Ben Lineton

A state space model for cochlear mechanics

The stability of a linear model of the active cochlea is difficult to determine from its calculated frequency response alone. A state space model of the cochlea is presented, which includes a discretized set of general micromechanical elements coupled via the cochlear fluid. The stability of this time domain model can be easily determined in the linear case, and the same framework used to simulate the time domain response of nonlinear models. Examples of stable and unstable behavior are illustrated using the active micromechanical model of Neely and Kim. The stability of this active cochlea is extremely sensitive to abrupt spatial inhomogeneities, while smoother inhomogeneities are less likely to cause instability. The model is a convenient tool for investigating the presence of instabilities due to random spatial inhomogeneities. The number of unstable poles is found to rise sharply with the relative amplitude of the inhomogeneities up to a few percent, but to be significantly reduced if the spatial variation is smoothed. In a saturating nonlinear model, such instabilities generate limit cycles that are thought to produce spontaneous otoacoustic emissions. An illustrative time domain simulation is presented, which shows how an unstable model evolves into a limit cycle, distributed along the cochlea.

Fluid coupling in a discrete model of cochlear mechanics

A discrete model of cochlear mechanics is introduced that includes a full, three-dimensional, description of fluid coupling. This formulation allows the fluid coupling and basilar membrane dynamics to be analyzed separately and then coupled together with a simple piece of linear algebra. The fluid coupling is initially analyzed using a wavenumber formulation and is separated into one component due to one-dimensional fluid coupling and one comprising all the other contributions. Using the theory of acoustic waves in a duct, however, these two components of the pressure can also be associated with a far field, due to the plane wave, and a near field, due to the evanescent, higher order, modes. The near field components are then seen as one of a number of sources of additional longitudinal coupling in the cochlea. The effects of non-uniformity and asymmetry in the fluid chamber areas can also be taken into account, to predict both the pressure difference between the chambers and the mean pressure. This allows the calculation, for example, of the effect of a short cochlear implant on the coupled response of the cochlea.

A wave finite element analysis of the passive cochlea.

Current models of the cochlea can be characterized as being either based on the assumed propagation of a single slow wave, which provides good insight, or involve the solution of a numerical model, such as in the finite element method, which allows the incorporation of more detailed anatomical features. In this paper it is shown how the wave finite element method can be used to decompose the results of a finite element calculation in terms of wave components, which allows the insight of the wave approach to be brought to bear on more complicated numerical models. In order to illustrate the method, a simple box model is considered, of a passive, locally reacting, basilar membrane interacting via three-dimensional fluid coupling. An analytic formulation of the dispersion equation is used initially to illustrate the types of wave one would expect in such a model. The wave finite element is then used to calculate the wavenumbers of all the waves in the finite element model. It is shown that only a single wave type dominates the response until this peaks at the best place in the cochlea, where an evanescent, higher order fluid wave can make a significant contribution.

Comparing two proposed measures of cochlear mechanical filter bandwidth based on stimulus frequency otoacoustic emissions

It has been hypothesized that the sharpness of the cochlear mechanical filter is related to two measures based on stimulus frequency otoacoustic emissions (SFOAEs). The first is the group delay of the SFOAE; the second is the bandwidth of the SFOAE two-tone suppression isoinput tuning characteristic. A corollary of this is that natural variability in cochlear mechanical bandwidth within a population would lead to a positive correlation between these two SFOAE-based measures of tuning within that population. To test this prediction, SFOAE group delay and SFOAE two-tone suppression isoinput tuning characteristics were measured in a sample of 16 audiometrically normal subjects. Contrary to the prediction, no statistically significant correlation was found between the two bandwidth measures. Cochlear model simulations were used to aid the interpretation of this result. These suggested that a positive correlation between the two measures is expected, but that it may well be too weak to detect with the given sample size, due to the influence on the SFOAE measures of random inhomogeneities in basilar membrane impedance.

The effect of suppression on the periodicity of stimulus frequency otoacoustic emissions: Experimental data

In a companion paper [Lineton and Lutman, J. Acoust. Soc. Am. 114, 859-870 (2003)], changes in the spectral period of stimulus frequency otoacoustic emissions (SFOAEs) during self-suppression and two-tone suppression were simulated using a nonlinear cochlear model based on the distributed roughness theory of otoacoustic emission generation [Zweig and Shera, J. Acoust. Soc. Am. 98, 2018-2047 (1995)1. The current paper presents the results of an experimental investigation of SFOAE suppression obtained from 20 human subjects. It was found that, in most subjects, the spectral period increased during self-suppression, but reduced during high-side two-tone suppression. This pattern of results is in close agreement with the predictions of the cochlear model, and therefore strongly supports the distributed roughness theory of Zweig and Shera. In addition, the results suggest that the SFOAE spectral period is sensitive to changes in the state of the cochlear amplifier.

