Normal vision develops rapidly during the first 2 years of life and continues more slowly until about age 9. During that time, a child's eyes and brain learn how to work together to see. Practice is the most important part of this learning process. Constant use of both eyes gives a child's eyes and brain the practice they need to develop normal eyesight.

If there is a problem with vision in one eye, it disrupts this learning process, and the brain stops developing normal vision for that eye. This can lead to amblyopia.