Plasticity Within The Language System

It is reasonable to assume that the rules and principles that govern the development of the sensory systems also guide the development of language-relevant brain systems. Next, we briefly review facts about the normal development of language and the capacity for plasticity in the language system after altered experience and brain lesions.

A. Facts about the Normal Development of Language

Developmental studies indicate several milestones during language acquisition. By 12 weeks, most infants produce vowellike sounds called cooing. By 20 weeks, vocalizations begin to include more consonant sounds, a stage termed babbling. Whereas initially these vocalizations are very different from the sounds of the language environment, they come to resemble the syllables of the surrounding language by 8 months of age. Isolated words are produced around 1 year; these usually include common nouns that describe everyday objects or frequent social words such as hello. Sometime during the second year, vocabulary size begins to grow at a dramatic pace and short, 2-word sentences appear. These sentences, although very brief, already display considerable structural information. Thus, young learners of English typically say "Daddy eat'' or "eat pizza,'' showing sensitivity to the subject-object distinction. Between 3 and 5 years of age, patterns of grammatical development occur, including syntactic and morphological developments such as over-regularization (goed vs went), question formation, negation, and passive. The finding that these milestones are shared in all learners all over the world supports the proposal that language learning has a significant biological basis.

Indeed, several reports indicate that the milestones of language development are extremely robust in the presence of significant variations in experience. For example, the same stages oflanguage development are observed across variations in the mother's speech. Children that are carried on their mother's backs and seldom talked to show the same milestones of language development as children raised in a more nurturing environment. Similarly, deaf children acquiring a visuospatial language also exhibit the same milestones. This is not to say that language experience does not guide language development. It is clear that the language environment determines which kind of language is learned. But a striking feature of language development (like visual development) is that the effects of experience are constrained by the age of the learner. For example, learners exposed to language late in life (whether second-language learners or individuals deprived of language exposure early in life) do not display the same level of proficiency as early learners. Thus, as in other domains, this research indicates that the effect of experiential influences during language development are constrained by the maturational status of the learner.

A few studies have begun to chart the changes in brain organization as children acquire their primary language and to ask whether different maturational constraints may exist for different types of processing as has been reported earlier for most sensory domains. One of the best-documented dichotomies in language processing is that between lexical-semantic information and grammatical information. For example, nouns and verbs (e.g., table, run) that convey semantic information elicit a different pattern of brain activity (as measured by ERPs) than do function words that convey grammatical relationships (e.g., within, out, the) in normal, right-handed, monolingual adults. Similarly, sentences that are semantically nonsensical (but grammatically intact) elicit a different pattern of

ERPs than do sentences that contain a violation of syntactic structure (but that leave the meaning intact). These results are consistent with several other types of evidence that imply two separate systems within language, one for lexical-semantic and one for grammatical processing. Specifically, the studies available indicate a greater role for more posterior temporal-parietal systems in the left hemisphere for lexical-semantic processing; frontal-temporal systems within the left hemisphere are implicated for grammatical processing. How does this functional specialization arise during development? ERPs for open- and closed-class words have been compared in infants and young children from 20 to 42 months of age (Fig. 5). All children understood and produced both the open- and closed-class words presented. At age 20 months, ERPs in response to open- and closed-class words did not differ. However, both types of words elicited ERPs that differed from those elicited by unknown and backward words. These data suggest that in the earliest stages of language development, when children are typically speaking in single-word utterances or beginning to put two words together, open- and closed-class words elicit similar patterns of brain activity. At 28-30 months of age, when children typically begin to speak in short phrases, ERPs for open- and closed-class words elicited different patterns of brain activity. However, the more mature left hemisphere asymmetry for closed-class words was still not observed. By 3 years of age, most children speak in sentences and use closed-class words appropriately to specify grammatical relations, so that like adults, ERPs from 3-year-olds displayed a left hemisphere asymmetry to closed-class words. The results across the three groups are consistent with the hypotheses that, initially, open-and closed-class words are processed by similar brain systems and that these systems become progressively more specialized with increasing language experience.

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Conquering Fear In The 21th Century

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