Microslicing of the resin-embedded cochlea in comparison with the surface preparation technique for analysis of hair cell number and morphology

A Comprehensive Study on Sudden Deafness for Analyzing Their Clinical Characteristics and Prognostic Factors

Background: To study the factors associated with the prognosis of patients with sudden deafness to facilitate clinical treatment and improve efficacy. Methods: A total of 414 patients with sudden deafness treated in Zhenjiang First People's Hospital from January 2020 to December 2022 were chosen. Relevant data were gathered and the effectiveness of treatment was assessed by comparing hearing test results before and after hospital admission and divided into effective and ineffective groups, and the effectiveness of each factor was analyzed using univariate analysis, Spearman's correlation analysis, and multifactor logistic regression. Results: The 2 groups had significant differences in age, presence of tinnitus, degree of hearing loss, and triglyceride levels. Spearman's rank correlation analysis showed a negative correlation between hearing threshold of at least 81 dB at 250 to 8000 Hz, the low-density lipoprotein (LDL), triglyceride levels, and the prognosis (r < 0, P < .001). A positive correlation exists between high-density lipoprotein levels and prognosis (r > 0, P < .001). Receiver operating characteristic curve showed LDL level, age, and time since disease onset appears to be highly predictive. Multivariable logistic regression analysis showed that age >47 years, LDL >2.93 mmol/L, and time to presentation >10 days after disease onset are at higher risk for poor prognosis. Conclusion: Factors that influence the prognosis of patients with sudden deafness include age, tinnitus symptoms, high LDL levels, and the type of hearing curve. Early intervention and targeted treatment should be given to high-risk patients to improve the outcome of sudden deafness in clinical practice.

Clinical features and prognosis of pediatric idiopathic sudden sensorineural hearing loss: A bi-center retrospective study

Objective

Limited research has focused on the clinical features of sudden sensorineural hearing loss (SSNHL) in pediatric patients. This study is aimed to investigate the relationship between clinical features and the baseline hearing severity and outcomes of SSNHL in the pediatric population.

Method

We conducted a bi-center retrospective observational study in 145 SSNHL patients aged no more than 18 years who were recruited between November 2013 and October 2022. Data extracted from medical records, audiograms, complete blood count (CBC) and coagulation tests have been assessed for the relationship with the severity (the thresholds of the initial hearing) and outcomes (recovery rate, hearing gain and the thresholds of the final hearing).

Results

A lower lymphocyte count (P = 0.004) and a higher platelet-to-lymphocyte ratio (PLR) (P = 0.041) were found in the patient group with profound initial hearing than in the less severe group. Vertigo (β = 13.932, 95%CI: 4.082–23.782, P = 0.007) and lymphocyte count (β = −6.686, 95%CI: −10.919 to −2.454, P = 0.003) showed significant associations with the threshold of the initial hearing. In the multivariate logistic model, the probability of recovery was higher for patients with ascending and flat audiograms compared to those with descending audiograms (ascending: OR 8.168, 95% CI 1.450–70.143, P = 0.029; flat: OR 3.966, 95% CI 1.341–12.651, P = 0.015). Patients with tinnitus had a 3.2-fold increase in the probability of recovery (OR 3.222, 95% CI 1.241–8.907, P = 0.019), while the baseline hearing threshold (OR 0.968, 95% CI 0.936–0.998, P = 0.047) and duration to the onset of therapy (OR 0.942, 95% CI 0.890–0.977, P = 0.010) were negatively associated with the odds of recovery.

Conclusions

The present study showed that accompanying tinnitus, the severity of initial hearing loss, the time elapse and the audiogram configuration might be related to the prognosis of pediatric SSNHL. Meanwhile, the presence of vertigo, lower lymphocytes and higher PLR were associated with worse severity.

