Outer hair cell electromotility and otoacoustic emissions

Hypotonic swelling of salicylate-treated cochlear outer hair cells

The outer hair cell (OHC) is a hydrostat with a low hydraulic conductivity of Pf=3x10(-4) cm/s across the plasma membrane (PM) and subsurface cisterna that make up the OHC's lateral wall. The SSC is structurally and functionally a transport barrier in normal cells that is known to be disrupted by salicylate. The effect of sodium salicylate on Pf is determined from osmotic experiments in which isolated, control and salicylate-treated OHCs were exposed to hypotonic solutions in a constant flow chamber. The value of Pf=3.5+/-0.5x10(-4) cm/s (mean+/-s.e.m., n=34) for salicylate-treated OHCs was not significantly different from Pf=2.4+/-0.3x10(-4) cm/s (mean+/-s.e.m., n=31) for untreated OHCs (p=.3302). Thus Pf is determined by the PM and is unaffected by salicylate treatment. The ratio of longitudinal strain to radial strain epsilonz/epsilonc=-0.76 for salicylate-treated OHCs was significantly smaller (p=.0143) from -0.72 for untreated OHCs, and is also independent of the magnitude of the applied osmotic challenge. Salicylate-treated OHCs took longer to attain a steady-state volume which is larger than that for untreated OHCs and increased in volume by 8-15% prior to hypotonic perfusion unlike sodium alpha-ketoglutarate-treated OHCs. It is suggested that depolymerization of cytoskeletal proteins and/or glycogen may be responsible for the large volume increase in salicylate-treated OHCs as well as the different responses to different modes of application of the hypotonic solution.

A validation and potential clinical application of multivariate analyses of distortion-product otoacoustic emission data

OBJECTIVE

To test the generalizability of multivariate analyses of distortion-product otoacoustic emission (DPOAE) data. Previously published multivariate solutions were applied to a new set of data to determine if test-performance improvements, evident in previous reports, are retained. An additional objective was to provide an alternative approach for making multivariate dichotomous decisions of hearing status in the clinic, based on DPOAE measurements.

DESIGN

DPOAE level and noise were obtained in 345 ears of 187 subjects. Approximately one third of the subjects had normal hearing, whereas the remainder had hearing loss, ranging from 25 to more than 120 dB HL. DPOAE data were collected at each of nine frequencies. After data collection, clinical decision theory, in combination with univariate (DPOAE level and signal-to-noise ratio [SNR]) and multivariate (logistic regression) analyses, was used to construct relative operating characteristic (ROC) curves and to generate ROC curve areas. In addition, test performance was assessed by fixing the false-alarm rate and comparing different approaches to analyses in terms of their failure rates as a function of magnitude of hearing loss. The DPOAE test results were compared with either single-frequency or multifrequency gold standards. The multivariate solutions were taken from previously published work (Dorn et al., 1999; Gorga, et al., 1999).

RESULTS

DPOAE level and SNR resulted in roughly equivalent test performance (ROC curve areas and failure rates among ears with hearing loss), although DPOAE level performed better for frequencies above 1 kHz, and SNR performed better for frequencies at 0.75 and 1 kHz. Multivariate analyses resulted in better test performance for nearly all conditions, compared with the univariate approaches that used either DPOAE level or SNR. The improvements in test performance were greatest for the frequencies at which the univariate analyses performed poorest (0.75 kHz, 1 kHz, and 8 kHz). Less difference was observed between univariate and multivariate approaches when multifrequency gold standards were used; however, even for the multifrequency cases, multivariate analyses generally resulted in better performance. An approach that might facilitate the interpretation of multifrequency DPOAE measurements in the clinic is described.

CONCLUSIONS

Previously described multivariate analyses were robust in that they improved test performance when applied to an entirely new set of DPOAE data. This, in turn, suggests that the previously described multivariate solutions may have clinical utility in that they are expected to improve test performance at no additional cost in terms of data-acquisition or data-analysis time. In addition to demonstrating that these solutions generalized to new data, an alternative approach to interpreting multifrequency DPOAE measurements is provided that includes the advantages of using multivariate analyses. This new metric may be useful when DPOAEs are used for screening purposes.

Summary of evidence pointing to a role of the dorsal cochlear nucleus in the etiology of tinnitus

Evidence has accumulated in the last decade that the dorsal cochlear nucleus (DCN) may be an important site in the etiology of tinnitus. This evidence comes from a combination of studies conducted in animals and humans. This paper will review the key findings, as follows. 1) Direct electrical stimulation of the DCN leads to changes in the loudness of tinnitus. This suggests that the loudness of tinnitus may be linked to changes in the level of neural activity in the DCN. 2) Exposure to tinnitus inducers, such as intense sound or cisplatin, causes neural activity in the DCN to become chronically elevated, a condition known as neuronal hyperactivity. 3) This hyperactivity is very similar to the activity that is evoked in the DCN by sound stimulation, suggesting that the hyperactivity represents a code that signals the presence of sound, even when there is no longer any sound stimulus. 4) Noise-induced hyperactivity in the DCN is correlated with tinnitus. Behavioral studies have demonstrated that animals exposed to the same intense sound that causes hyperactivity in the DCN develop tinnitus-like percepts. The correlation between the level of hyperactivity and the behavioral index of tinnitus was found to be statistically significant. 5) The DCN is a polysensory integration center, and electrophysiological studies have shown that both spontaneous activity and hyperactivity of neurons in the DCN can be modulated by stimulation of certain ipsilateral cranial nerves, such as the sensory branch of the trigeminal nerve. This ipsilateral modulation of DCN activity offers a plausible explanation of how tinnitus, when perceived on one side, can be modulated by certain manipulations of the head and neck on the side ipsilateral to the tinnitus, but rarely on the contralateral side. 6) The DCN exhibits various forms of neuronal plasticity that parallel the various forms of plasticity that characterize tinnitus. These findings collectively strengthen the view that the DCN may be a key structure that should be included as a target of anti-tinnitus treatment.

Modeling distortion product otoacoustic emission input/output functions using segmented regression

Distortion product otoacoustic emissions (DPOAEs) are low-level acoustic signals, the detection of which involves extraction from a background of noise. Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] described a method for modeling the presence and growth of these responses. While improving growth function parameter estimation, this technique excludes a significant fraction of the data (especially low-level responses), and relies on ad hoc model fit acceptance criteria. The statistical difficulties associated with these limitations are described, and a weighted segmented linear regression model that avoids them is proposed. A simple test is presented for the presence of DPOAE growth. This technique is compared to that of Boege and Janssen in a dataset of 9 556 input/output (I/O) functions collected over 4 years on 866 ears from 379 construction apprentices and 63 age-matched controls. Comparisons are made on the entire dataset and within audiometric hearing loss categories. Segmented regression avoids the statistical pitfalls of the previous method, allows estimation of the threshold and slope of auditory response on a far greater number of I/O functions, and improves estimation of these parameters in this dataset. The potential for this method to yield more sensitive metrics of hearing function and compromise is discussed.

Functional role of GABAergic innervation of the cochlea: phenotypic analysis of mice lacking GABA(A) receptor subunits alpha 1, alpha 2, alpha 5, alpha 6, beta 2, beta 3, or delta

The olivocochlear efferent system is both cholinergic and GABAergic and innervates sensory cells and sensory neurons of the inner ear. Cholinergic effects on cochlear sensory cells are well characterized, both in vivo and in vitro; however, the robust GABAergic innervation is poorly understood. To explore the functional roles of GABA in the inner ear, we characterized the cochlear phenotype of seven mouse lines with targeted deletion of a GABA(A) receptor subunit (alpha1, alpha2, alpha5, alpha6, beta2, beta3, or delta). Four of the lines (alpha1, alpha2, alpha6, and delta) were normal: there was no cochlear histopathology, and cochlear responses suggested normal function of hair cells, afferent fibers, and efferent feedback. The other three lines (alpha5, beta2, and beta3) showed threshold elevations indicative of outer hair cell dysfunction. Alpha5 and beta2 lines also showed decreased effects of efferent bundle activation, associated with decreased density of efferent terminals on outer hair cells: although the onset of this degeneration was later in alpha5 (>6 weeks) than beta2 (<6 weeks), both lines shows normal efferent development (up to 3 weeks). Two lines (beta2 and beta3) showed signs of neuropathy, either decreased density of afferent innervation (beta3) or decreased neural responses without concomitant attenuation of hair cell responses (beta2). One of the lines (beta2) showed a clear sexual dimorphism in cochlear phenotype. Results suggest that the GABAergic component of the olivocochlear system contributes to the long-term maintenance of hair cells and neurons in the inner ear.

