From: Martin Striz (metastriz@yahoo.com)
Date: Tue Feb 01 2005 - 17:59:07 MST
--- "Eliezer S. Yudkowsky" <sentience@pobox.com> wrote:
> Phil Goetz wrote:
> > The problem is harder than is suggested by the 200Hz
> > figure. That is a spike-firing frequency.
> > In many cases, we believe info is encoded in the
> > firing rate,
>
> I have previously disputed this assumption, not least because of its
> consequence. (Ten steps is just ridiculous...) Neurologists can easily
> count spikes. Decoding information is much harder. Thus, I suspect that
> many complex encodings are reported in journals as "firing rates" because
> that's what neurologists know how to measure and they don't know what's
> being computed, let alone how to decode it. For some brain areas, our
> knowledge of neurobiology may be equivalent to measuring the "firing rates"
> of transistors in a computer with no idea of what the transistors encode.
> In other areas, especially vision, we have some idea of what is being
> computed and so we can do better than that.
Neurons "compute" it by having protein complexes with specific voltage-gated
thresholds for activation. In the case of the neurons that they're describing,
if you watch a trace of the voltage potential across the membrane in the
dendrites, it drops off more slowly than in other neurons, so that a rapid
succession of spikes is additive, and it takes several spikes (the exact number
varies) for activation. I'm not sure what the reason for this is, whether it's
impossible to do with the classic neuron model, or just needless
complexification. I can propose a mechanism for why it exist: some neurons
act as signal-unifying centers, so that the brain deems a particular set of
signals worthy of a response only when a sufficient number of signals from
disparate areas comes in.
Martin
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