Is the middle ear the first filter of frequency selectivity?

Abstract

Introduction and objectivesThe cochlea has traditionally been considered as the first frequency selection filter in the auditory pathway due to the contraction of its external ciliated cells. Yet, much evidence has emerged from work carried out during experiments with animals, some of which is anatomical (connections between the auditory pathway and motor nuclei of the middle ear muscles); other work is physiological, which indicates that the middle ear might be the first filter through which specific sounds from noisy environments may initially be isolated. Material and methodsIn cooperation with the Department of Mechanical Engineering at the Technical School of Industrial Engineering at the University of Valladolid (UVa), we developed and refined a new admittance meter capable of evaluating changes in impedance that occur in the human middle ear depending on frequency. Using this device, we measured variation in impedance in 7 otologically healthy volunteers submitted to a varied range of sound environments. ResultsWe found that hearing impedance is not constant, but rather that the attention offered by the examined subjects when following a conversation in a noisy environment leads to variations in hearing impedance at high frequencies. ConclusionsIn the light of these findings, we feel that the middle ear does not play a merely passive role in hearing but rather that the contraction of the intratympanic muscles makes variations in impedance possible such that the resonance frequency of the ear shifts towards higher frequencies. This thus enhances sound discrimination in noisy environments.