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From: "Sergio Navega" <firstname.lastname@example.org>
Subject: Re: Analog vs Digital (reposted)
Date: 28 Jan 1999 00:00:00 GMT
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Neil Rickert wrote in message <email@example.com>...
>>The point is that the output is not an analog value. It is the same
>>whether the threshold is exceeded by a lot or just a little, and
>>nothing when the threshold is not exceeded. It is a discrete event, a
>>signal that whatever the cell was looking for has occured. It is a
>>binary digital signal, rather than a proportionate analog signal.
>I think you should hesitate before jumping to such conclusions. The
>current state of knowledge is such that we really cannot be sure
>whether neurons are digital devices or analog devices. We do know
>that they react about as you described. But we really don't know for
>sure how they represent information. If the information is in the
>firing rate, rather than in the event of firing, then your
>assumptions might be seriously off.
I agree partially with Neil here.
I would like to add that today there are at least two currents
regarding how information is represented by neurons (commonly
known as "neural code problem"). On the first, researchers study
individual neurons and try to find probabilistic and statistical
models of information coding, with some successes obtained mainly
in the neurons more close to sensory inputs. I guess this is
Modlin's preferred position.
The other group is studying the behavior of populations of neurons,
where aspects such as synchrony are more relevant. The spikes produced
by a single neuron, then, are irrelevant when compared to the
operation of the ensemble that this neuron is a part of. Although
there's a chasm between these two approaches, and although the first
approach is more traditional and more substantiated (the latter is
more recent), there's a growing suspicion that the second model
is more cognitively plausible.
So things about neurons are, unfortunately, unsettled. If I had
to choose between the two approaches, I would pick the latter,
because it can, finally, provide a plausible "bridge" between
low level aspects and high-level cognitive functions.
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