Steve Lehar slehar at
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
(O)((O))(((               slehar at               )))((O))(O)
(O)((O))(((    Steve Lehar Boston University Boston MA     )))((O))(O)
(O)((O))(((    (617) 424-7035 (H)   (617) 353-6741 (W)     )))((O))(O)

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