Are there critical periods in the development of every brain function?
Probably not. In the case of visual development, certain abilities are more at-risk than others when a young child’s vision is impaired by eye-crossing or other visual problems (such as congenital cataracts). Thus, two visual abilities–acuity (the perception of fine detail) and binocularity (the coordinated use of both eyes), which is especially important for depth perception–do depend on normal visual experience as a child, whereas two other visual abilities–color and peripheral vision–are not impaired by visual problems in early life. A similar distinction holds for language development: certain skills (including grammar and phonology–the ability to perceive and produce individual speech sounds) are more sensitive than others (such as vocabulary size) to a child’s experience with language in the first few years of life. We know much less about the development of other mental skills, such as emotional functioning, mathematical ability, or musical skill. If their development is co
