Effect of contralateral acoustic stimulation on the growth of click-evoked otoacoustic emissions in humans.

Input-output (I/O) functions of click-evoked otoacoustic emissions (CEOAEs) were obtained over a 12 dB range for 64 normally hearing adult listeners with and without contralateral broadband noise (BBN). Contralateral acoustic stimulation (CAS) is a convenient way of suppressing responses to ipsilateral stimuli, probably acting via the medial olivocochlear system (MOCS). The present study shows that this contralateral sound suppression of CEOAEs is largest at low stimulus levels. In fact, the curves obtained under CAS approach the curves obtained without CAS as stimulus level rises. I/O slope analysis for the whole study population (n = 64) showed a slight but significant rise in slope with BBN, which may be interpreted as I/O function decompression. A loss of contralateral suppression effect at high ipsilateral stimulus levels was found in both very low and very high amplitude CEOAE subjects, despite the fact that I/O slopes differed between these two groups, whereas rise in slope under contralateral stimulation failed to be found for these same 2 groups of 16 subjects each. These findings clearly indicate that the MOCS is mostly functional at low sound levels, and suggest that the study of CEOAE I/O slope alteration under CAS may help specify one form of MOCS action on cochlear functioning.