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Research gives lazy eye theory a workout

In a study that challenges conventional thinking about the condition known as lazy eye, researchers at MIT's Picower Institute for Learning and Memory show that it's the quality, not the quantity, of images and light striking the retina that causes one eye to lose function.

The study will appear in the May issue of the Journal of Neurophysiology.

Amblyopia, or lazy eye, is a developmental disorder characterized by poor or blurry vision in an eye that is structurally normal. The problem is caused by either no transmission or poor transmission of visual images to the brain for a sustained period during early childhood. Amblyopia has been estimated to affect 1 percent to 5 percent of the population.

"It's been known for a long time that if you are born with cataracts in one eye, you will go blind in that eye," said study co-author Mark Bear, Picower Professor of Neuroscience. "Depriving one eye of crisp images rapidly causes cortical neurons to lose responsiveness to the deprived eye."

While it was thought that inactivity caused the neurons associated with the deprived eye to wither -- a case of "use it or lose it" -- Bear and colleagues at Brown University report that a blurry image is worse than no image at all.

The conventional treatment for lazy eye is to wear a patch over the good eye in the hope that the weaker eye will get stronger. "It's a zero-sum game," Bear said, "because as the weak eye gets stronger, the strong eye gets weaker. The challenge is to promote recovery of the weak eye without impairing the other eye."

Clinicians have debated the value of allowing some light to penetrate the patch to activate neurons in the retina. However, Bear's new results indicate that the best way to preserve connections from a patched good eye is to eliminate as much activity in that eye as possible.

In the study, the researchers tested vision in animal subjects with normal vision, comparing two days of lid closure in one eye with two days of blur using an overcorrecting contact lens. They found that the overcorrecting contact lens was just as bad as closing the eyelid for weakening connections in the developing brain.

"The quality rather than the quantity of retinal illumination is the key factor here," Bear said.

This work is supported by the National Eye Institute and the Howard Hughes Medical Institute.

A version of this article appeared in MIT Tech Talk on April 26, 2006 (download PDF).

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