Quantification of audiogram fine-structure as a function of hearing threshold

Transient otoacoustic emissions and audiogram fine structure in the extended high-frequency region

OBJECTIVE

Previous studies at conventional audiometric frequencies found associations between the ripple depth seen in audiogram fine structure (AFS) and amplitudes of both transient evoked otoacoustic emissions (TEOAEs) and overall hearing threshold levels (HTLs). These associations are explained by the cochlear mechanical theory of multiple coherent reflections of the travelling wave apically by reflections sites on the basilar membrane and basally by the stapes.

DESIGN

The aim was to investigate whether a similar relationship is seen in the extended high-frequency (EHF) range from 8-16 kHz. Measurements from 8-16 kHz were obtained in normal-hearing subjects comprising EHF HTLs, EHF TEOAEs using a double evoked paradigm, and Bekesy audiometry to assess AFS ripple depth and spectral periodicity.

STUDY SAMPLE

Twenty eight normal-hearing subjects participated.

RESULTS

Results showed no significant correlation between AFS ripple depth and either frequency-averaged EHF HTLs or EHF TEOAE amplitudes. The amplitude of AFS ripple depth was also lower than that seen in the conventional frequency region and spectral periodicity in the ripple more difficult to discern.

CONCLUSION

The results suggest a weaker interference pattern between forward and reverse cochlear travelling waves in the most basal region compared to more apical regions, or a difference in cochlear mechanical properties.

Modeling cochlear dynamics: interrelation between cochlea mechanics and psychoacoustics

A model of the cochlea was used to bridge the gap between model approaches commonly used to investigate phenomena related to otoacoustic emissions and more filter-based model approaches often used in psychoacoustics. In the present study, a nonlinear and active one-dimensional transmission line model was developed that accounts for several aspects of physiological data with a single fixed parameter set. The model shows plausible excitation patterns and an input-output function similar to the linear-compressive-linear function as hypothesized in psychoacoustics. The model shows realistic results in a two-tone suppression paradigm and a plausible growth function of the 2f(1)-f(2) component of distortion product otoacoustic emissions. Finestructure was found in simulated stimulus-frequency otoacoustic emissions (SFOAE) with realistic levels and rapid phase rotation. A plausible "threshold in quiet" including finestructure and spontaneous otoacoustic emissions (SOAE) could be simulated. It is further shown that psychoacoustical data of modulation detection near threshold can be explained by the mechanical dynamics of the modeled healthy cochlea. It is discussed that such a model can be used to investigate the representation of acoustic signals in healthy and impaired cochleae at this early stage of the auditory pathway for both, physiological as well as psychoacoustical paradigms.

Testing the binaural equal-loudness-ratio hypothesis with hearing-impaired listeners

The primary purpose of the present experiment was to test whether the binaural equal-loudness-ratio hypothesis (i.e., the loudness ratio between monaural and binaural tones presented at the same Sound Pressure Level, SPL, is independent of SPL) holds for hearing-impaired listeners with bilaterally symmetrical hearing losses. The outcome of this experiment provided a theoretical construct for modeling loudness-growth functions. A cross-modality matching task between string length and tones was used to measure three loudness functions for eight listeners: two monaural (left and right) and one binaural. A multiple linear regression was performed to test the significance of presentation mode (monaural vs binaural and left vs right), level, and their interaction. Results indicate that monaural loudness functions differ between the ears of two listeners. The interaction between presentation mode (binaural/monaural) and level was significant for one listener. Although significant, these differences were quite small. Generally, the binaural equal-loudness-ratio hypothesis appears to hold for hearing-impaired listeners. These data also indicate that loudness-growth functions in two ears of an individual are more similar than loudness-growth functions in ears from different listeners. Finally, it is demonstrated that loudness-growth functions can be constructed for individual listeners from binaural level difference for equal-loudness data.

Automatic screening and detection of threshold fine structure

Audiograms measured with a high frequency resolution often show quasi-periodic ripples of up to 15 dB in normal-hearing listeners. This fine structure of the threshold in quiet is commonly associated with the active processes in the cochlea. Therefore its absence is discussed in the literature as an indicator of cochlear vulnerability. In order to enable a quick detection and an objective quantification of threshold fine structure, two instruments are introduced and evaluated in this article: (1) a high-resolution tracking method for measuring fine structure ('FINESS'), and (2) an automatic fine-structure detector ('FINESS-detector'). The method is tested on 22 subjects for its reliability, its accuracy, and drifts with frequency by analysing test/retest experiments and by comparing the measured thresholds to results from a reference procedure. The results indicate that FINESS and the FINESS-detector are suitable techniques for the measurement and detection of threshold fine structure that may help to investigate further into whether fine structure is a sensitive tool for the detection of an early hearing loss.

Distortion product otoacoustic emissions: cochlear-source contributions and clinical test performance

It has been proposed that the clinical accuracy of distortion product otoacoustic emissions (DPOAEs) is affected by the interaction of distortion and reflection sources contributing to the response. This study evaluated changes in dichotomous-decision test performance and threshold-prediction accuracy when DPOAE source contribution was controlled. Data were obtained from 205 normal and impaired ears with L(2) ranging from 0 to 80 dB SPL and f(2)=2 and 4 kHz. Data were collected for control conditions (no suppressor, f(3)) and with f(3) presented at three levels that previously had been shown to reduce the reflection-source contribution. The results indicated that controlling source contribution with a suppressor did not improve diagnostic accuracy (as reflected by relative operating characteristic curve area) and frequently resulted in poorer test performance compared to control conditions. Likewise, correlations between DPOAE and behavioral thresholds were not strengthened when using the suppressors to control source contribution. While improvements in test accuracy were observed for a subset of subjects (normal ears with the smallest DPOAEs and impaired ears with the largest DPOAEs), the lack of improvement for the larger, unselected subject group suggests that DPOAEs should be recorded in the clinic without attempting to control the source contribution with a suppressor.