Basic Neuron Questions

John H. johnh at faraway.xxx
Fri Apr 11 00:17:18 EST 2003


Richard, Didier,

It's simple, it aint, ...

I've learnt enough about neuro to appreciate Popper's comment, "We may
differ in what we know but in our infinite ignorance we are all equal."

So then, recalling,

"Cortical neurons receive 3000-10,000 synaptic contacts, 85% of which are
asymmetric and     hence presumably excitatory...Absent any inhibition, a
neuron ought to produce an action potential    whenever 10-40 input spikes
arrive within 10-20 msec of each other...The neuron computes quantities from
large numbers of synaptic input, yet the excitatory drive from only 10-40
inputs,     discharging at an average rate of 100 spikes/sec, should cause
the postsynaptic  neuron to     discharge near 100 spikes/sec.  If as few as
100 excitatory inputs are active (of the 3000 available)    the postsynaptic
neuron should discharge at a rate of 200 spikes/sec.  It is a wonder, then,
that the    neuron can produce any graded spike output at all."

Shadlen, M.N., & Newsome, W.T. 1998.  The variable discharge of cortical
neurons: implications for               connectivity, computation, and
information coding. Journal of Neuroscience, 18, 3870-3896

"The role of neurons in these computations has evolved conceptually from
that of a simple integrator of synaptic inputs until a threshold is reached
and an output pulse is initiated, to a much more sophisticated processor
with mixed analog-digital logic and highly adaptive synaptic elements."

Koch, C., & Segev, I. 2000. The role of single neurons in information
processing. Nature Neuroscience,  33,  Suppl: 1171-1177.

Now I have no hope of understanding this and will gladly leave it to you
bods. Please feel free to elaborate. I'm an amateur, be gentle with me. On
the other hand, if you want a fight ... .



John H.

"r norman" <rsnorman_ at _comcast.net> wrote in message
news:incb9vkhq5unqqc48ed611939f8g25hp39 at 4ax.com...
> On Thu, 10 Apr 2003 15:44:25 +0000 (UTC), "Didier A. Depireux"
> <didier at rai.isr.umd.edu> wrote:
>
> >John H. <johnh at faraway.xxx> wrote:
> >> Just for once, could someone say, "it's really quite straightforward."
I
> >> wonder how often lecturers in neuro related jazz feel threatened by
their
> >> students ... .
> >
> >So in a sense "it's really quite straightforward." The way you phrased
your
> >question prevented me from saying that!
> >
> >What I mean is that, people have been arguing about whether it's a time
code
> >or a rate code. And the truth seems to be a lot simpler, and neither rate
> >nor time codes. It's in between, one spike per relevant time-scale.
> >
> >If an engineer had been asked to design a system using spikes to
communicate,
> >he would have chosen either a rate or a timing code of sorts. The system
> >designed by evolution is both more complex (because the relevant
time-scale
> >will depend on what is being encoded) and a lot simpler (you can use the
> >same coding scheme for any sensory input). It's economical (precise
timing
> >requires a high metabolic rate, population code requires a lot of
neurons)
> >and quite flexible.
> >
> >I don't know your background, John. But maybe you don't appreciate how
very
> >very little we actually know about the brain. Our current brain research
is
> >to a large extent like butterfly collecting. We arrange the facts we
observe
> >according to their colors, size, shape, and put them in different boxes.
But
> >at a fundamental level we have no idea.
> >
> > Didier
>
> Sorry to jump in so late on this thread, but there is a certain noise
> level here that makes it very difficult to follow the few posts that
> do actually  make sense.
>
> Didier's comments are quite sensible, but I disasgree that 'it is
> really quite straightforward."  The problem is, as Didier says, that
> the nervous system was not actually designed -- it sort of just came
> out that way by evolution (the "intelligent-design folks
> notwithstanding).
>
> It is easy for an engineer to take things like look sort of like nerve
> cells and create at least conceptual schemes where binary data can be
> transmitted by the details of timing in a spike train.  The problem is
> in showing that anything remotely like that actually happens in a
> flesh and blood nervous system.  Even putting aside the conceptual
> difficulties of how to do it, the experimental difficulties of finding
> a preparation on which to test any hypothesis and then actually carry
> out the work is currently out of the question.
>
> Certainly there are specific cases where timing of nerve activity,
> even to the sub-millisecond level, is critical.  And certainly there
> are cases where a few interpolated spikes in a relatively steady train
> of a constant frequency can cause large changes in response through
> short term facilitation. But as a general rule, does critical timing
> really count or is only a rough running average of frequency count?
> For now, the latter seems like the usual way of coding.  Anyone who
> suggests otherwise has a heavy burden of proof to show that the timing
> mechanism is actually a general phenomenon in real, live brains, not
> in conceptual models.
>
>





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