Effect of interstimulus interval on evoked otoacoustic emissions

In order to explore extensively the effect of interstimulus interval, including very short interstimulus intervals, on evoked otoacoustic emissions (EOAEs), several EOAE recordings were carried out using pairs of clicks: a suppressor click preceded the stimulus click generating an EOAE, with various intervals between the two clicks. EOAEs elicited by two clicks separated by intervals under 8-9 ms had significantly smaller amplitudes than EOAEs evoked by the stimulus alone. The amplitude decay correlated with the interclick interval, and was about 40% when the interclick interval decreased from 12 to 1 ms. This phenomenon has been noted before but not precisely quantified. It might reflect an adaptive mechanism within the outer hair cells, which has been previously described, or else mechanical interactions on the basilar membrane. The delay in EOAE decrease is of the same order as the first phase of neural adaptation, known as 'rapid adaptation', and these thus may prove to be correlated.