Re: cochlear discussions

[Andrew Bell <bellring@xxxxxxxxxxxxxxxx>]

Dear Neil and List:

I strongly agree that the combination of tectorial membrane and OHC provides
the basis for frequency analysis in the mammalian ear.

Yes, auditory science has been "labouring under the possibly false belief
that the frequency selective properties of mamallian ear are due to the
mechanical properties of the basilar membrane."

Let me see if I can emulate your succinct style, and provide 5 good reasons
why the basilar membrane is not an appropriate structure for frequency
analysis.

1. Its combined mass and stiffness do not vary by the required factor of
10^6 (to provide a response over 3 decades of frequency).

2. In some bats, the mass and stiffness are not even tonotopically
organised, so that there is a so-called "paradoxical change in stiffness"
(Vater, 1988) in which the stiffness of the 70-111 kHz region is much
_lower_ than that in the adjacent 60-70 kHz region.

3. The BM has interfering structures, such as blood vessels, running along
its length.

4. It is sometimes found, along with a completely formed (and presumably
functional) cochlear partition, sitting on bone.

5. In birds and crocodiles, it is found with holes penetrating it,
short-circuiting the differential pressure that is supposedly required to
generate a traveling wave.

Nevertheless, it does possess graded mass and stiffness. Braun (Hear. Res.
78 [1994], 98-114) suggested that the BM was designed to absorb excess
cochlear vibration (at high SPLs), which is a good suggestion.


Andrew.


-----Original Message-----
From: AUDITORY Research in Auditory Perception
[mailto:AUDITORY@LISTS.MCGILL.CA]On Behalf Of Neil Todd
Sent: Sunday, 16 July 2000 4:37
To: AUDITORY@LISTS.MCGILL.CA
Subject: Re: cochlear discussions


Dear All

I apologise if my own evolutionary perspective has fallen on deaf
ears (excuse pun) due to use of highly technical jargon. However, I
do believe that in this case it is of interest to the general list
readership. So let me summarise this view in a less technical manner.

1. The travelling wave phenomenon has evolved independently at least
three times during the course of evolution.

2. The principles of parsimony and evolutionary consistency would
suggest that a single theory should account for all examples of the
same phenomenon.

3. The essence of  Lighthill theory is that a travelling wave can be
described by a chain of masses and springs (if you like mechanical
analogies) or inductances and capacitances (if you like electrical
analogies).

4. The implication of Lewis's (1988) interpretation of the three
structures in 1. (amphibian papilla (AP),  amphibian sacculus (S) and
mamallian cochlear (C)) from the Lighthill perspective can be
summarised as follows.

structure        springs         masses

AP                hair-cells      tectorium
S                   hair-cells      otoconial membrane
C                  OHCs          tectorial membrane

5. Conclusion. Irrespective of whether one goes for the TW theory
or the resonance theory (I am agnostic on this), evolutionary
consistency points to the interaction between the OHCs and the
tectorial membrane as being the important elements in the frequency
selective properties of the mamallian ear. This is also consistent
with the efferent system playing an active role in the selectivity,
since in part the spring-like (two-way transductive) property of the
OHCs may be neurobiological, rather than purely mechanical.

6. Surely this is of interest to all list members, not least because
many of us have been labouring under the possibly false belief that
the frequency selective properties of mamallian ear are due to the
mechanical properties of the basilar membrane.

I rest my case.

Neil Todd