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.