Review of Whole Head Experimental Cochlear Promontory Vibration with Bone Conduction Stimulation and Investigation of Experimental Setup Effects.

Bone conduction sound transmission in humans has been extensively studied using cochlear promontory vibrations. These studies use vibration data collected from measurements in live humans, whole cadavers, and severed cadaver heads, with stimulation applied either at an implant in the skull bone or directly on the skin. Experimental protocols, methods, and preparation of cadavers or cadaver heads vary among the studies, and it is currently unknown to what extent the aforementioned variables affect the outcome of those studies. The current study has two aims. The first aim is to review and compare available experimental data and assess the effects of the experimental protocol and methods. The second aim is to investigate similarities and differences found between the experimental studies based on simulations in a finite element model, the LiUHead. With implant stimulation, the average cochlear promontory vibration levels were within 10 dB, independent of the experimental setup and preparations of the cadavers or cadaver heads. With on-skin stimulation, the results were consistent between cadaver heads and living humans. Partial or complete replacement of the brain with air does not affect the cochlear promontory vibration, whereas replacing the brain with liquid reduces the vibration level by up to 5 dB. An intact head-neck connection affects the vibration of the head at frequencies below 300-400 Hz with a significant vibration reduction at frequencies below 200 Hz. Removing all soft tissue, brain tissue, and intracranial fluid from the head increases the overall cochlear promontory vibration level by around 5 dB.

Consequences of Mastoidectomy on Bone Conducted Sound Based on Simulations in a Whole Human Head.

HYPOTHESIS: The aim of this study is to investigate how a mastoidectomy surgery affects bone conduction (BC) sound transmission using a whole head finite element model. BACKGROUND: Air conduction (AC) and BC hearing thresholds are normally used to evaluate the effect of an ear surgery. It is then assumed that the BC hearing thresholds are unaffected by the surgery. Moreover, BC hearing aids are used in cases of unilateral or conductive hearing loss in heads that have undergone a mastoidectomy surgery. Given the invasiveness of the surgery, the BC hearing sensitivity may be altered by the surgery itself. METHODS: Two types of mastoid surgery, canal wall up and canal wall down, with and without obliteration, were simulated in a whole head finite element model for BC stimulation, the LiUHead. The evaluations were conducted for two different methods of applying the BC sound, at the skin surface (B71 transducer) and directly at the bone (BC hearing aid). RESULTS: The results showed that a mastoidectomy surgery increased the cochlear vibration responses with BC stimulation. The increase was less than 5 dB, except for a canal wall down surgery which gave an increase of up to 8 dB at frequencies close to 10 kHz. The increase was greater at the ipsilateral cochlea compared with the contralateral cochlea. CONCLUSION: A mastoidectomy surgery increases the vibration at both cochleae for BC stimulation and the increase generally improved with frequency. Obliteration of the surgical cavity does not influence BC sound transmission.

Power Dissipation in the Cochlea Can Enhance Frequency Selectivity.

The cochlear cavity is filled with viscous fluids, and it is partitioned by a viscoelastic structure called the organ of Corti complex. Acoustic energy propagates toward the apex of the cochlea through vibrations of the organ of Corti complex. The dimensions of the vibrating structures range from a few hundred (e.g., the basilar membrane) to a few micrometers (e.g., the stereocilia bundle). Vibrations of microstructures in viscous fluid are subjected to energy dissipation. Because the viscous dissipation is considered to be detrimental to the function of hearing-sound amplification and frequency tuning-the cochlea uses cellular actuators to overcome the dissipation. Compared to extensive investigations on the cellular actuators, the dissipating mechanisms have not been given appropriate attention, and there is little consensus on damping models. For example, many theoretical studies use an inviscid fluid approximation and lump the viscous effect to viscous damping components. Others neglect viscous dissipation in the organ of Corti but consider fluid viscosity. We have developed a computational model of the cochlea that incorporates viscous fluid dynamics, organ of Corti microstructural mechanics, and electrophysiology of the outer hair cells. The model is validated by comparing with existing measurements, such as the viscoelastic response of the tectorial membrane, and the cochlear input impedance. Using the model, we investigated how dissipation components in the cochlea affect its function. We found that the majority of acoustic energy dissipation of the cochlea occurs within the organ of Corti complex, not in the scalar fluids. Our model suggests that an appropriate dissipation can enhance the tuning quality by reducing the spread of energy provided by the outer hair cells' somatic motility.

Power dissipation in the subtectorial space of the mammalian cochlea is modulated by inner hair cell stereocilia.

The stereocilia bundle is the mechano-transduction apparatus of the inner ear. In the mammalian cochlea, the stereocilia bundles are situated in the subtectorial space (STS)--a micrometer-thick space between two flat surfaces vibrating relative to each other. Because microstructures vibrating in fluid are subject to high-viscous friction, previous studies considered the STS as the primary place of energy dissipation in the cochlea. Although there have been extensive studies on how metabolic energy is used to compensate the dissipation, much less attention has been paid to the mechanism of energy dissipation. Using a computational model, we investigated the power dissipation in the STS. The model simulates fluid flow around the inner hair cell (IHC) stereocilia bundle. The power dissipation in the STS because of the presence IHC stereocilia increased as the stimulating frequency decreased. Along the axis of the stimulating frequency, there were two asymptotic values of power dissipation. At high frequencies, the power dissipation was determined by the shear friction between the two flat surfaces of the STS. At low frequencies, the power dissipation was dominated by the viscous friction around the IHC stereocilia bundle--the IHC stereocilia increased the STS power dissipation by 50- to 100-fold. There exists a characteristic frequency for STS power dissipation, CFSTS, defined as the frequency where power dissipation drops to one-half of the low frequency value. The IHC stereocilia stiffness and the gap size between the IHC stereocilia and the tectorial membrane determine the characteristic frequency. In addition to the generally assumed shear flow, nonshear STS flow patterns were simulated. Different flow patterns have little effect on the CFSTS. When the mechano-transduction of the IHC was tuned near the vibrating frequency, the active motility of the IHC stereocilia bundle reduced the power dissipation in the STS.