Résumé
Language acquisition is a concerted action of innate language acquisition mechanisms that are neurologically wired, and of language input. Language input has to arrive during a time window called "the critical period for first language acquisition", otherwise language may not be acquired properly. A critical period is a notion that is known for various species, and for various cognitive and sensory systems. In all species and across systems it is a short, early period that needs a trigger for its onset, it remains active and plastic for a period of time, and then closes. In first language acquisition, the trigger for opening of the critical period is exposure to natural language. Once this critical period closes – language can no longer be acquired in the same way.
I will present evidence from individuals who were not exposed to sufficient language input during the first year of life and from individuals whose brain developed abnormally during the first year of life due to vitamin deficiency. Both cases, of insufficient input and of abnormal brain development during the first year of life, the critical period for the acquisition of syntax of the first language, result in atypical development of syntax, even if later they are exposed to language.
