title: Longitudinal spread of mechanical excitation through tectorial membrane traveling waves.
abstract: The mammalian inner ear separates sounds by their frequency content, and this separation underlies important properties of human hearing, including our ability to understand speech in noisy environments. Studies of genetic disorders of hearing have demonstrated a link between frequency selectivity and wave properties of the tectorial membrane (TM). To understand these wave properties better, we developed chemical manipulations that systematically and reversibly alter TM stiffness and viscosity. Using microfabricated shear probes, we show that (i) reducing pH reduces TM stiffness with little change in TM viscosity and (ii) adding PEG increases TM viscosity with little change in TM stiffness. By applying these manipulations in measurements of TM waves, we show that TM wave speed is determined primarily by stiffness at low frequencies and by viscosity at high frequencies. Both TM viscosity and stiffness affect the longitudinal spread of mechanical excitation through the TM over a broad range of frequencies. Increasing TM viscosity or decreasing stiffness reduces longitudinal spread of mechanical excitation, thereby coupling a smaller range of best frequencies and sharpening tuning. In contrast, increasing viscous loss or decreasing stiffness would tend to broaden tuning in resonance-based TM models. Thus, TM wave and resonance mechanisms are fundamentally different in the way they control frequency selectivity.
authors.affiliation: Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA 02139; Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139;
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Sellon
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139.
authors.forename: Shirin
authors.initials: S
authors.lastname: Farrahi
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139;
authors.forename: Roozbeh
authors.initials: R
authors.lastname: Ghaffari
authors.affiliation: Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA 02139; Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139; Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139 freeman@mit.edu.
authors.forename: Dennis M
authors.initials: DM
authors.lastname: Freeman
mesh_terms: D000818:Animals; D003051:Cochlea / Q000502:physiology; D008954:Models, Biological; D013680:Tectorial Membrane / Q000502:physiology*; D014783:Viscosity
keywords: cochlear mechanics; resonance; tectorial membrane; traveling waves; viscoelastic materials