Re: Traveling waves or resonance?

["Richard F. Lyon" <DickLyon@xxxxxxx>]

At 2:50 PM +0200 10/20/04, Martin Braun wrote:
> Richard F. Lyon wrote on October 20:
>  > Ren's statement that "In sensitive cochleae, the cochlear partition
>  > vibration at a given location shows a maximum response to a stimulus
>  > at the CF, falls off quickly at frequencies above or below the CF,
>  > and forms a sharp peak in magnitude transfer functions" has been
>  > interpreted by some as saying that the response is fairly "symmetric"
>  > about CF.  But the data do not show that, as Fig. 4C makes most
>  > clear.
>
> Dick, Fig. 4C is not the right one to explore the symmetry question. The
> most straightforward figure in this respect is Fig. 1A, as all unbiased
> readers will see immediately:
If we find any of those unbiased readers we should ask them.
Meanwhile, we have to work it out among ourselves.

I pointed out Fig. 4C because it has a logarithmic vertical scale and
shows enough dynamic range (over 30 dB) to show the asymmetry at 60
dB SPL pretty clearly.  Here it is:
http://www.pnas.org/cgi/content/full/99/26/17101/F4

> Full text article:
> http://www.pnas.org/cgi/content/full/99/26/17101
> Figure 1 only:
> http://www.pnas.org/cgi/content/full/99/26/17101/F1
> Here we can see that the curves for 10, 20, 30, 40, and 50 dB show
> symmetrical peaks. From 60 to 90 dB then, they are increasingly asymmetrical
> with longer and shallower slopes towards the basal side.
The active traveling wave models have a variable "tip" and a fixed
"tail", you might call them, and the bigger the peak, or the bigger
difference between tip and tail, the more symmetric it will appear in
the tip region.

Fig. 1 doesn't have enough dynamic range to show the smaller
asymmetry at low levels, but that's not sufficient reason to conclude
that the shape is not part of the same continuum of asymmetric shapes
that the traveling wave models predict.

> This figure accurately reflects a condition of local resonance up to
> ca 60 dB, and an additional basilar membrane traveling wave above ca
> 60 dB.
The "local resonance" is already shifting in position and in gain well
below 60 dB SPL, so there's a lot more going on at low levels than
"resonance".  I agree that Fig. 1 accurately reflects what is going
on, but it doesn't imply resonance or a split between two different
mechanisms or modes as far as I can see.

> All this has been discussed in detail on the Cochlea List:
> http://mimosa1.incubator.uiuc.edu/pipermail/cochlea/2002-December/001546.html
> http://mimosa1.incubator.uiuc.edu/pipermail/cochlea/2002-December/001550.html
I read those already, but it's pretty much the same limited cast of
characters as here, so I thought I would keep my remarks in this
thread, in reply to Andrew Bell's paper on resonance.  I accept
that we have at least a four-way split over how to interpret Ren's
experiment.  I'm not saying that any of you is wrong, but that
Ren's data are also very much consistent with the traveling wave
models.  Furthermore, those models can accommodate local resonances
that interact with and determine the wave.  So I don't see why we need
such a dichotomy between the approaches, when they can be integrated.

Dick