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href="http://serendip.brynmawr.edu/bb/neuro/neuro98">Biology 202
1998 Second Web Reports
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Strabismus

Jeremy Hirst

A camera lens focuses patterns of light onto film which records the image exactly. If the lens is out of focus or partially covered, a b lurry or obscured picture will result. The film is a recording device, it does not interpret and select what it portrays. Images from a camera are objective in a very literal sense. Seeing, however, is not such a seamless process. Our eyes work similarly to a camera in that they have a lens which focuses a real image on our retina, a light sensitive sheet of cells. This retinal image is a portrayal of the world as it truly is. The image which we see, however, is not this image. By considering a normal vis ual property as well as an uncommon ocular disorder the process of formulating our visual sense will be investigated. There is a difference between the picture recorded on film and that recorded by our brains. For purposes of this paper, the term "retina l image" is used as an analogy to a photographic image (one without interpretation by the brain). The phrase "brain image" refers to the retinal image post-brain interpretation. The brain image is the image which would be described by the person, the imag e which is thought of as seeing.

This phenomena of brain interpretation is easily demonstrated by discovering one's blind spot. (A good self experiment is described at 1) The blind spot results from an area of the retina which does n ot have photoreceptor nerve cells, the optic nerve head. Yet, even without this seemingly vital information, the brain is able to supply us with a complete brain image. The brain has filled in the blind spot with an image which it believes makes sense. Ha ving explored the blind spot, one can understand that what is captured in the brain image may be very different from what is truly present as the retinal image.

Another less common example of the way the brain interprets the world is what occurs in a condition called strabismus or crossed eyes. As defined by the American Academy of Ophthalmology (2) strabismus is a visual defect in which the eyes are misaligned and point in different directions. One eye may look straight ahead, whil e the other eye turns inward, outward, upward, or downward. You may always notice the misalignment, or it may come and go. A second disorder, amblyopia, commonly goes along with strabismus. Again, from the American Academy of Ophthalmology, Amblyopia is p oor vision in an eye that did not develop normal sight during early childhood. It is sometimes called lazy eye. (7) The reason these two conditions are so closely linked relates to how the brain interprets the information received from th e two separate eyes.

One cause of strabismus is the inadequate coordination of the six muscles which operate to move the eye. Other less common forms of strabismus result from defective refraction, damaged nerves which connect the photoreceptors to the brain, and damage to the nerves which control the eye muscles.(4) These multiple disorders all have the same effect, presenting conflicting, amblyopic, images to the brain.

Normal vision occurs by a coordinated synthesis of the retinal images into a single brain image. If, however, one of the eyes does not transmit a coordinated or useful image the brain may choose to ignore this image when conducting its synthesis. The region of the brain which is thought to first bring the two retinal images together is the visual cortex (VC). The VC has cells which grow based on visual stimuli into regions called ocular dominance columns. At birth the VC and its ocular dominance columns are not completely established. If there is a problem wi th one eye that prevents stimuli from reaching, or being utilized by the brain then these columns do not develop, leaving the eye functionally blind.(5)

A lazy, or wandering eye, results when the retinal images from one eye are not i ncorporated into the brain image and the eye is free to drift. If the VC did use the two images the child would see double, because the images would not fit together properly. Instead, the VC learns to only use one eye, the eye with better alignment or be tter vision. As a result the ocular dominance column for the misaligned eye is not developed, the cells never mature and grow into an operating unit. Thus, if treatment is not begun before the VC fully develops the problem eye will never be used for norma l vision. (7) (6)

A misaligned eye will look anatomically perfect upon inspection. It will have all the proper components to gather and transmit focused retinal images to the brain, yet these images are not used. Th is observation has led researchers to conclude that these vision problems stem from faults within the brain. It has been shown with animal models that if the VC does not receive adequate input from the eye during development, the cells which would be used to process these signals are not connected. An interesting example of this is comparing childhood cataracts (clouding of the eyes lens) with adult cataracts. With surgery the cloudy lens can be replaced with a new, clear lens, restoring the eye to an ana tomical condition which should allow it to see. With children who do not receive this surgery until after their VC has fully developed the eye is still unable to see. Adults, however, can see again once the surgery is complete. The difference in recovery is due to the VC. In children it developed without the ability to use the eye, because the signals coming from it (cloudy or non-existent) were not useful and thus ignored. Adults already had a fully developed VC when their cataracts developed and thus on ly had to regain anatomical use of their eye again, so the VC could once again use the clear signals coming from the fixed eye. (6)(3) (5)

This accommodation by the brain to generate a sensible brain image appears to be related to the blind spot phenomena. The brain fills in missing information from the blind spot and ignores the information coming from the defective eye. Both of these actions are done by the brain to formulate a sensible, useful ima ge of the world around us. In some way the brain chooses what the world is and presents only this interpretation as the brain image.

Studies have found that if strabismus is not treated in early childhood the normal use of one eye will be permanently lost. This seems to imply that the brain can learn to use the poor eye for this period of time, after which it has fully developed its pattern of generating vision. (7) (6) (3) Treatment of strabismus ut ilizes this characteristic of the brain, that it can develop in such a way that it learns to use the bad eye. By putting an eye patch over the good eye the young childs brain will be forced to use the bad eye for seeing. The retinal images from the bad ey e, even though they may be confusing, will be used by the VC. In this fashion the early plasticity of the developing nervous system is taken advantage of. As the ocular dominance columns are being developed the path which would have been ignored is forced to develop. Adult onset strabismus, however, results in double vision because the brain can not simply ignore the one image that does not make sense. This type of strabismus can not be treated with patching, as their brains have already developed a patte rn which uses both eyes to comprise the brain image. (4) (5)

This exploration of strabismus has shown that the brain is a very active participant in determining our perception of the world around us. Visual informati on received by the eye is only part of the story, the brain takes these images and selects what is useful, discarding (strabismus) as well as adding (blind spot) information, thus, constructing our visual experience.

List of Web Sites Used in Th is Paper:

1. Blind spots Explanation and easy self experiment to explore the blind spot.

2. Strabismus FAQs Defines Stabismus in easy to understand terms, discusses how normal eyes work together, compares to how misaligned eyes are used to see, diagnosing strabismus, and treatment.

3. Smith Kettlewell Inst/Takuji Kasamatsus Lab A scientific type paper which covers various projects on vision, talks about the cellular level manifestations of strabismus.

4. Crossed Eyes (Strabismus) Good extensive yet general sight on strabismus, sketches of the disorder.

5. The Urgent Need to Use Both Eyes Substantial s ight on strabismus with a graphic. Neurological components discussed. Many links to other sights on the senses.

6. Amblyopia A letter from a person with amblyopia, and a response fro m a doctor.

7.Amblyopia FAQs Defines amblyopia in easy to understand terms, offers information on the development of normal vision, discusses various causes of amblyop ia, and modes of treatment.

Another interesting web sites on vision and strabismus: This site, titled, fun with strabismus has numerous examples of the various types of strabismus.