Re: mechanical cochlear model

[Daniel TAFT <DTAFT@xxxxxxxxxxxxx>]

Hi Andrew,

Here comes a naïve question, but aren't A and B kind of the same thing? A = delays with filters; B = filters with delays? End result looks like a wave either way, whether the energy comes from the stimulus or from the system.

And anyway I still don't understand why both can't be going on. Energy comes in from the wave front via the middle ear lever and propagates along in series. And additional energy is put in by outer hair cells, especially at low sound pressure levels.

I also have concerns about three nice rows of resonators. The pictures I've seen of biological cochleae look pretty messy, squiggly, and far from neatly organized to me. Maybe your model can handle this, I don't know, my math ability is good up to cos^2(x) + sin^2(x) = 1 and a bit rusty beyond that. :)

Kind regards,
Daniel.


-----Original Message-----
From: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Andrew Bell
Sent: Tuesday, 9 March 2010 4:04 PM
To: AUDITORY@xxxxxxxxxxxxxxx
Subject: Re: [AUDITORY] mechanical cochlear model

Of course if you start out with traveling wave assumptions, you'll end up
with a traveling wave. 

What I and a few others have been saying is that there are serious anomalies
in recent observations, and so we need to re-examine the basic assumptions.
You can keep on adding refinements if you like, like epicycle upon epicycle,
but you may be missing a simpler alternative. Remember that exciting a bank
of tuned elements leads to something that looks like a traveling wave.

There are three main assumptions that I think need revision.

1. The system is not incompressible. The cochlear fluids are, but the outer
hair cells themselves contain a compressible material. This is an ideal
pressure detection scheme: when the stapes pushes in, energy is funnelled
directly to the OHCs.

2. The round window offers appreciable resistance to deformation (compared
to that of the OHC's).

3. At low SPLs, the active system (the OHCs) are more sensitive to the fast
pressure wave than they are to differential pressure. After all, their
stereocilia sit in the wrong direction to detect a longitudinal traveling
wave.

Couple those assumptions with the idea that there are highly tuned OHC
resonant elements (the three rows together form a bank of SAW resonators),
then we come back to a resonance theory of hearing, and this is worth
exploring. The SAW model makes predictions which explain otoacoustic
emissions, phase delays, and a range of other psychoacoustic measures (see
Bell & Maddess 2009, or if you don't have access to it, ask me for a
reprint).
 
Bell, A. and Maddess, T. (2009) Tilt of the outer hair cell lattice: origin
of dual tuning tips and cochlear bandwidth. In: Concepts and Challenges in
the Biophysics of Hearing, ed. N. P. Cooper and D. T. Kemp (World
Scientific, Singapore), 310-318.

___________

Definitions:
___________ 

A. Traveling wave: a coupled fluid-mechanical wave that carries energy. The
stimulus travels through the bank of tuned elements IN SERIES.

B. Resonance: a bank of highly tuned independent elements that are
near-instantaneously excited IN PARALLEL. The apparent wave front produced
carries no energy. 



Andrew.


Andrew Bell
Research School of Biology (RSB)
College of Medicine, Biology and Environment
The Australian National University
Canberra, ACT 0200, Australia