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Re: Wasn't v. Helmholtz right? (BM/neural tuning)


I take your point and I agree with your comment that Narayan et al. could
have made the curves to match at their base.  In that case, neural
thresholds would be lower than the BM threshold.  However, even if you did
that, their data show that BM response is as sharply tuned as AN response.
The bandwidths of their tuning curves are comparable at the tip, even
though their thresholds may be different.  Assuming that the BM and AN
tuning curves are made to match at their tails, one could then argue that
the role of intermediate structures (e.g., OHC, IHC, etc) could be only to
"amplify" the BM response, but NOT to sharpen the tuning of the system.
Any comments on this?

-- Enrique

At 09:06 PM 22/06/2000 +1000, you wrote:
>Dear Enrique and List:
>I appreciate your close interest in this important subject and your positive
>feedback. I am glad you have pointed to the paper of Narayan et al, for it
>offers instructive insight into how easily data can be misinterpreted.
>The key to understanding this paper is to recognise that the tuning tip of
>the BM response was _made_ to correspond with the neural tuning tip (see
>their footnote 33). That is, the BM tuning curve has an arbitrary position
>on the Y-axis relative to the neural curve, and in Fig. 1 the two curves
>have been made to correspond at their tips. This relativity can be easily
>understood when one recognises that the two Y-axes are not commensurate: the
>neural 'threshold' is defined in terms of spike-rate increase on the
>spontaneous rate (e.g., a notional 10% increase), whereas the mechanical
>threshold relates to the sensitivity (noise floor in units of nm
>displacement) of the measuring apparatus and in Narayan et al's case is
>measured in terms of dB above the noise floor. Now at neural threshold (13
>dB SPL in the case of 'A' and 0.5 dB in 'B'), the BM 'threshold' is made to
>match its companion by adjusting down the quoted sensitivity of the
>instrument - so in A the BM 'threshold' is set to 2.74 nm, while in B it is
>set to 0.26 nm. Therefore a better (less misleading) way of plotting the BM
>sensitivity would be relative to the noise floor, which is 0.5 um/s or
>about -25 dB SPL (footnote 32). Note also that the neural 'threshold' is
>also arbitrary, and will move up and down on the Y-axis depending on the
>criterion used for spike-rate increase (e.g. it will move down if a 5%
>increase is used as the criterion; up if a 20% increase is decided on).
>Once you see that the two curves are arbitrarily placed with respect to each
>other, you understand that explanations based on divergent behaviour of the
>curves are arbitrary too. [For simplicity, let us confine our discussion to
>the BM displacement and neural curves; the BM velocity curve is a
>complicating 'adjustment factor' (a high-pass filter of 6 dB/octave) that
>can be ignored if we keep to the idea that stereocilia are deflected by
>solid structures, not fluid movements.]
>IF the two curves are aligned at their tips, then one begins talking of the
>tail of the neural curve being "less sensitive" by 15-20 dB than the tail of
>the BM curve. But, logically, there is a alternative explanation: IF the
>curves are aligned at the tails (say at the 80 dB SPL level), then the tip
>of the _BM_ curve is 15-20 dB _less sensitive_ than the neural curve!
>Indeed, the latter is the explanation I favour, as I hope the following
>makes clear.
>At moderate levels (80 dB SPL) the two curves are measuring similar activity
>(the whole-scale movement up and down of the partition and its excitation of
>the IHCs). At low levels, however, they are measuring different things: the
>IHC are efficiently detecting the ripples originating from the OHC SAW
>resonator close by, whereas relatively little movement is being communicated
>to the BM (what's more, the BM and the relatively large (10-30 um) beads
>sitting on it are more or less summing the activity of both OHC2 (in phase)
>and OHC1/3 (anti-phase)). It therefore appears, correctly, that the BM is
>less sensitive than the IHCs.
>We have therefore come to a completely different, but equally valid,
>interpretation of the Narayan data. The authors say that differences between
>the neural and BM curves are evidence that "certain transformations do
>intervene between BM vibration and auditory nerve excitation." Because they
>have aligned the tips, they search for differences in the tails, and find it
>in terms of high-pass filtering and lack of a high-frequency plateau. My
>alternative view sees identity in the tails and looks
>for transformations in the tips.
>Matching the tails by raising the BM data by 15-20 dB also calls for a
>reinterpretation of the observed plateau, which the
>authors see in the BM data but not in the neural data. However, the maximum
>data-point on the neural high-frequency slope is at 100 dB (in A) or 90 dB
>SPL (in B), only some 0 dB (in A) or 10 dB (B) above the BM plateau; if the
>BM curve is raised 15-20 dB, it may actually coincide with a similar plateau
>in the neural curve, but there is no neural data at high enough levels
>to show it. If there were a plateau in the neural curve at about 110 dB SPL,
>it would provide confirmation to my alternative explanation (we would want
>to align at the plateaus, wouldn't we?).
>-----Original Message-----
>From: AUDITORY Research in Auditory Perception
>[mailto:AUDITORY@LISTS.MCGILL.CA]On Behalf Of Enrique A. Lopez-Poveda
>Sent: Tuesday, 20 June 2000 5:30
>Subject: Re: Wasn't v. Helmholtz right?
>Dear Andrew and List,
>Like Ben Hornsby, I have been following your discussion very closely.  I
>have also read your paper.  I think your model is an excellent piece of
>work that leads to many questions that are worth exploring.  There is one
>thing, however, that I don't understand.  If BM motion is not the direct
>"cause" of IHC excitation, how do you explain, for instance, the
>relationship described by Shyamla Narayan, S., Temchin, AN, Recio, A, and
>Ruggero, MA [Science 282: 1882-1884] between frequency tuning of BM and
>auditory nerve fibres in the same cochleae?  The relationship occurs at
>threshold and is almost perfect particularly at the tip of the tuning curve
>where, according to your model, BM plays the "least" important of its roles.
>-- Enrique
>Dr. Enrique A. Lopez-Poveda
>Profesor Asociado de Bases Físicas de la Medicina
>Facultad de Medicina               Tel. +34-967599200 ext.2749
>Universidad de Castilla-La Mancha  Fax. +34-967599272 / 04
>Campus Universitario               http://emedica.med-ab.uclm.es
>02071 Albacete -- Spain
Dr. Enrique A. Lopez-Poveda
Facultad de Medicina               Tel. +34-967599200 ext.2749
Universidad de Castilla-La Mancha  Fax. +34-967599272 / 04
Campus Universitario               http://emedica.med-ab.uclm.es
02071 Albacete -- Spain