[From Bill Powers (961024.0910 MDT)]
Bill Benzon --
As you know, lots of attention has been
given to just how one can see lines and edges and objects and such, how a
perceptual system can construct such things out of the flow of physical
energies which impinge on the sensors. I'm interested in what kind of
account we need to explain why/how we experience the world we do given the
But, in what sense are you interested in "the way we experience the world."
Do you want to do some Husserlian phenomenology based on introspection?
energies available to our sensors.
No, I'm talking about _extrospection_. Looking at the world that you see
around you. Any theory of perception has to explain why that world appears
as it does, in present-time perception. No matter what else a theory says,
if it implies that you can't see edges and lines, it's wrong. You can.
I don't understand the thrust of your initial question. Hubel & Weisel see
lines and edges, you do, I do. What follows from that? Certainly we don't
see Fourier transforms of visual objects [unless you're looking at the
appropriate technical articles --
That's what I'm talking about. Hubel % Weisel found places in the brain
where there are cells that output a signal when a line of a specific
orientation is seen. That signal is a perceptual signal representing that
line (or that orientation). On the way to generating this signal there may
be all sorts of intervening neural mechanisms that do things like
autocorrelations or Fourier transforms or anything else you might like to
conjecture about, transforming an array of point-illuminations on the retina
into this perceptual signal. But you don't perceive a line in a particular
orientation unless this signal is present.
(Actually, the perception of a line per se probably happens at a lower
level; strictly speaking, what Hubel and Weisel found were _orientation_
signals. A line is recognized as a line-object regardless of its orientation).
More later, got to take car in to get snow tires on. Big snow coming.
Best,
Bill P.
Bill Powers (961024.0910 MDT) sez:
No, I'm talking about _extrospection_. Looking at the world that you see
around you. Any theory of perception has to explain why that world appears
as it does, in present-time perception. No matter what else a theory says,
if it implies that you can't see edges and lines, it's wrong. You can.
Sure. I never implied any such thing.
That's what I'm talking about. Hubel % Weisel found places in the brain
where there are cells that output a signal when a line of a specific
orientation is seen. That signal is a perceptual signal representing that
line (or that orientation). On the way to generating this signal there may
be all sorts of intervening neural mechanisms that do things like
autocorrelations or Fourier transforms or anything else you might like to
conjecture about, transforming an array of point-illuminations on the retina
into this perceptual signal. But you don't perceive a line in a particular
orientation unless this signal is present.
H&W's detectors where, I think, in "primary sensory cortex." There's not
much, if anything, going on between them and the LGN. What I'm saying is
that those particular cells are right smack dab in the middle of a region
that's calculating autocorrelations or Fourier transforms over LGN input
and that the signals from those cells are bits and pieces from somewhere in
that process.
Those cells are nowhere near a "language area" which might "emit" the words
"line" and "edge." One point of my Hubel-Wiesel-Husel thought experiment
is that those words are "attached" to something else. Perhaps (here I'm
following a line suggested by John Platt years ago) the eye tracks along
the edge of the stimulus figure (which happens to be a short straight line
with, of course, some non-zero thickness) and the word "edge" is associated
with that particular scan path (and not directly with the low-level
perceptual analysis which, however, might play a role in regulating the
tracking behavior).
Snow, already? Ah, the Rockies.
later,
bill b
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William L. Benzon 518.272.4733
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