September 25, 1997
CRITICAL PERIOD IN BRAIN DEVELOPMENT DISCOVERED;
Babies can be spared vision problems as a result
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Kate Egan,404-727-7709
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Newborn babies may avoid lifelong vision problems thanks to a discovery in monkeys at the Yerkes Primate Research Center of Emory University.
Yerkes scientists have found that a dramatic reorganization of brain cells occurs in infant monkeys in the first three weeks of life, corresponding in humans to the first three months. These neural connections turn out to be the building blocks of a healthy visual system, allowing for a baby's sudden ability to see three-dimensionally, and as the months and years go by, to avoid a series of irreversible visual defects.
Infants make these vital connections only with the help of visual stimuli from the world around them. If a baby fails to receive normal visual input in the first three months--usually due to undetected visual defects at birth, such as cataracts--the baby will never recover the brain cell connections, and will suffer one or more of the vision deficits for life.
These results are reported in Perception (vol. 26), in the proceedings of the Child Vision Research Society in by Dr. Ron Boothe, Ph.D., Chief of the Visual Science division at Yerkes.
Dr. Boothe calls this critical three-month window the "neonatal" sensitive period----as compared with the "classical postnatal" sensitive period which lasts for several years after birth, and was described more than 10 years ago by vision scientists who won the Nobel prize for their work.
According to Dr. Boothe, the irreversible deficits involve problems with controlling eye movements, such as strabismus, in which the visual axes of the eyes are not parallel, leaving the child cross-eyed; latent nystagmus, a rhythmical oscillation of the eyeball; dissociated vertical deviations, in which a covered eye drifts upward; and deficits in motion processing, which impairs vision of movements going in a certain direction
The deficits range in severity from one person to another. Aside from the aesthetic problems involved with being cross-eyed, for instance, the deficits make performing many activities a bit riskier, says Dr. Boothe, because they are oculomotor problems affecting depth perception. "They would probably keep someone from becoming a fighter pilot, or a race car driver," he explains.
Currently, no treatment exists for the deficits once they appear. However, they are preventable. The clinical message is clear: newborns and neonates should be screened carefully and treated immediately for cataracts and other problems.
"Eye doctors have certainly seen these disorders in the past, and it was always in children who also had other vision problems, such as cataracts," notes Dr. Boothe. Scientists had assumed that cataracts and the deficits were caused by the same thing, perhaps some unknown genetic abnormality. But the new findings now reveal that the deficits are actually a separate, neurologic problem occurring during development, rather than a genetic one.
It was serendipitous that Dr. Boothe discovered that something quite significant must be happening in the brain during the neonatal period. He was working with rhesus monkeys to find treatments for cataracts when he noticed that permanent deficits in eye movements developed when he simulated cataracts experimentally in neonatal (less than a month old) monkeys----but not when these conditions were induced just a few weeks later. Dr. Boothe's observance of these functions in rhesus monkeys, which have a visual system almost identical to humans, led him next to relate the findings to the clinical literature on human babies with cataracts.
For years, Dr. Boothe has been working closely with clinical ophthalmologists who treat babies with cataracts----and who will now be screening to catch the problem as early in life as possible.
Infantile cataracts are fairly common, with an average of 4 cases per 10,000 American infants born each year. Their presence can be quite subtle, resembling a cloudy looking eye, and often making it difficult to detect early. In babies, usually only one eye is affected. If left untreated, a cataract will result in blindness in that eye.
The transition between the neonatal sensitive period, which produces these deficits, and the later classical sensitive period, which does not, correspond to the age when three-dimensional vision or stereovision emerges.
Stereovision is unique in its rapid onset; all the other developmental visual functions, such as visual acuity (sharpness, clarity of vision) develop very slowly, during the first few years of life. The findings describing the rapid emergence of stereovision in monkeys are reported in the most recent issue of Vision Research by Dr. Boothe and Yerkes researcher Dr. Cynthia O'Dell.
The Yerkes Primate Center is part of the Woodruff Health Sciences Center of Emory University, and is the oldest scientific institute in the world dedicated to primate research. Its programs cover a wide range of biomedical and behavioral sciences.
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