Cochlear tuning estimates from level ratio functions of distortion product otoacoustic emissions.

Objective: Distortion product otoacoustic emission (DPOAE) levels plotted as a function of stimulus frequency ratio demonstrate a bandpass shape. This bandpass shape is narrower at higher frequencies compared to lower frequencies and thus has been thought to be related to cochlear mechanical tuning.Design: However, the frequency- and level-dependence of these functions above 8 kHz is largely unknown. Furthermore, how tuning estimates from these functions are related to behavioural tuning is not fully understood.Study Sample: From experiment 1, we report DPOAE level ratio functions (LRF) from seven normal-hearing, young-adults for f2 = 0.75-16 kHz and two stimulus levels of 62/52 and 52/37 dB FPL. We found that LRFs became narrower as a function of increasing frequency and decreasing level.Results: Tuning estimates from these functions increased as expected from 1-8 kHz. In experiment 2, we compared tuning estimates from DPOAE LRF to behavioural tuning in 24 normal-hearing, young adults for 1 and 4 kHz and found that behavioural tuning generally predicted DPOAE LRF estimated tuning.Conclusions: Our findings suggest that DPOAE LRFs generally reflect the tuning profile consistent with basilar membrane, neural, and behavioural tuning. However, further investigations are warranted to fully determine the use of DPOAE LRF as a clinical measure of cochlear tuning.

Relationship Between Behavioral and Stimulus Frequency Otoacoustic Emissions Delay-Based Tuning Estimates.

Purpose The phase delay of stimulus frequency otoacoustic emissions (SFOAEs) has been proposed as a noninvasive, objective, and fast source for estimating cochlear mechanical tuning. However, the implementation of SFOAEs clinically has been thwarted by the gaps in understanding of the stability of SFOAE delay-based tuning estimates and their relationship to behavioral measures of tuning. Therefore, the goals of this study were (a) to investigate the relationship between delay-based tuning estimates from SFOAEs and simultaneously masked psychophysical tuning curves (PTCs) and (b) to assess the across- and within-session repeatability of tuning estimates from behavioral and OAE measures. Method Three sets of behavioral and OAE measurements were collected in 24 normal-hearing, young adults for two probe frequencies, 1 and 4 kHz. For each participant, delay-based tuning estimates were derived from the phase gradient of SFOAEs. SFOAE-based and behavioral estimates of tuning obtained using the fast-swept PTC paradigm were compared within and across sessions. Results In general, tuning estimates were sharper at 4 kHz compared to 1 kHz for both PTCs and SFOAEs. Statistical analyses revealed a significant correlation between SFOAE delay-based tuning and PTCs at 4 kHz, but not 1 kHz. Lastly, SFOAE delay-based tuning estimates showed better intra- and intersession repeatability compared to PTCs. Conclusions SFOAE phase-gradient delays reflect aspects of cochlear mechanical tuning, in that a frequency dependence similar to that of basilar membrane tuning was observed. Furthermore, the significant correlation with PTCs at 4 kHz and the high repeatability of SFOAE-based tuning measures offer promise of an objective, nonbehavioral assay of tuning in human ears.

Modeling the dependence of the distortion product otoacoustic emission response on primary frequency ratio.

When measured as a function of primary frequency ratio r = f2/f1, using a constant f2, distortion product otoacoustic emission (DPOAE) response demonstrates a bandpass shape, previously interpreted as the evidence for a cochlear "second filter." In this study, an alternate, interference-based explanation, previously advanced in variants, is forwarded on the basis of experimental data along with numerical and analytical solutions of nonlinear and linear cochlear models. The decrease of the DPOAE response with increasing and decreasing ratios is explained by a diminishing "overlap" generation region and the onset of negative interference among wavelets of different phase, respectively. In this paper, the additional quantitative hypothesis is made that negative interference becomes the dominant effect when the spatial width of the generation (overlap) region exceeds half a wavelength of the DPOAE wavelets. Therefore, r is predicted to be optimal when this condition is matched. Additionally, the minimum on the low-ratio side of the DPOAE curve is predicted to occur as the overlap region width equals one wavelength. As the width of the overlap region depends on both tuning and ratio, while wavelength depends on tuning only, an experimental method for estimating tuning from either the width of the pass band or the optimal ratio of the DPOAE vs. ratio curve has been theoretically formulated and evaluated using numerical simulations. A linear model without the possibility of nonlinear suppression is shown to reasonably approximate data from human subjects at low ratios reinforcing the relevance of the proposed negative interference effect. The different dependence of the distortion and reflection DPOAE components on r as well as the nonmonotonic behavior of the distortion component observed at very low ratios are also in agreement with this interpretation.