Basis Function Approaches for Two Dimensional Cochlear Models

The human cochlea possesses the amazing ability of analyzing audio signals. The structures and mechanisms behind its characteristic response to sound stimuli has been an active area of research for decades. It has been demonstrated that mathematical cochlear modeling poses a promising alternative to discover the elusive activities in an in vivo cochlea. However, despite the successful application of numerical methods such as the Wentzel-Kramers-Brillouin (WKB) method, finite difference method (FDM) and finite element method (FEM), the critical effects of the choice of basis functions have not been studied exclusively for the numerical solutions of cochlea models. This work presents the numerical solution procedures to two types of cochlear models using the basis function collocation approach. Accuracies and effectiveness of basis functions are evaluated by comparing simulation results with past experiment and physiological data. The time-domain solutions in response to various audio inputs are also shown. The cochlear model demonstrates sound processing abilities which are qualitatively comparable to physiological data. It is hoped that the results in this work would help in laying the foundation for future cochlear model solutions and cochlea-based audio signal processor

Detailed Simulation of the Cochlea: Recent Progress Using Large Shared Memory Parallel Computers

We have developed and are refining a detailed three-dimensional computational model of the human cochlea. The model uses the immersed boundary method to calculate the fluid-structure interactions produced in response to incoming sound waves. An accurate cochlear geometry obtained from physical measurements is incorporated. The model includes a detailed and realistic description of the various elastic structures present. Initially, a macro-mechanical computational model was developed for execution on a CRAY T90 at the San Diego Supercomputing Center. This code was ported to the latest generation of shared memory high performance servers from Hewlett Packard. Using compiler generated threads and OpenMP directives, we have achieved a high degree of parallelism in the executable, which has made possible to run several large scale numerical simulation experiments to study the interesting features of the cochlear system. In this paper, we outline the methods, algorithms and software tools that were used to implement and fine tune the code, and discuss some of the simulation results

A Comprehensive Three-Dimensional Model of the Cochlea

The human cochlea is a remarkable device, able to discern extremely small amplitude sound pressure waves, and discriminate between very close frequencies. Simulation of the cochlea is computationally challenging due to its complex geometry, intricate construction and small physical size. We have developed, and are continuing to refine, a detailed three-dimensional computational model based on an accurate cochlear geometry obtained from physical measurements. In the model, the immersed boundary method is used to calculate the fluid-structure interactions produced in response to incoming sound waves. The model includes a detailed and realistic description of the various elastic structures present. In this paper, we describe the computational model and its performance on the latest generation of shared memory servers from Hewlett Packard. Using compiler generated threads and OpenMP directives, we have achieved a high degree of parallelism in the executable, which has made possible several large scale numerical simulation experiments that study the interesting features of the cochlear system. We show several results from these simulations, reproducing some of the basic known characteristics of cochlear mechanics.Comment: 22 pages, 5 figure

A ratchet mechanism for amplification in low-frequency mammalian hearing

The sensitivity and frequency selectivity of hearing result from tuned amplification by an active process in the mechanoreceptive hair cells. In most vertebrates the active process stems from the active motility of hair bundles. The mammalian cochlea exhibits an additional form of mechanical activity termed electromotility: its outer hair cells (OHCs) change length upon electrical stimulation. The relative contributions of these two mechanisms to the active process in the mammalian inner ear is the subject of intense current debate. Here we show that active hair-bundle motility and electromotility can together implement an efficient mechanism for amplification that functions like a ratchet: sound-evoked forces acting on the basilar membrane are transmitted to the hair bundles whereas electromotility decouples active hair-bundle forces from the basilar membrane. This unidirectional coupling can extend the hearing range well below the resonant frequency of the basilar membrane. It thereby provides a concept for low-frequency hearing that accounts for a variety of unexplained experimental observations from the cochlear apex, including the shape and phase behavior of apical tuning curves, their lack of significant nonlinearities, and the shape changes of threshold tuning curves of auditory nerve fibers along the cochlea. The ratchet mechanism constitutes a general design principle for implementing mechanical amplification in engineering applications.Comment: 6 pages, 4 figures, plus Supplementary Information. Animation available on the PNAS website (http://dx.doi.org/10.1073/pnas.0914345107)

Improving the stimulation selectivity in the human cochlea by strategic selection of the current return electrode

The hearing quality provided by cochlear implants are poorly predicted by computer simulations. A realistic cochlear anatomy is crucial for the accuracy of predictions. In this study, the standard multipolar stimulation paradigms are revisited and Rattay’s Activating Function is evaluated in a finite element model of a realistic cochlear geometry that is based on µ-CT images and a commercial lead. The stimulation thresholds across the cochlear fibers were investigated for monopolar, bipolar, tripolar, and a novel (distant) bipolar electrode configuration using an active compartmental nerve model based on Schwartz-Eikhof-Frijns membrane dynamics. The results suggest that skipping of the stimulation point from the vicinity of the cathodic electrode to distant fibers, especially to the low frequency (apical) region of the basilar membrane that is most critical to hearing, occurs more often with monopolar stimulation than other electrode configurations. Bipolar and tripolar electrodes near the apical region did not provide a large threshold margin either before the stimulation skips over distant fibers. On the other hand, the threshold margin could be improved by proper selection of the electrode for the return current with bipolar stimulation, a technique named here as distant bipolar. The results also demonstrate the significance of having a realistic cochlear geometry in computer models for accurate interpretation for multipolar stimulation paradigms. More selective and focal stimulation may be possible by designing the electrode carrier shape and positioning of the current return electrodes more strategically. This is needed particularly in the apical turn of the cochlea where the current stimulation methods are the least selective

Time Domain Computation of a Nonlinear Nonlocal Cochlear Model with Applications to Multitone Interaction in Hearing

A nonlinear nonlocal cochlear model of the transmission line type is studied in order to capture the multitone interactions and resulting tonal suppression effects. The model can serve as a module for voice signal processing, it is a one dimensional (in space) damped dispersive nonlinear PDE based on mechanics and phenomenology of hearing. It describes the motion of basilar membrane (BM) in the cochlea driven by input pressure waves. Both elastic damping and selective longitudinal fluid damping are present. The former is nonlinear and nonlocal in BM displacement, and plays a key role in capturing tonal interactions. The latter is active only near the exit boundary (helicotrema), and is built in to damp out the remaining long waves. The initial boundary value problem is numerically solved with a semi-implicit second order finite difference method. Solutions reach a multi-frequency quasi-steady state. Numerical results are shown on two tone suppression from both high-frequency and low-frequency sides, consistent with known behavior of two tone suppression. Suppression effects among three tones are demonstrated by showing how the response magnitudes of the fixed two tones are reduced as we vary the third tone in frequency and amplitude. We observe qualitative agreement of our model solutions with existing cat auditory neural data. The model is thus simple and efficient as a processing tool for voice signals.Comment: 23 pages,7 figures; added reference