Forward and reverse waves in the one-dimensional model of the cochlea

Consideration of a source of oto-acoustic emission in a cochlear model implies consideration of the types of waves that such a source can emit. One wave travels in the normal, forward, direction. As any other forward wave it undergoes little or no reflection and it eventually disappears completely because of dissipation. The other wave travels in the reverse direction and it appears to undergo appreciable reflection. In the present paper this phenomenon is studied via the use of two appropriately simplified long-wave models of the cochlea. One model, the exponential model, puts emphasis on the variation of the stiffness along the length of the basilar membrane. The second model concentrates on what happens in the region of resonance. The latter model turns out to have the largest predictive power for the problem at hand. Consideration of the flow of energy in the cochlear fluid brings forth the explanation why in the used model of the cochlea reflection conditions at the stapes have such a surprisingly small influence on the operating conditions of a potential source of emission.     

Could Tailored Chirp Stimuli Benefit Measurement of the Supra-threshold Auditory Brainstem Wave-I Response?

Auditory brainstem responses (ABRs) to broadband clicks are strongly affected by dyssynchrony, or “latency dispersion”, of their frequency-specific cochlear contributions. Optimized chirp stimuli, designed to compensate for cochlear dispersion, can afford substantial increase in broadband ABR amplitudes, particularly for the prominent wave-V deflection. Reports on the smaller wave I, however, which may be useful for measuring cochlear synaptopathy, have been mixed. This study aimed to test previous claims that ABR latency dispersion differs between waves I and V, and between males and females, and thus that using wave- and/or sex-tailored chirps may provide more reliable wave-I benefit. Using the derived-band technique, we measured responses from frequency-restricted (one-octave-wide) cochlear regions to energy-matched click and chirp stimuli. The derived-band responses’ latencies were used to assess any wave- and/or sex-related dispersion differences across bands, and their amplitudes, to evaluate any within-band dispersion differences. Our results suggest that sex-related dispersion difference within the lowest-frequency cochlear regions (< 1 kHz), where dispersion is generally greatest, may be a predominant driver of the often-reported sex difference in broadband ABR amplitude. At the same time, they showed no systematic dispersion difference between waves I and V. Instead, they suggest that reduced chirp benefit on wave I may arise as a result of chirp-induced desynchronization of on- and off-frequency responses generated at the same cochlear places, and resultant reduction in response contributions from higher-frequency cochlear regions, to which wave I is thought to be particularly sensitive.

     

Validity of the Liouville-Green (or WKB) method for cochlear mechanics

This article presents a comparison of Liouville-Green (LG) calculations and exact solutions of 2- and 3-dimensional cochlea models. The agreement is in general quite good. For certain choices of the model parameter's, however, the 2- and 3-dimensional LG solutions show appreciable errors in the region just beyond the location of maximum amplitude of the basilar membrane response. The origin of these errors appears to be the non-uniqueness of the (complex) LG wave number k(x) in 2- and 3-dimensional models: the ‘eikonal equation’ from which k(x) has to be solved has multiple roots. To study this problem somewhat deeper, the properties of the locus of k = k(x) formed when x is varied, are investigated. Erratic behaviour of the LG solution is found to occur when this root locus approaches one of the saddle points of a complex function of k — called Q(k) — which plays the major role in the eikonal equation. Apart from this specific problem, the LG approximation is very well suited to unravel the mechanisms governing wave propagation and attenuation in the cochlea. The analysis shows clearly why and how the response of the basilar membrane builds up to a maximum and which factors cause a turnover and a rapid decrease to occur, in both the long-wave and the short-wave cases. A special discussion is dedicated to the relation between the LG approximation and the absence of wave reflection in cochlea models of the type studied.     