Correction of the peripheral spatiotemporal response pattern: a potential new signal-processing strategy

The purpose of this paper is to introduce a new signal-processing strategy, spatiotemporal pattern correction (SPC), that is based on our knowledge of the level-dependent temporal response properties of auditory nerve fibers in normal and impaired ears. In normal-hearing listeners, tuning is sharp for low-level inputs and broadens as input level increases. When peripheral filters change their shape with input level, the phase properties of the filters and the latency of the filter's response also change. However, in listeners with hearing loss, tuning is broad for both low- and high-level inputs. Thus, there is little change in the phase properties of the filters for different input levels. The SPC strategy manipulates the temporal aspects of different frequency channels of sounds in an attempt to "correct" for this abnormal spatiotemporal response pattern of the impaired ear. Quality judgments and intelligibility measures of speech processed at various SPC strengths were obtained from a small group of normal-hearing listeners and listeners with hearing loss. In general, listeners with hearing loss preferred sentences with some degree of SPC processing, whereas normal-hearing listeners preferred unprocessed sentences. Benefit from SPC on the nonsense syllable test varied greatly across phonemes and listeners. A more comprehensive study of listeners with different degrees and configurations of hearing loss is needed to determine the amount of SPC benefit.

Quantitative indices for the assessment of the repeatability of distortion product otoacoustic emissions in laboratory animals

Distortion product otoacoustic emissions (DPOAE) can be used to study cochlear function in an objective and non-invasive manner. One practical and essential aspect of any investigating measure is the consistency of its results upon repeated testing of the same individual/animal (i.e., its test/retest repeatability). The goal of the present work is to propose two indices to quantitatively assess the repeatability of DPOAE in laboratory animals. The methodology is here illustrated using two data sets which consist of DPOAE subsequently collected from Sprague-Dawley rats. The results of these experiments showed that the proposed indices are capable of estimating both the repeatability of the true emission level and the inconsistencies associated with measurement error. These indices could be a significantly useful tool to identify real and even small changes in the cochlear function exerted by potential ototoxic agents.

Electromechanical models of the outer hair cell composite membrane

The outer hair cell (OHC) is an extremely specialized cell and its proper functioning is essential for normal mammalian hearing. This article reviews recent developments in theoretical modeling that have increased our knowledge of the operation of this fascinating cell. The earliest models aimed at capturing experimental observations on voltage-induced cellular length changes and capacitance were based on isotropic elasticity and a two-state Boltzmann function. Recent advances in modeling based on the thermodynamics of orthotropic electroelastic materials better capture the cell's voltage-dependent stiffness, capacitance, interaction with its environment and ability to generate force at high frequencies. While complete models are crucial, simpler continuum models can be derived that retain fidelity over small changes in transmembrane voltage and strains occurring in vivo. By its function in the cochlea, the OHC behaves like a piezoelectric-like actuator, and the main cellular features can be described by piezoelectric models. However, a finer characterization of the cell's composite wall requires understanding the local mechanical and electrical fields. One of the key questions is the relative contribution of the in-plane and bending modes of electromechanical strains and forces (moments). The latter mode is associated with the flexoelectric effect in curved membranes. New data, including a novel experiment with tethers pulled from the cell membrane, can help in estimating the role of different modes of electromechanical coupling. Despite considerable progress, many problems still confound modelers. Thus, this article will conclude with a discussion of unanswered questions and highlight directions for future research.

Protective effects of glucocorticoids on ischemia-reperfusion injury of outer hair cells

OBJECTIVE

This animal study aimed to investigate effects of glucocorticoids on the functional recovery of outer hair cells (OHCs) after transient ischemia.

METHODS

Distortion-product otoacoustic emission (DPOAE) was examined before, during, and after transient cochlear ischemia of 30 minutes using albino guinea pigs.

RESULTS

DPOAE decreased to noise level during ischemia. On recirculation, DPOAE initially recovered with time until 20 minutes after the onset of reperfusion, but thereafter, the DPOAE level gradually decreased toward the noise level in the control animals. Prednisolone and methylprednisolone significantly improved the DPOAE level 60 minutes after the onset of reperfusion.

CONCLUSIONS

The present findings suggest that glucocorticoids possess protective effects against ischemia-reperfusion injury of OHCs.

Photometric recording of transmembrane potential in outer hair cells

Cochlear outer hair cells (OHCs) are polarized epithelial cells that have mechanoelectrical transduction channels within their apical stereocilia and produce electromotile force along their lateral wall. Phase shifts, or time delays, in the transmembrane voltage occurring at different axial locations along the cell may contribute to our understanding of how these cells operate at auditory frequencies. We developed a method to optically measure the phase of the OHC transmembrane potential using the voltage-sensitive dye (VSD) di-8-ANEPPS. The exit aperture of a fibre-optic light source was driven in two dimensions so that a 24 microm spot of excitation light could be positioned along the length of the OHC. We used the whole-cell patch-clamp technique in the current-clamp mode to stimulate the OHC at the base. The photometric response and the voltage response were monitored with a photodetector and patch-clamp amplifier, respectively. The photometric response was used to measure the regional changes in the membrane potential in response to maintained (dc) and sinusoidal (ac) current stimuli applied at the base of the cell. We used a neutral density filter to lower the excitation light intensity and reduce phototoxicity. A sensitive detector and lock-in amplifier were used to measure the small ac VSD signal. This permitted measurements of the ac photometric response below the noise floor of the static fluorescence. The amplitude and phase components of the photometric response were recorded for stimuli up to 800 Hz. VSD data at 400-800 Hz show the presence of a small phase delay between the stimulus voltage at the base of the cell and the local membrane potential measured along the lateral wall. Results are consistent with the hypothesis that OHCs exhibit inhomogeneous membrane potentials that vary with position in analogy with the voltage in nerve axons.

Contralateral Suppression of Otoacoustic Emissions in Children with Specific Language Impairment

OBJECTIVE

The purpose of the investigation was to determine whether a group of children with specific language impairments (SLI) have reduced peripheral auditory processes thought to be associated with speech-in-noise intelligibility.

DESIGN

Transient evoked otoacoustic emissions (TEOAE) and their suppression by the efferent activity of the medial olivocochlear system (MOCS) in response to contralateral acoustic stimulation were used to compare these processes in 18 children with SLI and 21 controls.

RESULTS

The results revealed no group difference in TEOAE suppression effect or left/right asymmetry of TEOAE suppression effect.

CONCLUSIONS

These results suggest that children with SLI do not have auditory processing problems at this peripheral level casting doubt on a hypothesized relationship between strength of MOCS activity and language impairment.

Transient evoked otoacoustic emissions after vestibular nerve section in chinchillas

Transient evoked otoacoustic emissions are believed to be sensitive to the effects of the cochlear efferent system. The most well-known function of this system is inhibitory on cochlear response. It has been demonstrated that crossed medial efferent system section produces inhibitory control of the outer hair cells mechanisms responsible for non-linear transient evoked otoacoustic emissions generation. However, we suppose that the uncrossed medial efferent system plays a role in outer hair cell function too. We recorded the non-linear part of transient evoked otoacoustic emissions in 17 chinchillas before and after section of the vestibular nerve (crossed and uncrossed fibers). Responses at frequencies bands centered on 0.8, 1.6, 2.4, 3.2 and 4.0 kHz, as well as total emission responses, were analyzed. After vestibular nerve section, there were significant increases in the amplitudes of the 2.4- and 4.0 kHz responses and of the total response. These results indicate that the medial efferent system is important to maintain normal cochlear mechanics. Uncrossed medial efferent system and lateral efferent system seem to be not important in maintaining normal cochlear mechanics.

