What is a "critical period" in brain development?

   Pruning or selection of active neural circuits takes place throughout life, but is far more common in early childhood. Animal studies have shown that there are certain windows of time during which the young are especially sensitive to their environment: newborn mice must experience normal whisker sensation in the first few days of life or they will develop abnormal tactile sensitivity in the face region; cats must be allowed normal visual input during the first three months or their vision will be permanently impaired; and monkeys need consistent social contact during the first six months or they will end up extremely emotionally disturbed. Many of the same critical periods appear to hold for human development, although we are less certain about their exact length. Thus, babies also require normal visual input or they may suffer permanent impairment; children born with crossed or "lazy" eyes will fail to develop full acuity and depth perception if the problem is not promptly corrected.      

   Language skills depend critically on verbal input (or sign language, for babies with hearing impairments) in the first few years or certain skills, particularly grammar and pronunciation, may be permanently impacted. The critical period for language-learning begins to close around five years of age and ends around puberty. This is why individuals who learn a new language after puberty almost always speak it with a foreign accent.

 

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 comparable to vision and language, we may expect that some features will be subject to a critical period while others are not. One musical skill known as "perfect pitch"--the ability to identify a musical note without reference to a tuning note--seems to develop only in musicians who began their training before the age of seven (and then, not in all professional musicians). Similarly, a child's social-emotional development depends on a positive, nurturing attachment to a primary caregiver, based on the higher frequency of serious behavioral problems among children who were severely neglected during the first year or more of life, (such as the thousands of Romanian children reared in state-run orphanages). Comparable problems emerge among monkeys who are reared in isolation, and neuroscientists are beginning to understand how the lack of attachment in infancy alters development of emotional areas of the primate brain.

- From the "Zero To Three" website, www.zerotothree.org