Stimulus Frequency Otoacoustic Emission Delays and Generating Mechanisms in Guinea Pigs,Chinchillas, and Simulations

According to coherent reflection theory (CRT), stimulus frequency otoacoustic emissions (SFOAEs) arise from cochlear irregularities coherently reflecting energy from basilar membrane motion within the traveling-wave peak. This reflected energy arrives in the ear canal predominantly with a single delay at each frequency. However, data from humans and animals indicate that (1) SFOAEs can have multiple delay components, (2) low-frequency SFOAE delays are too short to be accounted for by CRT, and (3) “SFOAEs” obtained with a 2nd (“suppressor”) tone ≥2 octaves above the probe tone have been interpreted as arising from the area basal to the region of cochlear amplification. To explore these issues, we collected SFOAEs by the suppression method in guinea pigs and time-frequency analyzed these data, simulated SFOAEs, and published chinchilla SFOAEs. Time-frequency analysis revealed that most frequencies showed only one SFOAE delay component while other frequencies had multiple components including some with short delays. We found no systematic patterns in the occurrence of multiple delay components. Using a cochlear model that had significant basilar membrane motion only in the peak region of the traveling wave, simulated SFOAEs had single and multiple delay components similar to the animal SFOAEs. This result indicates that multiple components (including ones with short delays) can originate from cochlear mechanical irregularities in the SFOAE peak region and are not necessarily indicative of SFOAE sources in regions ≥2 octaves basal of the SFOAE peak region. We conclude that SFOAEs obtained with suppressors close to the probe frequency provide information primarily about the mechanical response in the region that receives amplification, and we attribute the too-short SFOAE delays at low frequencies to distortion-source SFOAEs and coherent reflection from multiple cochlear motions. Our findings suggest that CRT needs revision to include reflections from multiple motions in the cochlear apex.     

大鼠单侧耳蜗损毁后下丘r-氨基丁酸及其神经元的变化

目的：研究大鼠单侧耳蜗损毁前后不同时期,下丘中r-氨基丁酸（gamma-aminobutyric acid,GABA）含量及其阳性神经元的分布变化,初步探讨去传入损伤后GABA在听觉中枢重组中的作用和意义。方法：健康SD大鼠随机分为4组：正常组和单侧耳蜗损毁后1周、2周及1个月组。于规定时间内检测并比较耳蜗损毁前后GABA含量及其阳性神经元的数量。结果：与正常组相比,损毁后1周,下丘中GABA含量及其神经元数量明显下降,差异有统计学意义（P〈0．05）;术后2周,下丘中GABA含量及其神经元数量稍上升但仍低于正常组（P〈0．05）;至术后1个月,下丘中GABA含量及其神经元数量上升,与正常组比较差异均无统计学意义（P〉0．05）。结论：单侧耳蜗损毁后,GABA含量及其阳性神经元数量在下丘中呈一明显的动态变化过程,说明GA—BA作为一种神经递质参与了耳蜗毁损后听功能的重组过程,提示GABA在单侧耳蜗损毁后听觉中枢的重组过程中可能起重要作用。     

X-平片与计算机X线摄影术对人工耳蜗植入手术效果的评价

目的 :探讨 X-平片及计算机 X线摄影术 (CR)作为人工耳蜗植入手术效果的评价手段。方法 :人工耳蜗植入术后行 X-平片与 CR检查 ,显示植入电极的形态和位置。结果 :15例 15耳人工耳蜗植入术后植入电极的形态、位置正常 ,植入深度平均为 2 3.0 5 mm,达到规定标准。结论 :使用 X-平片与 CR对人工耳蜗植入手术效果与术后康复效果进行评价 ,是一经济、简便、直观、有价值的方法。     

Can shape deformations of the organ of Corti influence the travelling wave in the cochlea?

Via their motile reactions outer hair cells may produce oscillatory power or amplify the fluid waves in the cochlea. When the cells move or change their lengths, the organ of Corti (OC) will change its shape. This paper describes the consequences of such shape changes for the physics of cochlear waves. The assertions posed are based on a mathematical derivation but the major conclusions can be grasped without following the mathematics in detail. There are two basic types of OC deformations. In the first type the net cross-sectional area of the OC is periodically varying, in the second type--the central subject of this paper--is is not. It is shown that for the latter type of OC shape deformation there is no interaction with the cochlear fluids, at least for long waves. Hence, in this case the outer hair cells cannot amplify cochlear waves. The other type of OC deformation has a better coupling with the cochlear waves. However, much of the pressure developed by oscillating hair cells is spent in the 'wrong' way, namely, by squeezing fluid lengthwise through the narrow channel of the OC. Therefore, in this mode power transfer from cochlear hair cells to cochlear fluids (and from there to the basilar membrane) would be very inefficient.     

