Cochlear partition anatomy and motion in humans differ from the classic view of mammals

Mammals detect sound through mechanosensitive cells of the cochlear organ of Corti that rest on the basilar membrane (BM). Motions of the BM and organ of Corti have been studied at the cochlear base in various laboratory animals, and the assumption has been that the cochleas of all mammals work similarly. In the classic view, the BM attaches to a stationary osseous spiral lamina (OSL), the tectorial membrane (TM) attaches to the limbus above the stationary OSL, and the BM is the major moving element, with a peak displacement near its center. Here, we measured the motion and studied the anatomy of the human cochlear partition (CP) at the cochlear base of fresh human cadaveric specimens. Unlike the classic view, we identified a soft-tissue structure between the BM and OSL in humans, which we name the CP “bridge.” We measured CP transverse motion in humans and found that the OSL moved like a plate hinged near the modiolus, with motion increasing from the modiolus to the bridge. The bridge moved almost as much as the BM, with the maximum CP motion near the bridge–BM connection. BM motion accounts for 100% of CP volume displacement in the classic view, but accounts for only 27 to 43% in the base of humans. In humans, the TM–limbus attachment is above the moving bridge, not above a fixed structure. These results challenge long-held assumptions about cochlear mechanics in humans. In addition, animal apical anatomy (inSI Appendix) doesn’t always fit the classic view.

Cochlear Place of Stimulation Is One Determinant of Cochlear Implant Sound Quality

Objective: Our aim was to determine the effect of acute changes in cochlear place of stimulation on cochlear implant (CI) sound quality. Design: In Experiment 1, 5 single-sided deaf (SSD) listeners fitted with a long (28-mm) electrode array were tested. Basal shifts in place of stimulation were implemented by turning off the most apical electrodes and reassigning the filters to more basal electrodes. In Experiment 2, 2 SSD patients fitted with a shorter (16.5-mm) electrode array were tested. Both basal and apical shifts in place of stimulation were implemented. The apical shifts were accomplished by current steering and creating a virtual place of stimulation more apical that that of the most apical electrode. Results: Listeners matched basal shifts by shifting, in the normal-hearing ear, the overall spectrum up in frequency and/or increasing voice pitch (F0). Listeners matched apical shifts by shifting down the overall frequency spectrum in the normal-hearing ear. Conclusion: One factor determining CI voice quality is the location of stimulation along the cochlear partition.

Imaging forward and Reverse Traveling Waves in the Cochlea

The presence of forward and reverse traveling wave modes on the basilar membrane has important implications to how the cochlea functions as a filter, transducer, and amplifier of sound. The presence and parameters of traveling waves are of particular importance to interpreting otoacoustic emissions (OAE). OAE are vibrations that propagate out of the cochlea and are measureable as sounds emitted from the tympanic membrane. The interpretation of OAE is a powerful research and clinical diagnostic tool, but OAE use has not reached full potential because the mechanisms of their generation and propagation are not fully understood. Of particular interest and deliberation is whether the emissions propagate as a fluid compression wave or a structural traveling wave. In this study a mechanical probe was used to simulate an OAE generation site and optical imaging was used to measure displacement of the inner hair cell stereocilia of the gerbil cochlea. Inner hair cell stereocilia displacement measurements were made in the radial dimension as a function of their longitudinal location along the length of the basilar membrane in response to a transverse stimulation from the probe. The analysis of the spatial frequency response of the inner hair cell stereocilia at frequencies near the characteristic frequency (CF) of the measurement location suggests that a traveling wave propagates in the cochlear partition simultaneously basal and apical (forward and reverse) from the probe location. The traveling wave velocity was estimated to be 5.9m/s - 8m/s in the base (near CF of 29kHz - 40kHz) and 1.9m/s - 2.4m/s in the second turn (near CF of 2kHz - 3kHz). These results suggest that the cochlear partition is capable of supporting both forward and reverse traveling wave modes generated by a source driving the basilar membrane. This suggests that traveling waves in the cochlear partition contribute to OAE propagation.

Acoustic Boundary Layer Attenuation in Ducts With Rigid and Elastic Walls Applied to Cochlear Mechanics

The cochlea is the most important part of the hearing system, due to the fact that it transforms sound guided through air, bone, and lymphatic fluid to vibrations of the cochlear partition which includes the organ of Corti with its sensory cells. These send nerve impulses to the brain leading to hearing perception. The work presents the wave propagation in rigid ducts filled with air or water including viscous-thermal boundary layer damping. In extension, a mechanical box model of the human cochlea represented by a rectangular duct limited by the tapered basilar membrane at one side is developed and evaluated numerically by the finite element method. The results match with rare experiments on human temporal bones without using the physically unfounded assumption of Rayleigh damping. A forecast on the concept of the traveling wave parametric amplification is given to potentially explain the high hearing sensitivity and otoacoustic emissions.