Re: Quick IHC question (Jont Allen )

Subject: Re: Quick IHC question
From:    Jont Allen  <jba(at)RESEARCH.ATT.COM>
Date:    Mon, 21 May 2001 09:55:45 -0700

Thomas Ulrich Christiansen wrote: > Text books say that IHC are "sensitive to shearing in one direction". > > My question is: Which direction does this correspond to in terms of pressure > at the tympanic membrane? > > A. Positive sound pressure > B. Negative sound pressure > C. This is not known > D. The question is irrelevant > > Thanks > > -thomas Thomas, The direction of motion that "depolarizes" the inner (or outer) hair cell is toward the tall cilia. This was best demonstrated by Hudspeth and Corey in 1977. It is widely accepted that the tectorial membrane shears against the cilia to cause this excitation to the cilia. This means that when the TM is moving radially outward, away from the core of the cochlea (Limbus), that the excitation is excitatory (depolarizing). When the pressure at the tympanic membrane is positive, it initiates a positive traveling wave down the basilar membrane. However due to the delay, one cannot say that an increased pressure at the TM gives rise to an increased pressure along the entire BM, at the same time. However this delay is not where the uncertainty is. If the pressure were positive across the BM at a location along the length of the cochlea, then one needs to know the impedance at that location, and frequency, to determine which direction it will move. For example, if the BM impedance is stiffness dominated, then Hooke's law will tell you that the BM will move in the direction of the force. If it were resistive, then the velocity is proportional to the force (pressure times area). This all leads to a more complex set of issues, but this is not the reason that we do not know the precise answer to your question. Kim et al. (1979, 1980) showed that the phase changes by 180 degrees, about 1/2 octave basal to the best frequency. They did this using population studies, of lots of neurons, to a few tones. In my view this 'proves' that modes in the micromechanics must play a role in sharpening the response of the BM traveling wave. The most likely place for these modes is in the tectorial membrane. See Allen 1980 for the first discussion of this view, as well as Allen 2001, for an up to date discussion. I have an electronic version of this paper available. Finally, in the most recent review works of Ruggero and his coworkers, they state that neural responses are 180 degrees out of phase with BM responses at the same location, at CF. They point out that this is at odds with direct excitation theories. As I see it, the preponderance of experimental evidence shows that choice A above is the correct answer to the question (taking the pressure reference point across the BM at CF, rather than at the stapes). Restated then, at the place corresponding to the best frequency for a given tone, as the pressure in the scala vestibule (upper chamber, connected to the stapes) increases relative to scala tympani (lower chamber, connected to the round window), the BM likely moves down (but not perfectly in phase, as you might expect from a stiffness dominated BM impedance), and the tectorial membrane moves radially outward, leading to an excitatory response. This movement of the tectorial membrane is the opposite that one would expect if it were rigidly coupled to the limbus. Finally, this question is at the heart of what we do not know about cochlear micromechanics, so every answer must be viewed critically, including of course, mine. You might want to study Dan Geisler's book "From sound to synapse" for his set of views, which tend to be very different from mine. Jont Allen References: ,author={Hudspeth, A.J. and Corey, D.P.} ,title={ Sensitivity, polarity, and conductance change in the response of vertebrate hair cells to controlled mechanical stimuli } ,journal=PNAS ,year={1977} ,volume={74} ,pages={2407--2411} ,month=jun ,number=6(at)inproceedings{Kim79, author={Kim, D.O. and Siegel, J.H. and Molnar, C.E.}, year={1979}, title={Cochlear Nonlinear Phenomena in Two-Tone Responses}, book={Models of the Auditory System and Related Signal Processing Techniques}, editor={Hoke, M. and DeBoer, E.}, booktitle={Scandinavian Audiology, Supplementum 9}, pages={63-82} (at)article{Kim80 ,title={Cochlear mechanics: {N}onlinear behavior in two-tone responses as reflected in cochlear-nerve-fiber responses and in ear-canal sound pressure} ,author={Kim, D. O. and Molnar, C. E. and Matthews, J. W.} ,journal=JASA ,year=1980 ,month=may ,volume=67 ,number={} ,pages={1704--1721} ,note={} } (at)article{Allen80, author = {Allen, J. B.}, title = {Cochlear micromechanics: {A} physical model of transduction}, journal = JASA, volume = {68}, number = {6}, pages = {1660-1670}, year = {1980} (at)incollection{Allen01b ,author={Allen, J.B.} ,title={Nonlinear Cochlear Signal Processing} ,booktitle={ Physiology of the Ear, Second Edition } ,publisher={Singular Thomson Learning} ,year=2001 ,editor={ Jahn, A.F. and Santos-Sacchi, J. } ,chapter={19} ,pages={393--442} ,address={ 401 West A Street, Suite 325 San Diego, CA 92101 } > > > -------------------------- > Thomas Ulrich Christiansen, > Oersted*DTU, Acoustic Technology, > Building 352, Technical University of Denmark > DK-2800 Lyngby, Denmark > Phone +45 4525 3968 (office), +45 4525 3930 (department) > Fax +45 4588 0577 > e-mail: tuc(at) > Web: -- Jont B. Allen AT&T Labs-Research, Shannon Laboratory, E161 180 Park Ave., Florham Park NJ, 07932-0971 973/360-8545voice, x7111fax,

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