A critical period in psychology refers to a specific time during development when the brain is particularly receptive to acquiring a skill or knowledge. When exposure occurs after this critical period has elapsed, it is much less effective.
Critical periods occur during early childhood, when the brain is extremely flexible in adapting to new types of information. As more information is taken in, the brain becomes entrenched in certain types of processing, making it difficult to acquire new information. For example, early visual experience is essential for typical development of visual processing. If visual experiences are blocked during the critical period of visual development, the brain reorganizes to use visual-processing tissue for other tasks.
A controversial type of critical period is the critical period for learning foreign languages. Infants and young children readily learn multiple languages with native-level proficiency. However, starting around puberty, this ability to learn a new language rapidly declines, with many people unable to ever reach native fluency. Classic interpretations of this phenomenon suggest that genes regulate language learning, decreasing the brain's receptiveness for learning new languages during puberty. However, others argue that the critical period is not an innate facet of the brain but rather that it emerges because of experience. As a person learns one language, it becomes difficult to learn another because of interference between the languages.