Attribute capture in the precedence effect for long-duration noise sounds

Listeners perceptually fuse the direct wave from a sound source with its reflections off nearby surfaces into a single sound image, located at or near the sound source (the precedence effect). This study investigated how a brief gap presented in the middle of either a direct wave or simulated reflection is incorporated into the fused image. For short (<9.5 ms) delays between the direct (leading) and reflected (lagging) waves, no sound was perceived from the direction of the lagging wave. For delays between 10 and 15 ms, both sounds were perceived, but the gap was heard only on the leading side. When the gap was only in the correlated lagging sound at short delays, it also was perceived as occurring on the leading side. Moreover, gap detection thresholds were the same for gaps in the leading and lagging sounds, suggesting that the perception of the gap was not suppressed, but rather incorporated into the leading sound. Finally, scalp event-related potentials were not associated with the precedence effect until the gap occurred. This suggests that cortical mechanisms are engaged to maintain fusion when attributes in direct or reflected waves change.     

Audiological performance after cochlear implantation in children with inner ear malformations

OBJECTIVES: To prove that cochlear implantation is a beneficial method of rehabilitation in deaf children with malformations of the inner ear. DESIGN: The evaluation of auditory responses to speech (EARS) test battery was performed on the children in this study after an average implant use of 3 years. RESULTS: Individual results of six children with inner ear anomalies receiving cochlear implants are presented in this study. Three of the patients showed an incomplete partition (Mondini dysplasia), one had a cochlear hypoplasia and two suffered from an intraoperative cerebrospinal fluid leak. The majority of the children in this study are successful implant users. Wherever possible, test scores are included and subjective case reports given. CONCLUSIONS: Results are similar to those in children with normal cochleas, therefore inner ear malformations found in as many as 20% of patients with congenital sensorineural hearing loss are no contraindication for cochlear implantation. Nevertheless, factors influencing the success of implantation are multiple, including a thorough preoperative radiological examination, a well-performed surgery and an individually tailored postoperative rehabilitation programme.     

Cochleovestibular Afferent Pathways of Trapezius Muscle Responses to Clicks in Human

Brief intense clicks cause short-latency cervical muscles microcontractions which are supposed to be of vestibular origin. Averaging these microcontractions allows myogenic vestibular evoked potentials (MVEP) to be obtained. MVEP from the trapezius muscles were investigated in normal subjects, cochleovestibular nerve-damaged patients and patients with a vestibular or a cochlear lesion. Muscular responses were recorded on right and left trapezius by averaging from surface electrodes following right and left monaural 100 dB hearing level click stimulation. In normal subjects, responses to monaural stimuli were bilateral, of equal amplitude and latency in left and right trapezia. Normal response consisted of four consecutive waves, labelled p13, n23, p32 and n40 according to their polarity (p, positive; n, negative) and mean peak latency in msec. In total unilateral cochleovestibular damaged patients, auditory stimulation of the affected side gave no MVEP either ipsilateral or contralateral to the stimulation. In the case of selective cochlear lesion, stimulation of the affected side gave MVEP which was present on ipsilateral and controlateral trapezius muscles. The four successive waves were present with a normal latency; however, amplitude was lower than that obtained after stimulation of the healthy ear. In the case of selective vestibular lesion, the four waves of MVEP were again present with normal latency but with reduced amplitude. Responses were present on both the ipsilateral and controlateral trapezius muscle. It is concluded that normal MVEP recorded on the trapezius muscles are bilateral and consist of four waves, the amplitude of which could depend on the simultaneous stimulation of both cochlear and vestibular afferents. In the case of unilateral cochlear and/or vestibular impairments responses were present on both ipsilateral and contralateral trapezius muscles. Latencies had normal values but amplitudes were reduced. MEVP recorded on trapezius muscles were absent in the case of total cochleovestibular damage.     

