Human Inner Ear Mechanics Studied With Experimental, Anatomical, and Computational Approaches

Abstract

Our understanding of human inner ear mechanics is mostly based on laboratory-animal studies. This thesis presents new findings specific to human hearing. Our methodological approach was threefold: We performed experiments in fresh human cadaveric temporal bones and live patients, carried out anatomical studies, and used mathematical models to advance our understanding of human inner ear mechanics. In Chapter 1, we investigate the impedance and stiffness of the human basilar membrane (BM) in the cochlear base and show that previous studies either underestimated or overestimated the BM stiffness by up to one order of magnitude. Chapter 2 looks beyond the BM and investigates the motion across the entire width of the cochlear partition (CP). We identify a soft-tissue structure in the CP in human—the “bridge”—that connects the BM and osseous spiral lamina (OSL) and has approximately the same width as the BM. We show that the bridge as well as the OSL moves considerably in humans. The motion of the bridge and OSL questions the applicability of the classic mammalian hearing model, based on laboratory-animal data, to humans. In Chapter 3, we characterize the anatomical microstructure of the bridge and OSL. Fibers traversing the bridge and the high porosity of the OSL could explain the nature of the bridge and OSL motion. Chapters 4 and 5 investigate the propagation of low-frequency sound and infrasound through the human middle ear and inner ear. In Chapter 4, we show that the middle ear limits sound energy propagated to the inner ear at low frequencies. A perturbation of the inner ear impedance by means of opening the semicircular canal changes the sound flow and sensitivity of the ear to low-frequency sound. We also characterize the impedance of semicircular canal dehiscence. Chapter 5 ties together experimental, clinical, and computational modeling results to propose how hearing-threshold shifts at low frequencies may be exploited to diagnose patients with a pathological defect of the semicircular canal. The thesis offers a comprehensive understanding of human passive cochlear mechanics in the cochlear base, as well as a comprehensive understanding of low-frequency sound propagation in the human inner ear.