Subject: Re: mechanical cochlear model From: Andrew Bell <andrew.bell@xxxxxxxx> Date: Sun, 7 Mar 2010 14:00:06 +1100 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>In addition to Martin's 2 pieces of evidence against the traveling wave model, we can add: 1. The peak of the traveling wave is unrealistically sharp. A) In a gerbil, Ren found that the peak occurred over a region extending less than 0.5 mm at 16 kHz (Ren 2002, PNAS 99, 17101). B) Russell & Nilsen saw a peak only 0.15-1 mm wide at 15 kHz in a guinea pig (R&N 1997, PNAS 94, 2660). C) Lonsbury-Martin & Martin found histologically a gap only 60-70 um wide due to damaging pure tone levels applied to monkeys' ears (L-M & M 1987, JASA 81, 1507). I quote Jont Allen's remark from 2001 that "the discrepancy in frequency selectivity between basilar membrane and neural responses has always been, and still is, the most serious problem for the cochlear modelling community." Jont, do you still feel that way? 2. The variation is stiffness is inadequate to tune the cochlea from 20 to 20000 Hz. Three decades of frequency calls for a million times variation in stiffness (more than between foam rubber and tungsten), and this is in contrast to measurements of 2 or 3 orders at most. See Naidu & Mountain 1998, Hear Res 124, 124. Bekesy found the value to be about a hundred-fold (p. 476 of Exp in Hearing). 3. Some workers (eg, Stenfelt) find that the spiral lamina is as flexible as the basilar membrane, removing another avenue by which tonotopic tuning can be achieved - because its width is about constant. (Stenfelt 2003, Hear Res 181, 131). 4. Cases are reported where a person has been found to possess holes in the basilar membrane - and the holes don't appear to affect hearing. In some birds, there is a naturally occurring shunt called the ductis brevis, which connects the upper and lower chambers at the basal end. Yet these birds hear perfectly well nonetheless. 5. People lacking a middle ear can still hear (and yet pressure on the oval and round windows should in these cases be equal). Together, all these anomalies cast doubt on the adequacy of the traveling wave model. I think the TW model, as currently framed, cannot work at low sound pressure levels, and have formulated a resonance model in which the effective stimulus is the fast pressure wave (not the pressure difference across the membrane) and where the OHCs are pressure sensors (they are compressible). I've mentioned various publications previously, but the most comprehensive one is my thesis. It discusses the above anomalies, and others, in some detail - the link is http://thesis.anu.edu.au/public/adt-ANU20080706.141018/index.html The traveling wave can drive some passive processes, but in terms of efficiency we need an active system, and I think that here we need living pressure sensors (outer hair cells) forming resonant elements. Andrew. Andrew Bell Research School of Biology (RSB) College of Medicine, Biology and Environment The Australian National University Canberra, ACT 0200, Australia > -----Original Message----- > From: AUDITORY - Research in Auditory Perception > [mailto:AUDITORY@xxxxxxxx On Behalf Of Martin Braun > Sent: Sunday, 7 March 2010 7:29 AM > To: AUDITORY@xxxxxxxx > Subject: Re: [AUDITORY] mechanical cochlear model > > > While the cochlear traveling wave has appeared in numerous > empirical reports > on real physical models and real biological animals, it's function in > hearing is not yet universally appreciated. Some people still > think that it > provides the well known frequency selectivity that we observe in the > auditory nerve. This view, however, has been proved wrong by > multiple direct > experimental evidence. Just consider two bodies of evidence: > > 1) Hearing sensitivity is not affected, when endolymphatic > hydrops presses > the basilar membrane flat upon the bony cochlear wall of the > scala timpani: > > http://www.neuroscience-of-music.se/Nageris.htm > > http://www.neuroscience-of-music.se/Xenellis.htm > > > 2) It is a well established observation for more than 50 > years that closure > of the round window does not affect hearing sensitivity. This > means that a > pressure difference across the basilar membrane and a > resulting traveling > wave cannot be a necessary condition of hair cell excitation. > Recently, > Perez et al. (2009) reported that closure of the round window > not only > leaves hearing sensitivity unchanged but increases cochlear > vulnerability at > high sound levels. This second new observation is a further > compelling > indication as to the real function of the cochlear traveling wave. > > http://www.neuroscience-of-music.se/Sohmer.htm > > > Martin > > > --------------------------------------------------------------------- > Martin Braun > Neuroscience of Music > S-671 95 Klässbol > Sweden > email: nombraun@xxxxxxxx > web site: http://www.neuroscience-of-music.se/index.htm >