Visual Processing and Visual Fields
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The act of seeing involves many separate processes and many different
parts of the brain, including the lymbic system and the cerebral
Most visual processing occurs in the occipital lobes of the cerebral
These lobes are located at the back of the skull.
Because so much visual processing is done here, this part of the
brain is often called the visual cortex.
Visual processing starts when light
enters the eye through the lens, which inverts the image.
Information that was on the viewer's left strikes the right side
of the cornea and is upside-down as well. Likewise,
information from the right
side of the scene goes to the left side of the eye.
Each eye sends information on the entire visual scene to the
optic chiasmus, an X-shaped bundle of nerves behind and between
the eyes (see the illustration on the left). There a remarkable thing
happens. All the information that was on the left side of the scene
gets shunted to the right cerebral hemisphere, where it
is processed by the
right occipital lobe. All the information from the right side of
the scene gets shunted to the left.
The occipital lobes do a great deal of processing of the information
they receive. Because there is so much information to process,
different parts of the visual cortex perform different parts of the
analysis. However, the tasks are divided in surprising ways.
There is one set of nerves for each possible orientation
of a line, for example. A separate set of nerves detects
corners and edges. Other parts of the visual cortex process color,
others detect motion, and still others
assemble lines and edges into simple shapes.
The neurons of the cerebral cortex pass their separate analyses
of the image on to other parts of the brain in the parietal
and temporal lobes, where they are assembled
and integrated with memory and emotion into a meaningful scene.
On the whole, it is a remarkable system, full of redundancies,
cross-checking, and cooperation among separate parts of the brain.
However, when the occipital lobes are damaged - by trauma, disease,
or stroke - the process breaks down. The eyes may work perfectly,
as may the assembly areas in other parts of the brain. But visual data
gets lost if it doesn't get properly processed by the visual cortex.
Brain Trauma and Visual Field Cuts
The right occipital lobe in the illustration
on the right has been damaged by a stroke.
Strokes often affect one hemisphere and not the other.
The person who survived this stroke will have difficulty
processing the left half of the visual field.
This is called a "left field cut."
He or she
does not see the word "liabilities" shown here,
but the word "abilities." The "li," which is "seen" only
by the right hemisphere, simply disappears.
The stroke survivor
is often completely unaware of this problem. The brain, being a
wonderful inventor of reality, constructs a "meaningful"
scene from the data that does get through, and never alerts
the viewer that there's something missing. That is, not until
something surprising and often unpleasant literally "comes out
of left field."
Visual Field Quadrants
The brain can repair itself to some degree. When a damaged
visual cortex recovers, it is often by subfields.
Each side of the visual field
is subdivided into an upper, middle, and lower "quadrant",
as shown on the left. (The term "quadrant" is somewhat misleading,
since there are six subfields, not four.)
As a person recovers from a stroke or other brain trauma, one or more
of these quadrants may recover independently of the others.
For example, if the lower left quadrant recovers, the person may be
able to see the number of fingers extended on a hand presented
below chin level. However, if the middle and upper quadrants have not
recovered, that same hand will "disappear" if it moves up
to nose level or is presented above the top of the head.