Nonlinear temporal interactions in click-evoked otoacoustic emissions.

Kapadia, Sarosh (2000). Nonlinear temporal interactions in click-evoked otoacoustic emissions. PhD thesis The Open University.



Click-evoked otoacoustic emissions (CEOAEs) may be reduced in amplitude by the presentation of “suppressor” clicks that either closely lead or follow the stimulus (“test”) clicks. This “click suppression” represents nonlinear temporal interaction between the test and suppressor clicks and/or the CEOAEs they evoke. Such suppression has not previously been studied in detail and the mechanisms giving rise to it are not understood. In particular, it is unclear whether click suppression may simply reflect the compressive nonlinearity of the CEOAE level function. It is also unclear whether the larger magnitude “rate suppression” observed in CEOAEs measured using streams of clicks at very high rates may be explained by a simple additive accumulation of click suppression.

The present study addresses these questions by detailed measurement of this suppression phenomenon in 20 normal adult ears, and establishes that:

1. Maximum suppression is generally obtained for suppressors presented up to 4 ms in advance of test clicks, contrary to expectation.

2. Suppression by suppressors that lead test clicks does not simply reflect CEOAE level function nonlinearity. It may, instead, arise from disturbance of the generator elements from their resting state prior to generation of the CEOAE.

3. Suppression by following suppressors behaves markedly differently from that by leading suppressors, and appears more closely related to level function nonlinearity.

4. Contrary to previous suggestions, suppression for both leading and following suppressors is insensitive to polarities of test and suppressor clicks.

5. Suppression does not accumulate in a simple, additive manner as has previously been suggested. Consequently, a more complex mechanism underpins the greater magnitude of rate suppression.

The parametric characterisation of click suppression presented may form the basis of models to explain this little-studied phenomenon. Further studies using tone bursts instead of clicks are recommended to help determine whether single-channel models are appropriate.

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