History
The critical period hypothesis was first proposed by Montreal neurologist Wilder Penfield and co-author Lamar Roberts in a 1959 paper Speech and Brain Mechanisms, and was popularised by Eric Lenneberg in 1967 with Biological Foundations of Language.
Lenneberg states that there are maturational constraints on the time a first language can be acquired. First language acquisition relies on neuroplasticity. If language acquisition does not occur by puberty, some aspects of language can be learnt but full mastery cannot be achieved. This was called the "critical period hypothesis."
An interesting example of this is the case of Genie. A thirteen-year-old victim of lifelong child abuse, Genie had been kept strapped to a potty chair and wearing diapers. She appeared to be entirely without language. Her father had judged her retarded at birth and had chosen to isolate her, and so she had remained until her discovery.
It was an ideal opportunity to test the theory that a nurturing environment could somehow make up for a total lack of language past the age of 12. She was unable to acquire language completely, although the degree to which she acquired language is disputed.
Detractors of the "Critical Period Hypothesis" point out that in this example and others like it (see Feral children), the child is hardly growing up in a nurturing environment, and that the lack of language acquisition in later life may be due to the results of a generally abusive environment rather than being specifically due to a lack of exposure to language.
Recently, it has been suggested that if a critical period does exist, it may be due at least partially to the delayed development of the prefrontal cortex in human children. Researchers have suggested that delayed development of the prefrontal cortex and a concomitant delay in the development of cognitive control may facilitate convention learning, allowing young children to learn language far more easily than cognitively mature adults and older children. This pattern of prefrontal development is unique to humans among similar mammalian (and primate) species, and may explain why humans—and not chimpanzees—are so adept at learning language.
Read more about this topic: Critical Period Hypothesis
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