Individual cochlear delays measured with tone-burst-evoked otoacoustic emissions: A new method to resolve the OAE onset ambiguity in the time domain

Methods to estimate cochlear delay in humans have been traditionally based on either phase-derived group delays from otoacoustic emissions (OAEs), or auditory brainstem responses (ABR). These methods demonstrate large variability in cochlear delay estimates, and are derived from across subject averages. This work aims to assess the individual variability in cochlear delay. Tone-burst evoked otoacoustic emissions (TBOAEs) are used in this study to estimate cochlear delay. The OAE is analysed by separating the non-linear components of cochlear origin, and the linear reflection in the time domain. The observed latencies as a function of frequency are qualitatively similar across subjects. For the individual subjects, the delay for each tone-burst frequency is reproducible. Defining OAE latency as the time between the onset of the stimulus and the peak of the first OAE burst yields results in agreement with previous studies. However, care must be taken when comparing the results of previous studies. This is due to an ambiguity in the time domain regarding the true onset point of the OAE, and hence the derived cochlear travelling wave latency. The inter-subject variability explains the discrepancy observed in other studies e.g. using different stimulus paradigms. The relatively small within-subject variability suggests that the present method is a good approach for estimating cochlear delay. Comparing these results with estimates based on ABR, the assumption that OAE delay is twice the basilar membrane delay (as implied by the theory of coherent reflection) does not hold for frequencies below 2kHz.