Visual Field Tester: Visual Prcessing
Exploring Visual Processing and Visual Fields

Visual Processing and Visual Fields

The FieldTester User Interface

Running the FieldTester Applet

References, Links, and Credits

Visual Processing

The act of seeing involves many separate processes and many different parts of the brain, including the lymbic system and the cerebral cortex. Most visual processing occurs in the occipital lobes of the cerebral cortex. 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.
Lou Ceci
Glyphic Technology
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