Cochlear Microphonic Potential Research Articles

The cochlear hook region detects harmonics beyond the canonical hearing range.

Ultrasound, or sound at frequencies exceeding the conventional range of human hearing, is not only audible to mice, microbats, and dolphins, but also creates an auditory sensation when delivered through bone conduction in humans. Although ultrasound is utilized for brain activation and in hearing aids, the physiological mechanism of ultrasonic hearing remains unknown. In guinea pigs, we found that ultrasound above the hearing range delivered through ossicles of the middle ear evokes an auditory brainstem response and a mechano-electrical transduction current through hair cells, as shown by the local field potential called the cochlear microphonic potential (CM). The CM synchronizes with ultrasound, and like the response to audible sounds is actively and nonlinearly amplified. In vivo optical nano-vibration analysis revealed that the sensory epithelium in the hook region, the basal extreme of the cochlear turns, resonates in response both to ultrasound within the hearing range and to harmonics beyond the hearing range. The results indicate that hair cells can respond to stimulation at the optimal frequency and its harmonics, and the hook region detects ultrasound stimuli with frequencies more than two octaves higher than the upper limit of the ordinary hearing range.

Recognition of unilateral auditory neuropathy in children

Objective: This study aimed to compare the audiological characteristics between children with unilateral auditory neuropathy (UAN) and single-sided deafness (SSD) to establish a valid basis for the differential diagnosis of children with UAN. Methods: A retrospective analysis was conducted on audiological and imaging evaluations of children with UAN and SSD who were treated at Beijing Children's Hospital of Capital Medical University between May 2015 and June 2023. There were 17 children with UAN, comprising 10 males and 7 females, with an average age of 4.7 years. Additionally, there were 43 children with SSD, consisting of 27 males and 16 females, with an average age of 6.5 years. Audiological assessments included Auditory brainstem response (ABR), Steady-state auditory evoked potential (ASSR), Behavioural audiometry, Cochlear microphonic potential (CM), Distortino-product otoacoustic emission (DPOAE), and acoustic immittance test. The results of the audiological assessment and imaging phenotypic between the two groups of children were compared and analyzed by applying SPSS 27.0 statistical software. Results: (1) The UAN group (77.8%) had a significantly higher rate of ABR wave IIIL than the SSD group (20.9%) (P<0.01). The PA thresholds at 500 Hz and 1 000 Hz of children with SSD were higher than those of children with UAN, while the ASSR thresholds at 500 Hz, 1000 Hz, 2 000 Hz, and 4 000 Hz of children with SSD were significantly higher than those of children with UAN (P<0.05). (2) The degree of hearing loss in both UAN and SSD children was predominantly complete hearing loss. The percentage of complete hearing loss was significantly higher (χ²=4.353, P=0.037) in the SSD group (93.0%, 40/43) than in the UAN group (63.6%, 7/11). However, the percentage of profound hearing loss was significantly higher in the UAN group (27.3%, 3/11) than in the SSD group (2.3%, 1/43) (Fisher's exact test, P=0.023). In terms of hearing curve configuration, the percentage of flat type was significantly higher in the SSD group (76.7%, 33/43) than in the UAN group (36.4%, 4/11). The proportion of the UAN group (27.3%, 3/11) was significantly higher than that in the SSD group (2.3%, 1/43) in ascending type (P<0.05). There were no statistically significant differences in the hearing curves of the declining type and other types between the two groups (P>0.05). (3) The proportion of imaging assessment without abnormality was significantly more common in the UAN group (81.8%) than in the SSD group (37.1%) (χ²=6.695, P=0.015). Conclusions: Compared to children with SSD, the occurrence of wave IIIL on the ABR test was significantly more common in children with UAN. The percentage of ascending hearing curves was significantly higher in children with UAN than in children with SSD. ASSR thresholds were significantly lower in children with UAN. The normal imaging phenotype was significantly more common in children with UAN than in children with SSD.

