Characterizing the bounce phenomenon via spontaneous otoacoustic emissions.

Intense sound stimulation can transiently affect various read-outs of cochlear sensitivity after stimulus cessation, for which the term bounce-phenomenon (BP) has been coined. To better understand stimulus characteristics determining the nature of the BP (amplitude, polarity, duration) across a wide frequency range, we measured spontaneous otoacoustic emissions (SOAEs, N = 21 human ears; tested frequency range: 0.5-12 kHz) before and right after application of an intense sound stimulus [3 types of transposed-bandpass noise (TBP-noise) centered at fc = 1300 Hz, and a 30-Hz pure tone; all at 85-phons for 90 s] and investigated: (1) The effect of the stimulus AC component on the BP [30-Hz AC vs smooth TBP-noises]. (2) The effect of stimulus bandwidth on the BP (1-ERB vs 4-ERB wide TBP-noises). (3) The relation between stimulus spectral content and BP characteristics across different SOAE frequency regions. It was found that: (a) All stimuli evoked BPs (b) the BP was not enhanced for the AC stimulation; oppositely it was less acute than for the smooth (continuous) stimulation. (c) A wider stimulus bandwidth did not enhance the BP. (d) Biphasic and more intense BPs mostly occurred relatively basal from each stimulus spectral content (i.e., above fc for TBP-noises), with the 30-Hz tone eliciting the most intense BPs in the lower SOAE frequency region. Results suggest that both the cochlear partition displacement amplitude and its longitudinal in-phase alignment are important for the homeostatic mechanisms thought to be behind the BP, likely explaining why low-frequency tones are most effective BP elicitors across a wide frequency range.