Re: mechanical cochlear model

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

At 8:59 PM -0500 3/9/10, David Mountain wrote:
> ...  When this energy reaches the beginning of 
> the peak region, some process, usually called 
> the "cochlear amplifier" and involving outer 
> hair cells, takes over and shapes the peak 
> response.

David, I like your definition and description, 
with one caveat:  it is often said that the 
"cochlear amplifier" is restricted to a narrow 
region before the peak, or it's described as 
"taking over", or as a different mechanism from 
the traveling wave.  That's OK, but I think it's 
perhaps more useful to say that the mechanism 
operates everywhere, and contributes to the 
physics that defines the wave equation and its 
dispersion relation.

The nonlinear properties of the mechanism that 
adds energy to the traveling wave gives rise to 
important nonlinear properties, including 
compression and distortion products.  But for the 
analysis of the traveling wave, it is useful to 
treat as linear, or approximately linearized at a 
given operating level.  Then its properties can 
be described simply, in term of how the gain or 
forces relate to frequency and wavelength.  In 
the region more basal than an octave before best 
place, the wavelength is quite long, and the 
energy moves through quite quickly, mostly away 
from the membrane, so the effect of the cochlear 
amplifier is necessarily very small.  The effect 
comes on gradually, and is greatest where the 
wavelength is short and the energy is 
concentrated near the membrane.  The effect can 
be described very simply as a 
negative-resistance-like part of the membrane 
impedance, in impedance-based models.

It's certainly true what you say, that we don't 
yet understand the micromechanical mechanisms of 
the cochlear amplifier, though we're a lot closer 
than we were ten years ago.  But there's ample 
evidence that its effect can be fit into a 
traveling wave model, and that such models do a 
credible job of fitting both neural and 
mechanical data simultaneously.  To the extent 
that there are remaining discrepancies, there are 
lots of people paying explicit attention to 
those, and trying to resolve them.  For example, 
if a high-frequency plateau is seen in mechanical 
data, and not in neural data, what does that 
mean, and how can it be resolved?  The problem is 
being addressed head-on, several ways, not being 
ignored as some would suggest.  It's a tiny 
effect, in terms of BM motion and energy, but 
understanding it is likely to help complete our 
understanding of the whole system.

Generally speaking, we've seen a convergence of 
experimental data, neural, mechanical, even 
psychophysical, and physics-based models that 
describe how it all fits together, over the last 
40 years (measuring back to Rhode's 1971 
mechanical measurements that showed the kind of 
nonlinearity near the peak that was already well 
known in neural data).

If people hadn't been questioning our 
understanding, our data, and our interpretations 
all along the way, we wouldn't be in the 
relatively good situation we're in today.  A few 
years ago in a talk I criticized Helmholtz and 
Ohm for their support of the phase-blind spectral 
analysis point of view of cochlear function; I 
realized later that I was off-base on this.  Like 
Bekesy and others that came since, they were 
operating half-blind, with limited apparatus, 
technique, and outlook, and doing their best in 
good faith to advance the science.  So I 
sensitized myself to such criticism of our 
forebears, which is why I sometimes speak up 
sounding a bit annoyed when I see unwarranted 
accusations like Martin's "Bekesy and his 
followers succeeded in neglecting all unwelcome 
evidence."  The historical evidence shows 
otherwise, and it's not a constructive 
interpretation of where the field has been or is 
headed, even if it can be shown to be true that 
certain individuals and groups were temporarily 
misled by adhering too long to a particular 
conception when the data were trying to take them 
elsewhere.

David, I count you and Jont and some others here 
as among those who drove progress over the last 
few decades, and who influenced my own progress 
in understanding this system, through your work 
and your generous help.  And I'm sure you 
understand that when I argue with you, it's part 
of addressing the issues that the data and models 
bring to our attention. I look forward to more 
from you, Jont, Reinhart, and others, and hope 
that those with alternative ideas to contribute 
will be able to do so without falling into that 
negativity thing that I fell into myself with 
Helmholtz and Ohm.

Dick