Contrary to accepted dogma, a University of Maryland researcher has developed a method that allows an adult to recover function in an eye that has been damaged and dysfunctional throughout life.
In a paper in the August 12 issue of Nature Neuroscience, Elizabeth Quinlan, associate professor of biology at the University of Maryland, and members of her research team, show that complete visual deprivation promotes subsequent recovery in an eye that has had severely compromised vision from birth to adulthood. It is the first demonstration that visual experience can regulate the recovery of function in amblyopic adults.
The finding may open new avenues for the treatment of amblyopia, a disorder of the eye characterized by poor vision in an eye that is otherwise physically normal. Amblyopia is estimated to affect between one and five percent of the world's population.
The research also demonstrates that synaptic plasticity, the ability of synaptic contacts between neurons to be strengthened or weakened in response to specific patterns of activity, can be reversibly regulated by experience throughout lifetime.
The research was performed in the Quinlan lab by graduate student Hai-Yan He, postdoctoral fellow Baishali Ray, and undergraduate Katie Dennis.
Early in postnatal life, the brains of all animals, including humans, possess a heightened sensitivity to environmental stimuli. Such postnatal sensory experience sculpts the connections between neurons and allows the proper development of sensory and motor function.
For example, visual experience in early postnatal life is required for the proper maturation of the visual system. An injury in either eye early in development will significantly compromise the development of normal vision.
In fact, if a child is born with a cataract in one eye, or any of a number of other restrictions that make the input into the two eyes unequal, amblyopia will develop. In amblyopia, the brain learns to ignore input from the weak or damaged eye and respond only to signals coming in from the strong eye.
The asymmetry impairs the development of three-dimensional vision, and vision in the compromised eye can deteriorate to the point of blindness. It previously has been demonstrated repeatedly that amblyopia can be reversed only if the damaged eye is repaired early in postnatal life.
Clinical studies demonstrate that the probability of recovery of function in amblyopia caused by unilateral cataract depends on the age at which the cataract is removed. The probability of full recovery is highest when the cataract is removed during the first three years of life.
“That is consistent with a large body of evidence that the juvenile mammalian cortex is more ‘plastic' than that of adults,” said Quinlan, “and can reorganize synaptic connections more easily in response to manipulations of sensory and motor experience.”