slehar at park.bu.edu
Sat Aug 10 09:56:25 EST 1991
> In your original posting you implied that backprojections in the
> visual cortex influenced the functionality of the computational models
> you described. However, my impression is that in the neuroscience
> community there is very little consensus regarding the functionality
> of feedback in the visual cortex.
> Was there any specific physiological findings that formed the basis
> for this model? Note, I am NOT saying that feedback between visual
> areas is
This model is based primarily on visual illusions and other
psychophysical findings, rather than on detailed study of
neuroanatomy, i.e. the model is confirmed by successful reproduction
and prediction of visual illusions. The structure of the model is
however consistent with known neurophysiology, and there has even been
some neurophysiological evidence for some kind of feedback, in that
illusory contours (non-existant edges that everyone sees in certain
figures) have been linked to signals in simple cells. How could the
simple cell fire to a nonexistant edge whose illusory existance could
only be determined by a more global view of nearby edges? This seems
to indicate a feedback interaction between the local simple cells and
the more global complex or hypercomplex cells.
> > | |
> > / \
> > ---- ----
> > ____ ____
> > \ /
> > | |
> > at the center of the "+", i.e. each line is made up of a pair of
> > edges, one light-to-dark, and the other dark-to-light. At this scale,
> > there are no points in the image without a specific orientation.
> I am confused. Why are you talking about edge detectors when you want
> to represent lines? Why can I not talk about receptive fields at coarsely
> quantized orientations that are selective for lines, i.e. inhibition on
> both sides? I would represent a curve by its local tangent estimate.
> Your `+' would be represented by two neurons each selective for lines
> at different orientations firing simutaneously. Also, when you are
Let me try this again. If you look at my figure above, each "-", "/",
"\" and "|" represents a single receptive field. At every point of
the image the choice is between "-\|/", and there is no point on the
image where a "X" or "+" receptive field would be more appropriate
than one of the "-\|/" field.
What you seem to be talking about is a "+" figure that is so small
that at the center crossing point it is covered by a single receptive
field. This figure would be so small that it would be unresolvable.
You are trying to resolve a form that is smaller than a single
receptive field! The only appropriate response to such a figure would
where the "o" represents ambiguous orientation. This is an
appropriate representation because at the very center of the cross
there is no locally detectable orientation- we only infer such
orientations on the basis of the nearby arms of the cross, i.e.
hyperacuity, and the central point would be just plain black. Now
according to this model, the nearby arms would influence the ambiguous
signal to become a "|" and a "-", and if each of these signals is
equally strong, then a "+" will result ("|" AND "-"), but
psychophysical evidence shows that these orientations compete, such
that any imbalance between the orientations will produce a stronger
local signal for one than the other.
> I would be happy to receive a copy, thanks.
I am sending you a copy now- maybe this will clear up some of your
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(O)((O))((( Steve Lehar Boston University Boston MA )))((O))(O)
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