Effects of stimulus repetition rate and frequency on the auditory brainstem response in normal cochlear-impaired, and VIII nerve/brainstem-impaired subjects

The effects of signal repetition rate and frequency on the auditory brainstem responses of normal listeners, of persons with cochlear lesions, and of persons with VIII nerve/brainstem lesions were evaluated. The normal group exhibited more waves I and II than did the cochlear and VIII/brainstem groups. The normal and cochlear groups exhibited more waves III and V than did the VIII nerve/brainstem group. The latency of wave I was not different among groups, whereas wave V was significantly later in the VIII nerve/brainstem group than in the other groups. Waves I, III, and V were later for 50/s than for 10/s. Waves I and III displayed shorter latencies for 4000 Hz than for 2000 Hz, whereas wave V displayed similar latencies for the two stimuli. In conclusion, cochlear pathology (less than or equal to 65 dB HL) does not prolong the latencies of waves I and V. A dual mechanism is discussed to explain the rate-dependent latency shift of wave V.     

Spectral Ripples in Round-Window Cochlear Microphonics: Evidence for Multiple Generation Mechanisms

The cochlear microphonic (CM) results from the vector sum of outer hair cell transduction currents excited by a stimulus. The classical theory of CM generation—that the response measured at the round window is dominated by cellular sources located within the tail region of the basilar membrane (BM) excitation pattern—predicts that CM amplitude and phase vary little with stimulus frequency. Contrary to expectations, CM amplitude and phase-gradient delay measured in response to low-level tones in chinchillas demonstrate a striking, quasiperiodic pattern of spectral ripples, even at frequencies >?5 kHz, where interference with neurophonic potentials is unlikely. The spectral ripples were reduced in the presence of a moderate-level saturating tone at a nearby frequency. When converted to the time domain, only the delayed CM energy was diminished in the presence of the saturator. We hypothesize that the ripples represent an interference pattern produced by CM components with different phase gradients: an early-latency component originating within the tail region of the BM excitation and two delayed components that depend on active cochlear processing near the peak region of the traveling wave. Using time windowing, we show that the early, middle, and late components have delays corresponding to estimated middle-ear transmission, cochlear forward delays, and cochlear round-trip delays, respectively. By extending the classical model of CM generation to include mechanical and electrical irregularities, we propose that middle components are generated through a mechanism of “coherent summation” analogous to the production of reflection-source otoacoustic emissions (OAEs), while the late components arise through a process of internal cochlear reflection related to the generation of stimulus-frequency OAEs. Although early-latency components from the passive tail region typically dominate the round-window CM, at low stimulus levels, substantial contributions from components shaped by active cochlear processing provide a new avenue for improving CM measurements as assays of cochlear health.     

Reflection- and Distortion-Source Otoacoustic Emissions: Evidence for Increased Irregularity in the Human Cochlea During Aging

Previous research on distortion product otoacoustic emission (DPOAE) components has hinted at possible differences in the effect of aging on the two basic types of OAEs: those generated by a reflection mechanism in the cochlea and those created by nonlinear distortion (Abdala and Dhar in J Assoc Res Otolaryngol 13:403–421, 2012). This initial work led to the hypothesis that micromechanical irregularity (“roughness”) increases in the aging cochlea, perhaps as the result of natural tissue degradation. Increased roughness would boost the backscattering of traveling waves (i.e., reflection emissions) while minimally impacting DPOAEs. To study the relational effect of aging on both types of emissions and address our hypothesis of its origin, we measured reflection- and distortion-type OAEs in 77 human subjects aged 18–76 years. The stimulus-frequency OAE (SFOAE), a reflection emission, and the distortion component of the DPOAE, a nonlinear distortion emission, were recorded at multiple stimulus levels across a four-octave range in all ears. Although the levels of both OAE types decreased with age, the rate of decline in OAE level was consistently greater for DPOAEs than for SFOAEs; that is, SFOAEs are relatively preserved with advancing age. Multiple regression analyses and other controls indicate that aging per se, and not hearing loss, drives this effect. Furthermore, SFOAE generation was simulated using computational modeling to explore the origin of this result. Increasing the amount of mechanical irregularity with age produced an enhancement of SFOAE levels, providing support for the hypothesis that increased intra-cochlear roughness during aging may preserve SFOAE levels. The characteristic aging effect—relatively preserved reflection-emission levels combined with more markedly reduced distortion-emission levels—indicates that SFOAE magnitudes in elderly individuals depend on more than simply the gain of the cochlear amplifier. This relative pattern of OAE decline with age may provide a diagnostic marker for aging-related changes in the cochlea.     

