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Re: Quick IHC question

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


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
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
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


,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 }

author={Kim, D.O. and Siegel, J.H. and Molnar, C.E.},
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},

,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.}
,note={} }

   author = {Allen, J. B.},
   title = {Cochlear micromechanics: {A} physical model of transduction},
   journal = JASA,
   volume = {68},
   number = {6},
   pages = {1660-1670},
   year = {1980}

,author={Allen, J.B.}
,title={Nonlinear Cochlear Signal Processing}
,booktitle={ Physiology of the Ear, Second Edition }
,publisher={Singular Thomson Learning}
,editor={ Jahn, A.F. and Santos-Sacchi, J.  }
,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@oersted.dtu.dk
> Web: www.dat.dtu.dk/~tuc

Jont B. Allen
AT&T Labs-Research, Shannon Laboratory, E161
180 Park Ave., Florham Park NJ, 07932-0971
973/360-8545voice, x7111fax, http://www.research.att.com/~jba