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Discovered back in 1978, by D. Kemp, oto-acoustic emissions (OAEs) were not fully explained until a few years later, after the OHC active mechanism had been understood. At least in the mammalian cochlea, OAEs reflect OHC electromotility. OHC contractions/elongations themselves vibrate cochlear fluids and the middle ear conducting mechanism transfers this vibration back to the air of the external auditory canal: there, the emissions can be registered by a microphone.
Schematic drawing of a probe for recording evoked OAEs
The probe contains
- 1 a speaker (red) emitting the stimulating sound
- 2 a microphone (blue) to record the returning OAEs.
Examples of evoked oto-acoustic emissions (normal subject)
OAEs in response to a click stimulation
The superimposition of two traces indicates the reproducibility of the recording. Actually yhry are remarkably constant: for the same type of stimulus in the same ear, the same OAEs will be recorded as long as the hearing is not impaired.
Oto-acoustic emissions and audiometry
Spontaneous oto-acoustic emisions (SOAEs)
Oto-acoustic emissions and efferent system
The medial efferent system, synapsing with OHCs, attenuates the electromotile properties of OHCs via slow contractions, thus reducing OAEs. Here are 3 examples demonstrating the phenomenon:
- a contra-lateral stimulation reduces OAEs in the opposite ear ;
- a similar effect is obtained by a direct application of acetylcholine, the main medial efferent neurotransmitter ;
- selective attention (either visual or auditory) reduces OAEs through an action of the medial efferent system driven by higher brain structures.
Thus, OAEs may be used to probe the activity of the medial efferent system.