Transformation of spatial sensitivity along the ascending auditory pathway

Locations of sounds are computed in the central auditory pathway based primarily on differences in sound level and timing at the two ears. In rats, the results of that computation appear in the primary auditory cortex (A1) as exclusively contralateral hemifield spatial sensitivity, with strong responses to sounds contralateral to the recording site, sharp cutoffs across the midline, and weak, sound-level-tolerant responses to ipsilateral sounds. We surveyed the auditory pathway in anesthetized rats to identify the brain level(s) at which level-tolerant spatial sensitivity arises. Noise-burst stimuli were varied in horizontal sound location and in sound level. Neurons in the central nucleus of the inferior colliculus (ICc) displayed contralateral tuning at low sound levels, but tuning was degraded at successively higher sound levels. In contrast, neurons in the nucleus of the brachium of the inferior colliculus (BIN) showed sharp, level-tolerant spatial sensitivity. The ventral division of the medial geniculate body (MGBv) contained two discrete neural populations, one showing broad sensitivity like the ICc and one showing sharp sensitivity like A1. Dorsal, medial, and shell regions of the MGB showed fairly sharp spatial sensitivity, likely reflecting inputs from A1 and/or the BIN. The results demonstrate two parallel brainstem pathways for spatial hearing. The tectal pathway, in which sharp, level-tolerant spatial sensitivity arises between ICc and BIN, projects to the superior colliculus and could support reflexive orientation to sounds. The lemniscal pathway, in which such sensitivity arises between ICc and the MGBv, projects to the forebrain to support perception of sound location.

Molecular basis of hair cell loss

Mechanisms that lead to the death of hair cells are reviewed. Exposure to noise, the use of ototoxic drugs that damage the cochlea and old age are accompanied by hair cell death. Outer hair cells are often more susceptible than inner hair cells, partly because of an intrinsically greater susceptibility; high frequency cells are also more vulnerable. A common factor in hair cell loss following age-related changes and exposure to ototoxic drugs or high noise levels is the generation of reactive oxygen species, which can trigger intrinsic apoptosis (the mitochondrial pathway). However, hair cell death is sometimes produced via an extracellular signal pathway triggering extrinsic apoptosis. Necrosis and necroptosis also play a role and, in various situations in which cochlear damage occurs, a balance exists between these possible routes of cell death, with no one mechanism being exclusively activated. Finally, the numerous studies on these mechanisms of hair cell death have led to the identification of many potential therapeutic agents, some of which have been used to attempt to treat people exposed to damaging events, although clinical trials are not yet conclusive. Continued work in this area is likely to lead to clinical treatments that could be used to prevent or ameliorate hearing loss.

Alignment of sound localization cues in the nucleus of the brachium of the inferior colliculus

Accurate sound localization is based on three acoustic cues (interaural time and intensity difference and spectral cues from directional filtering by the pinna). In natural listening conditions, every spatial position of a sound source provides a unique combination of these three cues in "natural alignment." Although neurons in the central nucleus (ICC) of the inferior colliculus (IC) are sensitive to multiple cues, they do not favor their natural spatial alignment. We tested for sensitivity to cue alignment in the nucleus of the brachium of the IC (BIN) in unanesthetized marmoset monkeys. The BIN receives its predominant auditory input from ICC and projects to the topographic auditory space map in the superior colliculus. Sound localization cues measured in each monkey were used to synthesize broadband stimuli with aligned and misaligned cues; spike responses to these stimuli were recorded in the BIN. We computed mutual information (MI) between the set of spike rates and the stimuli containing either aligned or misaligned cues. The results can be summarized as follows: 1) BIN neurons encode more information about auditory space when cues are aligned compared with misaligned. 2) Significantly more units prefer aligned cues in the BIN than in ICC. 3) An additive model based on summing the responses to stimuli with the localization cues varying individually accurately predicts the alignment preference with all cues varying. Overall, the results suggest that the BIN is the first site in the ascending mammalian auditory system that is tuned to natural combinations of sound localization cues.

Quantitative analysis of the expression of the glutamate-aspartate transporter and identification of functional glutamate uptake reveal a role for cochlear fibrocytes in glutamate homeostasis

