results for click delays less than 60 msec, nevertheless, were systematic and readily fitted by a sloping straight line (dB suppression versus time) reminiscent of the concept of threshold power integration. The onset of suppression may thus be characterized by a time constant. The delay of suppression also was found to be reduced by contralateral amplitude-modulated noise.

CONCLUSIONS

These findings reinforce a growing consensus in the literature that, despite initiation perhaps some milliseconds after onset of the contralateral stimulus, there is a substantial delay, i.e., in the tens of milliseconds, before maximal suppression is achieved. The exact time constant of this effect appears to depend upon the combination of probe and suppressor levels, including the temporality of the suppressor. These factors are likely to delimit the role/influence of this reflex in real-world function, favoring perhaps more-or-less sustained suppression that is activated in a time-varying sound environment.

Peripheral auditory asymmetry in infantile autism

Difficulty in filtering relevant auditory information in background noise is one of the features of autism. Auditory filtering processes can be investigated at the peripheral level as they are hypothesized to involve active cochlear mechanisms which are regulated by the efferent activity of the medial olivocochlear (MOC) system. The aim of the present work was therefore to assess these peripheral auditory processes in 22 children and adolescents with autism compared with age- and gender-matched normal controls. Active cochlear mechanisms were evaluated with transiently evoked otoacoustic emissions (TEOAEs) and MOC system efficiency was assessed via TEOAEs which are decreased when stimulating the contralateral ear with noise. The MOC system evaluation was performed on 18 of the 22 children. In both studies, results were analysed according to age (from 4 to 10 years and from 11 to 20 years). The main result concerns the asymmetry of the efferent system which differs in individuals with autism. Several neural processes might be hypothesized as involved in the results obtained as the MOC system which originates in the brainstem received regulating controls from upper brain structures including auditory cortex. Lateralization abnormalities at the auditory periphery may reflect indirectly a problem at a higher level of auditory processing. A second important result shows a decrease in TEOAE amplitude with age, in patients, that may correspond to a decrease in hearing sensitivity.

Distortion Product Otoacoustic Emissions: A Tool for Hearing Assessment and Scientific Study

Distortion product otoacoustic emissions (DPOAEs) reflect outer hnir cell integrity and cochlear function. When used appropriately in the audiology clinic, they are an effective diagnostic tool and can detect hearing loss with accuracy. DPOAEs are easily and rapidly recorded in newborns and children, and provide basic hearing screening information as well as detailed diagnostic information in cases of suspected hearing loss. In the past decade, solid guidelines hnve been established to select the most effective recording parameters, thereby optimizing the DPOAE's diagnostic potential. DPOAEs also provide hearing scientists with a frequency-specific and noninvasive probe of the cochlea and cochlear amplifier function. Sophisticated and complex DPOAE-based experimental paradigms hnve been developed and applied to address scientific questions about cochlear function in humans. One such paradigm, DPOAE ipsilateral suppression, has been used effectively in our laboratory to study the maturation of cochlear function in newborns. Because of its proven accuracy as a clinical tool for the detection of hearing loss and its extensive use as a scientific tool for cochlear exploration, DPOAEs are likely to enjoy continued popularity and application in both the audiology clinic and the hearing science laboratory.

