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.

The acoustically evoked potential with inverse tone burst stimulation compared to a model experiment.

'Inverse tone burst stimulation' refers to a brief interruption of a continuing tone. Stimulation by such an interruption affects the form and amplitude of the evoked potential. In tone interruption two distinct potentials, an off-potential by the break, and an on-potential by restoring the tone, can be discerned. The amplitudes of these two potentials show approximately equal values with tone breaks less than or equal to 400 ms. The phenomena of super-position with the shorter tone breaks determine the overall acoustically evoked potential (AEP) curve shape as a function of the duration of the break. Model experiments were carried out in which an averaged potential was being time-shifted by different amounts and added to the same potential which, however, was not being time-shifted. These model experiments simulated the phenomena of superposition and brought forth a lucid explanation.

[Frequency spectra and filtering of the early auditory evoked potential]. / Frequenzzusammensetzung und Filterung des frühen akustisch evozierten Potentials (FAEP).

We analysed the spectral content of click-evoked fast auditory evoked potentials (FAEPs) at stimulus levels of 80 dB nHL and were able to demonstrate the influence of the different frequency bands of the FAEP spectrum on the shape of the FAEP. The different kinds of filter and their typical effects are discussed, and some examples are demonstrated. Analogue high-pass filters with a steep slope change the shape of the FAEP markedly, because of the great phase distortion (non-linearity of the phase function). Therefore only one-pole filters (6 dB/octave) with a low cut-off frequency below about 70 Hz should be used. Higher cut-off frequencies can be used with digital zero-phase or linear-phase highpass filters. However, such frequency components which are essential for amplitudes and/or latencies of the FAEP should not be attenuated strongly by the filtering. Therefore, the low cut-off point should not exceed about 200 Hz.

Comparison between auditory brain stem responses evoked by rarefaction and condensation step functions and clicks.

In order to evaluate the influence of the trailing edge of clicks on the auditory brain stem response (ABR) in normal ears, rarefaction and condensation step functions (RS and CS) compared to rarefaction and condensation clicks (RC and CC) at an intensity of 70 dB nHL were used. Significant intraindividual differences could be found for the latencies and amplitudes in the RS-CS, RS-RC and RC-CC comparison. However, the mean values of the complete group of test subjects showed no significant differences for the latencies and amplitudes, except the significantly greater amplitudes of wave I and II for R versus C step and R versus C click. Only a tendency to shorter latency for wave VI with R versus C step and click was revealed. These results show that there was no essential influence of the trailing edge of the used R and C clicks on the ABR. The latency of the ABR with excitation of the cochlea by step or click function seemed to be mainly determined by the internal oscillation sequence in the cochlea and not by the stimulus polarity.

The acoustically evoked potentials in the case of stimulation by frequency modulation (FM) near the hearing threshold, compared with tone burst stimulation.

The present investigation concerned the possibility of obtaining higher AEP amplitudes at or near the hearing threshold by stimulation with frequency-modulated sinusoidal tones (FM stimulation) than by tone burst stimulation. With FM stimulation the frequency changes may lead to changes in the coupler sound pressure level in the headphone (additional amplitude or intensity modulation) as well as changes in sensation levels due to frequency-dependent hearing sensitivity which have to be compensated for. To ensure comparability of FM and tone burst stimulation it has also to be considered that the FM frequency increment does not substantially exceed the critical bandwidth of the human ear. With these conditions taken into account, no combination of FM stimulation parameters was found that resulted in a larger AEP amplitude than tone burst stimulation. With comparable stimulation parameters, the AEP amplitudes were identical for FM and tone burst stimulation.

Derived acoustically evoked brainstem responses by means of narrow-band and notched-noise masking in normal-hearing subjects.

Derived acoustically evoked brainstem responses determined by narrow-band and notched-noise masking at 2000 Hz frequencies and below were investigated and compared with those obtained in the use of the common high-pass masking technique. With all three masking methods a dominant late wave Wa could be detected at less than or equal to 1 600 Hz centre frequencies of the derived band and this wave is typical of the excitation of the apical section of the cochlea. The latency of this wave increases when the centre frequency is decreased at a constant stimulation level. For a constant centre frequency of the derived band the latency of wave Wa increases when the stimulation level of the click and in parallel to that the masking level is decreased. Wave Wa could be found near the hearing threshold in all three masking methods. The notched-noise masking method offers major advantages provided that full masking of the frequency regions above and below the notch frequency range is carried out. A direct frequency-specific stimulation takes place and there is no need for subtracting one response from a second one. This reduces the investigation time and the memory capacity of the measuring device. The advantages and applicability of the notched-noise masking method have to be re-checked in patients with different frequency-dependent loss of hearing.

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.

Brain-stem responses when masking with wide-band and high-pass filtered noise.

Investigations, using click stimuli masked by wide-band and high-pass filtered noise with cut-off frequencies of 2 600, 1 250 and 850 Hz were carried out. The latency shift of a response evoked by a click of 60 dB HL and wide-band masking at 80 dB HL is 0.2 - 0.3 ms for wave I and about 0.6 ms for wave V. The results obtained with high-pass masking at various cut-off frequencies differ distinctly from those found by other authors. For instance, when using high-pass masking noise of a lower limiting frequency of 850 Hz and levels of 85 and 90 dB no clear brain-stem responses were discernible. Therefore, the appearance of the brain-stem responses with masking noise at various frequency limits and at various levels was examined. With high-ass noise masking at decreasing cut-off frequencies from 1 800 Hz down to 600 Hz and increasing noise levels, wave V disappears with a slight latency shift, but subsequently to wave V an additional wave occurs which remains stable even in the case of high noise intensities. Further studies are necessary to clarify the properties of this additional wave.

[Treatment of experimental immune complex nephritis with indomethacin]. / Therapie der experimentellen Immunkomplexnephritis mit Indomethazin.

Functional, histological and immune-histological examination were performed in altogether 64 Wistar-rats, in order to control the effect of a therapy with 2 mg/kg per body weight indomethazine lasting 2 months at the model of an experimental immune complex nephritis. In 44 rats after presensibilisation an immune complex nephritis was performed by intraperitoneal injections with human serum albumin which were repeated three times a week. 24 glomerulonephritis animals and other 20 animals without glomerulonephritis were daily administered indomethazin through a tube probe, the remaining 20 animals with glomerulonephritis served as untreated control groups. The excretion function of the kidney was tested before the beginning of the experiment, 2 weeks after the beginning of the therapy and the regular serum injections, respectively, and before the end of the experiment by determination of the biological half-life period of 131J-hippuran. In every case one day before this the proteinuria during 24 hours was determined. At the end of the experiment the kidneys were examined histologically and immune-histologically. The results showed that indomethazin does not lead to a clear influence on the proteinuria in the immune complex nephritis of the rat. The excretion of 131J-hippuran was significantly restricted, whereas the histological and immune-histological preparations in the animals with foreign serum injections showed clear changes of the glomeruli in the sense of an early stage of the immune complex nephritis, however, they did not show any essential influence by indomethazin. That is, indomethazin had altogether no favourable effect on the immune complex nephritis of the rat.