The effect of an age-related hearing loss gene (Ahl) on noise-induced hearing loss and cochlear damage from low-frequency noise

Inbred C57BL/6J mice carry two copies of an age-related hearing loss gene (Ahl). It has been shown that these mice begin losing high-frequency hearing at two months. Several functional studies have reported that the Ahl gene renders mice more susceptible to noise-induced hearing loss (NIHL) than strains which do not carry this gene [e.g., Hear. Res. 93 (1996) 181; Hear. Res. 155 (2001) 82; J. Assoc. Res. Otolaryngol. 2 (2001) 233]. Johnson et al. [Hear. Res. 114 (1997) 83] developed a congenic B6.CAST-+Ahl mouse which carries the wild-type allele from Mus musculus castaneus at the Ahl locus. Five each of young C57BL/6J males and females, and B6.CAST-+Ahl males were exposed to a 4-kHz octave band of noise at 108 dB SPL for 4 h. Non-noise-exposed mice of the same strains and age served as controls. The noise-exposed mice were functionally tested for ABR thresholds and DPOAE levels pre-exposure and three times post-exposure: 0 days to determine the magnitude of temporary threshold shift (TTS); 6 days to determine rate of recovery; and 20 days to determine the magnitude of permanent threshold shift (PTS). At 20 days post-exposure, the animals underwent cardiac perfusion to fix their cochleae. The isolated cochleae were embedded in plastic and dissected into flat preparations. By phase-contrast microscopy, each cochlea was evaluated from apex to base to quantify the losses of hair cells, nerve fibers and stria vascularis and to localize stereocilia damage. Functional data from each mouse were aligned with the cytocochleogram using the frequency-place map of Ou et al. [Hear. Res. 145 (2000) 111; Hear. Res. 145 (2000) 123]. Sizable variation in the magnitude of TTS, PTS and hair-cell loss was found among mice of the same genetic strain. The congenic B6.CAST-+Ahl male mice had significantly less TTS immediately post-exposure than C57BL/6J males or females but not less PTS or hair-cell losses at 20 days post-exposure. These results indicate that, at one month of age, mice carrying two copies of the Ahl gene have an increased susceptibility to TTS from a low-frequency noise before they have any indication of age-related hearing or hair-cell loss. However, this appeared not to be the case for PTS. The Ahl gene appears to play a role in susceptibility to NIHL but, other genes as well as systemic and local factors must also be involved.

A model of real time monitoring of the cochlear function during an induced local ischemia

The aim of this study was to investigate the utility of distortion product otoacoustic emissions (DPOAEs) in intraoperative monitoring (IM) of cochlear ischemic episodes in animals during internal auditory artery (IAA) compression. The IAA was exposed using the posterior fossa approach and then compressed for 3 and 5 min intervals to effect ischemia. DPOAE amplitudes and phases were measured at 4, 8, and 12 kHz geometric mean frequency (GMF). In each monitored ear, laser-Doppler cochlear blood flow (CBF) was measured. All IAA compressions resulted in rapid decrease of DPOAE amplitude and CBF, with simultaneous DPOAE phase increase. DPOAE phase changes were found to increase consistently within several seconds of IAA compression, while corresponding DPOAE amplitudes changed more slowly, with up to 30-40 s delays. Following IAA release, DPOAEs at 12 kHz GMF were characterized by longer delays in returning to baseline than those measured at lower frequencies. In some cases, CBF did not return to baseline. In this animal model, DPOAEs were found to be sensitive measures of cochlear function during transient cochlear ischemic episodes, suggesting the utility of DPOAE monitoring of auditory function during surgery of cerebello-pontine angle tumors.

Changes in distortion of two-tone cochlear microphonic and otoacoustic emission signals during an acute endolymphatic hydrops in the guinea pig

An acute endolymphatic hydrops was induced by the injection of 1.1 mul of artificial endolymph into the scala media of guinea pig cochleas. This volume corresponds with an acute endolymphatic hydrops of 23%. During and after the injection, cochlear function was assessed by measuring the 2f(1)-f(2 )and f(2)-f(1) distortion products in cochlear microphonics (CMDP) and the 2f(1)-f(2) distortion product otoacoustic emission (DPOAE). A reversible pressure increase of 23 Pa and a relatively stable endocochlear potential (EP) were accompanied by a mean decrease in 2f(1)-f(2) DPOAE of only 3.4 dB. Similarly, the 2f(1)-f(2) CMDP amplitude change was minimal during and after the injection. The only substantial change was measured in the f(2)-f(1) CMDP amplitude. The measured range of distortion amplitudes during an acute endolymphatic hydrops can be related to small changes in the cochlear transducer operating point.

Evidence for a bipolar change in distortion product otoacoustic emissions during contralateral acoustic stimulation in humans

The aim of this study was to investigate the activity of the medial olivocochlear (MOC) efferents during contralateral (CAS) and ipsilateral acoustic stimulation (IAS) by recording distortion product otoacoustic emission (DPOAE) suppression and DPOAE adaptation in humans. The main question was: do large bipolar changes in DPOAE level (transition from enhancement to suppression) also occur in humans when changing the primary tone level within a small range as described by Maison and Liberman for guinea pigs [J. Neurosci. 20, 4701-4707 (2000)]? In the present study, large bipolar changes in DPOAE level (14 dB on average across subjects) were found during CAS predominantly at frequencies where dips in the DPOAE fine structure occurred. Thus, effects of the second DPOAE source might be responsible for the observed bipolar effect. In contrast, comparable effects were not found during IAS as was reported in guinea pigs. Reproducibility of CAS DPOAEs was better than that for IAS DPOAEs. Thus, contralateral DPOAE suppression is suggested to be superior to ipsilateral DPOAE adaptation with regard to measuring the MOC reflex strength and for evaluating the vulnerability of the cochlea to acoustic overexposure in a clinical context.

Inner ear drug delivery via a reciprocating perfusion system in the guinea pig

Rapid progress in understanding the molecular mechanisms associated with cochlear and auditory nerve degenerative processes offers hope for the development of gene-transfer and molecular approaches to treat these diseases in patients. For therapies based on these discoveries to become clinically useful, it will be necessary to develop safe and reliable mechanisms for the delivery of drugs into the inner ear, bypassing the blood-labyrinthine barrier. Toward the goal of developing an inner ear perfusion device for human use, a reciprocating microfluidic system that allows perfusion of drugs into the cochlear perilymph through a single inlet hole in scala tympani of the basal turn was developed. The performance of a prototype, extracorporeal reciprocating perfusion system in guinea pigs is described. Analysis of the cochlear distribution of compounds after perfusion took advantage of the place-dependent generation of responses to tones along the length of the cochlea. Perfusion with a control artificial perilymph solution had no effect. Two drugs with well-characterized effects on cochlear physiology, salicylate (5 mM) and DNQX (6,7-Dinitroquinoxaline-2,3-dione; 100 and 300 microM), reversibly altered responses. The magnitude of drug effect decreased with distance from the perfusion pipette for up to 10 mm, and increased with dose and length of application.

Audiometric predictions using stimulus-frequency otoacoustic emissions and middle ear measurements

OBJECTIVE

The goals of the study are to determine how well stimulus-frequency otoacoustic emissions (SFOAEs) identify hearing loss, classify hearing loss as mild or moderate-severe, and correlate with pure-tone thresholds in a population of adults with normal middle ear function. Other goals are to determine if middle ear function as assessed by wideband acoustic transfer function (ATF) measurements in the ear canal account for the variability in normal thresholds, and if the inclusion of ATFs improves the ability of SFOAEs to identify hearing loss and predict pure-tone thresholds.