Effects of PMCA2 mutation on DPOAE amplitudes and latencies in deafwaddler mice

The deafwaddler (dfw) mouse mutant is caused by a spontaneous mutation in the gene that encodes a plasma membrane Ca(2+) ATPase (type 2), PMCA2 (Street et al., 1998. Nat. Genet. 19, 390-394), which is expressed in cochlear and vestibular hair cells. Distortion product otoacoustic emission (DPOAE) amplitudes and latencies were examined in control mice, deafwaddler mutants, and controls treated with the drug furosemide. Furosemide causes a transient reduction of DPOAEs (Mills et al., 1993. J. Acoust. Soc. Am. 94, 2108-2122). We wanted to determine whether DPOAEs obtained in furosemide-treated mice were similar or different from results obtained in +/dfw mice. DPOAE amplitude and phase were measured as a function of f(2)/f(1) ratio. These data were converted into waveforms using inverse fast Fourier transform, and their average latency was used to estimate DPOAE group delay. Homozygous deafwaddlers did not produce DPOAEs. Heterozygous deafwaddlers (+/dfw) had increased DPOAE thresholds and reduced amplitudes at high frequencies, compared to controls. To the extent that DPOAEs depend on functional outer hair cells (OHCs), abnormal DPOAEs in +/dfw mice suggest that PMCA2 is important for OHC function at high frequencies. Similar to the effects of furosemide, the mutation reduced DPOAEs for low-level stimuli; in contrast to furosemide, the mutation altered DPOAEs elicited by high levels.

Effects of common topical otic preparations on the morphology of isolated cochlear outer hair cells

Otic drops are commonly used not only for otitis externa but also for otorrhea in the presence of tympanostomy tube or tympanic membrane perforation. Many studies have demonstrated the ototoxicity of common otic preparations such as Cortisporin otic drops. Recent studies have suggested the use of fluoroquinolone antibiotic drops as an alternative owing to their excellent antimicrobial coverage and no ototoxic effect. The purpose of this study was to assess the relative ototoxicity of four common otic preparations by direct exposure to isolated cochlear outer hair cells (OHCs). OHCs from adult chinchilla cochlea were exposed to standard bathing solution (control), Cortisporin, Cipro HC, Ciloxan, and Floxin. The cells were observed using an inverted microscope, and the images recorded in digital still-frame and video, and analyzed on the Image Pro-Plus 3.0 program. As measured by time to cell death and change in morphology of OHCs, Cortisporin was most toxic to OHCs. Among the fluoroquinolone drops, Floxin was more toxic than Ciloxan or Cipro HC.

Water permeability of cochlear outer hair cells: characterization and relationship to electromotility

The distinguishing feature of the mammalian outer hair cells (OHCs) is to elongate and shorten at acoustic frequencies, when their intracellular potential is changed. This "electromotility" or "electromechanics" depends critically on positive intracellular pressure (turgor), maintained by the inflow of water through yet uncharacterized water pathways. We measured the water volume flow, J(v), across the plasma membrane of isolated guinea pig and rat OHCs after osmotic challenges and estimated the osmotic water permeability coefficient, P(f), to be approximately 10(-2) cm/sec. This value is within the range reported for osmotic flow mediated by the water channel proteins, aquaporins. J(v) was inhibited by HgCl(2), which is known to block aquaporin-mediated water transport. P(f) values that were estimated for OHCs from neonatal rats were of the order of approximately 2 x 10(-3) cm/sec, equivalent to that of membranes lacking water channel proteins. In an immunofluorescence assay we showed that an anti-peptide antibody specific for aquaporins labels the lateral plasma membrane of the OHC in the region in which electromotility is generated. Using patch-clamp recording, we found that water influx into the OHC is regulated by intracellular voltage. We also found that the most pronounced increases of the electromotility-associated charge movement and of the expression of OHC water channels occur between postnatal days 8 and 12, preceding the onset of hearing function in the rat. Our data indicate that electromotility and water transport in OHCs may influence each other structurally and functionally.

Correlations among evoked potential thresholds, distortion product otoacoustic emissions and hair cell loss following various noise exposures in the chinchilla