There are several subtypes of fibrocyte in the spiral ligament and spiral limbus of the cochlea that may contribute to fluid homeostasis. Immunocytochemical data suggest that these fibrocytes possess the glutamate-aspartate transporter, GLAST, as do supporting cells around the hair cells. However, functional glutamate uptake has not been demonstrated in fibrocytes. We used confocal and post-embedding immunogold electron microscopy to confirm that GLAST is expressed in adult fibrocytes of CD-1 mice with a relative expression: spiral limbus fibrocytes>type II>V>IV>I spiral ligament fibrocytes. Because they were sparsely present in most samples, type III fibrocytes were assessed only in one sample where their GLAST levels were similar to type I. Type II, type V and spiral limbus fibrocytes have many fine cellular processes that increase their surface area, those of the latter two coming into direct contact with perilymph, and type V fibrocytes contain the most glutamate. These data imply that glutamate uptake occurs in the fibrocytes. We assessed uptake of D-aspartate (a glutamate analogue) together with GLAST expression immunocytochemically and electrophysiologically. D-aspartate accumulated into GLAST expressing fibrocytes in vitro and evoked currents blockable by the GLAST inhibitor D,L-threo-beta-benzyloxyaspartate (TBOA), similar to those of supporting cells around inner hair cells. Currents were strongest in spiral limbus fibrocytes, progressively lower in type V and type II fibrocytes, and were negligible in type I fibrocytes in accordance with the relative expression levels of GLAST. We conclude that in addition to their known homeostatic functions, fibrocytes, in particular spiral limbus, type II and type V fibrocytes play a role in glutamate homeostasis in the cochlea.

The dimensions and composition of stereociliary rootlets in mammalian cochlear hair cells: comparison between high- and low-frequency cells and evidence for a connection to the lateral membrane

The sensory bundle of vertebrate cochlear hair cells consists of actin-containing stereocilia that are thought to bend at their ankle during mechanical stimulation. Stereocilia have dense rootlets that extend through the ankle region to anchor them into the cuticular plate. Because this region may be important in bundle stiffness and durability during prolonged stimulation at high frequencies, we investigated the structure and dimensions of rootlets relative to the stereocilia in apical (low-frequency) and basal (high-frequency) regions of rodent cochleae using light and electron microscopy. Their composition was investigated using postembedding immunogold labeling of tropomyosin, spectrin, beta-actin, gamma-actin, espin, and prestin. The rootlets have a thick central core that widens at the ankle, and are embedded in a filamentous meshwork in the cuticular plate. Within a particular frequency region, rootlet length correlates with stereociliary height but between regions it changes disproportionately; apical stereocilia are, thus, approximately twice the height of basal stereocilia in equivalent rows, but rootlet lengths increase much less. Some rootlets contact the tight junctions that underlie the ends of the bundle. Rootlets contain spectrin, tropomyosin, and beta- and gamma-actin, but espin was not detected; spectrin is also evident near the apical and junctional membranes, whereas prestin is confined to the basolateral membrane below the junctions. These data suggest that rootlets strengthen the ankle region to provide durability and may contact with the lateral wall either to give additional anchoring of the stereocilia or to provide a route for interactions between the bundle and the lateral wall.

Differential distribution of beta- and gamma-actin in guinea-pig cochlear sensory and supporting cells

Sensory and supporting cells of the mammalian organ of Corti have cytoskeletons containing beta- and gamma-actin isoforms which have been described as having differing intracellular distributions in chick cochlear hair cells. Here, we have used post-embedding immunogold labelling for beta- and gamma-actin to investigate semiquantitatively how they are distributed in the guinea-pig cochlea and to compare different frequency locations. Amounts of beta-actin decrease and gamma-actin increase in the order, outer pillar cells, inner pillar cells, Deiters' cells and hair cells. There is also more beta-actin and less gamma-actin in outer pillar cells in higher than lower frequency regions. In hair cells, beta-actin is present in the cuticular plate but is more concentrated in the stereocilia, especially in the rootlets and towards the periphery of their shafts; labelling densities for gamma-actin differ less between these locations and it is the predominant isoform of the hair-cell lateral wall. Alignments of immunogold particles suggest beta-actin and gamma-actin form homomeric filaments. These data confirm differential distribution of these actin isoforms in the mammalian cochlea and reveal systematic differences between sensory and supporting cells. Increased expression of beta-actin in outer pillar cells towards the cochlear base may contribute to the greater stiffness of this region.

Time sequence of degeneration pattern in the guinea pig cochlea during cisplatin administration. A quantitative histological study

