Re: Cochlear travelling wave. An epiphenomenon?

[Antony Locke <A.Locke@xxxxxxxxxxx>]

Dear Neil, Andrew and List

Neil and Andrew refer to a comparative approach in our discussions on the
validity of the travelling wave model. I agree that in general the study
across species is, and has been, fruitful.  Unfortunately, my limited
knowledge doesn't span peripheral auditory systems across species; to
clarify I was considering cochlear mechanics as applied to humans+rodents;
apologies for any confusion. However, just because the travelling wave (TW)
is postulated, say, for the human cochlea doesn't necessarily say anything
about the mechanisms involved in other species; the configurations and
therefore the mechanisms involved may differ considerably.

Andrew pointed out:

1. "the crocodile ear and that of many birds incorporates a 'cochlear
shunt'", and

2. Tonndorf's post-mortem findings of large impedance discontinuities in
cochleae of normal hearing humans.

...and was interested to hear a TW explanation.

A couple of points are worth noting:-
Firstly, the role of the fast compressional (common-mode) wave in
initiating a TW.  Consider the case where the stimulus is not applied to
the stapes, but at the apex.  Von Bekesy did this and observed a TW
initiated at the conventional place (i.e. at the base) which propagated
towards the source.  This finding can be explained as a result of a fast
wave launched at the stimulus site, which travels towards the base, and due
to the impedance mismatch at the boundary, i.e. oval and round windows,
leads to an apical TW.  The argument also holds for bone conduction.  The
important point is the existence of a mismatch in the impedance presented
to the fast wave propagating through the scala vestibuli to that presented
to the fast wave in the scala typani; this leads to a pressure gradient
across the cochlear partition.  The effective site for launching the TW may
not be at the base, but just apical to the impedance discontinuity.

Secondly, the possibility of evanescent modes.  If evanescent modes exist
then tunnelling could arise.

This, though, is in the realm of speculation.

My original point posted to the list was concerned with the most basic
description of energy flow in the cochlea:-

Observation: a highly graded anisotropic cochlear partition (Voldrich, 1978).

Experiment: asymmetrically tuned cochlear partition responses with large
phase lags (approx. 4 - 5 cycles) (Rhode, 1971).

Analysis: dispersive hydromechanical waves - cochlear travelling waves
(Lighthill, 1991).

When compared to experiment, there are quantitative limitations to the TW
model in some regions of parameter space.  The limitations lead to
questions such as, how to include nonlinearity, what about 2 or 3 degrees
of freedom, the role of the tectorial membrane, is an active mechanism
necassary etc.  People have integrated additional elements to the TW model
with varying degrees of success in order to address these questions.  In
essence, the discussion over the past 2 weeks comes down to: are we willing
to disregard, or build upon, the TW model because of these limitations?

Kind regards
Antony Locke