Changes in cubic distortion product otoacoustic emissions (DeltaDPOAEs), evoked potential threshold shifts (TSs) and outer hair cell (OHC) losses were measured in a population of 95 noise-exposed chinchillas. Each animal was exposed to one of 23 different noises in an asymptotic threshold shift (ATS) producing paradigm or an interrupted noise paradigm which typically produced a toughening effect. Noises were narrow band (400 Hz) impacts with center frequencies of 0.5, 1.0, 2.0, 4.0 or 8.0 kHz presented 1 impact/s at peak SPLs of 109, 115, 121 or 127 dB. The duration of the exposures was 24 h/day for 5 days (ATS paradigm) or 6 h/day for 20 days (toughening paradigm). Based on a linear regression analysis of individual subject and group mean data, correlations among the following dependent variables were made: DeltaDPOAEs, ATS, toughening or TS recovery (TS(r)), permanent threshold shift (PTS) and OHC loss. Correlations among these metrics were generally highest for DPOAE primary frequency levels, L(1)=L(2)=70 dB. Correlation between DeltaDPOAE and TS(r) was typically low, while a considerably higher correlation was found between DeltaDPOAE and ATS. Correlations among the permanent measures of noise-induced effects, i.e. for DeltaDPOAE/PTS and DeltaDPOAE/OHC loss were typically poor when there was only a small or a moderate noise-induced effect (PTS<25 dB and DeltaDPOAE<20 dB). However, for PTS<25 dB the correlation between PTS and OHC loss was considerably better than the correlation between DeltaDPOAE and OHC loss. For more severe noise-induced changes there was generally a good correspondence between OHC loss, PTS and DeltaDPOAE metrics.

Identification of neonatal hearing impairment: distortion product otoacoustic emissions during the perinatal period

OBJECTIVES

1) To describe distortion product otoacoustic emission (DPOAE) levels, noise levels and signal to noise ratios (SNRs) for a wide range of frequencies and two stimulus levels in neonates and infants. 2) To describe the relations between these DPOAE measurements and age, test environment, baby state, and test time.

DESIGN

DPOAEs were measured in 2348 well babies without risk indicators, 353 well babies with at least one risk indicator, and 4478 graduates of neonatal intensive care units (NICUs). DPOAE and noise levels were measured at f2 frequencies of 1.0, 1.5, 2.0, 3.0, and 4.0 kHz, and for primary levels (L1/L2) of 65/50 dB SPL and 75/75 dB SPL. Measurement-based stopping rules were used such that a test did not terminate unless the response was at least 3 dB above the mean noise floor + 2 SDs (SNR) for at least four of five test frequencies. The test would terminate, however, if these criteria were not met after 360 sec. Baby state, test environment, and other test factors were captured at the time of each test.

RESULTS

DPOAE levels, noise levels and SNRs were similar for well babies without risk indicators, well babies with risk indicators, and NICU graduates. There was a tendency for larger responses at f2 frequencies of 1.5 and 2.0 Hz, compared with 3.0 and 4.0 kHz; however, the noise levels systematically decreased as frequency increased, resulting in the most favorable SNRs at 3.0 and 4.0 kHz. Response levels were least and noise levels highest for an f2 frequency of 1.0 kHz. In addition, test time to achieve automatic stopping criteria was greatest for 1.0 kHz. With the exception of "active/alert" and "crying" babies, baby state had little influence on DPOAE measurements. Additionally, test environment had little impact on these measurements, at least for the environments in which babies were tested in this study. However, the lowest SNRs were observed for infants who were tested in functioning isolettes. Finally, there were some subtle age affects on DPOAE levels, with the infants born most prematurely producing the smallest responses, regardless of age at the time of test.

CONCLUSIONS

DPOAE measurements in neonates and infants result in robust responses in the vast majority of ears for f2 frequencies of at least 2.0, 3.0 and 4.0 kHz. SNRs decrease as frequency decreases, making the measurements less reliable at 1.0 kHz. When considered along with test time, there may be little justification for including an f2 frequency at 1.0 kHz in newborn screening programs. It would appear that DPOAEs result in reliable measurements when tests are conducted in the environments in which babies typically are found. Finally, these data suggest that babies can be tested in those states of arousal that are most commonly encountered in the perinatal period.

