Effects of the conditioning click on click-evoked otoacoustic emission.

The effects of the preceding (conditioning) click on the evoked otoacoustic emission (EOAE) to the following (test) click were investigated in normally hearing adults. To overcome distortions due to superimposition of the test EOAE on the EOAE to the conditioning click, a special stimulation and response subtraction procedure was utilized. The conditioning stimulus was found to suppress the test EOAE. The suppression lessened with a decrease in the conditioning stimulus level and an increase in the time interval between the conditioning and the test stimuli. Nevertheless, the influence was traced even with the level of the conditioning stimulus as low as 5 dB SL, and lasted for the interval between the conditioning and the test stimuli, as long as 7.5 ms. An attempt at theoretical comprehension of the obtained results is made and their usefulness in differentiation of EOAEs from acoustic reflections is proposed.

Interrelation of spontaneous and evoked otoacoustic emissions.

The interrelation of spontaneous otoacoustic emissions (SOAEs) that can be synchronised by acoustic stimuli and transiently evoked otoacoustic emissions (TEOAEs) was studied in different experiments in normal ears. Click evoked TEOAEs mixed with synchronised SOAEs were investigated in time windows of 102.4 ms. Frequency spectra were obtained from the whole time window, the first and second half and from each quarter separately. Synchronised SOAEs recorded by time domain averaging show exactly the same frequencies in the TEOAE spectra of all analysed time window quarters compared to SOAEs recorded by frequency domain averaging. Moreover, synchronised SOAEs can influence the response shape of the TEOAE additionally. This is shown in long lasting responses exceeding the inter stimulus interval (ISI). In this case superimposing of consecutive single responses takes place and the shape of the averaged response depends on the exact ISI value used. These effects were investigated by varying the ISI in steps of 0.1 ms. The ISI variation has more pronounced effects on the response shape at short ISI values (50 ms) and low stimulus levels (10 dB HL). Thus the wave form changes are not accompanied by a change of the frequency content poor phase cancellation takes place. This is confirmed by simulated phase interaction. It is suggested that the phase of an already synchronised SOAE has significant effects on the re-synchronisation of the SOAE by the next click. Both influences should be considered in TEOAE recordings if synchronised SOAEs are present.

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.

Simultaneous application of electronystagmography (ENG) and photoelectronystagmography (PENG) in the case of thermal labyrinth examination.

At examination of normal persons, the ENG registration method was compared with that of PENG. Special examinations were executed with regard to the behaviour of the beat rates during the culmination phase of thermal labyrinth reactions by using each of both registering methods. By synchronous recording of PENG and ENG, it was possible to settle the discrepancy of the greater number of beats in ENG compared with those of PENG, while the examinations were executed at different times. The PENG-method does not exclude an optical fixation due to the necessary interior illumination, and this results in a retarding of the reaction and smaller beat rates. This fact should be considered when comparing the results of ENG and PENG.