thomas at McRCIM.McGill.EDU
Thu Aug 15 03:40:29 EST 1991
In article <SLEHAR.91Aug10105625 at park.bu.edu>, slehar at park.bu.edu (Steve Lehar) writes:
> > 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.
I assume you are referring to the work of Peterhans and von der Heydt who
found cells in area V2 (so can I assume that when you refer to feedback that
you are invoking horizontal connections within V2 as opposed to
backprojections from a `higher' area?) of macaque that responded
to `illusory contours'. However, I believe they also found that the
response time for these neurons was so fast that they ruled out any iterative
process. How do reconcile this? I am not saying I agree with them -- a
process that required 2 or 3 iterations might still be consistent with
You last sentence is puzzling. Why do you consider simple cells local and
complex cells global (note that the Hubel and Wiesel notion of hypercomplex
cells does not exist rather one can have simple cells and complex cells that
are either singlely or doublely end-stopped)? In terms of classical receptive
field they are both local operators. And both of their responses can be modulated by a surround as shown by: Blakemore, Robson, Allman, Van Essen etc.
> > 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 argue that any representation of a `+' would require an explicite coding of the discontinuity in orientation between the 2 lines. The
mechanism to achieve this would be having 2 orientated cells firing
simutaneously -- there is no ambiguity in orientation we simply require
multiple orientation labels at a point.
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