Basic Physiology of the Hearing Mechanism
The last 30 years has seen a remarkable development in our understanding of how the auditory system, particularly the peripheral system, processes complex sounds. Perhaps the most significant has been our understanding of the mechanisms underlying auditory frequency selectivity and their importance for normal and impaired auditory processing. Physiologically vulnerable cochlear filtering can account for many aspects of our normal and impaired psychophysical frequency selectivity.: For normal hearing, remarkable mechanisms in the organ of Corti, involving enhancement of mechanical tuning (in mammals probably by feedback of electromechanically generated energy from the hair cells), produce exquisite tuning, reflected in the tuning properties of cochlear nerve fibers. Recent comparisons of physiological (cochlear nerve) and psychophysical frequency selectivity in the same species indicate that the ear's overall frequency selectivity can be accounted for by thiscochlear filtering, at least in bandwidth terms.: Because this cochlear filtering is physiologically vulnerable, it deteriorates in deleterious conditions of the cochlea-hypoxia, disease, drugs, noise overexposure, mechanical disturbance-and is reflected in impaired psychophysical frequency selectivity. This is a fundamental feature of sensorineural hearing loss of cochlear origin, and is of diagnostic value. It can be shown to be impaired in the hearing of young people exposed to amplified popular music.: Cochlear filtering, particularly as reflected in the temporal patterns of cochlear fibers to complex sounds, is remarkably robust over a wide range of stimulus levels. Furthermore, cochlear filtering properties are a prime determinant of the -place- and -time- coding of frequency at the cochlear nerve level, both of which appear to be involved in pitch perception.
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