Otoacoustic Emission

Basic principle

An otoacoustic emission (OAE), first discovered by David Kemp in 1979, is a sound which is generated from within the inner ear by a number of different cellular mechanisms. OAEs disappears with inner ear damaged, so OAEs are often used as a measure of inner ear health.

There are two types of otoacoustic emissions: Spontaneous Otoacoustic Emissions (SOAEs) and Evoked Otoacoustic Emissions (EOAEs).

OAE is considered to be related to the amplification function of the cochlea. OAEs occur when the cochlear amplifier is too strong. Possibly outer hair cells enhance cochlear sensitivity and frequency selectivity and provide the energy. The outer hair cells have few afferent fibersbut receive extensive efferent innervation, whose activation decreases cochlear sensitivity and frequency discrimination.

Fig. 1 TEOAE of a healthy ear

EOAEs are currently evoked using two different methodologies. Transient EOAEs (TEOAE or TrEOAE) are evoked using a click stimulus that repeat at about 20 instances per second. 98% of the healthy ears have TEOAEs (0.5-4 kHz), but above 60 year only 35%. The evoked response from this type of stimulus covers the frequency range up to around 4 kHz. Transient potentials were the original methodology of examination used by Kemp. Distortion Product OAEs (DPOAE) are evoked using a pair of tones with particular intensity (usually either 65 - 55 dB or 65 for both) and frequency ratio (F2/F1). The evoked response from these stimuli occurs at a third frequency. This distortion product frequency is calculated based on the original F1 and F2. Inner ear damage diminishes the distortion product.

Applications

Middle, inner and retrocochlear disorders, tinnitus.

More Info

Last decade, OAEs became increasingly impoortnat in the clinical audiological practice. It is still uncertain if there is an interaction, e.g. at the level of the olivocochlear bundle, between the ears regarding OAEs. In general, contralateral stimulation does not provoke EOAEs. The frequency resolution, e.g. for speech, depends on very fast modulation of the incoming signal. Due to the neural distance, this modulation would lag behind if otoacoustic emissions in one ear would effect the opposite one.

Literature

"http://en.wikipedia.org/wiki/Otoacoustic_emission"

Probst R, Lonsbury-Martin BL, Martin GK. A review of otoacoustic emissions. J Acoust Soc Am. 1991, 89: 2027-67.