[Age-specific changes in otoacoustic emission]. / Vozrastnye izmeneniia otoakusticheskoi émissii.

The relation of age and external and middle ear parameters with otoacoustic emission (OAE) characteristics was studied. Ear canal equivalent volume (ECV), static compliance and tympanometric gradient were measured. Transient evoked OAE (TEOAE) and distortion product of OAE (DPOAE) were recorded. Measured parameters included: average TEOAE and DPOAE levels, TEOAE spectral peak level and DPOAE maximum level, TEOAE and DPOAE dominant frequencies as well as frequency corresponding to minimum DPOAE level. The analysis of variance demonstrated a significant interrelationship between subject's age and all TEOAE and DPOAE parameters. External and middle ear parameters had no significant influence on OAE. The only exception was ECV which had certain relation to TEOAE spectral peak frequency. The results obtained demonstrated that OAE age-related changes were predominantly determined by processes taking place within its generators system, i.e. at the outer hair cells level.

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.

Evoked otoacoustic emission: behaviour under the forward masking paradigm.

The behaviour of evoked otoacoustic emission (EOAE) has been studied in normally hearing adults under the conventional forward masking paradigm. The clicks and tone pips served as signals, and the noise bursts were used as maskers. At noise burst levels, defined as psychoacoustic masking thresholds of signals (postmasking threshold), attenuation of EOAEs was just noticeable but did not occur in all records. The EOAEs were not completely eliminated even at noise levels exceeding the post-masking threshold by 30 dB. Central or neural and peripheral or receptor mechanisms are suggested to be the constituents of the masking phenomenon. The receptor mechanism effectively joins the neural one at masker levels, well exceeding the thresholds of psychoacoustic masking. The increase in masker duration and decrease in interval between masker and signal seem to accentuate the neural mechanism. As a result, the difference between masker levels, leading to psychoacoustic masking of the signal and having marked attenuating effects upon the EOAEs, respectively, is increased.