It means that if the input of language does not take place after this particular time the individual will experience difficulty in obtaining a complete language. The proof for such a time is restricted, and assistance stems extensively from hypothetical disputes and similarities to other significant stages in biology like visual development, but nevertheless is accepted widely. Such a critical periods nature has however been the fiercest issues debated in cognitive science and psycholinguistics for decade. A Few writers have proposed an "optimal" or "sensitive" time other than a critical one. Some other writers dispute on the reasons that include cognitive mind of language factors and physical maturation. The critical periods duration varies as well greatly in various accounts.
In animals, a critical period refers to the biologically determined period during which learning must take place in order for development to happen. During a small window of time, the juvenile bird listens to and copies the song of a tutor; after the critical period closes, such copying is not possible. Blindfolded kitten do not develop normal vision but if the blindfold is removed before eight weeks normal vision can be restored.
In humans, if strabismus (crossed eyes) is not corrected in early childhood that is before the age of three a child will not develop normal binocular vision. The critical period for susceptibility to strabismus begins soon after birth and shows continued susceptibility to at least 4.6 years.
According to Eric Lenneberg, language acquisition is not possible at the age of two because the human brain is not sufficiently mature. After puberty normal language acquisition is not possible because the brain is physically mature. Lenneberg says that there exists maturational limitations on the period that a primary language can be obtained. The acquisition of the first language depends on neuroplasticity. If the acquisition