Adaptive noise cancellation in a multimicrophone system for distortion product otoacoustic emission acquisition

This study focuses on adaptive noise cancellation (ANC) techniques for the acquisition of distortion product otoacoustic emissions (DPOAEs). Otoacoustic emissions (OAEs) are very low level sounds produced by the outer hair cells of normal cochleas, spontaneously or in response to sound stimulation as a byproduct of a frequency and threshold sensitivity increasing mechanism. Current OAE recording systems rely on test probe noise attenuation and synchronous ensemble averaging for increasing signal-to-noise ratios (SNRs). The efficiency of an ANC algorithm for noise suppression was investigated using three microphones: one placed in the test ear, one in the nontest ear for internal noise reference; one near the subject's head for external noise reference. The system proposed was tested with simulations, off-line averaging and real-time implementation of the ANC algorithm. Simulation results showed that the technique had a potential noise reduction capability of 24 dB for complex multifrequency noise signals. Off-line results were positive, with a mean SNR improvement of 4.9 dB. Real-time results indicated that the use of an ANC algorithm in combination with standard averaging methods can reduce noise levels by as much as 10 dB beyond that obtained with standard noise reduction methods and probe attenuation alone.

On the spectral periodicity of transient-evoked otoacoustic emissions from normal and damaged cochleas

The spectral quasi-periodicity of transient-evoked otoacoustic emissions (TEOAE) is well acknowledged since Zwicker described a preferred spacing of 0.4 bark between consecutive peaks in the spectrum of otoacoustic emissions from normal ears. While there is scarce evidence of any anatomical reason for this regularity, several functional models of the cochlea have predicted that the structure of emission spectra reflects important characteristics of cochlear filters. In an attempt to check such predictions, the average regularity of TEOAE spectra was studied in three groups of human subjects, normally hearing adults, healthy neonates, and adults suffering from noise-induced hearing loss. Significant differences in emission periodicities were found. Around 1 kHz, the preferred spacing was close to 130 Hz in normally hearing adult ears and neonates. In contrast, no clear periodicity was found in the group of damaged ears, even though they had clinically normal pure-tone audiometry below 2 kHz. Around 4 kHz, the preferred spacing was close to 240 Hz in normal adults and neonates, whereas TEOAEs were absent in many impaired ears. A phenomenological model assuming that TEOAEs stem from the responses of a slightly disarrayed bank of highly tuned filters predicts that the filter width would be the same in healthy young adults and neonates. In contrast, ears suffering from high-frequency hearing loss could exhibit early damaged filters. The proposed method might provide an objective assessment of parameters otherwise difficult to evaluate, especially in neonatal cochleas.

Relationship between distortion product otoacoustic emissions and frequency modulation difference limens

OBJECTIVE

To explore the relationship between distortion product otoacoustic emissions (DPOAEs) and frequency modulation difference limens (FMDLs) in normal-hearing subjects.

DESIGN

Fifteen normal-hearing adult subjects (age range = 20 to 29 yr; mean age = 21 yr, 6 mo) participated in the study. Each subject met the following criteria: 1) hearing thresholds <20 dB HL bilaterally at frequencies 0.25 to 8 kHz; 2) type A tympanograms bilaterally; and 3) no history of otologic disease. To reduce gender-related differences, only female subjects were recruited as subjects. DPOAE amplitudes and FMDLs were measured in both ears of 15 female adults at 1, 2, and 4 kHz. Ears were classified on the basis of DPOAE amplitude into large DPOAEs and small DPOAEs.

RESULTS

Results indicated that ears with larger DPOAEs showed better frequency discrimination (smaller FMDLs) than did ears with smaller DPOAEs at 2 kHz.

CONCLUSIONS

Results of the current study support frequency discrimination models that predict a relationship between frequency selectivity and frequency discrimination.

An objective method of analyzing cochlear versus noncochlear patterns of distortion-product otoacoustic emissions in patients with acoustic neuromas

OBJECTIVES

To objectify the effects of retrocochlear disease on distortion-product otoacoustic emissions (DPOAEs) by developing a computer-based software strategy for classifying DPOAE patterns as cochlear or noncochlear and to evaluate the sensitivities of these techniques in a large series of patients with unilateral acoustic neuromas.

