Fluid coupling and waves in the cochlea

The cochlea plays an important role in human hearing. Its basic function is to map sounds of different frequencies onto corresponding characteristic positions on the basilar membrane, BM. When sounds enter the fluid-filled cochlea, deflections of the BM occur due to pressure differences between the cochlear fluid chambers. These deflections propagate along the cochlea to a frequency-dependent characteristic position and then decay away rapidly. The mechanics of the cochlea are modelled using both analytic and numerical models. In this thesis, the passive response of the cochlea is analysed, corresponding to its behaviour at high sound levels, to study the fluid coupling and waves in the cochlea.The fluid coupling is studied in 1D and 3D, uniform and non-uniform, uncoiled and coiled geometries, all with a passive basilar membrane. A ‘uniaxial model’, which is dependent on only a single dimension, is developed to represent the three-dimensional cochlea. The finite element method is also used to provide an independent check of the results from the analytic model.Analytic methods are used to predict waves due to different mechanisms in the passive cochlea, such as 1D and 3D fluid coupling and longitudinal BM dynamics. The wave finite element, WFE, method is then used to decompose the results of a full finite element model of the coupled cochlea into wave components. Results show that apart from the conventional slow wave, other additional types of wave in the passive cochlea do not appear to play a dominant role in normal passive cochlear function

Modelling the effect of round window stiffness on residual hearing after cochlear implantation

Preservation of residual hearing after cochlear implantation is now considered an important goal of surgery. However, studies indicate an average post-operative hearing loss of around 20 dB at low frequencies. One factor which may contribute to post-operative hearing loss, but which has received little attention in the literature to date, is the increased stiffness of the round window, due to the physical presence of the cochlear implant, and to its subsequent thickening or to bone growth around it. A finite element model was used to estimate that there is approximately a 100-fold increase in the round window stiffness due to a cochlear implant passing through it. A lumped element model was then developed to study the effects of this change in stiffness on the acoustic response of the cochlea. As the round window stiffness increases, the effects of the cochlear and vestibular aqueducts become more important. An increase of round window stiffness by a factor of 10 is predicted to have little effect on residual hearing, but increasing this stiffness by a factor of 100 reduces the acoustic sensitivity of the cochlea by about 20 dB, below 1 kHz, in reasonable agreement with the observed loss in residual hearing after implantation. It is also shown that the effect of this stiffening could be reduced by incorporating a small gas bubble within the cochlear implant

Finite element modelling of cochlear electrical coupling

The operation of each hair cell within the cochlea generates a change in electrical potential at the frequency of the vibrating basilar membrane beneath the hair cell. This electrical potential influences the operation of the cochlea at nearby locations and can also be detected as the cochlear microphonic signal. The effect of such potentials has been proposed as a mechanism for the non-local operation of the cochlear amplifier, and the interaction of such potentials has been thought to be the cause of the broadness of cochlea microphonic tuning curves. The spatial extent of influence of these potentials is an important parameter for determining the significance of their effects. Calculations of this extent have typically been based on calculating the longitudinal resistance of each of the scalae from the scala cross sectional area, and the conductivity of the lymph. In this paper, the range of influence of the electrical potential is examined using an electrical finite element model. It is found that the range of influence of the hair cell potential is much shorter than the conventional calculation, but is consistent with recent measurements

Prediction of mechanical effect due to a cochlear implant

The effect of a cochlear implant on residual, low frequency, hearing is complex and poorly understood. This research focuses on the mechanical effect of a cochlear implant on the cochlear mechanics by comparing the predicted basilar membrane, BM, response before and after the implantation. Audiograms measured from pre- and post-implant users are used as input of a computational model of the passive cochlea, proposed by Elliott et al. (Elliott et al., 2011), which are then used to study the mechanical effect of the implantation. In the model, a short cochlea implant, designed to electrically stimulate the basal regions at high frequencies while allowing normal hearing at low frequencies (Cochlear, 2008), is introduced into the lower cochlear fluid chamber. The active amplification of the cochlea is not considered, since a passive cochlear model whose response is not dependent on stimulus level can reasonably well represent the cochlea for subjects with hearing impairment. The results for the BM coupled response show that the volume change in the fluid chambers due to the implant has a negligible effect, less than about 0.1 dB, on the vibration of the modeled cochlea at low frequencies. A more extreme condition, in which the cochlear implant is assumed to touch the BM at some or whole basal positions and thus impeded its motion, is also studied. Although no travelling wave can propagate in the basal region in the latter case, the remainder of the cochlea is still coupled to the stapes by incompressible fluid. The BM response at low frequencies is relatively unaffected by the blocking of the BM motion in the basal region, although the effect is more dramatic for excitation frequency whose characteristic place is close to the end of the implant. Although this work does not model every aspect of the cochlear implantation, it does provide a way of predicting the possible mechanical effects of the implantation on the cochlear passive mechanics and the residual hearing

Near field fluid coupling between internal motion of the organ of Corti and the basilar membrane

