Power Dissipation in the Cochlea Can Enhance Frequency Selectivity

Abstract

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.

Two hydrodynamic effects allow strongly nonlinear cochlear response with level-independent admittance

“…Taken together, focusing and viscosity make the responses of the cochlear model much less sensitive to the fine-tuning of the active force (indeed, in our FEM and WKB simulations the net damping on the BM reaches negative values in the most active cases, which would mean instability, without viscosity, even without focusing), and they yield sharp profiles at low stimulus levels and large gain dynamics for both pressure and BM velocity, with a moderate nonlinear change of the admittance. As already observed by Prodanovic et al (2019), viscosity may paradoxically improve the cochlear tuning, because it helps suppressing the response within a narrow spatial region beyond its peak.…”

“…Because this study focuses on the fundamental physical nature of two hydrodynamic effects, we ignored the internal details of the OC. We note, however, that the actual movements of the complex structures of the OC within a viscous fluid are likely to increase the size of the viscous losses substantially, as shown by Prodanovic et al (2019), who take explicitly into account viscous losses in the OC. For this reason, we also performed simulations with a larger coefficient of viscosity, ten times that of water.…”

“…The large change of the phase lag and slope in the peak region, which is not consistent with physiological group delays (both neural and otoacoustic), and the large apical shift of the response peak with increasing G, suggests that the oversimplified schematization of the OC may have underestimated the effect of fluid viscosity. Indeed, fluid viscous losses provide (see also Prodanovic et al, 2019) a sharp cutoff to the growth of the TW in the peak region, limiting both the apical shift of the response and the group delay at the peak. Therefore, we repeated our simulations using a viscosity coefficient ten times larger than that of water.…”

“…Therefore, we repeated our simulations using a viscosity coefficient ten times larger than that of water. This may be considered as a rather crude way to account for viscous losses within the elements of the OC (Prodanovic et al, 2019). In Fig.…”

Mechanically facilitated micro-fluid mixing in the organ of Corti

“…where ν is the kinematic viscosity, ρ is the fluid density, and t is time. Fluid viscosity has been considered [15][16][17][18][19] or neglected [20][21][22][23] per the purpose of studies. To our knowledge, no previous study regarding cochlear fluids analyzed the effect of advection (the second term of Eq.…”