Contralateral suppression and stimulus rate effects on evoked otoacoustic emissions

Transiently evoked otoacoustic emissions (EOAE) were recorded in normal-hearing humans using pseudorandom pulse trains. This allowed the effect of increasing stimulus rate to be studied on EOAE amplitudes, input-output (I/O) functions and suppression by contralateral stimulation. EOAEs are very probably due to micromechanical properties of cochlear outer hair cells, and contralateral suppression is considered to result from olivo-cochlear efferent activation. EOAEs at various stimulus rates showed excellent reproducibility. Total EOAE amplitude diminished as interstimulus interval (ISI) decreased from 20 to 3 ms, but not for ISIs under 3 ms. The amplitude reduction was significant only on EOAE spectrum bands below 3.4 kHz. I/O functions, which kept a linear pattern, were steeper, and contralateral suppression was lower, with the highest stimulus rate (1111 c/s) relative to other rates. The EOAE decline with increasing stimulus rate might be due to incomplete recovery after adaptation of outer hair cells. The lower contralateral suppression at high stimulus rates suggests that crossed olivo-cochlear bundle action is lessened when outer hair cells are responding to a high-rate stimulus. An explanation may be that contralateral stimulation and a high-rate ipsilateral stimulus act via the same mechanisms, i.e., that high-rate stimulation activates an ipsilateral efferent loop.