Re: AW: Cochlear nonlinearity & TTS

[David Mountain <dcm@xxxxxx>]

 believe that the available data suggest that the cochlear amplifier only
has a significant effect over a small region at and basal to the peak of
the traveling wave.  See for example:

Cody AR (1992) Acoustic lesions in the mammalian cochlea: implications for
the spatial distribution of the 'active process'.  Hear Res. 1992
Oct;62(2):166-72. PMID: 1429258

I would rephrase Ramda's question to be the question of whether the
transformation between sound and auditory nerve firing pattern is
invertable.  I think the answer in the exact sense is that it is not
invertable.  It is my belief that we are dealing with a compression system
with some loss but one that preserves the features of biological
relevance.  Modern audio compression schemes take advantage of this fact
and throw away acoustic information that is not perceived or only barely
detectable by the listener.


--------------------------------------------------------------------

David C. Mountain, Ph.D.
Professor of Biomedical Engineering

Boston University
44 Cummington St.
Boston, MA 02215

Email:   dcm@xxxxxx
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On Thu, 18 Jan 2007, Ramdas Kumaresan wrote:

> Navid, Richard and the listees,
>
> I have heard a lot of speculation about the cochlear amplifier for many
> years. One of the questions that  I have wondered about
> as a signal processing engineer for many years, is with all the
> sophisticated  nonlinearities, delays, amplifiers, filters
> etc that are present in the auditory periphery, how does it "represent"
> an acoustic signal in the neural spike patterns
> that emanate from the auditory periphery? (I guess everyone  wonders
> about it.)
> Is it possible to reconstruct the acoustic signal if you were able to
> measure/monitor   the
> spike patterns  that are put out by all the auditory nerve fibers?  What
> is the reconstruction 'algorithm"?
> (I know about   Egbert deBoer's reconstruction method  for a single
> nerve fiber.) Is'n't the information about the signal
> distributed across many, many  nerve fibers? Should'nt the
> reconstruction take information from
> all nerve fibers and fuse them to reconstruct the signal? Just wondering
> aloud. RK
>
>
>
>
>
> Richard F. Lyon wrote:
>
> > At 9:17 AM -0800 1/16/07, Navid Shahnaz wrote:
> >
> >> Thank you Reinhart for your clarification. Does the cochlear
> >> amplifier works on both sides of the excitation pattern peak on the
> >> BM? or the amplifier operates  wore efficiently at a place that is
> >> just above or toward the apex from the point of disturbance created
> >> by travelling wave? Operationally this point may be an ideal point as
> >> it is less likely saturates the amplifier due to sharp slope of the
> >> travelling wave on the apical side.
> >> Cheers
> >> Navid
> >
> >
> > Navid,
> >
> > Both Monita and Reinhart have given good explanations, but let me add
> > a bit.
> >
> > The way I think of it, the active amplification is active everywhere,
> > but it competes with the passive loss mechanisms, and is only
> > significant at low enough levels.  The active loss mechanism (damping)
> > increases rapidly apically when a sine wave travels past a
> > characteristic place.  Because of the active gain, the response to a
> > sine wave can travel further before it damps out; from the "passive
> > peak" that Reinhart mentions, the peak response location can be
> > further apical, up to about a half octave worth of place further, when
> > the active amplification is significant, to the "active peak". The
> > "net" amplification is positive (in dB per mm or whatever) before the
> > response peak, and negative after the response peak, pretty much by
> > definition of peak.  That net includes the active gain, which
> > saturates, and the passive loss, which doesn't, so it's level dependent.
> >
> > In addition to the saturation that reduces the active gain at high
> > level, there is also efferent control that turns down the gain in
> > response to afferent response level and possibly other central control
> > signals.  This effect of efferent control of mechanical gain has been
> > directly demonstrated, but I don't recall exactly who/when/where to
> > cite right now.
> >
> > Dick
> >
>