Summary and Conclusions

In the previous chapter it was shown that the separate wave and place fixed DPOAE results can be qualitatively accounted for on the basis o f some reasonable assumptions about travelling wave behaviour in the cochlea and the mechanical nonlinear interaction which must take place. These assumptions have been formulated into a transmission line model consisting o f a wave fixed DP source in the region o f the primaries and randomly-spaced place fixed reflections in the region of the DP frequency place.

Good agreement between the model and experimental data was achieved indicating that the basic assumptions on which it is built can produce results that are compatible with DPOAE data. Differences between the model and experimental data could probably be further reduced by modifications to the modelling o f the phase curve o f the travelling wave and to the nonlinear interactions in the region o f the stimulus tone peaks but this would add little to the understanding gained.

This study has demonstrated that the apparently complex behaviour of DPOAE phase when primary frequencies are altered can be readily understood and even predicted on the basis o f a small number of reasonable assumptions about the cochlea.

The data do not support the concept o f two DP generator regions, but rather one generation region with two emission routes. They clearly illustrate that interference across a distributed source controls both the observed primary ratio dependence o f wave fixed DPOAEs and also the input to the reflection mechanism which is responsible for the place fixed DPOAE. When they are mixed, interference between direct and indirectly emitted DPOAEs results in additional DPOAE fine structure.

The careful separation o f DPOAE components in experimental data, as demonstrated here, should simplify the task of extracting consistent physiologically significant information from DPOAE data by removing interference between the wave and place fixed emissions. However the two emission modes may contain complementary information, so neither one should be discarded.

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H. Summary

a) Conclusions

The aim o f this study was to investigate the underlying mechanisms involved in otoacoustic emissions.

A close relationship was found between TEOAE and DPOAE in both level and phase- versus-frequency gradient (but only if a limited set of DPOAE stimulus parameters are employed). This result provided strong evidence that the TEOAE emission mechanism is closely linked to the DPOAE place fixed emission mechanism.

Detailed DPOAE frequency maps developed during this study supported the wave and place fixed emission mechanisms proposed by Kemp in 1986. In addition to this the lower and upper DPOAE sidebands were shown to be a continuation o f each other at small frequency ratios. The maps have demonstrated that not only is the upper sideband DPOAE site is linked to the DP frequency place, the place fixed part o f the lower sideband DPOAE is also linked to the DP frequency place. Two mechanisms for DPOAE ‘fine structure’ have been identified: (1) a mixing of wave and place fixed components, which disappears when the two components are separated and (2) interference effects involved in the place fixed emission, which are still present in the place fixed DPOAE component after separation from the wave fixed component.

In order to investigate the locations of DPOAE sources on the basilar membrane, measurements of DPOAE suppression by a third stimulus tone were obtained. The method was not helpful for this purpose because the suppression tuning curves were complex and did not positively identify the source o f the place fixed emission. This appeared to be because o f complications due to interference effects.

The wave and place fixed emission components were successfully separated by a Fourier transform and windowing method. The individual relationships which the wave and place fixed emissions have with the stimulus frequency and ratio have been demonstrated. The detail o f the place fixed phase component sheds new light on the possible mechanism for its emission. It appears that for the upper sideband the initial source for the place fixed emission is linked to the DP frequency place, and this is also the case with the lower sideband with a small frequency ratio. However, the lower sideband DP emission is linked to f] when stimulated with a wide frequency ratio.

Mechanisms o f otoacoustic emissions 166 The DPOAE mapping, the separated wave and place fixed DPOAE and the mathematical model all support a hypothesis for the creation of otoacoustic emissions. This consists of a single DP generator region which has variable directivity, depending largely on ^f% and which may direct the DP energy apically and basally. Energy directed apically requires a separate mechanism to reverse the direction o f propagation in order for the DP to be emitted to the ear canal. This hypothesis has been supported by a simple mathematical model which produced results containing many o f the features seen in experimental DPOAE.

This work has further clarified that instead o f there being many mechanisms o f otoacoustic emissions, all emissions can be identified with one o f two mechanisms.

b) Confidence in the findings o f this studv

In order to unwrap the DPOAE phase for the frequency maps it was necessary to insert 360° phase steps in the lower sideband phase data at small frequency ratios. This was necessary because o f the divergent phase behaviour o f this data set. This process can result in an incorrect number o f whole cycles being indicated between different parts of some o f the data field, but the orientation of the iso-phase contours is not compromised.

The calibration o f the different probe microphones agreed typically to within 3 dB. The calibration drifted by less than 1 dB over the course of the measurements. Ear canal standing waves raised calibration difficulties at high frequencies, these were minimised by optimising the probe fitting and by suspending the automatic frequency response correction in frequency regions where the ear canal frequency response was uneven. However, small departures from absolute calibration were not vital for the conclusions which have been drawn from the trends observed in this study.

The parts o f this study involving detailed DPOAE frequency maps were performed on only two ears. However the data contain many features that are known to be typical o f normally-hearing ears such as the divergence of phase behaviour o f the lower sideband and the frequency ratio giving the maximum level o f DPOAE. These and other features were seen in both ears investigated. There is therefore no reason to doubt that the results obtained here are typical o f normally-hearing ears in terms of the general patterns observed in the data. Fine details will differ with each ear.

Data were generally obtained over the frequency range 1 kHz to 4.1 kHz. It remains possible that different patterns may occur outside this frequency range.

Mechanisms o f otoacoustic emissions 167 Windowing effects may have arisen in the Fourier transformation process which was employed to separate the wave and place fixed DPOAE components. The windowing employed in the frequency domain was not ideal and will have added noise in the time domain. However this noise will be of a continuous nature and therefore has been quantified by observing a window o f data in the time domain after the DPOAE had died away, so the contents of this window were entirely noise. This noise estimate also includes the effect o f random noise present in the original data. The noise floor calculated in this way was generally below -15 dB SPL in the frequency domain. Narrow windowing in the time domain results in ‘smearing’ o f the data when returned to the frequency domain, but has the advantage of removing noise outside o f the selected time window. Some evidence of this ‘smearing’ was seen in the processed data but only at low level in regions of absent DP in the original data. The data were little changed by windowing if one emission was strongly dominating in the original data, demonstrating that the processing method retained the true overall data patterns.

cl Further studv

It would be of interest to observe the effect of mild hearing loss on the DPOAE frequency map, to see which parts o f the map are most sensitive to cochlear damage. To achieve this, subjects with mild hearing loss would be required, or ideally a subject with progressive hearing loss to be observed in a longitudinal study.

The prevalence of certain minor details in the data, for example the fine structure of the place fixed DPOAE and the tendency o f the upper sideband DPOAE to have occasional ‘favoured frequencies’ in which an emission can be emitted strongly even at very wide stimulus frequency ratios, could be explored by studying a larger number of normally-hearing subjects.

Mechanisms o f otoacoustic emissions 168