STUDY DESIGN

Development of a novel, software-based method of DPOAE analysis, which was evaluated with data obtained from a retrospective review of the results from audiometric tests performed in a series of patients.

METHODS

A computer-based software strategy was developed, using frequency-specific data from normal-hearing adults, for the purpose of distinguishing cochlear from noncochlear patterns of hearing loss, by determining the discrepancies between DPOAEs and behavioral audiometry. Preoperative pure-tone thresholds and DPOAEs from 97 patients with surgically confirmed acoustic neuroma were compared using an objective method and a standard, subjective technique that was considered to be the gold standard. The effects of bilateral hearing losses, such as noise-induced hearing loss and presbycusis, were accounted for during the analysis to isolate the effects of the tumors on hearing thresholds and DPOAEs.

RESULTS

Overall, 55 (57%) of the tumor ears were assigned to the cochlear group (i.e., DPOAEs consistent with hearing thresholds), 40 (41%) to the noncochlear group (i.e., DPOAEs inconsistent with hearing thresholds), and 2 (2%) to an indeterminate group, using the subjective technique for classifying DPOAEs. There was no significant difference in the categorization of the patients with acoustic neuroma when employing the objective strategy. The objective algorithm, when modified to maximize the number of noncochlear identifications, led to assignments of 36 (37%) to the cochlear, 57 (59%) to the noncochlear, and 4 (4%) to the indeterminate categories.

CONCLUSIONS

Subjective analysis of a large series patients with acoustic neuromas showed that the majority of ears with tumors demonstrated cochlear (57%), rather than non-cochlear (41%), patterns of DPOAEs. The computerized, software-based algorithm developed for differentiating cochlear from noncochlear patterns of DPOAEs in patients with retrocochlear disease had a maximum sensitivity of 59%. This value was significantly higher than that reported in previous studies.

Activation of medial olivocochlear efferent system in humans: influence of stimulus bandwidth

The activity of the medial olivocochlear bundle (MOCB) can be studied in humans through variations in the level of evoked otoacoustic emissions (EOAEs) elicited by contralateral acoustic stimuli (CAS). The present study sought to investigate how the activity of the MOC system at a given frequency, as measured through the contralateral suppression of tone-pip EOAEs, depends on the bandwidth of the contralateral stimulus. EOAEs were recorded in 155 normal-hearing subjects, successively with and without contralateral stimuli whose bandwidth, center frequency and level were systematically varied. We showed a clear dependence of contralateral EOAE suppression on bandwidth demonstrating increased suppression with increased bandwidth over about two octaves around the center frequency of the noise. This effect was obtained irrespective of whether contralateral noise energy was kept constant independently of bandwidth or not, which indicates a role of bandwidth per se in contralateral EOAE suppression. Results are interpreted in terms of a simple model of MOCB activation mechanisms including peripheral bandpass filtering, within-channel compression and across-channel spatial summation by the afferent paths. Complementary experiments suggested a greater effectiveness of increases in bandwidth on the upper than on the lower side and of frequency components akin to or remote from the test frequency than of intermediate bands. Finally, these results were complemented by detailed spectrum analyses of the EOAE level variations induced by the different noises, which revealed that whilst noise components close to or remote from the center frequency generally attenuated EOAE level, intermediate components could in some cases lead to a relative increase in EOAE level. These results can further be explained by assuming different positive and negative weights on the inputs to the spatial summation process depending on their position relative to the center frequency.