We investigated the key tissues that are implicated in cisplatin ototoxicity within the time window during which degeneration starts. Guinea pigs were treated with cisplatin at a dose of 2 mg/kg/day for either 4, 6, or 8 consecutive days. Histological changes in the organ of Corti, the stria vascularis and the spiral ganglion were quantified at the light microscopical level. Outer hair cell (OHC) loss started between 4 and 6 days of cisplatin administration, but is only significantly different from the non-treated group after 8 days of treatment. Midmodiolar OHC counts were comparable to the cytocochleogram data. The cross-sectional area of the stria vascularis did not differ from the non-treated group, nor did an endolymphatic hydrops develop during the course of treatment. Spiral ganglion cell (SGC) densities did not decrease. After 6 days, however, detachment of the myelin sheath of the type-I SGCs was seen in the lower basal turn, whereas after 8 days it was also present in the more apically located turns. Myelin sheath detachment is the result of perikaryal shrinkage and swelling of the myelin sheath. The present study confirms that cisplatin at a daily dose of 2 mg/kg has a detrimental effect on the OHCs as well as on the type-I SGCs. These intracochlear effects occur simultaneously; OHC loss and SGC shrinkage start between the fourth and sixth day of cisplatin administration and appear to develop in parallel. At this dose, no histological effect on the stria vascularis could be observed, although previous electrophysiological experiments demonstrated a clear effect on the endocochlear potential

Comparative distribution of glutamate transporters and receptors in relation to afferent innervation density in the mammalian cochlea

The local expression of proteins involved in handling glutamate may be regulated by the number and activity of synapses in regions of glutamatergic innervation. The systematically varying innervation of inner hair cells (IHCs) of the cochlea provides a model to test this suggestion. IHCs are glutamatergic and form a single row along the cochlear spiral. Along this row the number of afferent fibers terminating on IHCs increases toward the base, reaching a peak and thereafter declining. The afferents are segregated so that higher spontaneous rate fibers terminate on the pillar-cell side of the IHC and lower rate fibers terminate on the modiolar side. Using immunofluorescence and postembedding immunogold labeling, we investigated the distributions of the glutamate-aspartate transporter (GLAST or excitatory amino acid transporter 1), vesicular glutamate transporter (VGLUT1), and the AMPA receptor glutamate receptor 4 (GluR4) along the spiral. Immunofluorescent labeling for GLAST in IHC supporting cells increased in intensity to a peak in the region of 6-9 mm from the apex. Immunogold labeling for GLAST was greater overall in these cells in the 10 mm region than in the 1 mm region and also on the pillar-cell side of the IHC compared with the modiolar side. Immunogold labeling for GluR4 was confined to synaptic sites, represented by puncta in immunofluorescence. The relative numbers of puncta changed with a gradient similar to that of GLAST labeling. VGLUT1 labeling occurred in IHCs but showed no clear cochleotopic gradient. These data suggest that both the density of innervation and the activity levels of glutamatergic synapses may be involved in modulating regional expression of GLAST.

An immunogold investigation of the distribution of calmodulin in the apex of cochlear hair cells

Calmodulin is found in the mechanosensitive stereociliary bundle of hair cells where it plays a role in various calcium-sensitive events associated with mechanoelectrical transduction. In this study, we have investigated the ultrastructural distribution of calmodulin in the apex of guinea-pig cochlear hair cells, using post-embedding immunogold labelling, in order to determine in more detail where calmodulin-dependent processes may be occurring. Labelling was found in the cuticular plate as well as the hair bundle, the rootlets of the stereocilia being more densely labelled than the surrounding filamentous matrix. In the bundle, labelling was found almost exclusively at the periphery rather than over the centre of the actin core of the stereocilia, and was clearly associated with the attachments of the lateral links that connect them to their nearest neighbours. It was also found to be denser towards the tips of stereocilia compared to other stereociliary regions and occurred consistently at either end of the tip link that connects stereocilia of adjacent rows. The contact region between stereocilia that is found just below the tip link was also clearly labelled. These concentrations of labelling in the bundle are likely to indicate sites where calmodulin is associated with calcium/calmodulin-sensitive proteins such as the various myosin isoforms and the plasma membrane ATPase (PMCA2a) that are known to occur there, and possibly with the transduction channels themselves. At least one of the myosin isoforms, myosin 1c, is thought to be associated with slow adaptation, and PMCA2a with control of calcium levels in the bundle. The concentration of calmodulin in the contact region further supports the suggestion that this is a functionally distinct region rather than a simple geometrical association between adjacent stereocilia.