Fit for the frontline? A focus group exploration of auditory tasks carried out by infantry and combat support personnel

In order to preserve their operational effectiveness and ultimately their survival, military personnel must be able to detect important acoustic signals and maintain situational awareness. The possession of sufficient hearing ability to perform job-specific auditory tasks is defined as auditory fitness for duty (AFFD). Pure tone audiometry (PTA) is used to assess AFFD in the UK military; however, it is unclear whether PTA is able to accurately predict performance on job-specific auditory tasks. The aim of the current study was to gather information about auditory tasks carried out by infantry personnel on the frontline and the environment these tasks are performed in. The study consisted of 16 focus group interviews with an average of five participants per group. Eighty British army personnel were recruited from five infantry regiments. The focus group guideline included seven open-ended questions designed to elicit information about the auditory tasks performed on operational duty. Content analysis of the data resulted in two main themes: (1) the auditory tasks personnel are expected to perform and (2) situations where personnel felt their hearing ability was reduced. Auditory tasks were divided into subthemes of sound detection, speech communication and sound localization. Reasons for reduced performance included background noise, hearing protection and attention difficulties. The current study provided an important and novel insight to the complex auditory environment experienced by British infantry personnel and identified 17 auditory tasks carried out by personnel on operational duties. These auditory tasks will be used to inform the development of a functional AFFD test for infantry personnel.

Investigating the wave-fixed and place-fixed origins of the 2f1-f2 distortion product otoacoustic emission within a micromechanical cochlear model

The 2f(1)-f(2) distortion product otoacoustic emission (DPOAE) arises within the cochlea due to the nonlinear interaction of two stimulus tones (f(1) and f(2)). It is thought to comprise contributions from a wave-fixed source and a place-fixed source. The generation and transmission of the 2f(1)-f(2) DPOAE is investigated here using quasilinear solutions to an elemental model of the human cochlea with nonlinear micromechanics. The micromechanical parameters and nonlinearity are formulated to match the measured response of the cochlea to single- and two-tone stimulation. The controlled introduction of roughness into the active micromechanics of the model allows the wave- and place-fixed contributions to the DPOAE to be studied separately. It is also possible to manipulate the types of nonlinear suppression that occur within the quasilinear model to investigate the influence of stimulus parameters on DPOAE generation. The model predicts and explains a variety of 2f(1)-f(2) DPOAE phenomena: The dependence of emission amplitude on stimulus parameters, the weakness of experiments designed to quantify cochlear amplifier gain, and the predominant mechanism which gives rise to DPOAE fine structure. In addition, the model is used to investigate the properties of the wave-fixed source and how these properties are influenced by the stimulus parameters.

Click-evoked otoacoustic emissions (CEOAEs) recorded from neonates under 13 hours old using conventional and maximum length sequence (MLS) stimulation

Maximum length sequence (MLS) stimulation allows click evoked otoacoustic emissions (CEOAEs) to be averaged at very high stimulation rates. This enables a faster reduction of noise contamination of the response, and has been shown to improve the signal-to-noise ratio (SNR) of CEOAEs recorded from adult subjects. This study set out to investigate whether MLS averaging can enhance the SNR of CEOAEs recorded in newborns within the first day after birth, and so improve the pass rates for OAE screening in this period, when false alarm rates are very high. CEOAEs were recorded in a neonatal ward from 57 ears in 37 newborns ranging from 6 to 13h old, using both conventional (50/s) and high rate (5000/s) MLS averaging. SNR values and pass rates were compared for responses obtained within equal recording times at both rates. MLS averaging produced an SNR improvement of up to 3.8dB, with the greatest improvement found in higher frequency bands. This SNR advantage resulted in pass rate improvement between 5% and 10%, depending on pass criterion. A significant effect of age was found on both SNR and pass rate, with newborns between 6 and 10h old showing significantly lower values than those tested between 10 and 13h after birth, as well as a much greater improvement due to MLS averaging. The findings show that MLS averaging can reduce false alarm rates by up to 15% in very young neonates in a neonatal ward setting.

Effect of contralateral acoustic stimulation on cochlear tuning measured using stimulus frequency and distortion product OAEs

Abstract Objective: To study whether a change in cochlear tuning, measured using OAEs, could be detected due to contralateral activation of the efferent system using broadband noise. Design: Cochlear tuning measures based on SFOAE phase gradients and SFOAE-2TS ‘Q’ were used to test this hypothesis. SFOAE magnitude and phase gradient were measured using a pure-tone sweep from 1248 to 2496 Hz at 50 dB SPL. 2TS curves of SFOAE were recorded with a suppressor frequency swept from 1120 to 2080 Hz at 50 dB SPL. DPOAE f2-sweep phase gradient was also obtained to allow comparisons with the literature. All three assays were performed across with- and no-CAS conditions. Study sample: Twenty-two young, normal-hearing adults. Results: CAS did not produce a statistically significant change in the tuning metric in any of the OAE methods used, despite producing significant reductions in the OAE magnitude. Conclusion: It is unknown whether this insensitivity to CAS is due to an insensitivity of these three measures to cochlear mechanical tuning. The results suggest that any changes in tuning induced by CAS that may occur are small and difficult to detect using the OAE measurement paradigms used here.

An Electromechanical Model for the Cochlear Microphonic

The first of the many electrical signals generated in the ear, nerves and brain as a response to a sound incident on the ear is the cochlear microphonic (CM). The CM is generated by the hair cells of the cochlea, primarily the outer hairs cells. The potentials of this signal are a nonlinear filtered version of the acoustic pressure at the tympanic membrane. The CM signal has been used very little in recent years for clinical audiology and audiological research. This is because of uncertainty in interpreting the CM signal as a diagnostic measure, and also because of the difficulty of obtaining the signal, which has usually required the use of a transtympanic electrode. There are however, several potential clinical and research applications for acquisition of the CM. To promote understanding of the CM, and potential clinical application, a model is presented which can account for the generation of the cochlear microphonic signal. The model incorporates micro‐mechanical and macro‐mechanical aspects of previo...