The impressive spiral construction of the cochlea (top left) serves only to make the hearing organ more compact. The really important physical feature of the cochlea is the gradually tapering basilar membrane which runs the length of the spiral and carries the organ of Corti with its sensory haircells (lower right). This elastic membrane receives the sound energy delivered to the cochlear fluid by the middle ear. All sounds entering the cochlea result in a ripple wave along the basilar membrane which travels from base to apex. These waves travel hundreds of times slower than sound in air, taking several milliseconds to complete a journey of a few millimetres over the sensory haircells. Each individual frequency component wave grows in intensity as it travels, eventually reaching a peak before coming to a complete stop at a unique place on the basilar membrane (see figure caption below).

The peaking of cochlear travelling waves is crucial to the hearing process. It serves to separate excitation at different frequencies - rather as a prism separates the colors of light (right). Paralleling the eye, the cochlea acts to mould the raw material of sensation, in this case sound, into an image which can be read as a spatial pattern by the array of sensory cells and translated into neural code. The cochlear ‘image’ projected along the organ of Corti physically represents the external sound environment mapped according to the size of sound sources. Large objects radiating low frequencies are focused at the apex, and high frequency sounds typically radiated from small structures come to focus as the base.

The sensitivity and resolution of the ear depend on two things. One is the size and sharpness of cochlear travelling wave peaks - much as visual acuity depends on the sharpness of focus of the eye. The second is the efficiency of transduction to the auditory nerve. Sound image quality in the ear appears to depend on the health of the outer three rows of haircells, while the single inner row is responsible for the transduction and neural encoding (lower right). Without active outer haircell function, sound energy is lost from the travelling wave before it peaks. Peaks broaden and are of reduced size. Outer haircells generate replacement vibration which sustains and even amplifies the travelling wave, resulting in higher and sharper peaks of excitation to the inner haircells.

Most of the sound vibration generated by the outer haircells becomes part of the forward travelling wave, but a fraction escapes. It then travels back out of the cochlea to cause secondary vibrations of the middle ear and the ear drum. The whole process can take 3 to 15 milliseconds. These cochlear driven vibrations are the source of Otoacoustic Emissions.

Important as OAEs are for probing cochlear function it is ludicrous to suggest that auditory threshold can be reliably measured by OAEs. The crucial mechanism of transduction in the inner haircell is not involved in OAE generation. Furthermore, the mechanism for the escape of energy resulting in OAEs plays no part in the hearing process. This factor certainly accounts for much of the wide variation in the intensity of the OAE responses between individuals and across frequency in the same individual. We would not expect - and we do not observe - more than a 30% correlation between OAEs level and audiometric threshold - far too small for clinical use. But we do observe a very high correlation between the existence of OAEs and audiometric thresholds falling within the normal range. The implication is clear. Most cochlear pathologies involve outer haircell disorder, making OAEs an ideal frequency specific screening test for cochlear function.

This article has been extracted from the publication ‘Understanding & Using Otoacoustic Emissions’, written by Professor David Kemp and published by Otodynamics, and is reproduced with the author’s permission. Copyright remains with the author.