Amblyopia, a condition in which the vision of one eye is greatly reduced, is the most common cause of visual impairment in children, affecting three to five of every 100 children worldwide. The condition occurs when the two eyes do not work together properly, usually because of lazy eye or a cataract. If left untreated, vision will not develop correctly in the brain even though the eyes are healthy.
The cat visual system is very similar to humans. Studying how cats learn to see has helped doctors rehabilitate the vision of children with amblyopia more effectively.
Jandee McLaughlin, 2012.
How is this possible? When most animals and humans are first born, the brain connections necessary for vision have not fully developed. Scientists studying the normal development of vision in monkeys and cats realized that how well an adult animal could see depended on the animal’s past visual experience. The same applies to humans. For a child to form the precise connections necessary for proper vision, he must have normal visual experiences.
David Hubel, MD, and Torsten Wiesel, MD, winners of the 1981 Nobel Prize, learned from animal experiments that there is a time window, or a critical period, early in life, when the visual pathways are forming and remain open to change. Experiments using monkeys and cats helped determine that treatment for amblyopia in humans had the best outcome when it was started before the age of eight. During this period of time, visual experiences guide the development of the circuits. After the critical period comes to an end, the circuits cannot be easily modified. As a result, there is no cure for amblyopia in adulthood.
This work made it clear that interventions for amblyopia and other visual impairments had to be done early in life to effectively rehabilitate a child’s vision. More recent work using mice has started to reveal what factors change in the brain to prevent rewiring after a certain age. Removing these breaks seems to allow for changes later in life, and can restore vision in older amblyopic mice. These experiments are helping scientists understand brain development in animals, offering hope for finding a cure for amblyopia in adulthood.