Critical Period

This article is about a critical period in an organism's or person's development. See also America's Critical Period.

In general, a critical period is a limited time in which an event can occur, usually to result in some kind of transformation. In developmental psychology and developmental biology, a critical period is a phase in the life span during which an organism has heightened sensitivity to exogenous stimuli that are compulsory for the development of a particular skill. If the organism does not receive the appropriate stimulus during this "critical period", it may be difficult, ultimately less successful, or even impossible, to develop some functions later in life. The general idea is that failure to learn a particular skill allows the cortical areas normally allocated for that function to fall into disuse; as a result these unused brain areas will eventually adapt to perform a different function and therefore will no longer be available to perform other functions. The concurrence of critical periods for the auditory, visual, and vestibular systems suggests that the time period may be universal for emergent sensory systems.

This is fundamentally different than the sensitive period, which is a more extended period of time during development when an individual is more receptive to specific types of environmental stimuli, usually because nervous system development is especially sensitive to certain sensory stimuli. This makes the individual more predisposed to learning.

For example, the critical period for the development of a human child's binocular vision is thought to be between three and eight months, with sensitivity to damage extending up to at least three years of age. Further critical periods have been identified for the development of hearing and the vestibular system. There are critical periods during early postnatal development in which imprinting can occur, such as when a greylag goose becomes attached to a parent figure within the first 36 hours after hatching. A young chaffinch must hear an adult singing before it sexually matures, or it will never properly learn the highly intricate song. These observations have led some to hypothesise a critical period for certain areas of human learning, particularly language acquisition.

Confirming the existence of a critical period for a particular ability requires evidence that there is a point after which the associated behavior is no longer correlated with age and ability stays at the same level (in text citation). Those who are exposed to the stimuli after the critical period should perform significantly worse than those who were exposed to the same stimuli at the appropriate time. Some experimental research into critical periods has involved depriving animals of stimuli at different stages of development while other studies have looked at children deprived of certain experiences due to illness (such as temporary blindness), or social isolation (such as feral children). Many of the studies investigating a critical period for language acquisition have focused on deaf children of hearing parents.