Research on the variation in distortion product otoacoustic emissions in patients with auditory neuropathy during the natural course of the disease

Objective: The purpose of this study was to investigate the characteristics of distortion product otoacoustic emissions (DPOAE) in patients with auditory neuropathy (AN). The factors affecting DPOAE elicitation rate of each frequency, elicitation rate of each ear and change rate of first and last diagnosis in the natural course were analyzed. Methods: The sample was obtained from the Multicenter Study on Clinical Diagnosis and Intervention of AN (registration number: ChiCTR2100050125), and the diagnostic criteria for AN were based on the Chinese Clinical Practice Guidelines of Auditory Neuropathy (version 2022). Patients with bilateral AN who underwent 2 or more DPOAE tests were screened and divided into infant groups (≤3 years old) and non-infant groups (>3 years old) according to the age of detection, and the trend of DPOAE elicitation rate of each frequency, elicitation rate of each ear and change rate in the natural course of disease were analyzed, in order to explore the relevant influencing factors. Results: A total of 165 patients (330 ears) with AN were included in the study. The overall DPOAE elicitation rate per ear was 77.0%±29.4% at the initial diagnosis and 65.1%±35.2% at the final diagnosis, with a reduction observed in the elicitation rate of 171 ears (51.82%). In the infant group, there were 49 cases (98 ears), including 28 males and 21 females, whose found age ranged from 0 to 3 years old, with a median age of 0.7 years. DPOAE elicitation rate per ear was 57.9%±35.5% in the initial diagnosis, and 32.4%±32.1% in the final diagnosis, with a reduction observed in the elicitation rate of 69 ears (70.41%). In the non-infant group, there were 116 cases (232 ears), including 59 males and 57 females, ranging in found age from 3.9 to 40 years old, with a median age of 14 years old. DPOAE elicitation rate per ear was 84.6%±23.4% in the initial diagnosis, and 78.3%±27.1% in the final diagnosis, with a reduction observed in the elicitation rate of 102 ears (43.97%). Age was found to be correlated with DPOAE changes by multicategorical unordered logistic regression analysis (B=-0.224, OR=0.799, P<0.001). Conclusions: The elicitation rate of DPOAE in AN patients decreases or even disappears with increasing disease duration; The rate of DPOAE extraction is found to be lower in infant patients with auditory neuropathy (AN) compared to non-infant AN patients. Additionally, it is observed that the decrease in DPOAE extraction rate is more pronounced in infant AN patients as the disease progressed, as compared to non-infant AN patients. DPOAE and cochlear microphonic potentials should be fully combined for accurate diagnosis, and regular follow-up should be conducted to understand the natural course of the disease and give personalized guidance and assistance.

Preserved Auditory Steady State Response and Envelope-Following Response in Severe Brainstem Dysfunction Highlight the Need for Cross-Checking.

Commercially available auditory steady state response (ASSR) systems are widely used to obtain hearing thresholds in the pediatric population objectively. Children are often examined during natural or induced sleep so that the recorded ASSRs are of subcortical origin, the inferior colliculus being often designated as the main ASSR contributor in these conditions. This report presents data from a battery of auditory neurophysiological objective tests obtained in 3 cases of severe brainstem dysfunction in sleeping children. In addition to ASSRs, envelope-following response (EFR) recordings designed to distinguish peripheral (cochlear nerve) from central (brainstem) were recorded to document the effect of brainstem dysfunction on the two types of phase-locked responses. Results obtained in the 3 children with severe brainstem dysfunctions were compared with those of age-matched controls. The cases were identified as posterior fossa tumor, undiagnosed (UD), and Pelizaeus-Merzbacher-Like Disease. The standard audiological objective tests comprised tympanograms, distortion product otoacoustic emissions, click-evoked auditory brainstem responses (ABRs), and ASSRs. EFRs were recorded using horizontal (EFR-H) and vertical (EFR-V) channels and a stimulus phase rotation technique allowing isolation of the EFR waveforms in the time domain to obtain direct latency measurements. The brainstem dysfunctions of the 3 children were revealed as abnormal (weak, absent, or delayed) ABRs central waves with a normal wave I. In addition, they all presented a summating and cochlear microphonic potential in their ABRs, coupled with a normal wave I, which implies normal cochlear and cochlear nerve function. EFR-H and EFR-V waveforms were identified in the two cases in whom they were recorded. The EFR-Hs onset latencies, response durations, and phase-locking values did not differ from their respective age-matched control values, indicating normal cochlear nerve EFRs. In contrast, the EFR-V phase-locking value and onset latency varied from their control values. Both patients had abnormal but identifiable and significantly phase-locked brainstem EFRs, even in a case with severely distorted ABR central waves. ASSR objective audiograms were recorded in two cases. They showed normal or slightly elevated (explained by a slight transmission loss) thresholds that do not yield any clue about their brainstem dysfunction, revealing the method's lack of sensitivity to severe brainstem dysfunction. The present study, performed on 3 sleeping children with severe brainstem dysfunction but normal cochlear responses (cochlear microphonic potential, summating potential, and ABR wave I), revealed the differential sensitivity of three auditory electrophysiological techniques. Estimated thresholds obtained by standard ASSR recordings (cases UD and Pelizaeus-Merzbacher-Like Disease) provided no clue to the brainstem dysfunction clearly revealed by the click-evoked ABR. EFR recordings (cases posterior fossa tumor and UD) showed preserved central responses with abnormal latencies and low phase-locking values, whereas the peripheral EFR attributed to the cochlear nerve was normal. The one case (UD) for which the three techniques could be performed confirms this sensitivity gradient, emphasizing the need for applying the Cross-Check Principle by avoiding resorting to ASSR recording alone. The entirely normal EFR-H recordings observed in two cases further strengthen the hypothesis of its cochlear nerve origin in sleeping children.