DESIGN

The total suppressed SFOAE signal and its corresponding noise were recorded in 85 ears (22 normal ears and 63 ears with sensorineural hearing loss) at octave frequencies from 0.5 to 8 kHz, using a nonlinear residual method. SFOAEs were recorded a second time in three impaired ears to assess repeatability. Ambient-pressure ATFs were obtained in all but one of these 85 ears and were also obtained from an additional 31 normal-hearing subjects in whom SFOAE data were not obtained. Pure-tone air and bone conduction thresholds and 226-Hz tympanograms were obtained on all subjects. Normal tympanometry and the absence of air-bone gaps were used to screen subjects for normal middle ear function. Clinical decision theory was used to assess the performance of SFOAE and ATF predictors in classifying ears as normal or impaired, and linear regression analysis was used to test the ability of SFOAE and ATF variables to predict the air conduction audiogram.

RESULTS

The ability of SFOAEs to classify ears as normal or hearing impaired was significant at all test frequencies. The ability of SFOAEs to classify impaired ears as either mild or moderate-severe was significant at test frequencies from 0.5 to 4 kHz. SFOAEs were present in cases of severe hearing loss. SFOAEs were also significantly correlated with air conduction thresholds from 0.5 to 8 kHz. The best performance occurred with the use of the SFOAE signal-to-noise ratio as the predictor, and the overall best performance was at 2 kHz. The SFOAE signal-to-noise measures were repeatable to within 3.5 dB in impaired ears. The ATF measures explained up to 25% of the variance in the normal audiogram; however, ATF measures did not improve SFOAEs predictors of hearing loss except at 4 kHz.

CONCLUSIONS

In common with other OAE types, SFOAEs are capable of identifying the presence of hearing loss. In particular, SFOAEs performed better than distortion-product and click-evoked OAEs in predicting auditory status at 0.5 kHz; SFOAE performance was similar to that of other OAE types at higher frequencies except for a slight performance reduction at 4 kHz. Because SFOAEs were detected in ears with mild to severe cases of hearing loss, they may also provide an estimate of the classification of hearing loss. Although SFOAEs were significantly correlated with hearing threshold, they do not appear to have clinical utility in predicting a specific behavioral threshold. Information on middle ear status as assessed by ATF measures offered minimal improvement in SFOAE predictions of auditory status in a population of normal and impaired ears with normal middle ear function. However, ATF variables did explain a significant fraction of the variability in the audiograms of normal ears, suggesting that audiometric thresholds in normal ears are partially constrained by middle ear function as assessed by ATF tests.

Lateralization of the effects of the benzodiazepine drug oxazepam on medial olivocochlear system activity in humans

Benzodiazepines (Bzd) are known to interact with GABAergic inhibitory neurotransmission. Previous research on their effect on human auditory efferent pathways--through evoked otoacoustic emissions suppression by contralateral acoustic stimulation (CAS)--indicated a decrease in medial olivocochlear (MOC) efferent system inhibitory activity, after oral intake of oxazepam--representative of the Bzd drug class. To date, this pharmacological effect was only assessed in the right ear. Since a leftward asymmetry of Bzd receptors localization in human auditory cortex has been described recently, we explored in this study the hypothesis of an asymmetrical action of Bzd on MOC efferent functioning. The results revealed a significant difference of Bzd effect probing the right ear versus the left ear, with CAS-induced suppression being less effective in the right than left ear after oxazepam intake. This finding raises the question of possible neurochemical left-right asymmetry in the descending auditory pathways. The potential localization of this asymmetry is discussed.

Sensorineural hearing loss in patients with cerebral palsy after asphyxia and hyperbilirubinemia

The location of lesions causing hearing loss in patients with cerebral palsy due to asphyxia or neonatal hyperbilirubinemia has remained unclear. We performed behavioral audiometry, distortion product otoacoustic emission (DPOAE) and auditory brainstem evoked response (ABR) in six patients with cerebral palsy due to asphyxia or neonatal hyperbilirubinemia in order to determine the lesion location causing their hearing impairment. In all cases, behavioral audiometry revealed a threshold elevation of 50-75 dB and ABR were no response. DPOAE were totally absent in five patients and normal in one patient. Our study suggests that lesions causing hearing loss potentially include the organ of Corti especially at the outer hair cells and the cochlear nerve.

Tinnitus as a plastic phenomenon and its possible neural underpinnings in the dorsal cochlear nucleus

Tinnitus displays many features suggestive of plastic changes in the nervous system. These can be categorized based on the types of manipulations that induce them. We have categorized the various forms of plasticity that characterize tinnitus and searched for their neural underpinnings in the dorsal cochlear nucleus (DCN). This structure has been implicated as a possible site for the generation of tinnitus-producing signals owing to its tendency to become hyperactive following exposure to tinnitus inducing agents such as intense sound and cisplatin. In this paper, we review the many forms of plasticity that have been uncovered in anatomical, physiological and neurochemical studies of the DCN. Some of these plastic changes have been observed as consequences of peripheral injury or as fluctuations in the behavior and chemical activities of DCN neurons, while others can be induced by stimulation of auditory or even non-auditory structures. We show that many parallels can be drawn between the various forms of plasticity displayed by tinnitus and the various forms of neural plasticity which have been defined in the DCN. These parallels lend further support to the hypothesis that the DCN is an important site for the generation and modulation of tinnitus-producing signals.

A method for intracochlear drug delivery in the mouse

The confluence of two rapidly emerging research arenas - development of mouse models of human deafness and inner ear drug therapy for treatment and prevention of hearing loss - provides an opportunity for unprecedented approaches to study and treat deafness. Toward such goals, we have developed a method for intracochlear drug delivery in the mouse. The bulla was exposed using a ventral approach and the stapedial artery cauterized. An opening made into the inferior-medial aspect of the bulla, where the basal cochlear wall fuses with tympanic bulla, provided direct access to the scala tympani without separately opening the bulla or elevating auditory response thresholds. Cochlear responses, assayed by frequency-specific effects on ABRs and DPOAEs, were stable with infusion (1 microl/h) of an artificial perilymph solution (80 min). The glutamate receptor antagonist, CNQX (100 microM; 175 min), reduced ABR responses without affecting DPOAEs. Salicylate (5mM; 165 min) altered both. Both drugs had greatest effects at high frequencies, but distributed throughout the cochlea and were reversible. The safe delivery of drugs into the cochlea by this approach has immediate application in the study and treatment of various forms of human hearing loss that can be modeled in the mouse.

Cochlear function in mice following inhalation of brevetoxin-3

Brevetoxin-3 was shown previously to adversely affect central auditory function in goldfish. The present study evaluated the effects of exposure to this agent on cochlear function in mice using the 2f(1)-f(2) distortion-product otoacoustic emission (DPOAE). Towards this end, inbred CBA/CaJ mice were exposed to a relatively high concentration of brevetoxin-3 (approximately=400 microg/m(3)) by nose-only inhalation for a 2-h period. Further, a subset of these mice received a second exposure a day later that lasted for an additional 4 h. Mice exposed only once for 2 h did not exhibit any notable cochlear effects. Similarly, mice exposed two times, for a cumulative dose of 6 h, exhibited essentially no change in DPOAE levels.

