67 Otoacoustic Emissions
Using modern computing technology and processing, signal averaging techniques and miniature microphones, cochlear outer hair cell vibrations can be detected in the external auditory meatus as otoacoustic emissions (OAEs). They were first described by David Kemp in 1978, in response to sound stimulation, and represent an objective measure of cochlear function.
67.1 Physiology
The cochlea provides an elegant mechanism for transforming the physical properties of sound into electrical neural impulses. Sound vibrations pass from the environment through the external and middle-ear systems to cause vibrations of the cochlear perilymph. These vibrations produce travelling waves in the basilar membrane. As a result of the gradient of width, thickness and consequently stiffness, of the basilar membrane, these travelling waves reach maximal amplitude at specific points along the cochlea. High frequencies are represented at the basal turn and low frequencies at the apical portion. These traveling waves are detected as a result of shearing forces on two separate hair cell systems in the organ of Corti: the inner (IHCs) and outer (OHCs) hair cells. The inner hair cells are purely sensory and are responsible for detecting these vibrations and producing neural impulses to allow them to be perceived by the central nervous system as sound. The outer hair cells also detect these vibrations but, in contrast, have a motor function. The outer hair cells at the region of maximal travelling wave amplitude vibrate in synchrony with the stimulating signal whilst the OHCs on either side of this region suppress vibration of the basilar membrane. This mechanism allows the fine-tuning, and non-linear response, found in the healthy cochlea. These outer hair cell vibrations can be detected in the external auditory meatus as sounds and have been labelled: otoacoustic emissions (originally described as “cochlear echoes”).
67.2 Types of Otoacoustic Emissions
Four classes of OAE exist:
1.
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