Behaviour of delayed evoked otoacoustic emission under forward masking paradigm

The behaviour of delayed evoked otoacoustic emission (DEOAE) has been studied in normally hearing adults under a conventional forward-masking paradigm, and subjective measurements were carried out additionally for comparison. The clicks served as signals and the noise bursts were used as masker. In different experimental sets, signal and masker intensity, masker duration, and the interval between masker and maskee were altered. At masker levels corresponding to the subjective post-masking threshold of the clicks, the DEOAE was unaffected, i.e. had no noticeable alteration, compared with click stimulation without masking. Even at higher masker levels the inaudible clicks elicited clearly discernible DEOAEs. The forward-masking detection threshold of DEOAE ('DEOAE post-masking threshold') was reached at masker levels approximately 35 dB above the subjective post-masking threshold. The gap between subjective and DEOAE post-masking threshold vis-à-vis the masker level was also evident at different masker durations and different time intervals between masker and maskee. Central neural and peripheral receptor mechanisms are suggested to be the constituents of the masking phenomenon. The neural mechanism is involved at low masker levels. The receptor mechanism effectively joins the neural one at masker levels exceeding the threshold of psychoacoustic masking. The progressive increase in the number of auditory units from the periphery to the centre in the hearing system, linked with an increase in inhibition, can help to explain these effects.