Auditory brainstem response and cochlear microphonic potentials evoked by the bone-conducted ultrasound above the hearing range in guinea pigs.

Auditory brainstem response and cochlear microphonic potentials evoked by the bone-conducted ultrasound above the hearing range in guinea pigs.

КОМПЛЕКСНАЯ ДИАГНОСТИКА СЛУХА У ДЕТЕЙ С ЗАБОЛЕВАНИЕМ СПЕКТРА АУДИТОРНЫХ НЕЙРОПАТИЙ В РЕСПУБЛИКЕ БЕЛАРУСЬ

Auditory Neuropathy Spectrum Disorders, ANSD is a state of the auditory system in which the patient has otoacoustic emissions (OAEs) and/or cochlear microphonic potential (CMP), and auditory brainstem responses (ABRs) are absent or significantly reduced. In contrast to sensorineural hearing loss, where the outer and inner hair cells of the cochlea are damaged, in ANSD the outer hair cells are preserved. The complexity of ANSD detection and the variety of clinical symptoms of this disorder make it difficult to differentiate this disease from sensorineural hearing loss. Purpose. Improvement of the differential diagnosis of ANSD and sensorineural hearing loss by defining the main anamnestic and audiological criteria. Materials and methods. The research included 2 groups of patients: the core group - 48 patients with bilateral ANSD and the control group - 30 patients with bilateral chronic sensorineural hearing loss. All patients underwent assessment of the anamnesis data, impedance measurement, tone threshold audiometry, registration of OAE (TEOAE), registration of ABR with allocation of CMP, registration of stationary auditory evoked potentials. Results. In patients with ANSD the incidence of prematurity, low birth weight, and hyperbilirubinemia is significantly higher (p &lt;0.05) than in patients with sensorineural hearing loss. In patients with ANSD the 35delG mutation in the GJB2 gene was not identified, in contrast to patients with sensorineural hearing loss (genetic factor was confirmed in 23.3%) (p &lt;0.05). Newborn hearing screening on both sides was conducted in 15 children (31.3%) with ANSD and only 1 child (3.3%) with sensorineural hearing loss (p &lt;0.05). In 8 patients (16.7%) with ANSD was recorded TEOAE during the initial audiological diagnosis, in contrast to sensorineural hearing loss (TEOAE was not recorded in 100% of patients) (p &lt;0.05). In contrast to sensorineural hearing loss, the ABR thresholds in ANSD did not correspond to the behavioral hearing thresholds. CMP was recorded on the right and left ear in all patients (100%) with ANSD and remained unchanged during the observation process, in contrast to patients with sensorineural hearing loss, in whom CMP was not recorded on both sides (p &lt;0.05). Conclusions. The structure of the main risk factors for hearing loss in children with ANSD and sensorineural hearing loss is different. In the differential diagnosis with sensorineural hearing loss, the possibility of TEOAE disappearance in ANSD should be taken into account. Registration of ABRs with the allocation of CMP is the most informative method of ANSD diagnostics. CMP registration should be conducted for all patients who do not have ABR peaks at the maximum stimulus intensity, regardless of the OAE registration results.