Investigation of potential effects of cellular phones on human auditory function by means of distortion product otoacoustic emissions

Outer hair cells (OHC) are thought to act like piezoelectric transducers that amplify low sounds and hence enable the ear's exquisite sensitivity. Distortion product otoacoustic emissions (DPOAE) reflect OHC function. The present study investigated potential effects of electromagnetic fields (EMF) of GSM (Global System for Mobile Communication) cellular phones on OHCs by means of DPOAEs. DPOAE measurements were performed during exposure, i.e., between consecutive GSM signal pulses, and during sham exposure (no EMF) in 28 normally hearing subjects at tone frequencies around 4 kHz. For a reliable DPOAE measurement, a 900-MHz GSM-like signal was used where transmission pause was increased from 4.034 ms (GSM standard) to 24.204 ms. Peak transmitter power was set to 20 W, corresponding to a specific absorption rate (SAR) of 0.1 W/kg. No significant change in the DPOAE level in response to the EMF exposure was found. However, when undesired side effects on DPOAEs were compensated, in some subjects an extremely small EMF-exposure-correlated change in the DPOAE level (< 1 dB) was observed. In view of the very large dynamic range of hearing in humans (120 dB), it is suggested that this observation is physiologically irrelevant.

Predicting severity of cochlear hair cell damage in adult chickens using DPOAE input-output functions

Distortion product otoacoustic emissions (DPOAE) were recorded from the ear canal of aged broiler chickens which have been shown to present with age-related cochlear degeneration [Hear. Res. 166 (2002) 82]. We describe the relationship between the shape of the DPOAE input-output (I/O) function and the type of hair cell damage present at and between the cochlear frequency places of the DPOAE primary tones (f1 and f2). The mid stimulus level compressive growth of the mean DPOAE I/O functions is reduced in a graded fashion relative to the severity of hair cell damage. However, individual DPOAE I/O functions within most hair cell damage groups show large variability from this characteristic. Various least squares regression models were used to predict hair cell density from indices derived from the DPOAE I/O function (area, threshold and slope). The results showed that no simple linear relationship exists between hair cell density and the DPOAE I/O function indices. Multivariate binary logistic regression used DPOAE I/O function indices to predict membership in hair cell damage groups. The logistic model revealed that DPOAE threshold can be used to predict the occurrence of severe/total hair cell damage with good specificity though poor sensitivity.

Evaluation of distortion products produced by the human auditory system

During the simultaneous monaural presentation of two primary tones, distortion products can be measured acoustically in the ear canal (DPOAEs) and electrically as auditory evoked potentials (DPAEPs). The purpose of this investigation was to elucidate the sources of nonlinearity within the human auditory system responsible for generating quadratic (QDT) and cubic (CDT) difference tones. Measurements of DPOAEs and DPAEPs were obtained from 24 normal-hearing adults (12 male) in conditions with and without presentation of a 60 dB SPL contralateral noise. The effects of primary-tone signal duration and mode of presentation on measurements of DPAEPs were also examined. Results indicated that overall, both acoustic and electric distortion products were suppressed during presentation of a contralateral noise. Increases in the duration of the primary tones caused increases in DPAEP amplitudes. A greater proportion of individuals exhibited DPAEPs with monotic compared to dichotic presentation of the primary tones. The findings of the investigation supported the conjecture that a cochlear nonlinearity produced CDT acoustic and electric distortion products. Evidence concerning the origin of the QDT DPAEP was inconclusive, and contributions from both cochlear and neural nonlinear sources could not be ruled out.

Effects of 900 MHz electromagnetic fields exposure on cochlear cells' functionality in rats: Evaluation of distortion product otoacoustic emissions

In recent years, the widespread use of mobile phones has been accompanied by public debate about possible adverse consequences on human health. The auditory system is a major target of exposure to electromagnetic fields (EMF) emitted by cellular telephones; the aim of this study was the evaluation of possible effects of cellular phone-like emissions on the functionality of rat's cochlea. Distortion Products OtoAcoustic Emission (DPOAE) amplitude was selected as cochlea's outer hair cells (OHC) status indicator. A number of protocols, including different frequencies (the lower ones in rat's cochlea sensitivity spectrum), intensities and periods of exposure, were used; tests were carried out before, during and after the period of treatment. No significant variation due to exposure to microwaves has been evidenced.

The significance of the calcium signal in the outer hair cells and its possible role in tinnitus of cochlear origin

Finely tuned changes in intracellular Ca(2+) concentration modulate a variety of cellular functions in eukaryotic cells. The cytosolic Ca(2+) concentration is also tightly controlled in the outer hair cells (OHCs), the highly specialized receptor and effector cells in the mammalian auditory epithelium, which are responsible for high sensitivity and sharp frequency discrimination in hearing. OHCs possess a complex system of transporters, pumps, exchangers, channels and binding proteins to develop and to halt the regulatory Ca(2+) signal. The crucial role of elevated intracellular Ca(2+) concentration in OHCs is to increase the efficacy of the electromechanical (electromotile) feedback via remodeling of the cortical cytoskeleton. Anomalies in the Ca(2+) signaling pathway may lead to hypersensitivity of the cochlear amplifier and subsequently trigger tinnitus of cochlear origin. This review describes the dynamics of Ca(2+) signaling in the OHCs and a model that may convey a putative mechanism of development of subjective idiopathic cochlear tinnitus.

Intraoperative Monitoring of Cochlear Function Using Distortion Product Otoacoustic Emissions (DPOAEs) in Patients with Cerebellopontine Angle Tumors

HYPOTHESIS

Intraoperative monitoring by distortion-product otoacoustic emissions reflects the cochlear function changes in the real-time domain during removal of cerebellopontine angle tumors.

BACKGROUND

Cerebellopontine angle tumor surgery is associated with a significant risk of damaging internal auditory canal contents. Although monitoring facial nerve function intraoperatively has already been effectively developed, such efficacious monitoring of auditory function remains to be established. The aim of this study was to investigate the utility of distortion-product otoacoustic emissions for intraoperative monitoring of the cochlear function in humans during removal of cerebellopontine angle tumors.

METHODS

Continuous intraoperative monitoring of distortion-product otoacoustic emissions was performed in 20 of 62 patients undergoing surgical removal of cerebellopontine angle tumors. All of these 20 patients, who underwent the retrosigmoid approach, had distortion-product otoacoustic emissions present preoperatively. Depending on the amplitude and frequency band at which distortion-product otoacoustic emissions were present, monitoring was carried out at 2.0 to 6.0 kHz with primary stimulus tone amplitudes of 60 to 70 dB sound pressure level.

RESULTS

In patients operated on for cerebellopontine angle tumors, various patterns of distortion-product otoacoustic emission amplitude reductions and recoveries were observed. Distortion-product otoacoustic emissions recorded from the basal part of the cochlea (i.e., high frequencies) changed earlier and more profoundly than those from the middle and apical sections (i.e., lower frequencies). In some cases, cochlear function was affected irreversibly as reflected by loss of distortion-product otoacoustic emissions. Microcoagulation of small vessels, tumor debulking, and compression or stretch of the internal auditory canal contents were found to be procedures affecting distortion-product otoacoustic emissions. The status of distortion-product otoacoustic emissions at the conclusion of tumor dissection correlated with postoperative hearing levels.

CONCLUSION

Distortion-product otoacoustic emissions were used to monitor in the real-time domain auditory function during cerebellopontine angle tumor removal operations. The status of distortion-product otoacoustic emissions at the conclusion of the operations was related to postoperative hearing.

Assessment of auditory function in rhesus monkeys (Macaca mulatta): effects of age and calorie restriction

Age-related alterations in auditory function were evaluated in adult male rhesus monkeys (Macaca mulatta) involved in a long-term study evaluating the effects of caloric restriction (CR) on aging. We assessed 26 monkeys in a control group fed a low fat, high fiber diet at approximately ad libitum levels and 24 monkeys in a CR group that were fed the same diet reduced in amount by 30% compared to age- and weight-matched controls. The following measures of auditory function were obtained while monkeys were maintained under anesthesia: (1) distortion product otoacoustic emissions (DPOAEs); (2) auditory brainstem responses (ABRs); and (3) middle latency responses (MLRs). All DPOAE measures and peak II amplitude significantly decreased with age, while peak IV latency and ABR threshold significantly increased with age. We found no significant effects of CR on any auditory parameters examined.

