Do click-evoked otoacoustic emissions have frequency specificity?

Simultaneous suppression of tone burst-evoked otoacoustic emissions--effect of level and presentation paradigm

There is conflict in the literature over whether individual frequency components of a transient-evoked otoacoustic emission (TEOAE) are generated within relatively independent "channels" along the basilar membrane (BM), or whether each component may be generated by widespread areas of the BM. Two previous studies on TEOAE suppression are consistent with generation within largely independent channels, but with a degree of interaction between nearby channels. However, both these studies reported significant suppression only at high stimulus levels, at which the "nonlinear" presentation paradigm was used. The present study clarifies the separate influences of stimulus level and presentation paradigm on this type of suppression. TEOAEs were recorded using stimulus tone bursts at 1, 2 and 3 kHz and a complex stimulus consisting of a digital addition of the three tone bursts, over a range of stimulus levels and both "linear" and "nonlinear" presentation paradigms. Responses to the individual tone bursts were combined offline and compared with responses to the complex stimuli. Results clearly demonstrate that TEOAE suppression under these conditions is dependent upon stimulus level, and not upon presentation paradigm. It is further argued that the data support the "local" rather than "widespread" model of TEOAE generation, subject to nonlinear interactions between nearby generation channels.

Nonlinear temporal interactions in click-evoked otoacoustic emissions. II. Experimental data

Click-evoked otoacoustic emissions (CEOAEs) are reduced in amplitude by the presentation of 'suppressor' clicks that either closely lead or follow the stimulus ('test') clicks. A model described in a companion paper (Kapadia and Lutman, Hear. Res. 146 (2000) 89-100) shows that such nonlinear temporal interactions, as previously reported, may be explained in terms of the compressive non-linearity of the CEOAE input-output (I-O) function. This paper presents the results of a detailed parametric investigation into such nonlinear interactions, studied in 12 normal adult ears over a wide range of test and suppressor click levels and inter-click intervals. The results differ from those generated by the model in a number of respects. Principally, maximum suppression is generally obtained for suppressors presented in advance of test clicks, rather than co-incident with the test clicks. The amount of advance depends systematically on the two click levels. The measured suppression can also exceed the theoretical maximum allowed by the model. It is concluded that the nonlinear temporal interactions measured do not simply reflect CEOAE I-O function non-linearity. They may, instead, arise from disturbance of the generator elements from their resting state prior to generation of the CEOAE. These results may also have general implications relating to cochlear responses to transient stimuli and indicate the potential of CEOAEs in probing aspects of cochlear mechanics.