A theory of neurolinguistic development.

This article offers a developmental theory of language and the neural systems that lead to and subserve linguistic capabilities. Early perceptual experience and discontinuities in linguistic development suggest that language develops in four phases that occur in a fixed, interdependent sequence. In each phase of language, a unique ontogenetic function is accomplished. These functions have proprietary neural systems that vary in their degree of specialization. Of particular interest is an analytical mechanism that is responsible for linguistic grammar. This mechanism is time-locked and can only be turned on in the third phase. Confirming evidence is provided by children who are delayed in the second phase of the language learning process. These children store insufficient lexical material to activate their analytic mechanism. Inactivation behaves like damage, shifting language functions to homologous mechanisms in the nondominant hemisphere, thereby increasing functional and anatomical symmetry across the hemispheres. This atypical assembly of neurolinguistic resources produces functional but imperfect command of spoken language and may complicate learning of written language. The theory thus offers a different role for genetics and early experience, and a different interpretation of neuroanatomic findings, from those entertained in most other proposals on developmental language disorders.