Otoacoustic emission

An otoacoustic emission (OAE) is a sound which is generated from within the inner ear. Having been predicted by Thomas Gold in 1948, their existence was first demonstrated experimentally by David Kemp in 1978 and they have since been shown to arise by a number of different cellular mechanisms within the inner ear. Numerous studies have shown that OAEs disappear after the inner ear has been damaged, so OAEs are often used in the laboratory and the clinic as a measure of inner ear health. Broadly speaking, there are two types of otoacoustic emissions: Spontaneous Otoacoustic Emissions (SOAEs) and Evoked Otoacoustic Emissions (EOAEs). As their names suggest, SOAEs arise spontaneously and EOAEs require an evoking stimulus.

Relations
OAE's are considered to be related to the amplification function of the cochlea. In the absence of external stimulation, the activity of the cochlear amplifier increases, leading to the production of sound. Several lines of evidence suggest that, in mammals, outer hair cells are the elements that enhance cochlear sensitivity and frequency selectivity and hence act as the energy sources for amplification. The outer hair cells make only token projections via afferent fibers to the central nervous system. However, they receive extensive efferent innervation, whose activation decreases cochlear sensitivity and frequency discrimination.

Evoking OAE's
OAEs are currently evoked using two different methodologies. Transient-evoked OAEs (TEOAEs or TrOAEs) are evoked using a click (broad frequency range) or toneburst (brief duration pure tone) stimulus. The evoked response from a click covers the frequency range up to around 4 kHz, while a toneburst will elicit a response from the region that has the same frequency as the pure tone. Distortion product OAEs (DPOAEs) are evoked using a pair of primary tones $$f_1$$ and $$f_2$$ with particular intensity (usually either 65 - 55 dB or 65 for both) and ratio ($$f_1\mbox{ }:\mbox{ }f_2$$). The evoked responses from these stimuli occur at frequencies ($$f_{dp}$$) mathematically related to the primary frequencies, with the two most prominent being $$f_{dp}=2f_1-f_2$$ (the "quadratic" distortion tone, most commonly used for hearing screening) and $$f_{dp}=f_2-f_1$$ (the "cubic" distortion tone, or simple difference tone).

Clinical Importance of OAE's
Otoacoustic emissions are clinically important because they are the basis of a simple, non-invasive, test for hearing defects in newborn babies and in children who are too young to cooperate in conventional hearing tests. Many western countries now have national programmes for the universal hearing screening of newborn babies. The primary screening tool is a test for the presence of a click-evoked OAE. Otoacoustic emissions also assist in differential diagnosis of cochlear and higher level hearing losses (e.g., auditory neuropathy).