Immunohistochemical and morphological studies on the human fetal cochlea: a comparative view on methods

The preservation of morphology and antigenicity can vary uncontrollably with human fetuses since these rely heavily on immediate fixation of the temporal bone following spontaneous abortion. Once good fixation is established, there is the question of the approach taken for morphologic and immunohistochemical studies. To achieve maximal preservation for the purpose of studying normal and pathologic fetal cochleae, commonly used preparation methods for analyzing the cochlea were reviewed and compared for both immunohistochemical and morphologic studies. Cochleae obtained after spontaneous abortion ranged from the 9th gestational week to birth. Four different methods were compared for morphologic study: the block surface method; a microslicing technique; paraffin; and celloidin sectioning. For immunohistochemical study, three methods were compared: pre-embedding; paraffin; and frozen sectioning. For morphologic preservation, the block surface method gave best overall results, showing good representation of the fetal cochlea for surface preparation, light, and electron microscopy. Celloidin sectioning was also found to show good light microscopic results for both the middle and inner ear. To achieve optimal results, preservation quality, fixation procedures, and antibody all contribute to the efficacy of a methods choice.

Ultrastructural localisation of spectrin in sensory and supporting cells of guinea-pig organ of Corti

Spectrin is a cytoskeletal protein found in the cortex of many cell types. It is known to occur in cochlear outer hair cells (OHCs) with previous immunoelectron microscopical studies showing that it is located in the cuticular plate and the cortical lattice. The latter is a network of filaments associated with the lateral plasma membrane that is thought to play a role in OHC motility. Spectrin has also been found in inner hair cells (IHCs) and supporting cells using immunofluorescent techniques, but its ultrastructural distribution in these cells has not yet been described. This has, therefore, been investigated using a monoclonal antibody to alpha-spectrin in conjunction with pre- and post-embedding immunogold labelling for transmission electron microscopy. Labelling was found in a meshwork of filaments beneath the plasma membranes of both IHCs and supporting cells and, in pillar cells, close to microtubule/microfilament arrays. It was also found in association with the stereocilia of OHCs and IHCs and, as expected, in the cortical lattice and cuticular plate of OHCs. Thus, spectrin is a general component of cytoskeletal structures involved in maintaining the specialised cell shapes in the organ of Corti and may contribute to the mechanical properties of all the cell types examined.

Coding for auditory space in the nucleus of the brachium of the inferior colliculus in the ferret

Coding for auditory space in the nucleus of the brachium of the inferior colliculus in the ferret. J. Neurophysiol. 78: 2717-2731, 1997. The nucleus of the brachium of the inferior colliculus (BIN) projects topographically to the deeper layers of the superior colliculus (SC), which contain a two-dimensional map of auditory space. In this study, we have used broadband stimuli presented in the free field to investigate how auditory space is represented in the BIN of the ferret. Response latencies and temporal firing patterns were comparable with those in the SC, and both properties showed some variation with stimulus location. We obtained spatial response profiles at two sound levels (5-15 and 25-35 dB above unit threshold). A large proportion of azimuth profiles (41% in the suprathreshold condition, 80% in the near-threshold condition) presented a single peak, indicating that they were tuned to single regions in space. For some of these units, the preferred speaker position varied considerably with sound level. The remaining units showed predominantly either broad "hemifield" or spatially ambiguous "bilobed" response profiles. At suprathreshold sound levels, the preferred azimuths of the tuned cells were ordered topographically along the rostrocaudal axis of the BIN, although this representation is considerably more scattered than that in the SC. In contrast to the SC, we observed no systematic variation in the distribution of near-threshold best azimuths, which were instead concentrated around the interaural axis in the contralateral hemifield. The azimuth tuning of individual units in the BIN was generally broader at both sound levels than that in the SC. Many units also were tuned for the elevation of the sound source (48% for supra-, 77% for near-threshold stimulation), but there was no evidence for topographic order in the distribution of preferred elevations within the BIN. These results suggest that the BIN sends inputs to the SC that are already selective for sound azimuth and elevation and that show some degree of topographic order for sound azimuth. These inputs then presumably are sharpened and their topography refined by a mechanism that is likely to involve convergence of other inputs and activity-dependent fine tuning of terminal connections, to result in a precise two-dimensional map of auditory space in the SC.

