Norman Allan
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chapter four, part two


Analysis of Visual Information
in Mammalian Neural Networks

                                                   retina.gif (112716 bytes)
In the eye,                     
in the retina,
we see three
of neurons
and we could
consider these
to be three
of the visual field: ~.

first:        rods and cones

second: "intermediary level"

third:    : "ganglion level"

emrod.gif (98551 bytes)rod.gif (32082 bytes)

The first order of
neurons are the rods
and cones themselves
and these give us
a pixilated point by point
mapping of the visual field
(with some "coding" for colour:
which we'll ignore for now)

images/nret.gif (46867 bytes)                                               
The second neural level
(which is again in the retina)
is formed by the "bipolar"
and the "horizontal cells"
and these are processing
information so that at
the third neural level,
the "ganglion cell" layer,
information is organized
as "surround contrast" fields
- that is to say if you shine
a light at the centre of its
own tiny little visual field
it will increase (or decrease)
its rate of firing, but if you
shine a light on the
periphery of its field it will do the opposite, ngraph.gif (19800 bytes)
decrease (or  increase), its rate of firing. 
The up-shot is that the ganglion cells
responds to points of light, or to edges.
(but not to a solid block of light extending across its visual field).
hubelgang1.jpg (12802 bytes)
     from Hubel's "The Visual Cortex of the Brain" Scientific American November 1963

So in the "ganglion cell" layer of the retina             
we again have a representation of light
in the eye's visual field, but it has been
processed through "surround contrast"
to highlight spots  edges, contrasts.
And of course the two synaptic relays
through which this processing has occurredngen.gif (21266 bytes)
takes time.  So we have iterations
of the visual fields falling down time.

In the forth order neuron, in the lateral geniculate bodies (LGB),
visual information is processed to
integrate information from the two eyes,
depth parallax stuff, stereoscopic vision,
and is organized in six overlapping leafs (visual fields)
three from the contralateral eye, leafs 1, 4 and 6;
three from the ipsilateral eye, leafs 2, 3 and 5.

The visual fields are again contrast surround fields
though rather larger than those of the ganglion cells.

The 4th order neurons of the LGB
project through the visual tract
to cells in the occipital visual cortex.
occipital visual "area 17" in the cat  
   npath.gif (35226 bytes)

In the cortex fifth order neurons   nvpath.gif (20811 bytes)
that Hubel and Weisal call "simple cortical cells"
respond to bars and edges in the visual field.
Simple cortical cells receive input
from 4th order neurons (contrast surround fields)
arranged along a linear axis in the visual field.
hubelsimp2.jpg (25582 bytes)
from Hubel's "The Visual Cortex of the Brain" Scientific American November 1963

In area 17 there are also
sixth order neurons
that Hubel andWiesal call
"complex cortical cells"  hubelsimple1.jpg (22586 bytes)
that are responding
to moving bars or edges,
which we may imagine
are wired from the "simple cells"

The illustration of "simple cortical cells" to the right is from Hubel's
"The Visual Cortex of the Brain" Scientific American November 1963

Cortical area 17 is arranged in columns
such that all the cells (simple and complex)
within the column are responding
to bars and edges with one particular orientation,
as is illustrated below.

Beyond area 17 our simple and complex cells appear to be projecting to occipital visual cortex area 18, though the visual field here is flipped (vertically).   What properties in the visual field that cells in visual 18 are responding to is not known.

we should of course note here
that the cortex too is layered

What is germane to our discussion
is the fact that we have all the successive iteration of the visual field, each a little different as the information is processed, each a little delayed (by synaptic relay), and no doubt, all of them resonating together.


Chapter 4: neural nets


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                      hubelcol.jpg (14945 bytes)