Re: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr

[Andrew Bell <bellring@xxxxxxxxxxxxxxxx>]

Dear Neil and List:

Thankyou for reminding us that there are good reasons for believing the TW
in amphibians isn't necessary. These cases are instructive, for they can
help us gauge what conditions are necessary for setting up a TW.

It is of particular interest that the crocodile ear and that of many birds
incorporates a 'cochlear shunt' - in particular, a hole through the basilar
membrane at the oval-window end called the ductis brevis (Kohlloffel, Hear.
Res. 13 (1984), 77-81). In some birds such as goose the duct is very wide.
In terms of TW models, it is difficult to see how the required 'differential
pressure' can be maintained, and the TW propagate, with such a short circuit
in place.

Similar short circuits have been noted in humans. Tonndorf (Acta Oto-laryng.
50 (1959), 171) relates a case where a person had 'open communication'
between scala vestibuli of the first turn and scala typani of the second
turn (observed post mortem), yet the person's audiogram before death showed
that hearing for frequencies below the locus of the lesion was normal.
Tonndorf also mentions cases where subjects had portions of their BM
completely ossified, but they still heard normally.

To me, the only possible explanation of this evidence is a common-mode
pressure response of the partition. I would certainly like to hear a TW
explanation.



Andrew Bell







-----Original Message-----
From: AUDITORY Research in Auditory Perception
[mailto:AUDITORY@LISTS.MCGILL.CA]On Behalf Of Neil Todd
Sent: Friday, 30 June 2000 7:34
To: AUDITORY@LISTS.MCGILL.CA
Subject: Re: Cochlear travelling wave. An epiphenomenon? Re: Cochlear tr


Dear List

If I may be so bold as to add an evolutionary perspective to this
fascinating
discussion. This is not at all an area of speciality of my own, but I happen
to have some familiarity with the literature due to my own interest in the
acoustic
sensitivity of the sacculus (Hear Res. 141, 180-188, 2000). If the cochlear
TW is an epiphenomenon,
it is not unique in the evolutionary history of hearing. The amphibian ear
posesses at least
four distinct end organs which have an acoustic sensitivity, two
perilymphatic structures,
the basilar papilla (BP) and the amphibian papilla (AP),  and two otolith
structures, the
saccular (S) and lagena (L) maculae. Of these curiously the AP and S appear
to have a high
order TW property but the BP appears to be a simple resonance struture.
Lewis and Lombard
(1988) speculate that "If hair cells are indeed bidirectional
transducers,...., then energy
can be taken out of them by mechanical reactances, in which case, the
hair-cells themselves
could be the shunt resonances [in the critical layer resonance model]. The
tectorium of
the AP and the otoconial membrane of the saccule, could provide the
complementary reactive
coupling between the shunt resonances. We therefore have putative travelling
wave structures
in both end organs." This may have some relevance to the cochlear TW. We
should of course
exercise some caution in comparing the highly derived structures of extant
species (the
ancestral Devonian amphibian of mammals and frogs may have had hearing more
like a fish)
but the fact that the TW (epi)phenomenon has independently evolved a number
of times would
seem to lend support to the TM theory, since clearly a BM is not essential.


Cheers

Neil Todd


Lewis, E.R. and Lombard, R.E.  In Fritzsch, B., Ryan, M.J., Wilcynski, W.,
Hetherington,
T.E., Walkowiak, W. (Eds.) The Evolution of the Amphibian Auditory System.
Wiley and Sons,
New York.   (1988)


see also

DB Webster, RR Fay, and AN Popper (Eds) The Evolutionary Biology of Hearing.
Springer-Verlag, New York. (1992).