Similarity in loudness and distortion product otoacoustic emission input/output functions: implications for an objective hearing aid adjustment

The aim of the present study was to compare distortion product otoacoustic emissions (DPOAEs) to loudness with regard to the potentiality of DPOAEs to determine characteristic quantities of the cochlear-impaired ear and to derive objective hearing aid parameters. Recently, Neely et al. [J. Acoust. Soc. Am. 114, 1499-1507 (2003)] compared DPOAE input/output functions to the Fletcher and Munson [J. Acoust. Soc. Am. 5, 82-108 (1933)] loudness function finding a close resemblance in the slope characteristics of both measures. The present study extended their work by performing both loudness and DPOAE measurements in the same subject sample, and by developing a method for the estimation of gain needed to compensate for loss of cochlear sensitivity and compression. DPOAEs and loudness exhibited similar behavior when plotted on a logarithmic scale and slope increased with increasing hearing loss, confirming the findings of Neely et al. To compensate for undesired nonpathological impacts on the magnitude of DPOAE level, normalization of DPOAE data was implemented. A close resemblance between gain functions based on loudness and normalized DPOAE data was achieved. These findings suggest that DPOAEs are able to quantify the loss of cochlear sensitivity and compression and thus might provide parameters for a noncooperative hearing aid adjustment.

Identification of unique transcripts from a mouse full-length, subtracted inner ear cDNA library

A small-scale full-length library construction approach was developed to facilitate production of a mouse full-length cDNA encyclopedia representing approximately 250 enriched, normalized, and/or subtracted cDNA libraries. One library produced using this approach was a subtracted adult mouse inner ear cDNA library (sIEa). The average size of the inserts was approximately 2.5 kb, with the majority ranging from 0.5 to 7.0 kb. From this library 22,574 sequence reads were obtained from 15,958 independent clones. Sequencing and chromosomal localization established 5240 clusters, with 1302 clusters being unique and 359 representing new ESTs. Our sIEa library contributed 56.1% of the 7773 nonredundant Unigene clusters associated with the four mouse inner ear libraries in the NCBI dbEST. Based on homologous chromosomal regions between human and mouse, we identified 1018 UniGene clusters associated with the deafness locus critical regions. Of these, 59 clusters were found only in our sIEa library and represented approximately 50% of the identified critical regions.

Protective effect of corticosteroid against the cytotoxicity of aminoglycoside otic drops on isolated cochlear outer hair cells

OBJECTIVES

Otic drops are commonly used not only for otitis externa but also for otorrhea in the presence of tympanic membrane perforation or tympanostomy tube. Many studies demonstrated the ototoxicity of aminoglycoside. In our previous study, we observed that gentamicin (GM), when activated with liver extract, demonstrated significant cytotoxicity. The purpose of this study was to assess the protective effect of corticosteroid against the cytotoxicity of GM and tobramycin drops using isolated cochlear outer hair cells (OHCs) in vitro with liver extract.

METHODS

OHCs from adult chinchilla cochleae were exposed to standard bathing solution, liver extract alone, and aminoglycoside otic drops with and without corticosteroid and liver extract. All experiments were performed at an osmolality of 305 +/- 5 mOsm, at room temperature, and for up to 60 minutes. The images of OHCs were recorded using an inverted microscope and analyzed on the Image Pro-Plus 3.0 program. Time to cell death and change of cell length were measured and analyzed.

RESULTS

The time to cell death and percent change in cell length observed was significantly longer in the GM + liver extract + dexamethasone group than the GM + liver extract group (P <.05). The Tobradex + liver extract group showed an insignificant increase in percent change of cell length (P >.05) and significantly increased time to cell death than the tobramycin + liver extract group (P <.05).

CONCLUSION

This study demonstrated that dexamethasone significantly reduced aminoglycoside cytotoxicity.

On a possible prognostic value of otoacoustic emissions: a study on patients with sudden hearing loss

Otoacoustic emissions (OAE) and pure tone audiogram (PTA) were examined in 26 ears of 25 patients suffering from sudden hearing loss from the 1st day to up to 505 days following the drop of hearing to test the hypothesis whether the OAEs are capable of delivering predictive information about the recovery process. The patients were selected from 50 candidates according to the following criteria: one or both ears exhibited a systematic and significant recovery of pure tone threshold in at least one frequency, OAEs were detectable and PTA available, a conductive hearing loss was excluded, and the auditory brainstem responses (ABR) yielded no signs of retrocochlear disorders. Transitory evoked otoacoustic emissions (TEOAE) and distortion product otoacoustic emissions (DPOAE) were measured under constant stimulus and recording conditions in three to nine sessions. The relation between OAE level and actual pure tone threshold was subject to a regression analysis. The correlation between both parameters is small but significant. Even smaller correlations are observed if the OAE level is related to former hearing loss, whereas the correlation improves if the OAE level is compared to the pure tone threshold measured in a later session. The slopes of individual trajectories that connect the successive results of one ear in a plane defined by hearing loss and OAE level show a remarkable accumulation around zero, i.e., in many cases the OAEs remain unchanged even if the hearing loss decreases. The comparison of the OAE levels measured at an early stage with later audiograms shows that there are only a small number of cases with small initial emissions and good final threshold or large initial emissions and bad final threshold. This means that small initial OAEs end up with a remaining final hearing deficit, whereas a high OAE level immediately after drop of threshold correlates with good outcome. The reliability of an individual prediction based on the OAE level combined with the threshold after sudden hearing loss and the consequences for the physiologic mechanisms underlying the sudden hearing loss remain to be proved in further investigations.

Mechanisms of Mammalian Otoacoustic Emission and their Implications for the Clinical Utility of Otoacoustic Emissions

We review recent progress in understanding the physical and physiological mechanisms that generate otoacoustic emissions (OAEs). Until recently, the conceptual model underlying the interpretation of OAEs has been an integrated view that regards all OAEs as manifestations of cochlear nonlinearity. However, OAEs appear to arise by at least two fundamentally different mechanisms within the cochlea: nonlinear distortion and linear reflection. These differences in mechanism have be used to construct a new taxonomy for OAEs that identifies OAEs based on their mechanisms of generation rather than the details of their measurement. The mechanism-based taxonomy provides a useful conceptual framework for understanding and interpreting otoacoustic responses. As commonly measured in the clinic, distortion-product and other evoked OAEs comprise a mixtures of emissions produced by both mechanisms. This mixing precludes any fixed correspondence with the conventional, measurement-based nomenclature. We discuss consequences of the taxonomy for the clinical measurement and interpretation of OAEs.

Effect of platelet activating factor with and without receptor antagonist (WEB2170) on morphology of isolated cochlear outer hair cells

Platelet activating factor (PAF), generated from biologically active phospholipids, has been implicated as a potent inflammatory mediator and has been shown to be involved in many pathological processes, especially in inflammation and allergy. It has been suspected that PAF may be one of the inflammatory mediators in middle ear effusion that can induce sensorineural hearing loss, as observed in chronic otitis media. The PAF receptor antagonist WEB2170 has been studied extensively, and its inhibitory effects against various PAF actions are well proven in otologic systems. The purpose of our study was to determine the effect of superfusion of PAF and WEB2170 on morphological changes in isolated cochlear outer hair cells (OHCs). Isolated OHCs from adult chinchilla cochleas were exposed to albumin-phosphate-buffered saline solution (1 mg/mL), WEB2170 (5 mg/30 mL), PAF (1 micromol/L), or both PAF (I micromol/L) and WEB2170 (5 mg/30 mL). All experiments were performed at an osmolality of 305 +/- 5 mOsm at room temperature for 30 minutes. The cells were observed with an inverted microscope; the images were stored and analyzed on the Image Pro-Plus program. The OHCs exposed to control albumin-phosphate-buffered saline solution or to WEB2170 did not show any significant change in cell shape or length. The cells exposed to 1 micromol/L of PAF showed ballooning and significant shortening of the mean cell length in 15 to 20 minutes. These morphological changes in OHCs can be prevented by pretreating OHCs with WEB2170. This study demonstrated that exposure to PAF causes morphological changes in isolated OHCs that can be prevented by the PAF receptor antagonist WEB2170.

