Otoacoustic emissions, travelling waves and cochlear mechanisms
Abstract
The phenomenon of otoacoustic emissions is discussed in relation to the question—is the cochlear travelling wave actively enhanced? Distinctions are made between active loss reduction and true amplification, and between different types of emission source. The dynamic control exercised by the cochlea over the level of mechanical activity is analysed. An adaptive mechanism, capable of oscillatory transient behaviour is found both acoustically and psychophysically in response to strong low frequency stimulation. A feedback model is presented and used to predict aspects of emission behaviour. cochlea, otoacoustic emission, travelling wave, amplification
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An outer hair cell-powered global hydromechanical mechanism for cochlear amplification
2022, Hearing ResearchIt is a common belief that the mammalian cochlea achieves its exquisite sensitivity, frequency selectivity, and dynamic range through an outer hair cell-based active process, or cochlear amplification. As a sound-induced traveling wave propagates from the cochlear base toward the apex, outer hair cells at a narrow region amplify the low level sound-induced vibration through a local feedback mechanism. This widely accepted theory has been tested by measuring sound-induced sub-nanometer vibrations within the organ of Corti in the sensitive living cochleae using heterodyne low-coherence interferometry and optical coherence tomography. The aim of this short review is to summarize experimental findings on the cochlear active process by the authors’ group. Our data show that outer hair cells are able to generate substantial forces for driving the cochlear partition at all audible frequencies in vivo. The acoustically induced reticular lamina vibration is larger and more broadly tuned than the basilar membrane vibration. The reticular lamina and basilar membrane vibrate approximately in opposite directions at low frequencies and in the same direction at the best frequency. The group delay of the reticular lamina is larger than that of the basilar membrane. The magnitude and phase differences between the reticular lamina and basilar membrane vibration are physiologically vulnerable. These results contradict predictions based on the local feedback mechanism but suggest a global hydromechanical mechanism for cochlear amplification.
This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.
The development of audiology - From Helmholtz until today
2021, Zeitschrift fur Medizinische PhysikAnlässlich des 200. Geburtstags von Hermann von Helmholtz gibt der Artikel einen Rückblick über die Entwicklung der Audiologie in den letzten zwei Jahrhunderten. Die moderne Audiologie versteht sich als multidisziplinäres Fachgebiet, das sich mit der Erforschung der Ursachen von Hörstörungen und aller damit zusammenhängenden Phänomene, insbesondere Prävention/Protektion, Diagnostik und Behandlung von Hörstörungen sowie der Rehabilitation schwerhöriger und gehörloser Menschen befasst. Eine komplette Abhandlung aller dieser Bereiche würde den Rahmen eines Zeitschriftenartikels sprengen. Deshalb fokussiert sich der Überblick auf zwei dieser Teilgebiete, nämlich die Hörforschung und diagnostische Verfahren, die hier stellvertretend für die anderen, gleichermaßen bedeutenden Teilgebiete der Audiologie stehen sollen. Der Artikel schließt mit einer kurzen Betrachtung zur Entwicklung der fachwissenschaftlichen Organisationsstrukturen und einem Fazit.
On occasion of the 200th anniversary of the birthday of Hermann von Helmholtz the article presents a historical review on the development of audiology over the past two centuries. Modern audiology is considered to be a multidisciplinary field addressing clinical work and research on all aspects of hearing impairment, particularly prevention/protection, diagnostics and treatment of hearing problems as well as rehabilitation of patients with any degree of hearing loss. To cover all these subjects comprehensively would go beyond the scope of a journal article. Therefore, the review focuses on two of these domains, namely hearing research and diagnostic procedures as representatives for the other areas of audiology being equally important. The paper concludes with a brief look into the development of the organizational structures of audiology and a summary.
Twin study of neonatal transient-evoked otoacoustic emissions
2020, Hearing ResearchOur knowledge of which physiological mechanisms shape transient evoked otoacoustic emissions (TEOAEs) is incomplete, although thousands of TEOAEs are recorded each day as part of universal newborn hearing-screening (UNHS). TEOAE heritability may explain some of the large TEOAE variability observed in neonates, and give insights into the TEOAE generators and modulators, and why TEOAEs are generally larger in females and right ears.
The aim was to estimate TEOAE heritability and describe ear and sex effects in a consecutive subset of all twins that passed UNHS at the same occasion at two hospitals during a six-year period (more than 30 000 neonates screened in total). TEOAEs were studied and TEOAE level correlations compared in twin sets of same-sex (SS, 302 individual twins, 151 twin pairs) and opposite-sex (OS, 152 individual twins, 76 twin pairs). A mathematical model was used to estimate and compare monozygotic (MZ) and dizygotic (DZ) intra-twin pair TEOAE level correlations, based on the data from the SS and OS twin sets.
For both SS and OS twin pairs TEOAE levels were significantly higher in right ears and females, compared to left ears and males, as previously demonstrated in young adult twins and large groups of neonates. Neonatal females in OS twin pairs did not demonstrate masculinized TEOAEs, as has been demonstrated for OAEs in young adult females in OS twin pairs. The within-twin pair TEOAE level correlations were higher for SS twin pairs than for OS twin pairs, whereas the within-pair correlation coefficients could not be distinguished from zero when twins were randomly paired. These results reflect heredity as a key factor in TEOAE level variability. Additionally, the estimated MZ within-twin pair TEOAE level correlations were higher than those for DZ twin pairs. The heritability estimates reached up to 100% TEOAE heritability, which is numerically larger than previous estimates of about 75% in young adult twins.