Inner hair cell dysfunction in Klhl18 mutant mice leads to low frequency progressive hearing loss.

Age-related hearing loss in humans (presbycusis) typically involves impairment of high frequency sensitivity before becoming progressively more severe at lower frequencies. Pathologies initially affecting lower frequency regions of hearing are less common. Here we describe a progressive, predominantly low-frequency recessive hearing impairment in two mutant mouse lines carrying different mutant alleles of the Klhl18 gene: a spontaneous missense mutation (Klhl18lowf) and a targeted mutation (Klhl18tm1a(KOMP)Wtsi). Both males and females were studied, and the two mutant lines showed similar phenotypes. Threshold for auditory brainstem responses (ABR; a measure of auditory nerve and brainstem neural activity) were normal at 3 weeks old but showed progressive increases from 4 weeks onwards. In contrast, distortion product otoacoustic emission (DPOAE) sensitivity and amplitudes (a reflection of cochlear outer hair cell function) remained normal in mutants. Electrophysiological recordings from the round window of Klhl18lowf mutants at 6 weeks old revealed 1) raised compound action potential thresholds that were similar to ABR thresholds, 2) cochlear microphonic potentials that were normal compared with wildtype and heterozygous control mice and 3) summating potentials that were reduced in amplitude compared to control mice. Scanning electron microscopy showed that Klhl18lowf mutant mice had abnormally tapering of the tips of inner hair cell stereocilia in the apical half of the cochlea while their synapses appeared normal. These results suggest that Klhl18 is necessary to maintain inner hair cell stereocilia and normal inner hair cell function at low frequencies.

Analyses of audiological examination in children without auditory brainstem response

Objective: To investigate residual hearing of children severe and profound sensorineural deafness in whom wave V was not found in auditory brainstem response(ABR) testing, and to emphasize the importance of objective audiological tests. Methods: Two hundred and fifty-two children who were admitted to the Second Affiliated Hospital of Zhengzhou University between January 2015 and April 2018, with an average age of 20 months from 72 days to 4 years, received a full battery of objective audiological tests consisting of distortion product otoacoustic emission(DPOAEs), tympanometry, auditory brainstem responses(ABRs), 40 Hz auditory event related potential(40 HzAERP) and auditory steady-state response(ASSRs).There were 159 males(318 ears) and 93 females(186 ears). Residual hearing obtained by 40 HzAERP、ASSR of 252 children with sensorineural deafness was studied in relation to the absence of wave V in click ABR. SPSS 16.0 software was used to analyze the data. Results: Four hundred and forty-four ears of 504 ears have residual hearing of different degrees at different frequencies(88.1%),60 ears (11.9%) were found in whom responses was not found in 40 HzAERP、ASSR testing; Seventy-two ears(14.3%) in 38 patients were tested cochlear microphonic potentials (CMs). Conclusion: In children hearing evaluations,a full battery of objective audiological tests could better investigate residual hearing; The CMs were tested could provide the Audiotery Neuropathy diagnosis in infants with OAEs and ABR absent.

Simultaneous measurement of electrocochleography and ear canal pressure in normal-hearing adults

The auditory system is regulated by various adaptive mechanisms that modify sound as it passes upstream to the cortex. Studying these adaptive mechanisms, such as the middle ear muscle (MEM) reflex and the medial olivocochlear (MOC) reflex, reveals how the normal auditory system adjusts to challenging listening tasks and how abnormal reflexes may contribute to perceptual difficulties experienced by patients with sensorineural hearing loss. Studies in laboratory animals reveal that MOC and MEM reflexes result in neural antimasking at the output of the auditory nerve. Antimasking effects at the level of the middle ear and at the level of the cochlear hair cells should carry over to effects observed at the output of the auditory nerve. Antimasking due to MEM and MOC reflexes has been difficult to study in humans since measurement approaches are limited to non-invasive techniques with only a single output variable (e.g., ear canal pressure). In these studies, we assess MEM and MOC function by simultaneous measurements of ear canal pressure and electrocochleography, including the cochlear microphonic potential and compound action potential. Simultaneous measurements are expected to reveal the antimasking effects of MEM and MOC reflexes to middle ear, hair cell, and auditory nerve responses.

