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What is Strabismus?
by Dr. Jeffrey Cooper. © 2001-2016

Vision Development and Critical Periods - Binocular Vision, Strabismus, Amblyopia

The development of keen binocular (two-eyed) vision with resultant stereopsis (3D vision and depth perception) is a result of genetics and appropriate development of the binocular visual system during the early formative years. The ability to see 20/20, focusing ability (accommodation), eye muscle coordination (aiming or alignment) and stereopsis are all developed by 6 months of age in humans.

In the 1960s, Nobel prize winning research using monkeys and cats, which have stereoscopic (3D) vision similar to ours, improved our understanding of binocular vision development. In the 21st century, however, numerous scientific studies on successful treatment of amblyopia (lazy eye) in older children and adults has disproven the earlier conclusions about critical periods drawn from that 1960's research.

Details on the Influencial 1960s Research on Critical Periods of Vision Development
In the early 1960's two Nobel Prize winners, Hubel and Weisel from Harvard, recorded activity of cortical cells. In a group of eloquent studies they covered one of the cat's eyes, stabilized the movement of the viewing eye. Then they inserted an electrode in a cell in the visual cortex of the brain of a cat, amplified the signal, and recorded the output of various cells that they tested. Lastly, they moved a light around until the cell responded. Specifically, they studied cells in the area in the occipital cortex that was known to be associated with vision. Each cell responded to a different location in space.

In addition, the cells responded to different types of light, e.g. some cells responded to a bar of light moving left to right while others responded to light moving up and down. First they recorded from one eye and then the other. They found that 80% of the cells responded to the input from one eye (binocular cells) while the other 20% of the cells only responded to the input from either eye (monocular cells). Binocular cells are necessary for the two eyes to work together and are the basis for depth perception or stereopsis. This was an important breakthrough since they demonstrated the location and characteristic of stereopsis in the brain.

Then they altered the cat's visual experience. They patched one eye for weeks on end, blurred an eye with contact lenses, and/or made the cat artificially strabismic (eye turn). Afterwards they recorded the effect of these procedures by measuring the responses of the cells in the visual cortex. These altered visual experiences changed how the cells fired. Cells that use to respond to the input from either eye now only responded to the input from one eye. Thus all the cells became monocular. Actually, the cells of the "good eye" inhibited the responses of the "bad eye". With special techniques neuro-physiologists also developed techniques to measure visual acuity, color vision, depth perception, etc. in the cats and monkeys. Vision was reduced and there was a loss in depth perception.

Autopsies of the animals demonstrated the cells associated with seeing and binocular vision became atrophic (smaller in size). These results only occurred if the disruption in the visual experience happened early in life. Thus, early visual experience changed how the cells responded, what the animals saw, and how the cells looked. Altering visual experience via patching, blurring of vision or surgery did not effect older animals as much. The period of time in which the cells changed from alteration of visual experience is known as the critical period. It is of important interest to note that intense Vision Therapy after the end of the critical period still resulted in improvement in vision and binocularity in these animals. Thus, the critical period is only the time of maximum neurological plasticity.

These animal studies taught us how the visual system develops and works. They show that altering the visual experience of the cat or monkey's life during the first few years of life, critical period, has a great impact on future development. This same phenomenon happens in humans. Strabismus (eye turn) and amblyopia (reduction in vision because one eye is deprived clear single vision during the critical period) could be experimentally created in animals and studied. These models help us learn how amblyopia and strabismus develop and must be treated. Research suggests that the maximum critical period in humans is from just after birth to 2 years of age. Any disruption of binocular vision from 6 mos. to approximately 4 years will result in strabismus and/or amblyopia. Thus, every infant without an apparent problem should have their first examination between 9 mos. to one year of age. Up to the first 6 mos. of age intermittent strabismus is a normal developmental milestone. By 9 mos. of age the system is in place. Young babies are also easy to exam. Age 2 is neurologically late and a difficult time to examine the young totter. If everything is normal at that 9 mos. examination, the next examination should be in kinden-garden.

Infant Eye Examination

The best chance of success of eliminating the effects of amblyopia or constant strabismus occurs before the age of two. However, this does not preclude success in patients older than 6 years of age. There are numerous studies that demonstrate that treatment after the age of 6 is very successful. One study compared treatment before age 6 to treatment after age 6. They found no statistical difference between the two groups. As a matter of fact, loss of an eye in patients over the age of 65 who were never treated for their amblyopia experienced a spontaneous improvement in vision in over one-third of the cases. Thus, every attempt should be made to improve strabismus and amblyopia though treatment might not be as effective after the age of six and definitely requires more work. Also, remember that if an eye turn occurs only some of the time (intermittent), the cells of the brains do not develop the changes associated with constant eye turns. It doesn't take much stimulation to maintain function.

An analogy to understanding the relationship of treatment of eye muscle anomalies would be learning to speak a second language. During the period of neurological development, around the first year of life, language development is natural and spontaneous. Children raised in families that speak two languages from birth automatically learn both languages. However, if the second language is introduced in later school years language development is longer and more arduous. Remember, people learn languages in their sixties and seventies. The same is true of visual development. Its easier to develop normal vision during the critical period, but with work many people can develop normal binocular vision in later years. It is never too late to try.

One should realize that if surgery is used during the critical period then the goal must be perfect alignment. One must also realize that good cosmetic alignment is often not achieved until two or three surgical procedures are performed. Even if the eyes are almost straight but not perfectly straight then they can not work together. Effective surgical intervention requires not only perfect alignment but also an Optometrist who knows how to make the two eyes work together with lenses, prism, and Vision Therapy. Surgery, if necessary, must be coordinated with an Optometrist if the goal is for improved vision or binocular vision. If therapy is not initiated immediately (during the critical period) after surgery the chances of success diminish. Surgery, which does not achieve almost perfect alignment, is cosmetic only.

Dr. Jeffrey Cooper
Written: 2000

What is Strabismus?
Dr. Jeffrey Cooper
Dr. Jeffrey Cooper FAAO, FCOVD, our expert on strabismus, answers your questions in a What is Strabismus?. Check out the fun bits on the evolution of human vision.

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