Middle ear influence on otoacoustic emissions. I: noninvasive investigation of the human transmission apparatus and comparison with model results

Evoked otoacoustic emissions (EOAEs) are generated within the cochlea in response to external sounds, and they can be acoustically detected in the external auditory meatus after backward propagation through the middle ear. In addition to being used to probe the cochlear mechanisms, they are expected to be sensitive to minute changes in middle ear impedance. Systematic measurements of the changes in the vectorial components of EOAEs were carried out after various manipulations of the human middle ear in order to characterize the influence of stiffness and inertia of the stapes and tympanic-membrane systems. For this purpose, stapedius muscle contractions were elicited by high-level contralateral sound, controlled changes in middle ear pressure (range +/-100 daPa) were produced and the tympanic membrane was loaded with water droplets. A computer model of the middle ear network was implemented using a standard lumped-element electric analog of the middle ear (Zwislocki's model). Forward and backward transmission changes were simulated and model predictions were compared to experimental data. Apart from the case of positive middle ear pressures, a close qualitative correspondence was found between model and real-ear results. Each of the effects was characterized by a unique pattern of phase and magnitude changes as a function of frequency, in relation to the mechanical characteristics of the involved subsystem (i.e. stapes stiffness, tympanic-membrane stiffness or mass) and its resonance properties. Owing to their high sensitivity, EOAEs could be helpful for an accurate individual multifrequency analysis of middle ear impedance by comparisons under rest and test conditions.

Olivocochlear efferent vs. middle-ear contributions to the alteration of otoacoustic emissions by contralateral noise

The medial olivocochlear efferent bundle is the key element of a bilateral efferent reflex activated by sound in either ear and acting directly on cochlear outer hair cells (OHC) via numerous cholinergic synapses. It probably contributes to regulating the mechanical activity of the cochlea. Otoacoustic emissions, being sounds emitted by the cochlea as a reflection of its activity and suppressed by efferent activation, are increasingly considered to be the privileged tool for a noninvasive assessment of the efferent reflex. However, confounding effects on otoacoustic emissions can occur. A primary influence is middle-ear muscle reflex activation, which shares common features with the effects of cochlear efferent activation. We report a systematic comparison of the responses of human otoacoustic emissions to efferent activation by low-level noise in the contralateral ear to various middle-ear manipulations (reflex contractions of the stapedius muscle induced by high-level contralateral noise; moderate middle-ear pressure changes). The profiles of level and phase changes of otoacoustic emissions as a function of frequency were highly specific to the origin of the effects. The changes induced by middle-ear manipulations matched the predictions computed from a standard lumped-element middle-ear model, with one or two peaks around the resonance frequency(ies) of the involved subsystem, stapes or tympanic membrane. In contrast, the efferent effect was completely different, exhibiting a broadband-level suppression associated with a small phase lead. We propose that a careful vector analysis of otoacoustic emission modifications enables the identification of the contribution of the efferent reflex without ambiguity even when it is mixed with middle-ear effects. Thereby, otoacoustic emissions can be used more reliably as noninvasive probes of efferent olivocochlear function.

Ototoxicity caused by aminoglycosides is ameliorated by melatonin without interfering with the antibiotic capacity of the drugs

The production of free radicals seems to be involved in the mechanisms of ototoxicity. Aminoglycosides produce ototoxicity, which can be determined through distortion product otoacoustic emissions (OAEs) that measure the activity of the outer hair cells of the organ of Corti. An ototoxic chart was obtained in rats using gentamicin or tobramycin. Together with this treatment, the animals ingested melatonin in the drinking water, or melatonin was injected subcutaneously or intramuscularly. The distortion product OAEs were determined over a prolonged period of time for each of the groups. The effect of melatonin on the antibiotic capacity of the aminoglycosides used was also studied. Antibiograms inoculated with Escherichia coli or Pseudomonas aeruginosa and treated with gentamicin or tobramycin in the presence or absence of melatonin at quantities from pharmacological to physiological doses were performed. The ototoxicity produced by gentamicin and tobramycin was maximal from days 3 to 5 post-treatment, returning to normal values in 2 wk. When melatonin was present, the recovery was at day 5 post-treatment, independently of the means of administration of the pineal product. The antibiograms showed that melatonin had no effect on the antibiotic capacity. It is concluded that the ototoxicity caused by gentamicin and tobramycin is ameliorated by melatonin and that the pineal hormone does not interfere with the antibiotic capacity of these antibiotics.

