Dear AUDITORY List people:
I have been reading with respect the many extended comments on Mechanical Cochlear Model.� I have been delighted to see the intense interest that the discussion has stimulated.� I have a few points to make for consideration by the list.
1) So much of the discussion is naturally focused on the cochlear peripheral system as though it existed as an analyzer in itself.� This reductionist perspective separates the peripheral from the whole animal, thereby often missing the functional value of sensor parts as components of a complete working system (animal).� I have in mind the title of the 1967 Zwicker & Feldtkeller book �Das Ohr Als Nachrichtenemphanger�. For me the big idea of their approach was considering the sensory system as-a-whole-process, neural as well as peripheral. The extraordinary sensitivity (as in receiver terms � �noise figure�) for auditory performance implies some very special system properties that require some internal form of parametric amplification to achieve what seems to be better than 3 dB noise figure considering animal body temperature.� How this is accomplished by animal auditory means is a tale not yet fully told.
2) There is an important signal-processing concept with respect to time domain behavior of �receiver systems�.
A signal path can be constructed from a chain of very highly resonant sections so that for extremely slowly varying signals it would appear to be flat in frequency response.� The behavior of such resonant element network with transient signals is an altogether different matter � transients just don�t pass through such a network.� The exceptional time domain abilities in auditory perception require something more like a transversal process than resonant ones. Nature is known to be an Ocam�s Razor devotee and it would seem to be heavily weighted towards transversal processing whether peripheral or combined with CNS resources. This may well include some resonant elements however of limited Q.� From this perspective, -� what might be the transducer parts of such systems?�� Bregman�s Auditory Scene Analysis sets the stage for more integrated perspective on the auditory perceptual apparatus.� A recent study has shown the ability of humans to resolve acoustic rise time differences of 5 microseconds or less.� (Kunshar 2008)�
3) The development of the telephone has greatly altered concepts of auditory communication.� People tend to neglect the extent to which such marginal and disturbed sound tools depend upon aural skills and memory rather than the accuracy of reproduction.� Even more neglected is the effects of sound of wavelengths greater than one meter in providing recognizable and comfortable sonic sense of place.� Such spectrum is received more via whole body sense than by the diminished long wavelength sensitivity of human ears.� There have been few efforts to examine human long-wavelength sound perception.
Mitchell Cotter,� CTO
AIR ISOSYS TECHNOLOGIES
With a background in fluid mechanics perhaps my perspective on the traveling wave helps the discussion.
I don't think there is a question whether or not there is a traveling wave in the cochlea. Fluid mechanics dictates that there has to be one.
The confusion comes - I think - from comparing the basilar memebrane with a string where the energy is passed on through the string and it is that same string which is showing the movement. In this respect the comparison with surface waves on water is much appropriate. The fluid-air interface is showing the movement, but it is the underlying fluid which passes
on the motion. Imagine a pond surface covered with ducks. Imagine it to be covered so densely you cannot see the water surface. When the water is set in motion (not neccessarily at its
surface), the ducks will move. This motion will look like a wave and I guess everyone would agree with the use of the term travelling wave in this case. The energy causing the
ducks to move is not passed on from one duck to the other, but stems from the motion of
Likewise in the cochlea the BM motion is caused by motion of the fluid. The fact that we
have fluid on both sides of the BM, whereas in the example we have fluid below and air on
top can be shown (mathematically) to be of no consequence for the principle. Also the
fact that in the example the restoring force acting on the ducks is gravity, whereas in
the cochlea it is the BM stiffness does not affect this story.
The main problem with the resonator/resonance theory (at least in the versions I know) is
that the motion of neighbouring resonators is independent. In the example neighouring ducks can not move independently because their motion is linked through the motion of the underlying (continous) water.
Complicating factor in the discussion is perhaps that in the cochlea, the restoring force being stiffness combined inevitably with mass, we automatically get resonators. So in my view it
is not a question of resonance OR travelling wave. It has to be a bit of both.
Fluid mechanics dictates that there is a travelling wave on the basilar membrane unless cochlear fluid is unlike any other fluid I know. The question that may remain is whether this wave motion is what causes the effective stimulation of haircells. But there should not be a question whether or not there is a traveling wave, even if it has not been shown
definitively in measurements.
The problem I see with a compression wave being the stimulus and the haircells acting as pressure sensors is that. This assumes that the haircells will be compressed by a pressure acting on them form the outside. However, the haircells are filled with fluid themselves and there will be no pressure difference between the inside and outside of the cell. This implies that the cell wil not deform and I do not quite see how the sensor would then operate. (But the fact that I don't see it does not mean it impossible, of course...).
The references to texts already given by dr Frosch and others are excellent and I don't have much else to add.
All the best,
Peter van Hengel