Chapter 9: Hearing: Physiology & Psychoacoustics

Hearing: Physiology & Psychoacoustics exploration of auditory physiology and psychoacoustics examines the transformation of physical sound energy into meaningful neural representations. Sound is defined as pressure fluctuations traveling through a medium, with its primary characteristics being frequency, which dictates perceived pitch, and amplitude, which influences perceived loudness. The human auditory system is remarkably sensitive, utilizing the pinna and ear canal to funnel vibrations toward the tympanic membrane. In the middle ear, the ossicles—consisting of the malleus, incus, and stapes—provide essential amplification by leveraging mechanical force and concentrating pressure from the large eardrum onto the much smaller oval window. This mechanical energy enters the fluid-filled cochlea, where traveling waves displace the basilar membrane. The organ of Corti serves as the site of transduction, utilizing specialized inner hair cells to convert mechanical movement into electrochemical signals through the bending of stereocilia and the action of tip links. Outer hair cells contribute an active mechanical feedback loop known as electromotility, which sharpens frequency tuning and increases sensitivity. The auditory nerve transmits this data using a dual-coding system: a place code based on the specific location of displacement along the tonotopic basilar membrane, and a temporal code involving phase locking and the volley principle for low-frequency sounds. Neural signals then traverse a complex pathway through the brainstem, including the cochlear nucleus and the superior olive—the critical site for sound localization—before ascending through the inferior colliculus and the medial geniculate nucleus of the thalamus to the primary auditory cortex. Psychoacoustic studies further illuminate these processes by measuring audibility thresholds and equal-loudness curves, as well as exploring phenomena like masking and critical bandwidths. The discussion concludes with an analysis of hearing impairments, ranging from conductive issues and age-related presbycusis to sensorineural damage and hidden hearing loss, alongside the technological functions of hearing aids and cochlear implants.