The CAPOS mutation in ATP1A3 alters Na/K-ATPase function and results in auditory neuropathy which has implications for management.

Cerebellar ataxia, areflexia, pes cavus, optic atrophy and sensorineural hearing impairment (CAPOS) is a rare clinically distinct syndrome caused by a single dominant missense mutation, c.2452G>A, p.Glu818Lys, in ATP1A3, encoding the neuron-specific alpha subunit of the Na+/K+-ATPase α3. Allelic mutations cause the neurological diseases rapid dystonia Parkinsonism and alternating hemiplegia of childhood, disorders which do not encompass hearing or visual impairment. We present detailed clinical phenotypic information in 18 genetically confirmed patients from 11 families (10 previously unreported) from Denmark, Sweden, UK and Germany indicating a specific type of hearing impairment-auditory neuropathy (AN). All patients were clinically suspected of CAPOS and had hearing problems. In this retrospective analysis of audiological data, we show for the first time that cochlear outer hair cell activity was preserved as shown by the presence of otoacoustic emissions and cochlear microphonic potentials, but the auditory brainstem responses were grossly abnormal, likely reflecting neural dyssynchrony. Poor speech perception was observed, especially in noise, which was beyond the hearing level obtained in the pure tone audiograms in several of the patients presented here. Molecular modelling and in vitro electrophysiological studies of the specific CAPOS mutation were performed. Heterologous expression studies of α3 with the p.Glu818Lys mutation affects sodium binding to, and release from, the sodium-specific site in the pump, the third ion-binding site. Molecular dynamics simulations confirm that the structure of the C-terminal region is affected. In conclusion, we demonstrate for the first time evidence for auditory neuropathy in CAPOS syndrome, which may reflect impaired propagation of electrical impulses along the spiral ganglion neurons. This has implications for diagnosis and patient management. Auditory neuropathy is difficult to treat with conventional hearing aids, but preliminary improvement in speech perception in some patientssuggests that cochlear implantation may be effectivein CAPOS patients.

Resistance of Gerbil Auditory Function to Reversible Decrease in Cochlear Blood Flow

The objective was to design in gerbils a model of reversible decrease in cochlear blood flow (CBF) and analyze its influence on cochlear function. In Mongolian gerbils injected with ferromagnetic microbeads, a magnet placed near the porus acusticus allowed CBF to be manipulated. The cochlear microphonic potential (CM) from the basal cochlea was monitored by a round-window electrode. In 13 of the 20 successfully injected gerbils, stable CBF reduction was obtained for 11.5 min on average. The CM was affected only when CBF fell to less than 60% of its baseline, yet remained >40% of its initial level in about 2/3 of such cases. After CBF restoration, CM recovery was fast and usually complete. Reduced CM came with a 35- to 45-dB threshold elevation of neural responses determined by compound action potentials. This method allowing reversible changes of CBF confirms the robustness of cochlear function to decreased CBF. It can be used to study whether a hypovascularized cochlea is abnormally sensitive to stress.

Using Cochlear Microphonic Potentials to Localize Peripheral Hearing Loss.

The cochlear microphonic (CM) is created primarily by the receptor currents of outer hair cells (OHCs) and may therefore be useful for identifying cochlear regions with impaired OHCs. However, the CM measured across the frequency range with round-window or ear-canal electrodes lacks place-specificity as it is dominated by cellular sources located most proximal to the recording site (e.g., at the cochlear base). To overcome this limitation, we extract the “residual” CM (rCM), defined as the complex difference between the CM measured with and without an additional tone (saturating tone, ST). If the ST saturates receptor currents near the peak of its excitation pattern, then the rCM should reflect the activity of OHCs in that region. To test this idea, we measured round-window CMs in chinchillas in response to low-level probe tones presented alone or with an ST ranging from 1 to 2.6 times the probe frequency. CMs were measured both before and after inducing a local impairment in cochlear function (a 4-kHz notch-type acoustic trauma). Following the acoustic trauma, little change was observed in the probe-alone CM. In contrast, rCMs were reduced in a frequency-specific manner. When shifts in rCM levels were plotted vs. the ST frequency, they matched well the frequency range of shifts in neural thresholds. These results suggest that rCMs originate near the cochlear place tuned to the ST frequency and thus can be used to assess OHC function in that region. Our interpretation of the data is supported by predictions of a simple phenomenological model of CM generation and two-tone interactions. The model indicates that the sensitivity of rCM to acoustic trauma is governed by changes in cochlear response at the ST tonotopic place rather than at the probe place. The model also suggests that a combination of CM and rCM measurements could be used to assess both the site and etiology of sensory hearing loss in clinical applications.