A longitudinal study of distortion product otoacoustic emission ipsilateral suppression and input/output characteristics in human neonates

Past work has shown that distortion product otoacoustic emission (DPOAE) (2f1-f2) ipsilateral suppression and input/output (I/O) characteristics are not adult-like in prematurely born neonates [Abdala, J. Acoust. Soc. Am. 110, 1465-1476 (2001)]. These age differences are most pronounced at f2 = 6000 Hz and have been interpreted to indicate a subtle immaturity in human cochlear function prior to term birth. It is still not clear, however, whether term-born neonates are completely adult-like in cochlear function. To study this question, DPOAE suppression and I/O functions for f2 = 6000 Hz were measured in a group of prematurely born neonates at weekly intervals over a period of 7-8-weeks, a group of normal-hearing adults, and during a one-time test session in a group of term-born neonates. Results show that there was no significant change in suppression tuning, suppression growth, and various I/O characteristics across test session for premature neonates, but there was an age-group effect; even once prematurely born neonates reached the equivalence of term-like status (38-40-weeks postconceptional weeks), they continued to show narrower suppression tuning than adults, shallower suppression growth for low-frequency side suppressor tones, and an elevated amplitude saturation plateau on the I/O function. Term-born neonates showed DPOAE results that were comparable to those measured from premature neonates and unlike adult findings. These results suggest that a subtle immaturity in cochlear function persists into the postnatal period.

Cochlear implantation in a patient with neurofibromatosis type 1 and profound hearing loss: evidence to support a cochlear site of lesion

OBJECTIVE

To determine the efficacy of cochlear implantation in an individual with neurofibromatosis type 1 and profound hearing loss and to determine, to the extent possible in a living subject, the site of lesion of the hearing loss in an individual with neurofibromatosis type 1.

STUDY DESIGN

Postoperative assessment of an adult male with neurofibromatosis type 1.

SETTING

The study was completed in the Departments of Otolaryngology and Audiology, University of Massachusetts Medical Center, Massachusetts.

PATIENTS

One adult male with neurofibromatosis type 1 and bilateral, chiefly profound, sensorineural hearing loss.

INTERVENTIONS

The subject underwent cochlear implantation as treatment of bilateral, chiefly profound sensorineural hearing loss.

MAIN OUTCOME MEASURES

Outcome measures include results of electrical auditory brainstem response testing, electrical compound action potential measures, and speech recognition scores.

RESULTS

Results of testing revealed expected latencies of waves II, III, and V of the electrical auditory brainstem response, measurable compound action potentials across virtually the entire length of the electrode array, and high levels of open-set, auditory-only speech understanding.

CONCLUSIONS

This is the first case report describing the outcome of cochlear implantation in a patient with neurofibromatosis type 1. Composite test results support the absence of VIIIth nerve degeneration or other anomalies, indicating a cochlear site of lesion for the hearing loss in this patient. Based on the results of this investigation, standard cochlear implantation should be considered a viable option and the first line of therapy for the treatment of bilateral, severe to profound sensorineural hearing loss in patients with neurofibromatosis type 1 and normal, VIIIth nerve radiographic findings.

Cochlear compression estimates from measurements of distortion-product otoacoustic emissions

Evidence of the compressive growth of basilar-membrane displacement can be seen in distortion-product otoacoustic emission (DPOAE) levels measured as a function of stimulus level. When the levels of the two stimulus tones (f1 and f2) are related by the formula L1 = 39 dB + 0.4 x L2 [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] the shape of the function relating DPOAE level to L2 is similar (up to an L2 of 70 dB SPL) to the classic Fletcher and Munson [J. Acoust. Soc. Am. 9, 1-10 (1933)] loudness function when plotted on a logarithmic scale. Explicit estimates of compression have been derived based on recent DPOAE measurements from the laboratory. If DPOAE growth rate is defined as the slope of the DPOAE I/O function (in dB/dB), then a cogent definition of compression is the reciprocal of the growth rate. In humans with normal hearing, compression varies from about 1 at threshold to about 4 at 70 dB SPL. With hearing loss, compression is still about 1 at threshold, but grows more slowly above threshold. Median DPOAE I/O data from ears with normal hearing, mild loss, and moderate loss are each well fit by log functions. When the I/O function is logarithmic, then the corresponding compression is a linear function of stimulus level. Evidence of cochlear compression also exists in DPOAE suppression tuning curves, which indicate the level of a third stimulus tone (f3) that reduces DPOAE level by 3 dB. All three stimulus tones generate compressive growth within the cochlea; however, only the relative compression (RC) of the primary and suppressor responses is observable in DPOAE suppression data. An RC value of 1 indicates that the cochlear responses to the primary and suppressor components grow at the same rate. In normal ears, RC rises to 4, when f3 is an octave below f2. The similarities between DPOAE and loudness compression estimates suggest the possibility of predicting loudness growth from DPOAEs; however, intersubject variability makes such predictions difficult at this time.

Preoperative speech and pure-tone audiometry in four types of patients with acoustic neuroma

PURPOSE

The goal of this study was to compare pure-tone and speech audiometry in 4 groups of patients with confirmed unilateral acoustic neuroma in which auditory brainstem responses and transient-evoked otoacoustic emissions were either both normal, abnormal, or one of the tests was abnormal.

MATERIAL AND METHODS

This study was realized during a preoperative assessment of 65 patients (29 men, 36 women) from 25 to 78 years of age suffering from unilateral acoustic neuroma. The assessment, preceded by tympanometry, included recordings of auditory brainstem responses, transient-evoked otoacoustic emissions, pure-tone audiometry, speech recognition thresholds, and speech discrimination.

RESULTS

Some discrepancies between objective and behavioral test results were noticed. Subjects with no otoacoustic emissions but present auditory brainstem responses did not show any significant difference in their speech scores as compared with subjects with both auditory brainstem responses and otoacoustic emissions.

CONCLUSIONS

In summary, this study showed that comparable audiometric findings in acoustic neuroma patients can be found regardless of the presence of transient otoacoustic emissions (TEOAEs). TEOAEs are a good screening tool and have been used, like auditory brainstem-evoked responses, as a predictive measure before hearing preservation procedures but cannot predict the audiogram or give information about speech perception. The findings confirm that even if auditory brainstem responses are an extremely useful diagnostic tool for identifying acoustic neuroma, this test provides only giving pieces of information regarding auditory abilities. Also, the pure-tone audiogram gives useful information but has to be used in conjunction with speech audiometry to get an accurate picture of the patient's true auditory abilities.

Electrophysiological findings in the Sprague-Dawley rat induced by moderate-dose carboplatin

Carboplatin is a second generation platinum-containing anti-tumor drug which selectively alters the micromechanical function of the inner hair cells (IHCs) of the organ of Corti in the chinchilla. Data from a recent study [Wake et al., Acta Otolaryngol. 116 (1996) 374-381], using the chinchilla model, have suggested that a moderate dose of carboplatin alters the efferent feedback loop gain of the OHCs. The present study was designed to evaluate the possible 'efferent feedback alteration mechanism' in the Sprague-Dawley rat using distortion product otoacoustic emissions (DPOAEs). A moderate dose of carboplatin (50 mg/kg body weight) was administered by a 30 min i.p. infusion. Pre- and 72-h post-treatment DPOAE and auditory brainstem response (ABR) recordings were acquired from a group of 12 rats. The animals were anesthetized with a ketamine-atropin anesthesia administered in two consecutive phases. The DPOAE responses (cubic distortion products) were recorded with four asymmetrical protocols: P1=60-50, P2=50-40, P3=40-30 and P4=30-20 dB SPL (sound pressure level), in the frequency range from 4.0 to 16 kHz. ABR responses were obtained for bipolar clicks and tone pips at the frequencies 8.0, 10.0, 20.0 and 30 kHz using stimuli in the range from 100 to 30 dB SPL. Significant ABR threshold shifts of 15 dB were observed at 30 kHz, and shifts of 10 dB at 20, 16 and 10 kHz. The comparison of pre- and post-treatment DPOAE responses did not reveal any significant changes for protocols P1, P2 and P4. Data from the P3 protocol indicated a decrease of the DPOAE amplitude. The findings from the rat model suggest that (a) moderate doses of carboplatin do not affect the efferent feedback loop OHC function and (b) the cochlear susceptibility to carboplatin across species is different, even at moderate-dose regimes.