Distortion product otoacoustic emissions and outer hair cell defects in the hyt/hyt mutant mouse

Thyroid hormone plays an important role in hearing development. Hereditary hypothyroidism is frequently associated with sensorineural hearing loss as identified in both animal models and human patients. Building upon our original demonstration of congenital deafness and hair cell abnormality in a hyt/hyt mouse model which carries an autosomal recessive mutation causing hereditary hypothyroidism, we investigated the functional capacity of the outer hair cell (OHC) system in these animals using distortion product otoacoustic emissions (DPOAEs). In particular, the amplitude and detection features of DPOAEs were correlated with measures of the auditory brainstem response (ABR) as well as the cellular structure and ultrastructure of the organ of Corti. Input-output (I/O) functions for the 2f(2)-f(1) DPOAEs were obtained for frequencies from 2 to 18 kHz. The thresholds were significantly higher and amplitudes were significantly lower in the homozygous mice (hyt/hyt) than in both heterozygous mice (hyt/+) and wild-type controls at DPOAE frequencies recorded above 6 kHz. Hearing thresholds were significantly elevated in the mutant compared to control mice. In addition, morphological studies revealed consistent inner ear defects in hyt/hyt animals including distortion of the tectorial membrane, dysplasia of the tunnel of Corti and distinct OHC abnormalities. The most striking histopathological finding was a contiguous membrane along the apices of all of the OHC stereocilia. Such ultrastructural changes in the stereocilia of the OHC may limit the deflection of the stereocilia and therefore affect an active cochlear function that produces otoacoustic emissions as well as cause a failure to evoke the normal action potentials in the auditory nerve. From both functional and morphologic evaluations, it was concluded that the OHC system is the most susceptible to the developmental effects of congenital hypothyroidism in the hyt/hyt mouse. The normal OHCs with well-developed ciliary bundles are crucial to maintain the activity of biological mechanisms within the cochlea.

Effects of cis-platinum chemotherapy on otoacoustic emissions: the development of an objective screening protocol. Third place--Resident Clinical Science Award 1998

To develop an objective, fast, and simply performed screening protocol for cis -platinum (CP) ototoxicity, we compared the efficacy of screening with distortion-product otoacoustic emissions (DPOAEs) with the outcome of both conventional and ultra-high-frequency (UHF) audiometry. Baseline audiometric and DPOAE testing was performed in 66 patients, 33 of whom met criteria for inclusion in the final database. Comparisons were made between baseline measurements and those recorded before subsequent CP infusions. Outcomes were analyzed clinically and with paired repeated-measures analysis of variance. Results indicated that DPOAEs and UHF were better measures than conventional audiometry. Further, DPOAEs may be better suited for screening older patients receiving CP chemotherapy because DPOAEs are as sensitive as UHF and are present in a greater number of these patients. Screening with DPOAEs may be enhanced by testing only in the 3- to 5.2-kHz range, thus decreasing testing time. Higher time averages to increase the signal-to-noise ratio and use of this narrower bandwidth might also allow for accurate bedside testing.

Age-related loss of distortion product otoacoustic emissions in four mouse strains

Changes in cochlear function in four inbred strains of mice, CBA/CaJ (CBA), C57BL/6J (C57), BALB/cByJ (BALB), and WB/ReJ (WB), previously used to study age-related hearing loss, were evaluated serially as a function of age with 2f(1)-f(2) distortion-product otoacoustic emissions (DPOAEs). DPOAE levels in response to equilevel primary tones for geometric-mean (GM) frequencies from 5.6 to 48.5 kHz were recorded systematically as DP-grams and response/growth or input/output (I/O) functions at monthly intervals from about 2 to 15 months of age. Over the approximate 13-month measurement period, CBAs showed robust and unchanged DPOAEs for all tested frequencies, while BALBs, C57s, and WBs showed strain-specific, age-related decreases in DPOAEs that progressed systematically from the high to low frequencies. Specifically, for the youngest WBs at 2 months of age, no DPOAEs were recordable for GM frequencies > or = 32 kHz, while C57s and BALBs reached the identical stage of cochlear dysfunction by 5 and 8 months, respectively. The differential decline in DPOAE activity shown for WB, C57, and BALB mice supports the notion that they represent unique animal models of age-related changes in cochlear function. In contrast, the unchanging DPOAEs for CBAs over the same time period indicate that this strain makes an effective control for normal cochlear function in the mouse, at least, up to 15 months of age.