A connexin30 mutation rescues hearing and reveals roles for gap junctions in cochlear amplification and micromechanics

Accelerated age-related hearing loss disrupts high-frequency hearing in inbred CD-1 mice. The p.Ala88Val (A88V) mutation in the gene coding for the gap-junction protein connexin30 (Cx30) protects the cochlear basal turn of adult CD-1Cx30A88V/A88V mice from degeneration and rescues hearing. Here we report that the passive compliance of the cochlear partition and active frequency tuning of the basilar membrane are enhanced in the cochleae of CD-1Cx30A88V/A88V compared to CBA/J mice with sensitive high-frequency hearing, suggesting that gap junctions contribute to passive cochlear mechanics and energy distribution in the active cochlea. Surprisingly, the endocochlear potential that drives mechanoelectrical transduction currents in outer hair cells and hence cochlear amplification is greatly reduced in CD-1Cx30A88V/A88V mice. Yet, the saturating amplitudes of cochlear microphonic potentials in CD-1Cx30A88V/A88V and CBA/J mice are comparable. Although not conclusive, these results are compatible with the proposal that transmembrane potentials, determined mainly by extracellular potentials, drive somatic electromotility of outer hair cells.

Noninvasive detection of alarming intracranial pressure changes by auditory monitoring in early management of brain injury: a prospective invasive versus noninvasive study

BackgroundIn brain-injured patients intracranial pressure (ICP) is monitored invasively by a ventricular or intraparenchymal transducer. The procedure requires specific expertise and exposes the patient to complications such as malposition, hemorrhage or infection. As inner-ear fluid compartments are connected to the cerebrospinal fluid space, ICP changes elicit subtle changes in the physiology of the inner ear. Notably, we previously demonstrated that the phase of cochlear microphonic potential (CM) generated by sound stimuli rotates with ICP. The aim of our study was to validate the monitoring of CM as a noninvasive method to follow ICP.MethodsNon-invasive measure of CM-phase was compared to ICP recorded invasively in a prospective series of patients with acute brain injury managed in a neuro-intensive care unit. The study focused on patients with varying ICP and normal middle-ear function.ResultsIn the 24 patients with less than 4 days of endotracheal ventilation and whose ICP fluctuated (50-hour data), we demonstrated close correlation between CM-phase rotation and ICP (average 1.26 degrees/mmHg). As a binary classifier, CM phase changes of 7–10 degrees signaled 7.5-mmHg ICP increases with a sensitivity of 83% and 19% fallout.ConclusionReference methods to measure ICP require the surgical placement of a pressure transducer. Noninvasive CM-based monitoring of ICP might be beneficial to early management of brain-injured patients with initially preserved consciousness and to the diagnosis of neurological conditions, whenever invasive monitoring cannot be performed.Trial registrationClinicalTrials.gov NCT01685476, registered on 30 August 2012.

Inner Hair Cell Loss Disrupts Hearing and Cochlear Function Leading to Sensory Deprivation and Enhanced Central Auditory Gain.

