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I would be extremely cautious when it comes to using OAE data to estimate
cochlear group delays. Both experiments and models suggest that OAEs are
not generated in a single place but are instead arise from a distributed
response with the result being a sum of components with different group
delays which can result in both summation and cancelation. Goodman et al.
discuss some of the problems associated with OAEs in their paper and
attempt to get around them by focusing on low sound levels, "good"
waveforms, and extrapolation. The problem with extrapolating the data to
lower sound levels is that it is unlikely that the group delay-SPL curve
is linear at low SPL. If the author's assumptions are correct, one would
expect that the curve would level off at low SPL as do the data from
direct BM measurements (see Fig. 7 in their paper). Unfortunately, their
data are not good enough to establish whether there is a low SPL
assymptote in the OAE group delay.
David C. Mountain, Ph.D.
Professor of Biomedical Engineering
I concur with the basic tenet of Dr Mountain's comments. Indeed, in the
Discussion section of the paper of Goodman et al (2004) we observed that at
low stimulus levels OAE delay presumably asymptotes to a constant value
(because the OAE is presumably arising from a spatially confined location).
However, I do not share Dr Mountain's concerns re extrapolation of the data
and making an inference from this extrapolation about cochlear delay. The
linear extrapolation shows the delay to be in the "ball-park" for a round
trip cochlear delay; deviation from a linear extrapolation associated with
the delay asymptoting to a constant value would not change this conclusion.
OAEs are an essential part of cochlear mechanics and are an expression of
cochlear delay (provided we understand adequately how they are generated).
It is not expected that such delays will correspond directly to
measurements on the basilar membrane except where such measurements can be
considered comparable i.e., where the OAE arises from a sufficiently
spatially confined region as to approximate the spatial limit of the
basilar membrane measurement. OAE measurements of cochlear delay supplement
(but do not supplant) in-situ measurements of cochlear delay, both being
essential as we continue to unravel the complexities of energy propagation
within the mammalian cochlea.