

Re: cochlear discussions

Dear Andrew and List

If I may add a couple more points to this discussion.

1. Another advantage of adopting an evolutionary perspective, as well as supporting
the case for the TM/OHC basis of selectivity, is that it enables us to see how the mammalian
cochlea may have developed from more primitive hearing structures.To give some specific examples.
List readers may be interested to know that during the course of evolution, in addition to the
mamallian cochlea (C) and its homologue the cochlear duct (CD) in birds and  crocodiles, at least 7
distinct structures have been implicated in hearing. These are the agnathan (jawless fish) common
macula (CM), three otolith organs,  i.e. the sacculus (S), utricle (U) and lagena (L),  and four
perilymphatic structures, the papilla neglecta (PN), the amphibian papilla (AP) and the basillar
papilla (BP).  Of these the first four have also been associated with motion sensing. In some species
the otolith organs appear to have a dual motion/acoustic sensing function whereas in others they
appear to be exclusively acoustic. The sacculus and lagena in particular are thought to be the
principal auditory structures in teleost fish.

2. The Lighthill model actually enables us to classify all in a simple manner. As I explained
in the previous message, the essense of the Lighthill model is that the travelling wave can be
described as the interaction of a chain of mass/spring circuits,

where a single (2nd order) element in the chain is

- m - .

so that a (higher order) chain is something like

- m - .- m - .- m - .- m - .- m - .
      |      |      |      |      |
      s      s      s      s      s
      |      |      |      |      |

On this basis we may classify each of the structures according to the following

ANIMAL             PRIMITIVE SENSOR (2nd order)      TRAVELLING WAVE SENSOR (high order)

agnathan fish        CM

teleost fish         S,  U, L

amphibians           PN, BP, L                          S, AP

birds                S,  L                              CD

crocs                S,  L                              CD

mammals              S                                  C

3. But this raises a question. The Lighthill model is surely also agnostic as to which particular
physical structures in the cochlea constitute the mass/spring elements of the travelling wave, since
between the "travelling wave" theory and the "resonance" theory? Or is it just an argument about
what are the essential structural elements of the cochlea?

4. References. One of the earlier complaints was that of obscure references. May I strongly
recommend to list readers the following.

Lewis, Leverenz and Bialek (1986) The Vertebrate Inner Ear. CRC Boca Raton, Florida.
(A must for every library).

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

See also

(1988) The Evolution of the Amphibian Auditory System. Wiley and Sons, New York.

Lighthill, J. (1981) Energy flow in the cochlea. J. Fluid Mechanics. 106, 149-203.


>Date:    Mon, 17 Jul 2000 09:47:00 +1000
>From:    Andrew Bell <bellring@SMARTCHAT.NET.AU>
>Subject: Re: cochlear discussions
>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