There are three times as many outer hair cells (OHC) as inner hair cells (IHC), yet IHC transmit virtually all acoustic information to the brain as they synapse with 90–95% of type I auditory nerve fibers. Here we review a comprehensive series of experiments aimed at determining how loss of the IHC/type I system affects hearing by selectively destroying these cells in chinchillas using the ototoxic anti-cancer agent carboplatin. Eliminating IHC/type I neurons has no effect on distortion product otoacoustic emission or the cochlear microphonic potential generated by OHC; however, it greatly reduces the summating potential produced by IHC and the compound action potential (CAP) generated by type I neurons. Remarkably, responses from remaining auditory nerve fibers maintain sharp tuning and low thresholds despite innervating regions of the cochlea with ~80% IHC loss. Moreover, chinchillas with large IHC lesions have surprisingly normal thresholds in quiet until IHC losses exceeded 80%, suggesting that only a few IHC are needed to detect sounds in quiet. However, behavioral thresholds in broadband noise are elevated significantly and tone-in-narrow band noise masking patterns exhibit greater remote masking. These results suggest the auditory system is able to compensate for considerable loss of IHC/type I neurons in quiet but not in difficult listening conditions. How does the auditory brain deal with the drastic loss of cochlear input? Recordings from the inferior colliculus found a relatively small decline in sound-evoked activity despite a large decrease in CAP amplitude after IHC lesion. Paradoxically, sound-evoked responses are generally larger than normal in the auditory cortex, indicative of increased central gain. This gain enhancement in the auditory cortex is associated with decreased GABA-mediated inhibition. These results suggest that when the neural output of the cochlea is reduced, the central auditory system compensates by turning up its gain so that weak signals once again become comfortably loud. While this gain enhancement is able to restore normal hearing under quiet conditions, it may not adequately compensate for peripheral dysfunction in more complex sound environments. In addition, excessive gain increases may convert recruitment into the debilitating condition known as hyperacusis.

Study of cochlear microphonic potentials in auditory neuropathy

IntroductionAuditory Neuropathy/Dyssynchrony is a disorder characterized by the presence of Otoacoustic Emissions and Cochlear Microphonic Potentials, an absence or severe alteration of Brainstem Evoked Auditory Potential, auditory thresholds incompatible with speech thresholds and altered acoustic reflexes. The study of the Cochlear Microphonic Potential appears to be the most important tool for an accurate diagnosis of this pathology. ObjectiveDetermine the characteristics of the Cochlear Microphonic in Auditory Neuropathy/Dyssynchrony using an integrative review. MethodsBibliographic survey of Pubmed and Bireme platforms and MedLine, LILACS and SciELO data banks, with standardized searches up to July 2014, using keywords. Criteria were established for the selection and assessment of the scientific studies surveyed, considering the following aspects: author, year/place, degree of recommendation/level of scientific evidence, objective, sample, age range, mean age, tests, results and conclusion. ResultsOf the 1959 articles found, 1914 were excluded for the title, 20 for the abstract, 9 for the text of the article, 2 for being repeated and 14 were selected for the study. ConclusionThe presence of the Cochlear Microphonic is a determining finding in the differential diagnosis of Auditory Neuropathy/Dyssynchrony. The protocol for the determination of Cochlear Microphonic must include the use of insert earphones, reverse polarity and blocking the stimulus tube to eliminate electrical artifact interference. The amplitude of the Cochlear Microphonic in Auditory Neuropathy/Dyssynchrony shows no significant difference from that of normal individuals. The duration of the Cochlear Microphonic is longer in individuals with Auditory Neuropathy/Dyssynchrony.

N-acetyl-cysteine prevents age-related hearing loss and the progressive loss of inner hair cells in γ-glutamyl transferase 1 deficient mice.

Genetic factors combined with oxidative stress are major determinants of age-related hearing loss (ARHL), one of the most prevalent disorders of the elderly. Dwarf grey mice, Ggt1dwg/dwg, are homozygous for a loss of function mutation of the γ-glutamyl transferase 1 gene, which encodes an important antioxidant enzyme critical for the resynthesis of glutathione (GSH). Since GSH reduces oxidative damage, we hypothesized that Ggt1dwg/dwg mice would be susceptible to ARHL. Surprisingly, otoacoustic emissions and cochlear microphonic potentials, which reflect cochlear outer hair cell (OHC) function, were largely unaffected in mutant mice, whereas auditory brainstem responses and the compound action potential were grossly abnormal. These functional deficits were associated with an unusual and selective loss of inner hair cells (IHC), but retention of OHC and auditory nerve fibers. Remarkably, hearing deficits and IHC loss were completely prevented by N-acetyl-L-cysteine, which induces de novo synthesis of GSH; however, hearing deficits and IHC loss reappeared when treatment was discontinued. Ggt1dwg/dwgmice represent an important new model for investigating ARHL, therapeutic interventions, and understanding the perceptual and electrophysiological consequences of sensory deprivation caused by the loss of sensory input exclusively from IHC.