Effect of hearing loss of cochlear origin on the auditory brain stem response.

Auditory brain stem response (ABR) testing is widely used to detect lesions of the auditory neural pathways. The ABR waves depend not only on the integrity of the neural pathways, but also on the condition of the cochlea. To properly interpret the ABR response, it is necessary to understand the effects of cochlear hearing loss on the ABR wave latencies. We studied two populations of subjects with cochlear hearing loss: one with varying degrees of high-frequency hearing loss and the other with varying degrees of flat configuration hearing loss. The degree of cochlear hearing loss was quantified in several different ways and subjected to one linear and three nonlinear regression analyses to test for accuracy in predicting ABR wave latencies and interpeak intervals (waves I, III, V, I-V, I-III, and III-V) for three click intensities. Hearing loss levels from 2 to 6 kHz, in particular 4 kHz, were superior to other audiometric test frequencies as predictors of ABR wave latencies for the group with the high-frequency losses. No particular characterization was found to be superior for the flat hearing loss configurations. From these results, modeled predictions of wave latencies as a function of degree and configuration of hearing loss were made. The modeled predictions are then used to suggest guidelines for interpretations of ABR results where hearing impaired patients are involved.     

Do Forward- and Backward-Traveling Waves Occur Within the Cochlea? Countering the Critique of Nobili et al.

The question of whether or not forward- and backward-traveling waves occur within the cochlea constitutes a long-standing controversy in cochlear mechanics recently brought to the fore by the problem of understanding otoacoustic emissions. Nobili and colleagues articulate the opposition to the traveling-wave viewpoint by arguing that wave-equation formulations of cochlear mechanics fundamentally misrepresent the hydrodynamics of the cochlea [e.g., Nobili et al. (2003) J. Assoc. Res. Otolaryngol. 4:478–494]. To correct the perceived deficiencies of the wave-equation formulation, Nobili et al. advocate an apparently altogether different approach to cochlear modeling—the so-called hydrodynamic or Greens function approach—in which cochlear responses are represented not as forward- and backward-traveling waves but as weighted sums of the motions of individual basilar membrane oscillators, each interacting with the others via forces communicated instantaneously through the cochlear fluids. In this article, we examine Nobili and colleagues arguments and conclusions while attempting to clarify the broader issues at stake. We demonstrate that the one-dimensional wave-equation formulation of cochlear hydrodynamics does not misrepresent long-range fluid coupling in the cochlea, as claimed. Indeed, we show that the long-range component of Nobili et al.s three-dimensional force propagator is identical to the hydrodynamic Greens function representing a one-dimensional tapered transmission line. Furthermore, simulations that Nobili et al. use to discredit wave-equation formulations of cochlear mechanics (i.e., cochlear responses to excitation at a point along the basilar membrane) are readily reproduced and interpreted using a simple superposition of forward- and backward-traveling waves. Nobili and coworkers critique of wave-equation formulations of cochlear mechanics thus appears to be without compelling foundation. Although the traveling-wave and hydrodynamic formulations impose strikingly disparate conceptual and computational frameworks, the two approaches ultimately describe the same underlying physics.     

