Re: cochlear discussions (Andrew Bell )

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

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(at)LISTS.MCGILL.CA]On Behalf Of Neil Todd Sent: Sunday, 16 July 2000 4:37 To: AUDITORY(at)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

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