Applications of distortion-product emissions to an otological practice

OBJECTIVES

The multiple clinical applications of distortion-product emissions (DPEs) to an otological practice are reviewed. Through an examination of studies involving thousands of infants, as well as adult ears, the relationships between measurements of DPEs and PTTs (PTTs) are examined. The cochlear physiology underlying generation of DPEs and the interpretation of these measurements are described in some detail. We study the contribution of phase measurements of DPEs to calculation of delay in cochlear traveling waves. The clinical applications of these measurements are described. The objective of this work is to demonstrate the clinical utility of DPEs through an examination of the correlation of the features of DPEs with PTTs in patients varying in age from newborn to the elderly.

STUDY DESIGN

METHODS

Experimental design and methodology involved careful prospective selection of large numbers of subjects in each age group, novel techniques for recording DPEs, and multivariate statistical analysis of the data. Three separate experiments encompass issues surrounding the age groups of adults, neonates, and elderly individuals.

RESULTS

The results show that current techniques allow a 90% correct prediction of auditory PTTs for frequencies varying from 1,000 to 6,000 Hz, for patients of all ages.

CONCLUSION

Techniques for measurement of DPEs currently represent an integral diagnostic component for an otological practice. The potential future application of measures of DPEs to diagnosis of tinnitus is described. With an extension of the technique for measuring DPEs to the phenomenon of reflectance, there may be promise of application to novel approaches to the treatment of tinnitus and the design of hearing aids.

Evaluation of cisplatin ototoxicity in a rat animal model

The ototoxic effects of cisplatin were evaluated by otoacoustic emissions and evoked auditory responses. A transient otoacoustic emissions protocol indicated no significant ototoxic effects in rats treated intravenously with 7.5 mg/kg/week for 2-weeks. A chronic 6-week treatment (2.5 mg/kg/week) monitored by 2F1-F2 distortion product emissions presented significant SNR alterations in a narrow range of frequencies (5.04-5.66 kHz). An acute treatment of 15 mg/kg, using slow 30-min intraperitoneal infusion, presented the highest DP and ABR alterations. The SNR at the 2F1-F2 frequencies 6.34, 7.13, and 7.56 kHz was found significantly decreased, and ABR latency measurements from 8-kHz burst stimuli verified these alterations.

Modeling otoacoustic emissions by active nonlinear oscillators

The phenomenology of spontaneous otoacoustic emissions (OAEs) is compared to theoretical predictions given by models in which they are produced by active nonlinear oscillators. Along with the well-known Van der Pol oscillator, a new active oscillator model is proposed and analyzed here. Numerical simulations and multi-scale analytical computation results are compared to experimental data of neonatal spontaneous and evoked OAEs. A simple analysis technique is proposed, in which the time evolution after a click stimulus of the amplitude of each spectral line corresponding to a spontaneous OAE is studied. Apart from a few stationary lines, an approximately exponential decay law, with characteristic damping coefficients in the 20-200 Hz range, was found to fit the data. These results are not compatible with a Van der Pol oscillator model, and show that some important aspects of the OAE phenomenology can be better explained by the proposed oscillator. Other interesting features of the spontaneous end evoked OAE phenomenology, such as spontaneous OAE suppression by external tones and the following recovery, as well as stimulus/response curves in the linear and nonlinear mode of acquisition, are also well reproduced by the proposed model.