Further efforts to predict pure-tone thresholds from distortion product otoacoustic emission input/output functions

Recently, Boege and Janssen [J. Acoust. Soc. Am. 111, 1810-1818 (2002)] fit linear equations to distortion product otoacoustic emission (DPOAE) input/output (UO) functions after the DPOAE level (in dB SPL) was converted into pressure (in microPa). Significant correlations were observed between these DPOAE thresholds and audiometric thresholds. The present study extends their work by (1) evaluating the effect of frequency, (2) determining the behavioral thresholds in those conditions that did not meet inclusion criteria, and (3) including a wider range of stimulus levels. DPOAE I/O functions were measured in as many as 278 ears of subjects with normal and impaired hearing. Nine f2 frequencies (500 to 8000 Hz in 1/2-octave steps) were used, L2 ranged from 10 to 85 dB SPL (5-dB steps), and L1 was set according to the equation L1 = 0.4L2 + 39 dB [Kummer et al., J. Acoust. Soc. Am. 103, 3431-3444 (1998)] for L2 levels up to 65 dB SPL, beyond which L1 = L2. For the same conditions as those used by Boege and Janssen, we observed a frequency effect such that correlations were higher for mid-frequency threshold comparisons. In addition, a larger proportion of conditions not meeting inclusion criteria at mid and high frequencies had hearing losses exceeding 30 dB HL, compared to lower frequencies. These results suggest that DPOAE I/O functions can be used to predict audiometric thresholds with greater accuracy at mid and high frequencies, but only when certain inclusion criteria are met. When the SNR inclusion criterion is not met, the expected amount of hearing loss increases. Increasing the range of input levels from 20-65 dB SPL to 10-85 dB SPL increased the number of functions meeting inclusion criteria and increased the overall correlation between DPOAE and behavioral thresholds.

Distortion product otoacoustic emission response characteristics in older adults

OBJECTIVE

The primary purpose of this study was to determine the distortion product otoacoustic emission (DPOAE) and noise response characteristics in a large sample of older adults. Another purpose was to evaluate how specific absolute DPOAE levels or DPOAE/Noise ratios differentiated hearing status in these individuals.

DESIGN

A cross-sectional design was utilized for this study. As a part of the Epidemiology of Hearing Loss Study (EHLS), DPOAEs were measured in 937 of the 3,429 participants aged 48 to 92 yr. The DPOAE and noise response characteristics were evaluated at 1,000, 2,000, 4,000, and 8,000 Hz. Absolute DPOAE level and DPOAE/Noise ratios were measured in the participants. The DPOAE data were compared with individual pure-tone frequencies (1,000, 2,000, 4,000, and 8,000 Hz) in the participants to investigate how DPOAE responses differentiated ears with normal hearing from impaired ears. Sensitivity, specificity, positive and negative predictive values, and accuracies were calculated for various absolute DPOAE levels and DPOAE/Noise ratios.

RESULTS

Due to the considerable overlap between DPOAE responses and the noise levels at 1,000 Hz, this frequency was not used for any analyses. Sensitivity and specificity were calculated for various DPOAE responses. Sensitivity and specificity varied by frequency for absolute DPOAE levels and DPOAE/Noise ratios. Receiver operator characteristic (ROC) analyses were used to determine which DPOAE responses differentiated normal hearing from hearing loss. The ROC analyses demonstrated that -6 dB SPL at 2,000 Hz, -14 dB SPL at 4,000 Hz, and -22 dB SPL at 8,000 Hz and a +9 dB DPOAE/Noise ratio at each of these frequencies yielded the highest discrimination.

CONCLUSIONS

Sensitivity and specificity varied by DPOAE response characteristics and frequency. The decision as to which DPOAE response criterion used should be based on careful consideration of objectives and the possible consequences of misdiagnosis. The results of this study support the use of DPOAEs as a clinical measure for older adults.

Cochlear toughening, protection, and potentiation of noise-induced trauma by non-Gaussian noise

An interrupted noise exposure of sufficient intensity, presented on a daily repeating cycle, produces a threshold shift (TS) following the first day of exposure. TSs measured on subsequent days of the exposure sequence have been shown to decrease relative to the initial TS. This reduction of TS, despite the continuing daily exposure regime, has been called a cochlear toughening effect and the exposures referred to as toughening exposures. Four groups of chinchillas were exposed to one of four different noises presented on an interrupted (6 h/day for 20 days) or noninterrupted (24 h/day for 5 days) schedule. The exposures had equivalent total energy, an overall level of 100 dB(A) SPL, and approximately the same flat, broadband long-term spectrum. The noises differed primarily in their temporal structures; two were Gaussian and two were non-Gausssian, nonstationary. Brainstem auditory evoked potentials were used to estimate hearing thresholds and surface preparation histology was used to determine sensory cell loss. The experimental results presented here show that: (1) Exposures to interrupted high-level, non-Gaussian signals produce a toughening effect comparable to that produced by an equivalent interrupted Gaussian noise. (2) Toughening, whether produced by Gaussian or non-Gaussian noise, results in reduced trauma compared to the equivalent uninterrupted noise, and (3) that both continuous and interrupted non-Gaussian exposures produce more trauma than do energy and spectrally equivalent Gaussian noises. Over the course of the 20-day exposure, the pattern of TS following each day's exposure could exhibit a variety of configurations. These results do not support the equal energy hypothesis as a unifying principal for estimating the potential of a noise exposure to produce hearing loss.

Cochlear function in ears with immunomediated inner ear disorder

The aim of the present study was to evaluate the performance of ears with inner ear disorder, responsive to immunosuppressive drugs, in advanced tests designed to assess primary cochlear functions (temporal integration, frequency selectivity, cochlear mechanics). The results of this study suggest that immunomediated inner ear disease results, in the acute clinical stage, in the development of endolymphatic hydrops, which increases the stiffness of the vibrating structures within the inner ear and causes dysfunctions of the outer hair cells. Our patients presented with upsloping or flat sensorineural hearing loss, absence of evoked otoacoustic emissions and distortion-product otoacoustic evoked emissions and abnormal temporal integration, frequency selectivity and cochlear mechanics. Following immunosuppressive treatment, hydrops recovered, hearing subsequently returned to normal, the audiometric curve became flat at low-to-middle frequencies and primary cochlear function tended to normalize. This study seems to support the usefulness of testing primary cochlear functions in order to monitor the clinical course of immunomediated inner ear disorders.

Hearing loss evaluation of Sjögren's syndrome using distortion product otoacoustic emissions

Sjögren's syndrome (SS) is a cell-mediated immune disorder primarily affecting the exocrine glands and hearing loss may be the first otological manifestation of this autoimmune disease. In order to assess the degree of sensorineural hearing loss in SS, 22 female patients were examined by means of standard audiometric tests (pure-tone audiometry, acoustic reflexes and impedance testing) and using distortion product otoacoustic emissions (DPOAEs). The results indicated that only 36.3% of the patients had mild sensorineural hearing loss. Hearing level and distortion product threshold estimates were found to be significantly correlated. No relationship was found between the duration of the disease and the DPOAE and hearing threshold variables. The data suggest that SS may not directly cause sensorineural hearing loss.

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.