Thomas Jelonek thomas at McRCIM.McGill.EDU
Fri Aug 9 00:46:55 EST 1991

In article <SLEHAR.91Aug8005639 at park.bu.edu>, slehar at park.bu.edu (Steve Lehar) writes:
> > Do you  have    any references on  a  functional   significance  for
> > backprojections   in    the    visual    cortex?   I    believe   to
> > neurophysiologists there  is still a  great deal  of   mystery as to
> > their functional significance.  I am not familiar with the Grossberg
> > model,  but   do  you  have any  references  from   the neuroscience
> > literature?
> There are two models relating to this issue, one is by Gail Carpenter,
> who together with Grossberg,  worked out the Adaptive Resonance Theory
> (ART) [1].  This is an abstract computational model to demonstrate the
> principle of resonant feedback in neural systems.  You see,  there are
> [material regarding ART model deleted]

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.  Certainly not in any computational sense -- there are
a few hints from the perspective of neuroanatomy that feedback may be
a mechanism to link visual pathways and visual areas.  Was there 
any specific physiological findings that formed the basis for this 
model? Note, I am NOT saying that feedback between visual areas is
unimportant, rather that there is almost no physiological evidence to 
tell us what that functionality is.

> > Why is it inconsistent to have 2 lines of different orientations at a 
> > point?  The visual scene is filled with such examples.
> I am talking about  edge detectors  at  the very limit  of resolution-
> i.e. the smallest edges you can make out.  Now although the  middle of
> this "+" symbol has a vertical and a horizontal line, in order for you
> to  see this shape,  your visual  system must  be able  to discern the
> boundaries of the  entire   form,  i.e. your oriented detectors   must
> perceive a pattern something like this...
> 	     |  |
> 	     /  \
> 	----     ----
> 	____     ____
> 	    \    /
> 	     |  |
> 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 
discussing `the very limit  of resolution' are you talking about
hyperacuity?  What do you mean by your last sentence?  Surely there are
points on an image that carry no orientation information, whether you 
are operating at hyperacuity or not.

> I have a text file  describing Grossberg's vision model  , the BCS, in
> an informal and intuitive  way, I would   be happy to send a   copy by
> email on request.

I would be happy to receive a copy, thanks.


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