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Re: Cochlea Amplifier models : a new list
Richard F. Lyon asked:
Where should I look to study the data on this idea that underlies your
modeling approach?
You can take any data on level dependence of neural responses. None of
them mirrors the half-octave shift of BM tuning.
I've looked at lots of data, and you must be interpreting it differently
from how I am. So if you have something specific that we can look at and
discuss, we can try to resolve that difference. Lacking that, I'll stick
with Ruggero's interpretation that says mechanical and neural are
essentially the same, not different.
OK, here it goes.
1) Mario Ruggero only compared mechanical and neural behavior at threshold.
Had he compared mechanical and neural behavior at high sound levels, he
would have seen the striking dissociation between the two.
2) The mechanical data (basilar membrane BM):
The literature is full of data showing the half octave shift of BM behavior
between low/medium sound levels and high sound levels. A freely available,
and recent, example is this one:
Fig. 1A of http://www.pnas.org/cgi/content/full/97/22/11744
Here we see that, at one single recording location, the BM reacted most
strongly to sound levels of 0, 20, and 40 dB, when the probe tones had a
frequency 9.5 kHz. At a sound level of 100 dB, however, the BM reacted most
strongly, when the probe tones had a frequency 6 kHz.
Interpretation: The low level data reveal the tuning of the outer hair cells
(OHC), whose motility primarily excites the adjacent inner hair cells (IHC)
and secondarily, as a side effect, cause local BM excursions. The high level
data reveal the tuning of the BM proper, the passive BM. The protective
effect of this passive BM tuning: high level sound of 9.5 kHz vibrates the
BM *basalward* of the place of the OHCs tuned to 9.5 kHz. This takes out
energy at the most critical frequency (9.5 kHz) that otherwise might damage
the OHCs.
3) The neural data (auditory nerve fiber):
If neural data would correspond to BM data, the figure above would have a
corresponding figure, where the fiber fired most strongly at sound levels of
0, 20, and 40 dB, when the probe tones had a frequency 9.5 kHz, but most
strongly at sound levels of 100 dB, when the probe tones had a frequency 6
kHz.
Now - and this is the crucial point - such figures do not exist. Did they
exist, you can be certain that Mario Ruggero would have published one,
together with the figure above. The experiments were done, and the figures
were plotted. But they do NOT show a parallel to BM behavior. Actually, I
know that Mario is aware of these figures!
For an example, take Fig. 7B in this paper:
http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=4215872&ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum
Here we see that at one single auditory nerve fiber recording site the
firing rate was strongest at the lowest sound levels of 40 and 50 dB, when
the probe tones had a frequency 6 kHz. At a sound level of 100 dB the firing
rate was *again* strongest, when the probe tones had a frequency 6 kHz. Most
interestingly, the data for the 100 dB probe tones show a second peak at 5.2
kHz. So, the neural data mainly reflect the OHC tuning, and secondarily also
reflect the passive BM tuning. At following stages of neural processing
these secondary peaks are then filtered out by lateral inhibition.
4) Conclusion:
The dissociation of BM tuning and auditory nerve fiber tuning is perfect and
obvious, and it has a physiological explanation. It is one of the many
pieces of evidence that demonstrate the *real* function of BM mechanics.
I hope these data can be of use. I would appreciate if Richard F. Lyon, or
somebody else from the list, could check the referenced data and confirm for
the list that they are real.
Martin
---------------------------------------------------------------------
Martin Braun
Neuroscience of Music
S-671 95 Klässbol
Sweden
web site: http://w1.570.telia.com/~u57011259/index.htm
----- Original Message -----
From: "Richard F. Lyon" <DickLyon@xxxxxxx>
To: <AUDITORY@xxxxxxxxxxxxxxx>
Sent: Wednesday, October 10, 2007 9:55 PM
Subject: Re: Cochlea Amplifier models : a new list
At 5:39 PM +0200 10/10/07, Martin Braun wrote:
I haven't been able to find the evidence for the assertion that neural
tuning remains unchanged in some way that differs from the sense in which
BM tuning remains unchanged;
[.......]
Where should I look to study the data on this idea that underlies your
modeling approach?
You can take any data on level dependence of neural responses. None of
them mirrors the half-octave shift of BM tuning.
I've looked at lots of data, and you must be interpreting it differently
from how I am. So if you have something specific that we can look at and
discuss, we can try to resolve that difference. Lacking that, I'll stick
with Ruggero's interpretation that says mechanical and neural are
essentially the same, not different.
Dick