The pressure distribution in each of the fluid chambers of the cochlea can be decomposed into a 1D, or plane wave, component and a near field component, which decays rapidly away from the excitation point. The transverse motion of the basilar membrane, BM, for example, generates both a 1D pressure field, which couples into the slow wave, and a local near field pressure, proportional to the BM acceleration, that generates an added mass on the BM due to the fluid motion. When the organ of Corti, OC, undergoes internal motion, due for example to outer hair cell activity, this motion will not itself generate any 1D pressure if the OC is incompressible and the BM is constrained not to move volumetrically, and so will not directly couple into the slow wave. This motion will, however, generate a near field pressure, proportional to the OC acceleration, which will act on the OC and thus increases its effective mass. The near field pressure due to this OC motion will also act on the BM, generating a force on the BM proportional to the acceleration of the OC, and thus create a “coupling mass” effect. By reciprocity, this coupling mass is the same as that acting on the OC due to the motion of the BM. This near field fluid coupling is initially observed in a finite element model of a slice of the cochlea. These simulations suggest a simple analytical formulation for the fluid coupling, using higher order beam modes across the width of the cochlear partition. It is well known that the added mass due to the near field pressure dominates the overall mass of the BM, and thus significantly affects the micromechanical dynamics. This work not only quantifies the added mass of the OC due its own motion in the fluid, and shows that this is important, but also demonstrates that the coupling mass effect between the BM and OC significantly affects the dynamics of simple micromechanical model

Modelling Cochlear mechanics

The cochlea plays a crucial role in mammal hearing. The basic function of the cochlea is to map sounds of different frequencies onto corresponding characteristic positions on the basilar membrane (BM). Sounds enter the fluid-filled cochlea and cause deflection of the BM due to pressure differences between the cochlear fluid chambers. These deflections travel along the cochlea, increasing in amplitude, until a frequency-dependent characteristic position and then decay away rapidly. The hair cells can detect these deflections and encode them as neural signals. Modelling the mechanics of the cochlea is of help in interpreting experimental observations and also can provide predictions of the results of experiments that cannot currently be performed due to technical limitations. This paper focuses on reviewing the numerical modelling of the mechanical and electrical processes in the cochlea, which include fluid coupling, micromechanics, the cochlear amplifier, nonlinearity, and electrical coupling

Prediction of inertial effects due to bone conduction in a 2D box model of the cochlea

A 2D box model of the cochlea has been used to predict the basilar membrane, BM, velocity and the fluid flow caused by two components of bone conduction: due to inertia of the middle ear and due to inertia of the cochlear fluids. A finite difference approach has been used with asymmetric fluid chambers, that enables an investigation of the effect of varying window stiffness, due to otosclerosis for example. The BM is represented as a series of locally reacting single degree of freedom systems, with graded stiffness along the cochlea to represent the distribution of natural frequencies and with a damping representative of the passive cochlea. The velocity distributions along the passive BM are similar for harmonic excitation via the middle ear inertia or via the fluid inertia, but the variation of the BM velocity magnitude with excitation frequency is different in the two cases. Excitation via the middle ear is suppressed if the oval window is assumed to be blocked, but the excitation via the cochlear fluids is still possible. By assuming a combined excitation due to both middle ear and fluid excitation, the difference between the overall response can be calculated with a flexible and a blocked oval window, which gives a reasonable prediction of Carhart’s notch

Finite element model of the active organ of Corti

The cochlear amplifier that provides our hearing with its extraordinary sensitivity and selectivity is thought to be the result of an active biomechanical process within the sensory auditory organ, the organ of Corti. Although imaging techniques are developing rapidly, it is not currently possible, in a fully active cochlea, to obtain detailed measurements of the motion of individual elements within a cross section of the organ of Corti. This motion is predicted using a two-dimensional finite element model. The various solid components are modelled using elastic elements, the outer hair cells as piezoelectric elements, and the perilymph and endolymph as viscous and nearly incompressible fluid elements. The model is validated by comparison with existing measurements of the motions within the passive organ of Corti, calculated when it is driven either acoustically, by the fluid pressure, or electrically, by excitation of the outer hair cells. The transverse basilar membrane motion and the shearing motion between the tectorial membrane and the reticular lamina are calculated for these two excitation modes. The fully active response of the basilar membrane to acoustic excitation is predicted using a linear superposition of the calculated responses and an assumed frequency response for the outer hair cell feedback.<br/

Change of cochlear micromechanics due to different types of hearing loss

The reasons for hearing loss are complex and currently the mechanics are not entirely clear. Outer hair cell (OHC) loss is believed to play an important role. Experimental observations shown that damage on OHCs due to ototoxic acid starts from the outermost row to the innermost row, whereas, loss of OHCs due to intense noise exposure occurs from the innermost row to the outermost row. Inspired by these experiments, this study employs the finite element method to develop a detailed model of a slice of the human cochlea including cochlear fine structures. OHC motility is implemented by applying forces at the two ends of the OHCs in response to stereocilia deflection, which are believed to be a key process in cochlear amplification. In this way, the effects of a loss of OHCs due to either intense noise exposure or ototoxic acid can be studied by suppressing forces on individual OHCs. Change of cochlear mechanical amplification and vibration patterns inside the organ of Corti due to different hearing loss mechanisms can thus be predicte