[Martin Taylor 2011.11.03.23.03]
[From Bill Powers (2011.11.03.1910 MDT)]
Rick Marken (2011.11.03.1210) --
RM: Thanks. But I
don’t see how
this is relevant to Martin’s question. I_think_ Martin is suggesting that perceptual signals that are
the
output of a perceptual function (which has a vector input) could itself be a vectors: the perceptual signal would be a vector.
BP: Yes, that's the implication: output vector = matrix times
input
vector.RM: I was just saying
that that
may be true but I can’t think of how to usesuch a signal in a control model and besides there is no data
that
seems to require that kind of change in model
architecture.
BP: There's no mystery -- we just treat the matrix operation as
the sum
of a lot of scalar operations, which is how matrix algebra is done
anyway. The matrix notation is a lot more compact than writing out
all
the details, but you get the identical result either way. Did you
look at
those two figures, 3.11 and 3.12, in B:CP?RM: Your
multicontroller is a
whole bunch of control systems eachcontrolling a scalar perceptual variable. I don't believe this
is
thearchitectural alternative Martin was suggesting.
BP: Sure it is. Look at the source code: all the operations that
would
appear in a matrix and vector treatment are there, but just
spelled out
in detail rather than relying on the systematic cycling of indices
and so
on that makes the matrix treatment easier to keep track of without
making
mistakes.Suppose we have three linear equations in three unknowns, x1, x2,
and x3,
the values of the three expressions being y1, y2, and y3. We can
writey1 := a11*x1 + a12*x2 + a13*x3 y2 := a21*x1 + a22*x2 + a23*x3 y3 := a31*x1 + a32*x2 + a33*x3 There are all the operations that go into these three equations.
If we
want to treat the equations separately, each one is simply a
scalar
equation. But we can also write__Y = A * X __ Where **Y** and **X** are vectors and **A** is the
matrix of
coefficients. Now we can do matrix algebra with this single
equation, but
it means exactly the same thing as the three equations above. If
you
expand the matrix equation you get the three scalar equations
back.In a somewhat different form, those could be three equations for
three
control systems sharing a common environment, each sensing the
same set
of environmental variables and acting on all of them. The y’s
would be
perceptual signals. That collection of control systems could be
written
in matrix notation in a similar way, with Y being a vector
standing for the three perceptual signals. X would be the
vector
of three reference signals, and there would also be vectors of
three disturbances and three environmental variables. Three, that
is, if
we want a fully-determined system.I happen to prefer the scalar way of writing (and programming) the
equations, because I get a better picture of what’s going on when
I can
see all the details. The matrix notation hides everything
interesting.
But sometimes, when dealing with fairly large and complicated
systems, I
try to use matrix algebra simply because it’s been boiled down to
some
simple rules which, if I follow them carefully, pretty much
guarantee
that I won’t make mistakes. Often the choice is a toss-up – when
I get
confused about subscripts I’m as likely to make a mistake in
implementing
the matrix algebra as I am in using the scalar version. But the
matrix
algebra can be very useful, as in that program I just posted in
which the
number of systems involved is adjustable and can be fairly
large.In short, the scalar/vector choice is simply a choice between
representations, and has nothing to do with the underlying
physical
architecture.Ok, Taylor and Kennaway, I await your judgement of all this with
trepidation.
I have no issue with the mathematical side of this, nor with your
final comment that it “is simply a choice between representations”.
That’s all independent of the question that I was originally puzzled
by, and that I am beginning to solidify in my mind, as you may have
seen in my last message to Rick.
The possibility of a largely distributed representation of many or
most controlled variables ties in with quite a lot of other things
that at first glance may seem quite independent about which I have
been thinking and corresponding with other people off-line. That’s
why I keep trying to rephrase my question so as to help make it more
clear both to me and to those who may read my meanderings. I don’t
plan to introduce those other topics here, at least not yet. I just
want to be satisfied as to whether there is any engineering or
physiological gotcha that prohibits a controlled variable being
represented in a distributed manner across the brain. The components
of the distribution may well be scalar, but the issue is whether it
is necessary for any or all of those scalar quantities to correspond
to what out conscious perception would consider a unitary property
of the environment.
Martin