Christian Breuninger

The effects of complex stapes motion on the response of the cochlea.

Hypothesis: The piston-like motion of the stapes footplate is the only effective stimulus to the cochlea, and rocking-like stapes motions have no effect on hearing. Background: Studies of the vibration of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. At low frequencies, the vibrations are predominantly piston-like, but they become increasingly rocking-like at middle and high frequencies. These complex vibrations can be decomposed into a translational, piston-like displacement and 2 rotational movements around the long and short axes of the stapes. The rotational components produce no net volume displacement of the cochlear fluid at some distance from the footplate. Therefore, according to the classic theory of hearing, the rotational motion is not transformed into cochlear activity and a hearing sensation. It was the goal of this study to test this hypothesis experimentally in guinea pigs. Methods: A piezoelectric 3-axis device was used to vibrate the stapes in various desired directions while simultaneously monitoring the actual motion of the stapes by a 3-dimensional laser Doppler interferometer and the cochlear activity by recording the compound action potential. Results: The collected data of the presented study cannot be explained by the current theory of hearing. Conclusion: The qualitative results provide supportive evidence that complex movements of the stapes footplate may lead to cochlear activity. Further experiments are necessary to confirm and quantify these effects.

MECHANICAL EXCITATION OF COMPLEX STAPES MOTION IN GUINEA PIGS

As observed in many studies the natural vibration pattern of the stapes on acoustic stimulation reveals a complex motion pattern dependent on the frequency of excitation. Whereas for low frequencies the motion is predominantly piston-like, significant rocking can be found for higher frequencies. The cochlea fluid is mechanically exited by the piston-like motion and two rotational movements along the short and long axis of the footplate. The rotational components produce no net volume flux of the cochlea fluid and, therefore, their influence on the hearing sensation is still an open question. To investigate the response of the cochlea on complex motion of the stapes footplate, different vibration pattern on the stapes have been mechanically applied in anesthetized guinea pigs. A test rig was built to position the subject, an actuator and a Laser Doppler Vibrometer adjustable by micro manipulators. A three-axis piezoelectric actuator has been designed and coupled to the stapes head of the surgically prepared guinea pig by a coupling rod. For capturing the effective motion of the stapes three-dimensional laser Doppler vibrometry was applied. The excitation procedure for arbitrary stapes motion consists of both an identification and a measurement phase to determine the transducer behavior and the electrophysiological measurements of the cochlea response on the applied motion. Spatial velocity of stapes and cochlea potentials on repeated transducer activation are captured simultaneously by the data acquisition system for appropriate post-processing. The task of driving arbitrary motion patterns yields a much higher complexity than classical one-dimensional consideration. The setup enables investigations in electrocochleography with different amounts of piston and rocking-like motions of the stapes.

Optimization of Mechatronic Systems using the Software Package NEWOPT/AIMS

Optimization of mechatronic systems is an interactive process which requires the iterative application of several different computer algorithms and decisions of the designer depending on the demands of the optimization problem. In this paper the software package NEWOPT/AIMS for optimization of multibody systems is presented. For the application to mechatronical systems the software is augmented by additional methods for sensitivity analysis and scalar optimization.The software package includes algorithms for simulation, sensitivity analysis and optimization. Sensitivities can be provided by the efficient semi-analytical adjoint variable method, automatic differentiation or finite differences. For multicriteria optimization the designer may choose from different strategies and algorithms.The interactive optimization process is supported by an interface designed for a simplified and intuitive usage of the software tool.The optimization procedure is demonstrated for two complex systems, a manipulator with parallel kinematics, and a model of a reconstruction of a damaged human middle ear.

THE EFFECTS OF COMPLEX STAPES MOTION ON THE RESPONSE OF THE COCHLEA IN GUINEA PIGS

Studies into the vibration modes of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. These complex vibrations can be expressed as one translational displacement and two rotational movements around the long and short axes of the stapes, known as the three elementary motions. According to the classical theory of hearing, the rotational motions induce no volume displacement of cochlear fluid and, therefore, no cochlear activity (i.e. hearing sensation). It is the goal of this study to verify this hypothesis. A custom-built, three-axis piezoelectric actuator, capable of eliciting any desired vibration mode, was coupled to the surgically prepared stapes superstructure of anesthetized guinea pigs. When producing different movement patterns, electrophysiological measurements of the cochlear potentials were simultaneously recorded. Mechanical stimulation of the stapes according to the three elementary motions delivered three cochlear potentials of different amplitude. The greatest potential resulted from translational motions. The results of the present study show a cochlear excitation in all performed movement patterns of the stapes. Hence, the prevailing hypothesis could not be verified.

ON THE OPTIMAL COUPLING OF AN IMPLANTABLE HEARING AID – MEASUREMENTS AND SIMULATIONS

The sound transfer is strongly dependent on the coupling of the prosthesis to the ossicles. This holds for both passive but in particular for active implants. Various principles of coupling and different designs of attaching the implant to ossicle are in use showing a different mechanical behavior concerning stiffness and damping. Pressing a driving rod against the ossicles means a unilateral coupling which needs a particular prestress. Due to the nonlinear transfer behavior of the coupling region and ossicular chain, the actual motions, e.g. of actuator, stapes and umbo are dependent on that prestress. An optimal coupling should allow a stapes motion sufficient to compensate a hearing loss, but should avoid a distorted sound transfer or feedback due to sound radiation from ear drum. The static adjustment influences the sound transfer of the dynamical system essentially. To find an optimal adjustment of the actuator, the dynamical behavior of the actuator itself, of the coupling region and the individual ossicular chain has to be regarded. During surgery the stiffness of the individual chain has to be estimated and the configuration when the actuator touches the ossicles has to be detected very carefully.

Mechanical Aspects in Human Hearing

For the description of the hearing process nonlinear models for normal, pathological and reconstructed ears were established based on multibody systems or finite elements. Nonlinearities are found in the constitutive equations of the ear drum, the ligaments and the coupling between implant and ossicles. Measurements in the clinical practice and in the lab are used to determine the dynamical behavior of the hearing organ and to derive the belonging model parameters. Simulations with various types of implants and different manners of incisions show the big influence of the points of attachment and the coupling conditions on the sound transfer. For actively driven implants the restricted coupling forces have to be regarded since distortion may lead to unacceptable results. The models allow virtual tests of passive and active implants to optimize their performance, to shorten clinical test series and an interpretation of injuries due to loud sound events.Copyright

Optimization of a reconstructed middle ear using an evolution strategy

Publisher Summary Optimization of complex multibody systems requires a great computational effort. Therefore, sophisticated optimization methods must be applied. This chapter presents a paper that describes an approach for the optimization of multibody systems in a systematic way and is applied to the optimization of a human middle ear. For the optimization, the software package called NEWOPT/AIMS is used, which includes several algorithms for the analysis and optimization of multibody systems. The goal of the optimization is to improve sound transfer for the reconstruction of a damaged middle ear by variation of insertion conditions of a passive middle-ear implant. The disadvantage of evolution strategies, which requires a high number of criteria evaluations, is reduced by calculating the criteria parallelly on different workstations. For this parallelization, a master–slave approach can be implemented and used for several large-scale optimizations. For the optimization of a reconstructed middle ear with respect to sound transfer to an inner ear, a criteria formulation in the frequency domain can be used for an efficient assessment of system behaviors. An improved hearing can be achieved by adaptation of insertion conditions of the implant.