Otoacoustic Emissions

In addition to receiving and processing sound stimuli, the ear is capable of producing sounds too. First described by British physicist David Kemp in 1978, otoacoustic emissions (OAEs) are low-level sounds that are generated within the cochlea of the inner ear. These weak vibrations are transmitted outwardly through the middle ear to the ear canal, [Page 710]where they can be recorded with a sensitive microphone. This entry describes the mechanism that creates OAEs and the clinical use of OAEs.

Otoacoustic emissions arise from the miniscule movements of the outer hair cells (OHCs) in the cochlea. The OHCs structurally resemble the inner hair cells, which are true sensory cells in that they convert vibrations into neuroelectrical responses the brain understands. The OHCs, on the other hand, act as tiny mechanical amplifiers. These cells are attached to the basilar membrane, which is a flexible ribbon of tissue that extends the entire length of the cochlea. When the ear is stimulated with sound, the basilar membrane undulates—moving up and down several thousand times per second. The OHCs, in turn, react by changing their length extremely rapidly, becoming shorter and longer via contraction and expansion, respectively. The push-pull action exerted by the OHCs boosts the basilar membrane's motion, and this enhanced vibration results in improved hearing sensitivity.

The typical paradigm for evoking OAEs involves presenting a series of brief acoustic clicks or a steady-state tone to the ear and then recording the ensuing sound. The click method produces transient evoked otoacoustic emissions (TEOAEs). Because the stimulus contains many frequencies, a large number of frequencies are present in TEOAE recordings. The tonal technique yields stimulus-frequency otoacoustic emissions (SFOAEs). SFOAEs are not in widespread use, as it is difficult to separate the response generated in the ear from the stimulus being presented. A more common way to elicit OAEs is by delivering not one but two tones simultaneously. The tones generate patterns of vibration that interact on the basilar membrane. Through this mixing process, the wave patterns are altered slightly and so-called distortion-product otoacoustic emissions (DPOAEs) are produced. Alternatively, spontaneous otoacoustic emissions (SOAEs) occur in the absence of external stimulation. SOAEs occur because the OHCs in a restricted region on the basilar membrane appear to vibrate continuously without provocation. Localized areas of the basilar membrane correspond to specific pitches, and thus SOAEs are nearly tonal in character.

As mentioned previously, the lengthwise motion of the OHCs plays a key role in generating all types of OAEs. At a more fundamental level, at least two different mechanisms may contribute to the production of OAEs. One underlying mechanism involves mechanical reflection. Slight irregularities are thought to exist at various locations along the basilar membrane. Perhaps, these imperfections serve as acoustic reflectors and inbound sound simply bounces off these microscopic flaws in essentially the same way light reflects off a mirror. The second mechanism is based on nonlinear distortion. The amplification provided by the OHCs depends on the level of the incoming sound—soft sounds are boosted more than loud sounds. One consequence of this compressive phenomenon is that stimulus vibrations become “distorted” such that frequencies not present in the original sound are introduced into the response. The best way to observe cochlear distortion is to deliver two tones and then look for more than two frequency components in the spectrum of otoacoustic response. Moreover, the mechanisms of reflection and distortion likely both contribute to the production of evoked OAEs. Work is currently underway to determine the relative contribution of each mechanism.

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