A test of a mechanism of simultaneous suppression of tone burst evoked otoacoustic emissions

Current theory of transient evoked otoacoustic emission (TEOAE) generation states that TEOAEs recorded using conventional time-windowing techniques arise via randomly distributed, “place-fixed” perturbations in cochlear mechanics. Place-fixed generation is consistent with the widely supported concept of local TEOAE generator channels tonotopically distributed along the basilar membrane (BM), within which a TEOAE component at frequency f, is evoked by a stimulus component at f. This one-to-one relationship between stimulus and TEOAE frequency components underpins the potential for using TEOAEs as a frequency specific measure of cochlear function. Inconsistent with the concept of local TEOAE generator channels, suppression of tone burst evoked otoacoustic emissions caused by the simultaneous presence of additional equi-level, higher frequency tone bursts has been reported, i.e. simultaneous suppression of tone burst evoked otoacoustic emissions (e.g. Yoshikawa et al, 2000). These results implicate a widespread TEOAE generation mechanism wherein a TEOAE component at f is generated at remote BM locations basal to f. This apparent breakdown in correspondence between stimulus and TEOAE components contradicts the use of TEOAEs as a frequency specific measure of cochlear function. However, it has been argued that this suppression phenomenon does not negate the existence of local TEOAE generator channels, but simply indicates the broad spread of BM excitation associated with tone bursts (Killan and Kapadia, 2006). In order to test this theory a simple model based solely on local TEOAE generator channels was developed and used to repeat the methodology employed by Yoshikawa et al (2000) for an extended range of tone burst frequencies (0.5, 1, 1.5, 2, 3 kHz) and levels (40, 50, 60, 70 dB p.e. SPL). Model predictions of suppression were compared to corresponding TEOAE suppression measurements from 14 normally hearing human ears. Results demonstrated that the model was able to make reasonable predictions of the suppression measurements obtained from human ears. However, the model demonstrated a tendency to understimate suppression for conditions when tone bursts were well separated in frequency. Results suggest that local TEOAE generation can account for simultaneous suppression of tone burst evoked otoacoustic emissions and therefore support the use of TEOAEs as a frequency specific assessment of cochlear function. References Killan E.C. & Kapadia S. 2006. Simultaneous suppression of tone burst-evoked otoacoustic emissions – Effect of level and presentation paradigm. Hear Res, 212, 65-73. Yoshikawa H., Smurzynski J. & Probst R. 2000. Suppression of tone burst evoked otoacoustic emissions in relation to frequency separation. Hear Res, 148, 95-106.