[From Bill Powers (960515.0500 MDT)]
I am very tardy in acknowledging and passing on some shocking news that
Gary Cziko told me about a week ago: Donald T. Campbell has died.
Those who have read the dedication of my '73 book know that Don Campbell
was a primary source of encouragement and practical help to me. He
persisted until he got me to write and finish the book and he helped me
find a publisher for it. He was a true scientist. He did not care what
you thought or said or what your status in life was; if you were doing
science he granted you the unconditional right to be heard and
respected, quite independently of what he himself believed. I have never
known anyone with such a clear vision of the great art through which
humanity explores the world. I am acutely sad about his sudden and
permanent absence.
Peter Cariani [960514.2000) --
Your reply to Ellery Lanier was helpful to me, too. It reminded me of
one aspect of your approach that had slipped my mind, your concept of
"multiplexing." I can see why this concept would be useful in building a
model of the type you want to build, but of course that sort of goal-
driven postulate is to be considered most carefully; one has to ask just
how badly one wants a model that is heterarchical, anarchic, etc. -- and
why.
For some reason your explanation, this time, helped me to see a way in
which our concepts are basically similar (despite the above). You are
seeing multiplexing as analogous to time-sharing or parallel frequency
channels, in which a single signal can carry multiple channels of
information. I would like to suggest that this is an unnecessarily
literal transfer of technology from electronics to neuroscience.
However, we are talking about the same phenomena and in some ways
proposing similar ways for the nervous system to deal with it.
Your description makes it seem as though there is some mechanism which
systematically creates independent codings of various aspects of the
basic input data and superimposes them in a temporal pattern of some
sort. That is what a multiplexer would do. The basic idea of
multiplexing assumes that the information to be sent in the multiplexed
channels is separate to begin with, and is then combined in some orderly
way that provides for later sorting out of the various messages when
they reach their destinations. At the destinations, presumably, the
appropriate inverse operations would be carried out, to demultiplex the
combined messages as appropriate to their various destinations.
It is, of course, quite possible to design a system like this. But to
design it, you have to set up a specific multiplexing-demultiplexing
algorithm, and a coding system, so the compression processes at the
source correspond properly to the decompression processes at the
destination(s) and the decoding corresponds to the coding. In other
words, there has to be a kind of master design behind the whole process,
which is set up to assure that it will work. And you have to start with
a set of separate messages.
Suppose, however, that there are no separate messages in the environment
waiting to be transmitted into the nervous system: no frequencies, no
pitches, no timbres, no phonemes, and so forth. Suppose there is simply
a total physical process going on, which is sampled by the sensory
receptors of an organism. What we end up with is a set of input signals
that, just as you suppose, carry all the information that the organism
can obtain about the world outside it. But there is no multiplexer,
because there are no separate messages already sorted out in the
environment and waiting to be transmitted to destinations in the brain.
Instead, what we have is a set of neural signals, emitted by sensors, in
which there is an enormous potential for finding order. That there is
order to be found is a basic postulate of science, but the order does
not spontaneously reveal itself. It must be constructed and extracted.
It's instructive to compare systems with different capacities for
finding order. When I say the word "telescope," my ears and my cat's
ears both generate a fairly similar set of neural signals. If we
examined those signals immediately after they are generated, we would
find complex waveforms. But somehow I hear the word "telescope," and my
cat does not (I hope that my point will not be obscured by factual
quibbles about what cats actually hear).
The point is that in the basic neural signals is all the information
that any nervous system could possibly extract, even a nervous system
far more competent than the human one. But we do not extract all that
information, nor do we all extract the _same_ information. What we
extract depends, to be sure, on what is there, but it depends even more
critically on the mechanisms we possess for extraction.
In my model, perceptual signals are generated by computing functions
that receive sets of input signals and produce output signals that vary
as a function of the input variations. The meaning of the output signals
is determined by the form of the computing function. For example, a
computing function that sums its inputs create a signal that is
invariant with respect to the amplitude of the individual inputs, as
long as those inputs vary in a way that leave their sum constant. A
computing function that is tuned to respond maximally to variations in
its input at a specific frequency creates an output that is invariant
with respect to which inputs are varying, as long as the result is a net
variation at the specific frequency. This could also be viewed as a
function that reports the presence of a predominant interval between
input events.
I see I'm going to have some trouble with this next part, but here goes:
Now, what is the difference between saying that the input signals
contain multiplexed channels carrying pitch, intensity, and timbre
information which is then demultiplexed by a receiving system, and
saying that there is an input function which computes a perceptual
signal representing pitch, intensity, or timbre on the basis of the
input signal? There are some very sharp differences, but in either case
we are saying that a perceptual signal results from extracting a certain
kind of information from the input data.
In the Cariani system, the same basic input data, multiplexed, can be
passed to any systems in the brain, so that many systems could
demultiplex the same subchannel, for different purposes. Is this
fundamentally different from saying that the first system which
constructs a perceptual signal out of the basic input also passes that
signal on to higher systems? There is a practical difference, of course:
in my system, the "demultiplexing" is done only once, with the resulting
signal being available as inputs to other systems. But in terms of
information available to higher systems, there is no difference. The
same information gets to the higher systems.
The basic difference between our schemes is that I do not assume pre-
existing information channels in the signals reaching any system. That
is, I do not assume that the world as represented at any level was
sorted out into "messages" before entering the nervous system. I do not
assume any _given_ partitioning of the external world into familiar
concepts. The multiplexing concept assumes that there are specific
identifiable kind of information already sorted into the various
subchannels, so that the partitioning is objectively established prior
to perception. I think that some strong arguments can be mounted against
this idea.
I see that I'm going to have to try again later. There's a very clear
idea lurking in here somewhere, having to do with alternate ways of
interpreting the same input information, and with the fact that
different kinds of information are the business of different levels of
interpretation. But perhaps if the idea were really clear, I would be
able to state it. This calls for a little nap to finish out the night.
ยทยทยท
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Best to all,
Bill P.