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VIEWPOINT
Language acquisition is one of the most fundamental human traits, and it is obviously the brain that undergoes the developmental changes. During the years of language acquisition, the brain not only stores linguistic information but also adapts to the grammatical regularities of language. Recent advances in functional neuroimaging have substantially contributed to systems-level analyses of brain development. In this Viewpoint, I review the current understanding of how the "final state" of language acquisition is represented in the mature brain and summarize new findings on cortical plasticity for second language acquisition, focusing particularly on the function of the grammar center.
A child acquires any natural languages within a few years, without the aid of analytical thinking and without explicit "grammar" instruction as usually taught in school. The origin of grammatical rules should thus be ascribed to an innate system in the human brain (1). The knowledge of and competence for human language is acquired through various means and modality types. Linguists regard speaking, signing, and language comprehension as primary faculties of language, i.e., innate or inherent and biologically determined, whereas they regard reading and writing as secondary abilities. Indeed, the native or first language (L1) is acquired during the first years of life through such primary faculties while children are rapidly expanding their linguistic knowledge (2). In contrast, reading and writing are learned with much conscious effort and repetition, usually at school. This ability may be influenced by cultural rather than biological factors. However, the existence of developmental dyslexies indicates that reading ability requires specific neural mechanisms (3), and a link between poor reading and impaired auditory resolution has been suggested (4). It is therefore crucial to understand how distinct linguistic faculties develop in the brain throughout various ages. Figure 1 illustrates the typical development of L1 faculties. This correlates with a massive increase in brain volume during the first years. Speech in infants develops from babbling at around 6 to 8 months of age, to the one-word stage at 10 to 12 months, and then to the two-word stage around 2 years. Note that sign systems are spontaneously acquired by both deaf and hearing infants in a similar developmental course (5), starting from manual silent "babbling" (6). However, these obvious developmental changes...