Evaluation of the cochlear amplifier using time-efficient acquisition of short-pulse DPOAE levels maps
Distortion-product optoacoustic emissions (DPOAEs) arise due to the nonlinear amplification of sound waves in the cochlea in response to two simultaneously presented tones of different frequencies (f2/f1 = 1.2). DPOAEs consist mainly of two components, a nonlinear-distortion component and a coherent-reflection component. DPOAE input/output functions enable the extrapolation of an estimated distortion-product threshold (EDPT) (Boege & Janssen, 2002) by utilizing the so-called scissor paradigm (Kummer et al., 1998). However, wave interference between the two components and deviation from optimal stimulus levels limit the accuracy of the recorded EDPTs. Here, a refined technique is proposed enabling EDPT acquisition without two-source interference by analyzing DPOAEs in level maps as a function of L1 and L2.
DPOAEs were recorded in five normal-hearing ears at five frequencies f2 = 1 – 5 kHz with twelve L1, L2-pairs using short-pulsed stimuli. A nonlinear least-squares fit yields individual DPOAE level maps, which enable the derivation of EDPTs by extrapolating the level-map ridge to the L1, L2-plane.
Short-pulse DPOAEs exhibit a distinct amplitude maximum for a given L2-value as a function of L1. Behavioural thresholds obtained by tracking audiometry show a significant correlation with the acquired EDPTs (r = 0.61, p = 0.0011) with an estimation error of 6.26 dB.
The reconstruction of individual level maps by means of a nonlinear least-squares fit of a model level map to a limited number of L1, L2-pairs enable a time-efficient and accurate acquisition of EDPTs without the need of a-priori determined optimal stimulus levels.
23. April 2019 (online)
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