High frequency transient-evoked otoacoustic emission measurements using chirp and click stimuli
2019, Hearing ResearchTransient-evoked otoacoustic emissions (TEOAEs) at high frequencies are a non-invasive physiological test of basilar membrane mechanics at the basal end, and have clinical potential to detect risk of hearing loss related to outer-hair-cell dysfunction. Using stimuli with constant incident pressure across frequency, TEOAEs were measured in experiment 1 at low frequencies (0.7–8 kHz) and high frequencies (7.1–14.7 kHz) in adults with normal hearing up to 8 kHz and varying hearing levels from 9 to 16 kHz. In combination with click stimuli, chirp stimuli were used with slow, medium and fast sweep rates for which the local frequency increased or decreased with time. Chirp TEOAEs were transformed into equivalent click TEOAEs by inverse filtering out chirp stimulus phase, and analyzed similarly to click TEOAEs. To improve detection above 8 kHz, TEOAEs were measured in experiment 2 with higher-level stimuli and longer averaging times. These changes increased the TEOAE signal-to-noise ratio (SNR) by 10 dB. Slower sweep rates were investigated but the elicited TEOAEs were detected in fewer ears compared to faster rates. Data were acquired in adults and children (age 11–17 y), including children with cystic fibrosis (CF) treated with ototoxic antibiotics. Test-retest measurements revealed satisfactory repeatability of high-frequency TEOAE SNR (median of 1.3 dB) and coherence synchrony measure, despite small test-retest differences related to changes in forward and reverse transmission in the ear canal. The results suggest the potential use of such tests to screen for sensorineural hearing loss, including ototoxic loss. Experiment 2 was a feasibility study to explore TEOAE test parameters that might be used in a full-scale study to screen CF patients for risk of ototoxic hearing loss.
No otoacoustic evidence for a peripheral basis of absolute pitch
2018, Hearing ResearchAbsolute pitch (AP) is the ability to identify the perceived pitch of a sound without an external reference. Relatively rare, with an incidence of approximately 1/10,000, the mechanisms underlying AP are not well understood. This study examined otoacoustic emissions (OAEs) to determine if there is evidence of a peripheral (i.e., cochlear) basis for AP. Two OAE types were examined: spontaneous emissions (SOAEs) and stimulus-frequency emissions (SFOAEs). Our motivations to explore a peripheral foundation for AP were several-fold. First is the observation that pitch judgment accuracy has been reported to decrease with age due to age-dependent physiological changes cochlear biomechanics. Second is the notion that SOAEs, which are indirectly related to perception, could act as a fixed frequency reference. Third, SFOAE delays, which have been demonstrated to serve as a proxy measure for cochlear frequency selectivity, could indicate tuning differences between groups. These led us to the hypotheses that AP subjects would (relative to controls) exhibit a. greater SOAE activity and b. sharper cochlear tuning. To test these notions, measurements were made in normal-hearing control (N = 33) and AP-possessor (N = 20) populations. In short, no substantial difference in SOAE activity was found between groups, indicating no evidence for one or more strong SOAEs that could act as a fixed cue. SFOAE phase-gradient delays, measured at several different probe levels (20-50 dB SPL), also showed no significant differences between groups. This observation argues against sharper cochlear frequency selectivity in AP subjects. Taken together, these data support the prevailing view that AP mechanisms predominantly arise at a processing level in the central nervous system (CNS) at the brainstem or higher, not within the cochlea.
Investigation of the 2f <inf>1</inf> −f <inf>2</inf> and 2f <inf>2</inf> −f <inf>1</inf> distortion product otoacoustic emissions using a computational model of the gerbil ear
2018, Hearing ResearchIn this work, a three-dimensional computational model of the gerbil ear is used to investigate the generation of the and distortion product otoacoustic emissions (DPOAEs). In order to predict both the distortion and reflection sources, cochlear roughness is modeled by introducing random inhomogeneities in the outer hair cell properties. The model was used to simulate the generation of DPOAEs in response to a two-tone stimulus for various primary stimulus levels and frequency ratios. As in published experiments, the DPOAEs are mostly dominated by the distortion component while the DPOAEs are dominated by the reflection component; furthermore, the influence of the levels and frequency ratio of the primaries are consistent with measurements. Analysis of the intracochlear response shows that the distortion component has the highest magnitude at all longitudinal locations for the distortion product (DP) while the distortion component only dominates close to the DP best place in the case of the DP. Decomposition of the intracochlear DPs into forward and reverse waves demonstrates that the DP generates reverse waves for both the distortion and reflection components; however, a reverse wave is only generated for the reflection component in the case of the DP. As in experiments in the gerbil, the group delay of the reflection component of the DPOAE is between and the forward group delay, which is consistent with the propagation of DP towards the stapes as slow reverse waves.