Further tests of the local nonlinear interaction-based mechanism for simultaneous suppression of tone burst-evoked otoacoustic emissions

Otoacoustic emissions from ears with spontaneous activity behave differently to those without: Stronger responses to tone bursts as well as to clicks

It has been reported that both click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) have higher amplitudes in ears that possess spontaneous otoacoustic emissions (SOAEs). The general aim of the present study was to investigate whether the presence of spontaneous activity in the cochlea affected tone-burst evoked otoacoustic emissions (TBOAEs). As a benchmark, the study also measured growth functions of CEOAEs. Spontaneous activity in the cochlea was measured by the level of synchronized spontaneous otoacoustic emissions (SSOAEs), an emission evoked by a click but closely related to spontaneous otoacoustic emissions (SOAEs, which are detectable without any stimulus). Measurements were made on a group of 15 adults whose ears were categorized as either having recordable SSOAEs or no SSOAEs. In each ear, CEOAEs and TBOAEs were registered at frequencies of 0.5, 1, 2, and 4 kHz, and input/output functions were measured at 40, 50, 60, 70, and 80 dB SPL. Global and half-octave-band values of response level and latency were estimated. Our main finding was that in ears with spontaneous activity, TBOAEs had higher levels than in ears without. The difference was more apparent for global values, but were also seen with half-octave-band analysis. Input/output functions had similar growth rates for ears with and without SSOAEs. There were no significant differences in latencies between TBOAEs from ears with and without SSOAEs, although latencies tended to be longer for lower stimulus levels and lower stimulus frequencies. When TBOAE levels were compared to CEOAE levels, the latter showed greater differences between recordings from ears with and without SSOAEs. Although TBOAEs reflect activity from a more restricted cochlear region than CEOAEs, at all stimulus frequencies their behavior still depends on whether SSOAEs are present or not.

Simultaneous suppression of tone burst-evoked otoacoustic emissions: Two and three-tone burst combinations

Previous investigations have shown that components of a tone burst-evoked otoacoustic emission (TBOAE) evoked by a 1 kHz tone burst (TB1) can be suppressed by the simultaneous presence of a 2 kHz tone burst (TB2) or a pair of tone bursts at 2 and 3 kHz (TB2 and TB3 respectively). No previous study has measured this "simultaneous suppression of TBOAEs" for both TB2 alone and TB2 and TB3 from the same ears, so that the effect of the additional presence of TB3 on suppression caused by TB2 is not known. In simple terms, three outcomes are possible; suppression increases, suppression is reduced or suppression is not affected. Comparison of previously reported simultaneous suppression data suggests TB3 causes a reduction in suppression, though it is not clear if this is a genuine effect or simply reflects methodological and ear differences between studies. This issue has implications for previously proposed mechanisms of simultaneous suppression of TBOAEs and the interpretation of clinical data, and is clarified by the present study. Simultaneous suppression of TBOAEs was measured for TB1 and TB2 as well as TB1, TB2 and TB3 at 50, 60 and 70 dB p.e. SPL from nine normal human ears. Results showed no significant difference between mean suppression obtained for the two and three-tone burst combinations, indicating the reduction of suppression inferred from comparison of previous data is likely a result of methodological and ear differences rather than a genuine effect.