Quantitative assessment of human cochlear function by evoked otoacoustic emissions

The amplitudes of evoked otoacoustic emissions (EOE) and their detection threshold were measured in 44 normal young adults and 118 patients with two categories of cochlear dysfunction, acoustic trauma and presbycusis. A different method was used for each category: detection of click EOE or of stimulus frequency emissions. A partial correlation and multivariate analysis was performed for both groups of results to investigate the relations between EOE threshold one pure tone audiometric thresholds (250 to 8000 Hz). Only one significant correlation was found, linearly relating EOE threshold and hearing threshold at 2 kHz (P less than 0.001), independently of the origin of cochlear dysfunction. It suggests that EOE threshold is not frequency-specific since the frequency of EOE at threshold was nearly always close to 1 kHz. A simple model is proposed, based on the assumption that EOE amplitudes and threshold are proportional to the total number of residual active sites in the organ of Corti, i.e. to the total length of active basilar membrane. It is shown that this model accounts for the results disclosed by the statistical analysis and fits the experimental data. It can be used for quantitatively predicting the residual cochlear activity of a patient. However, the EOE threshold is only sensitive to already important cochlear alterations and this parameter does not seem to allow a follow-up of early stages of cochlear dysfunction.     

Inner ear damage due to hyperlipidemia in the young and old guinea pigs]

Nineteen guinea pigs of one month and 11 of 30 months were given high fat diet for 3 months to produce hyperlipidemia; 16 of them followed by 3 months' normal diet to resume blood lipid level; 27 for control. The results showed that: The serum cholesterol level of the experimental groups was markedly elevated (P < 0.01) with fatty degeneration of liver. Damages in OHC, IHC, cells of stria vascularis and few myelin sheaths of cochlear nerve were seen in all experimental animals, and the reduction of cochlear damages was not seen in those animals with blood lipid level resumed to normal. Auditory dysfunction was very marked as shown by ABR in the old animals with hyperlipidemia as compared with controls (P < 0.01).     

Magnetic fields from the auditory cortex of a deaf human individual occurring spontaneously or evoked by stimulation through a cochlear prosthesis

In a postlingually deaf individual, the magnetic field evoked by stimulation through a cochlear prosthesis (extracochlear electrodes) as well as of the spontaneous magnetoencephalogram was measured over the hemisphere contralateral to the prosthesis (CP), and the results were compared with those obtained from normal-hearing subjects. The latency of the 2 best developed waves M100 and M200 turned out to be prolonged in the CP patient by approximately 40 ms. The amplitude of wave M100 was significantly diminished, while wave M200 was only poorly developed. Location and direction of the equivalent current dipole (ECD) calculated for wave M100 was in good agreement with normal data, whereas the dipole moment was only about one third of the average dipole moment found in normals. Furthermore, evidence was obtained for another magnetic field wave, preceding the delayed auditory wave M100, which exhibits the same latency, ECD location and direction as reported in the literature for the somatosensory evoked magnetic field. This wave probably results from stimulation, through the intratympanic electrodes, of somatosensory nerves innervating the tympanic cavity. A potential clinical application of neuromagnetic measurements is discussed: The calculation of the ECD moment from the auditory cortical magnetic field evoked by electrical stimulation at the promontory would allow to estimate, prior to CP implantation, the number of persisting, excitable nerve fibres.     

Autism and auditory brain stem responses

OBJECTIVE: To study a controversy that has been discussed for more than two decades: whether or not children with autism have abnormalities affecting the cochlear nerve or the auditory pathway in the brain stem and, if so, to describe these abnormalities. DESIGN: A group of 153 children and adolescents with autistic disorder were included in an investigation of auditory brain stem responses (ABR). Two thirds of this group, 101 individuals (75 boys, 26 girls), had normal hearing and they were selected for an in-depth ABR study. The results from the study group were compared with those of an age-matched comparison group. RESULTS: The III-V interpeak latency (IPL) was significantly prolonged in both boys and girls with autism, compared with the controls. The latencies of ABR waves I and V were also significantly lengthened in the study groups. The individual test results showed that more than half of this normal-hearing autistic disorder group (58%) had abnormalities of one or more of eight ABR parameters studied. The most common abnormalities were prolongation of wave V (38%), and of I-V IPL (28%). A lengthening of the I-V IPL was also recorded in 27% of 49 children who were difficult to test or who had hearing loss. Abnormal left-right differences of ABR latencies were found in 18% of autism cases with normal hearing. CONCLUSIONS: Possible causes of the reported ABR abnormalities, observed here as well as in other studies, are discussed. Brain stem lesion, occult cochlear dysfunction, and involvement of the cochlear efferent system are probable factors that can explain the ABR findings