Re: Get lost, Mr. Cochlea!! --- The Brain (Jont Allen )

[Ramdas Kumaresan <kumar@xxxxxxxxxxx>]

Dear Jont:

Jont Allen wrote in
http://sound.media.mit.edu/dpwe-bin/mhmessage.cgi/AUDITORY/postings/2001/135

>The ear IS similar to a floating point converter. The ear does not have

an infinite
>dynamic range or signal to noise ratio. This limited dynamic range
>shows up as masking. Do you disagree?



I don't know,  but masking of a weak signal due to an intense signal in
its neighbourhood, is it entirely due to what happens in the periphery?
(We know about asymmetry, spreading and shifting of excitation to higher

frequencies.)
What if the periphery still accurately (to the extent it is
allowable by timing jitter etc) represents the weak and intense signal
combo and the higher centers ignore the weak component, say, because
there  is much more precise  phase locking to the intense signal.
I am not too hot on the trail in masking.
Is it established that the information loss (masking) is entirely due to

the
periphery?

Jont Allen wrote:
>The auditory nerve signal is not about zero crossing.  Even zero
crossing
>are not exact, and would have jitter. But masking is NOT timing jitter.

We thought the classical theory of a neuron firing says that if the
membrane potential exceeds a threshold then it fires. If so, then it IS
some form of zero or level crossing detector. It is a question of how
the cochlear mechanics transforms the signal and presents it to the
neuron/haircell.

Zero-crossings, as descriptors of a signal,  have acquired
an undeserved  bad reputation. As we have pointed out in our
original post, the zero-crossings of a STIMULUS SIGNAL,
themselves are NOT of much use.
But there are ways to carry  reliably in zero-crossings (of other
related signals)
information about the temporal envelope and phase of a stimulus signal,
thereby implicitly, but completely representing  a signal.
This is our Main point.
Those familiar with speech signal processing  know
about what is called Line-Spectrum-Frequencies (LSFs)
originally proposed by Fumitada Itakura, which represent
the spectral envelope of a signal. These LSFs are used reliably and
successfully in speech coding, recognition etc. These are
indeed 'zero-crossings' that represent the
spectral envelope, except that these zero-crossings occur along
the frequency axis, instead of time axis. Thus, there is already
evidence
albeit in the other (frequency) domain that these
the zero-crossings are reliable.


On a lighter note, I asked Yadong Wang  (my grad student), two years
ago, to take a look at zero-crossings after reading your 1985 paper
in which you seemed to be saying that the auditory nerve signal IS based

on zero-crossings. (Jont B.Allen, "Cochlear Modeling", IEEE Acoustics,
Speech and Signal Processing Magazine,January 1985, p.3-28.)  Refer to
Figure 25
and Figure 26 in this paper. Quoting from captions of Figure 25:
"Based on the model of the haircell, we assume here that the information

is carried by the zero-crossings of the multitudinous narrow band
signals. This is because the hair cell cilia appear to act as a switch,
given moderate and high level signals, transforming the signals
to peak-clipped signal. In an infinitely peak-clipped signal the
the information is coded by the zero-crossings..."
It is heart breaking to see that you would abandon zero-crossings and us

midstream.

Rmadas Kumaresan
Yadong Wang

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Subject: Re: Get lost, Mr. Cochlea!! --- The Brain
From:    Jont Allen  <jba@RESEARCH.ATT.COM>
Date:    Tue, 27 Feb 2001 00:01:19 -0500

Yadong,

This is all very cute, and I dont want to be accused of not having a
sense of humor,
(clearly you do, and it is refreshing), but there is a thing called
masking.
Information is lost in the early auditory stages, due to neural coding.
The auditory nerve signal is not about zero crossing.  Even zero
crossing
are not exact, and would have jitter. But masking is NOT timing jitter.

The ear IS similar to a floating point converter. The ear does not have
an infinite
dynamic
range or signal to noise ratio. This limited dynamic range shows up as
masking.

Do you disagree?

Jont