Co-administration of the neurotrophic ACTH(4-9) analogue, ORG 2766, may reduce the cochleotoxic effects of cisplatin

In this study the effect of the neurotrophic ACTH(4-9) analogue, ORG 2766, on cisplatin cochleotoxicity was investigated with both light- and transmission electron microscopy. Guinea pigs were treated with either cisplatin+ORG 2766 (n = 11) or cisplatin + physiological saline (n = 9). All animals treated with cisplatin + physiological saline showed complete loss of outer hair cells (OHC) and degeneration of the organ of Corti in the basal cochlear turns, while partial OHC loss was found in the middle and apical turns. The inner hair cells (IHC) and other cochlear tissues were not affected. Eight animals from the group treated with cisplatin + ORG 2766 demonstrated similar pathological changes, but to a lesser degree, especially in the middle turns. The three remaining animals demonstrated no cochlear alterations at all, light-microscopically, and only minor subcellular changes in the OHCs at the ultrastructural level. Electrophysiologically, these three animals showed normals compound action potential (CAP) amplitudes at stimulus frequencies from 0.5 to 16 kHz and normal cochlear microphonics (CM) in the frequency range from 0.5 to 8 kHz. The other animals treated with cisplatin + ORG 2766 showed a severe loss in their CAPs and CM, except for one showing intermediate loss. All animals from the group treated with cisplatin alone showed a severe loss in their CAPs and CM. Endolymphatic hydrops was present in all animals from the cisplatin- and the cisplatin + ORG 2766-treated groups. These data indicate that daily, concomitant administration of ORG 2766 may reduce OHC loss and subsequent degeneration of the organ of Corti in cisplatin-treated guinea pig cochleas.

Kinematic analysis of shear displacement as a means for operating mechanotransduction channels in the contact region between adjacent stereocilia of mammalian cochlear hair cells

In sensory hair cells of the cochlea, deflection of the stereociliary bundle results in direct mechanical gating of mechanoelectrical transduction channels, a function generally attributed to the tip link running between the tips of short stereocilia and the sides of adjacent taller ones. However, immunocytochemical experiments indicate that the channels may not be associated with the tip link but occur just below it in a region of contact between the stereocilia. To determine whether transduction channels in this location could be operated during physiologically appropriate deflections as effectively by shear displacement as if they were associated with the tip link, a two dimensional kinematic analysis of relative motion between stereocilia has been performed assuming contact between stereocilia is maintained during deflection. Bundle geometry and dimensions were determined from transmission electron micrographs of hair cells from several frequency locations between 0.27 and 13.00 kHz in the guinea-pig cochlea. The analysis indicates that for a 10 nm deflection of the tallest stereocilia of both inner and outer hair cells, i.e. within the range of the maximum sensitivity of mammalian hair bundles, the average shear displacement in the contact region would be 1.6 nm, but that it increases systematically towards higher frequency regions for outer hair cells. This displacement is comparable in magnitude to tip-link elongation for individual stereociliary pairs.

The possible relationship between transient evoked otoacoustic emissions and organ of Corti irregularities in the guinea pig

Otoacoustic emissions are believed to arise from an active process associated with the outer hair cells in the mammalian organ of Corti. They have been attributed to the presence of impedance discontinuities on the basilar membrane which might be caused by hair cell irregularities. To test this hypothesis we have investigated the possible relationship between transient evoked otoacoustic emissions (TEOAEs) and anatomical integrity in the organ of Corti. Click-evoked TEOAEs have been measured from the ear canals of normal, pigmented guinea pigs using an Otodynamics ILO88 analyser. Emissions were present in 18 out of 19 animals tested and the major frequencies observed were consistently present in different measurements over periods of up to ten weeks provided recording conditions were satisfactory. The frequency spectra of the TEOAEs resembled those measured in humans but the latencies of the responses were considerably shorter. In one acute experiment, the TEOAEs were shown to be dependent on metabolic energy as they were lost rapidly following termination with an overdose of anaesthetic. In another case, evoked emissions of long duration (sustained) at about 1 kHz were obtained from both ears. All cochleae examined showed irregularities, especially patches of mainly apical outer hair cell loss of differing extents. However, there was no evidence that substantial lesions coincided consistently with the frequency regions corresponding to the major emissions. Nevertheless, it was noted that the total energy level of emissions was proportional to the total outer hair cell loss, except in one case, where the outer hair cell loss was substantial and the energy level of TEOAEs was considerably lower. Although there is no clear relationship between TEOAEs of specific frequencies and abnormalities at the corresponding cochleotopic location in the organ of Corti which could represent impedance discontinuities, the degree of irregularity may determine the overall emission level. This finding is consistent with the idea that emissions arise as a result of irregularity producing variations in the reflection coefficient.