Time domain visualisation of distortion product otoacoustic emissions

Abstract

Distortion product otoacoustic emissions (DPOAEs) are small sounds believed to be the combination of distortion and reflection energy produced by the cochlea in response to stimulation with simultaneous pure tones. Although used clinically, the exact generation mechanisms associated with DPOAEs are not clear; this is especially the case for 2f2-f1.

It is thought that 2f1-f2 and 2f2-f1 DPOAEs contain reflection (R) and distortion (D) energy; the D energy may arise from the region of overlap between the stimulus travelling waves while R energy is reflected from fixed points along the basilar membrane. Cochlear mechanics would suggest the two emissions cannot arise from the same cochlear location; current theories suggest the f2 characteristic place is important to 2f1-f2 while 2f2-f1 is generated more basally.

Theories of DPOAE generation have been investigated using various techniques , including time domain investigations. Time domain investigations have often used the phase gradient latency (PGL) technique but this has inherent limitations. An alternative approach is primary tone phase variation (PTPV) - a phase rotation method described by Whitehead et al. (1996) that allows for specific DPOAEs to be extracted.
This study has used PTPV to investigate DPOAEs generated by an ISVR developed cochlear model and DPOAEs recorded from 40 normal hearing ears (across two experiments); the overall aim of the study is to learn more about the generation of 2f1-f2 and 2f2-f1 emissions by viewing DPOAEs in the time domain with PTPV. Specific areas of interest are the effect of frequency ratio and primary ramping on the time course of 2f1-f2 and 2f2-f1, and how the time course of emissions varies with stimulus level and fine structure position. It is believed these areas will allow for theories of DPOAE generation to be tested.

Output from simulations and recorded emissions supports what is already known and theorised about DPOAE generation. It would appear that 2f1-f2 and 2f2-f1 are the product of multiple components originating from a distributed cochlear source. Temporal behaviour of emissions is influenced by stimulus parameters including level, frequency and ratio.

This investigation has contributed to knowledge by studying 2f1-f2 and 2f2-f1 emissions with the PTPV procedure and a novel onset latency estimation technique. Data was collected from a group of normal hearing participants and compared to output of a cochlear model.

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Identifiers

ORCID for Ben Lineton: orcid.org/0000-0003-4784-7762

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