October 13, 1997
Volume 50, No. 8
Newborn babies may avoid lifelong vision problems thanks to a discovery in monkeys at the Yerkes Center.
Yerkes scientists have found that a dramatic reorganization of brain cells occurs in infant monkeys during 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, the proceedings of the Child Vision Research Society, by Ron Boothe, chief of the Yerkes' Visual Science Division.
Boothe calls this critical three-month window the "neonatal" sensitive period as compared to 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 Boothe, these irreversible deficits involve problems 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 in a certain direction.
The deficits range in individual severity. Besides the aesthetic problems with crossed eyes, the condition makes performing many activities riskier, said Boothe, because it affects depth perception. "[Strabismus] would probably keep someone from becoming a fighter pilot or a race car driver," he explained.
Currently no treatment exists for the deficits once they appear, but they are preventable. The clinical message is clear: newborns 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," Boothe said. 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 that occur during development, rather than a genetic one.
It was serendipitous that Boothe made his discovery. Working with rhesus monkeys to find treatments for cataracts, he noticed that permanent deficits in eye movements developed when he simulated cataracts experimentally in monkeys less than a month old, but not when these conditions were induced a few weeks later. Since rhesus monkeys have a visual system almost identical to humans, he sought to relate his findings to clinical literature on human babies with cataracts.
Boothe has been working closely for years with clinical ophthalmologists who treat babies with cataracts-and who will now be screening their patients 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 births each year. Their presence can be quite subtle and difficult to detect early. In babies, usually only one eye is affected. If left untreated, a cataract will result in blindness in the eye it affects.
The transition between the neonatal sensitive period, which produces these deficits, and the later classical sensitive period, which does not, corresponds to the age when three-dimensional vision or stereovision emerges.
Stereovision is unique in its rapid onset; all other developmental visual functions, such as clarity, 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 Boothe and Yerkes colleague Cynthia O'Dell.
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