ATLANTA -- In two separate studies, vision researchers at the Yerkes Primate Research Center have discovered that the visual experience of one eye influences the growth and subsequent quality of vision in the fellow eye. Previously it was believed that a problem existing in one eye -- a cataract or nearsightedness for instance -- would have no ill effect on the other, normal eye. These studies, reported in the January issue of Investigative Ophthalmology & Visual Science and the upcoming May issue of Vision Research add to the growing evidence that from infancy, visual development is influenced by a control system integrating the two eyes which is dependent on environmental, not merely genetic factors.
Discovering how this interocular control system works is a major step oward the prevention of problems like nearsightedness, or myopia -- a condition affecting approximately 25 percent of the U.S. population and costing billions of dollars per year in treatment. Preventing myopia during the developmental stage would enable more people to achieve accurate focus without wearing eyeglasses or having corrective surgery, the effects of which are not always permanent.
Though the work was done in monkeys, the results have eventual clinical applications for humans, according to Drs. Dolores Bradley and Alcides Fernandes, co-investigators of the studies. Dr. Bradley, a vision research scientist and Dr. Fernandes, a pediatric ophthalmologist, use monkeys as models for conditions affecting the visual system in children. Their findings suggest that infants should have their eyes checked in the first year of life so that a problem in one eye can be detected early and prevent disrupting the growth of the fellow eye. Parents often wait until their children reach school age to have their eyes checked, but by then subtle problems in one eye may have already damaged the healthy eye as well.
Dr. Bradley first studied a large group of 237 normal monkeys to develop a road map for exactly what "normal" visual development entails. "Our ultimate goal is to find a way to predict the course of normal eye growth -- to work out a schedule so that if an infant at six months or one year is not developing properly, clinicians could intervene to get him back on track," says Dr. Bradley. Such intervention, she says, could save a child from myopia, hyperopia (farsightedness) or strabismus (crossed-eyes,) all of which would otherwise affect him for life. Such problems inhibit depth perception, the ability to see fine detail, and general visual acuity, limiting an individual's activities.
Both monkeys and humans start life with their eyes not fully developed; the eyeball is too short for the eye's powerful optic capability. As babies grow older, the eyes grow in length so that by age one in monkeys and age four in humans, about 95% of the growth is complete. This is about a seven millimeter increase -- just the right length for normal vision with no refractive error.
Refraction refers to the deflection of a ray of light as it enters the eye. As it goes in the light is bent or refracted. With normal vision, the image of a distant target comes into focus on the surface of the retina, located at the back of the eye. In a myopic (nearsighted) eye, the image comes into focus at a point before the retina because the eye has grown too long. In a hyperopic (farsighted) eye, the image comes into focus at a point past the retina because the eye is too short. These terms describe refractive error -- the degree to which you have normal,myopic or hyperopic vision. Refractive error is determined by two measures: the length of the eye and the power of the eye's optical system, which includes the curve of the cornea.
When the Yerkes team examined the group of normal monkeys from newborns to adolescents they found that the more farsighted (short-eyed) the animals were at birth the faster their eyes grew. No matter where the monkeys started at birth, by the time they were 1.5 years of age, their eye growth was all tightly clustered at one specific stage of growth. The monkeys whose eyes were closer to that stage at birth didn't change much, while those further from it grew at a faster rate. "This suggests that there is indeed instruction from the environment and that it works hand in hand with genetics," says Dr. Bradley.
After examining the normal group, the team compared it with 15 neonatal monkeys that were reared in three different treatment groups until adolescence. For each group, one "treated" eye was allowed a different type of visual input, using specially made lenses. The fellow "untreated" eye had completely normal vision. The untreated eyes from the three test groups were then compared with monkeys of the same age who had normal vision in both eyes.
Results showed that the length of the normal, untreated eyes varied systematically according to the differences in the visual experience of the treated eyes. Before, it was thought that each eye had its own regulatory mechanism. "Now we see that they work together, so whichever corrective prescription you give to a child it may affect the development of both eyes," says Dr. Fernandes.
The finding does present the opportunity, however, to exercise at least some control over how our eyes develop. "Instead of having people wait to develop myopia and treat it in adulthood," says Dr. Bradley, "we hope that this type of study will allow us to find out very early in life if myopia is likely to occur later -- so we can correct the visual input to prevent it. That's why we study eye growth: to see if it can be influenced to prevent or reduce the need for glasses and slow down the inevitable progression of myopia as a function of age." For instance, a child who is myopic at age 7 is even more so at 20. If eyecare providers could know at 4 years of age that the child is going to be myopic at 7, they could impose some form of external correction to manipulate growth so the eyes don't grow as fast as they would have.
The surgeries available today to correct refractive error do so by altering the curvature of the cornea. The Yerkes lab is attacking the same problem with a different approach -- trying to influence the length of the eye. "It's just an earlier stage of intervention," says Dr. Bradley. The scientists point out, however, that clinical application is still in the future, and will require further investigation.
Future studies at Yerkes include trying to determine what mediates this link between the two eyes, in hopes of developing treatments for children with defects such as strabismus. Is the untreated eye changing just to keep the physical length of the eyes in lockstep? Or is the untreated eye reacting instead to the quality of visual input it receives? What is the nature of the mechanism?
Monkeys make the best model for answering these questions about human vision because, like us, their visual system is designed for the two eyes to act as one: both eyes face the front and work together in a complex binocular visual system to create a "cyclopean" view of the world. Thus, when one eye focuses the other one does too, and by the same degree. This is not always the case in other animals, such as chickens, which have frequently been used in vision studies. A chicken's eyes can move and grow and accomodate independently of one another. The studies reported here differ from previous primate experiments because former ones were conducted on one eye, using the other as a control. This is the first time that such treatments have been tested and then compared with a large population of monkeys with two normal eyes.
Thus, future models of eye growth will have to consider both the direct influence of visual input on the growing eye as well as the indirect influence coming from the fellow eye.
Collaborators on these studies include Ronald Boothe, Michael Lynn, and Margerete Tigges at the Yerkes Primate Center. Funding support was provided by the National Eye Institute and the National Center for Research Resources, which are both part of the National Institutes of Health.
The Yerkes Regional Primate Research Center is the oldest scientific institution dedicated to primate research. Its programs cover a wide range of biomedical and behavioral sciences.
Journal
Investigative Ophthalmology & Visual Science