Thomas Jelonek thomas at McRCIM.McGill.EDU
Wed Aug 7 01:49:45 EST 1991

In article <SLEHAR.91Aug3103428 at park.bu.edu>, slehar at park.bu.edu (Steve Lehar) writes:
> Feedback: For years it was a mystery  why the pathway from the lateral
> geniculate nucleus (first stage in the visual  pathway) to the primary
> visual  cortex (second stage)  is  actually  SMALLER than a reciprocal
> pathway  from visual cortex  BACK to   the lateral geniculate!  Indeed
> throughout the brain we see multiple  feedback  pathways.  What is the
> significance of these backwards connections?   Grossberg proposes that
> the  feedback allows for a resonant  matching  between lower level and
> higher level representations.  At each level of representation (within

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?  I know people have looked at cooling 
area V2 and examining its modulatory on V1. The result was inconclusive -
the greatest change being in the infragranular layers of V1 (response of V1
neurons increased with V2 being cooled and some neurons that were 
directionally selective in one direction became responsive to movement 
in both directions). In the supragranular layers of V1 the affect was
mostly to turn of the cell.  All together I believe only 1/3 of neurons 
were affected.

> each neural  layer) there are  certain  computational constraints that
> are   expressed within    that  layer   by excitatory or    inhibitory
> interactions.  For simple cells  in  visual  cortex, for instance,  an
> edge found at  one location at, say, 30  degrees, is inconsistant with
> an edge found at that SAME  location but a  different orientation, say
> 60   degrees.     The   cells   that  represent   these    conflicting
> representations experience a  mutually inhibitory relationship so that
> only one can  remain active even when both   receive some stimulation.

Why is it inconsistent to have 2 lines of different orientations at a 
point?  The visual scene is filled with such examples.


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