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

Subject: Re: Wasn't v. Helmholtz right? (BM/neural tuning)
From:    Andrew Bell  <bellring(at)SMARTCHAT.NET.AU>
Date:    Thu, 22 Jun 2000 21:06:25 +1000

Dear Enrique and List: I appreciate your close interest in this important subject and your posit= ive feedback. I am glad you have pointed to the paper of Narayan et al, for i= t offers instructive insight into how easily data can be misinterpreted. The key to understanding this paper is to recognise that the tuning tip o= f 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 positio= n 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 (1= 3 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 e= ach other, you understand that explanations based on divergent behaviour of t= he 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 t= he 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 ti= p 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 activ= ity (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: t= he IHC are efficiently detecting the ripples originating from the OHC SAW resonator close by, whereas relatively little movement is being communica= ted 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 phas= e) 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 betw= een the neural and BM curves are evidence that "certain transformations do intervene between BM vibration and auditory nerve excitation." Because th= ey 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 maxim= um data-point on the neural high-frequency slope is at 100 dB (in A) or 90 d= B SPL (in B), only some 0 dB (in A) or 10 dB (B) above the BM plateau; if t= he BM curve is raised 15-20 dB, it may actually coincide with a similar plat= eau 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 S= PL, it would provide confirmation to my alternative explanation (we would wan= t to align at the plateaus, wouldn't we?). Andrew. -----Original Message----- From: AUDITORY Research in Auditory Perception [mailto:AUDITORY(at)LISTS.MCGILL.CA]On Behalf Of Enrique A. Lopez-Poveda Sent: Tuesday, 20 June 2000 5:30 To: AUDITORY(at)LISTS.MCGILL.CA 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 cur= ve where, according to your model, BM plays the "least" important of its rol= es. -- Enrique ________________________________________________________________ Dr. Enrique A. Lopez-Poveda Profesor Asociado de Bases F=EDsicas de la Medicina Facultad de Medicina Tel. +34-967599200 ext.2749 Universidad de Castilla-La Mancha Fax. +34-967599272 / 04 Campus Universitario 02071 Albacete -- Spain ________________________________________________________________

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