Effects of salicylate on sound-evoked outer hair cell stereocilia deflections.

Hearing depends on sound-evoked deflections of the stereocilia that protrude from the sensory hair cells in the inner ear. Although sound provides an important force driving stereocilia, forces generated through mechanically sensitive ion channels and through the motor protein prestin have been shown to influence stereocilia motion in solitary hair cells. While a possible influence of prestin on mechanically sensitive ion channels has not been systematically investigated, a decrease in transducer currents is evident in solitary hair cells when prestin is blocked with salicylate, raising the question of whether a reduced prestin activity or salicylate itself affected the mechanotransduction apparatus. We used two- and three-dimensional time-resolved confocal imaging to visualize outer hair cell stereocilia during sound stimulation in the apical turn of cochlear explant preparations from the guinea pig. Surprisingly, following application of salicylate, outer hair cell stereocilia deflections increased, while cochlear microphonic potentials decreased. However, when prestin activity was altered with the chloride ionophore tributyltin, both the cochlear microphonic potential and the stereocilia deflection amplitude decreased. Neither positive nor negative current stimulation abolished the bundle movements in the presence of salicylate, indicating that the observed effects did not depend on the endocochlear potential. These data suggest that salicylate may alter the mechanical properties of stereocilia, decreasing their bending stiffness.

A novel mode of off-frequency hearing as a result of defective outer hair cells hair bundles unveiled by Nherf1-/- mice

Nherf1, a PDZ-domain-containing protein, was identified in the hair bundle in differentiating outer hair cells (OHCs). Nherf1-/- mice showed apparently mild hearing-threshold elevations at mid/high sound frequencies, associated to OHC hair-bundle shape anomalies, prominent in the basal cochlea. This mild impact on hearing sensitivity was discordant with the finding of almost non-responding OHCs in the basal cochlea as assessed by distortion-product otoacoustic emissions and cochlear microphonic potentials. Unlike normal mice, responses of Nherf1-/- mice to high-frequency test tones were not masked by tones of neighboring frequencies. Efficient masker tones displayed unusual characteristics: maximal efficiency at lower frequencies (up to two octaves lower than the test tone), and at low levels (up to 25 dB below test-tone level). This, and the relative growth of the masker and test tones, suggests that mid-high frequency tones of moderate intensity are detected off-frequency, in the functionally unaffected apical cochlear region. Our results establish that Nherf1 is critical for hair bundle morphogenesis and reveal a novel mode of off-frequency detection, probably involving the persistent contact between OHCs and the tectorial membrane. These findings suggest how to circumvent major pitfalls in hearing assessment of some patients, by avoiding misleading interpretations of hearing thresholds.

A mouse model for human deafness DFNB22 reveals that hearing impairment is due to a loss of inner hair cell stimulation

The gene causative for the human nonsyndromic recessive form of deafness DFNB22 encodes otoancorin, a 120-kDa inner ear-specific protein that is expressed on the surface of the spiral limbus in the cochlea. Gene targeting in ES cells was used to create an EGFP knock-in, otoancorin KO (Otoa(EGFP/EGFP)) mouse. In the Otoa(EGFP/EGFP) mouse, the tectorial membrane (TM), a ribbon-like strip of ECM that is normally anchored by one edge to the spiral limbus and lies over the organ of Corti, retains its general form, and remains in close proximity to the organ of Corti, but is detached from the limbal surface. Measurements of cochlear microphonic potentials, distortion product otoacoustic emissions, and basilar membrane motion indicate that the TM remains functionally attached to the electromotile, sensorimotor outer hair cells of the organ of Corti, and that the amplification and frequency tuning of the basilar membrane responses to sounds are almost normal. The compound action potential masker tuning curves, a measure of the tuning of the sensory inner hair cells, are also sharply tuned, but the thresholds of the compound action potentials, a measure of inner hair cell sensitivity, are significantly elevated. These results indicate that the hearing loss in patients with Otoa mutations is caused by a defect in inner hair cell stimulation, and reveal the limbal attachment of the TM plays a critical role in this process.