Subject: Re: mechanical cochlear model From: Peter van Hengel <pwj.vanhengel@xxxxxxxx> Date: Fri, 12 Mar 2010 15:07:02 +0100 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>--0016e6d7f07ab412af04819b0bee Content-Type: text/plain; charset=ISO-8859-1 Dear list, 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 the fluid. 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 --0016e6d7f07ab412af04819b0bee Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable <p>Dear list,<br>=A0<br>With a background in fluid mechanics perhaps my per= spective on the traveling wave helps the discussion.<br>=A0<br>I don't = think there is a question whether or not there is a traveling wave in the c= ochlea. Fluid mechanics dictates that there has to be one.</p> <p>The confusion comes - I think - from comparing the basilar memebrane wit= h 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 w= ith surface waves on water is much appropriate. The fluid-air interface is = showing the movement, but it is the underlying fluid which passes<br> 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<br>surface), the ducks will move. This m= otion will look like a wave and I guess everyone would agree with the use o= f the term travelling wave in this case. The energy causing the<br> ducks to move is not passed on from one duck to the other, but stems from t= he motion of<br>the fluid.<br>=A0<br>Likewise in the cochlea the BM motion = is caused by motion of the fluid. The fact that we<br>have fluid on both si= des of the BM, whereas in the example we have fluid below and air on<br> top can be shown (mathematically) to be of no consequence for the principle= . Also the<br>fact that in the example the restoring force acting on the du= cks is gravity, whereas in<br>the cochlea it is the BM stiffness does not a= ffect this story.<br> =A0<br>The main problem with the resonator/resonance theory (at least in th= e versions I know) is<br>that the motion of neighbouring resonators is inde= pendent. In the example neighouring ducks can not move independently becaus= e their motion is linked through the motion of the underlying (continous) w= ater.<br> =A0<br>Complicating factor in the discussion is perhaps that in the cochlea= , the restoring force being stiffness combined inevitably with mass, we aut= omatically get resonators. So in my view it<br>is not a question of resonan= ce OR travelling wave. It has to be a bit of both.</p> <p>Fluid mechanics dictates that there is a travelling wave on the basilar = membrane unless cochlear fluid is unlike any other fluid I know. The questi= on 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 no= t there is a traveling wave, even if it has not been shown<br> definitively in measurements.<br>=A0<br>The problem I see with a compressio= n wave being the stimulus and the haircells acting as pressure sensors is t= hat. This assumes that the haircells will be compressed by a pressure actin= g on them form the outside. However, the haircells are filled with fluid th= emselves and there will be no pressure difference between the inside and ou= tside of the cell. This implies that the cell wil not deform and I do not q= uite see how the sensor would then operate. (But the fact that I don't = see it does not mean it impossible, of course...). <br> =A0<br>The references to texts already given by dr Frosch and others are ex= cellent and I don't have much else to add.</p> <p>All the best,<br>Peter van Hengel</p> --0016e6d7f07ab412af04819b0bee--