difficult for late learners than learning that “cats chase mice” has a different meaning from “mice chase cats.” We see the same trend when we look at Genie, a child who experienced extreme deprivation for the first 13 years of her life. During this deprivation, Genie made essentially no progress in developing a communication system (she had, after all, no one to talk to). After discovery and rehabilitation, Genie was found to make progress in acquiring certain components of language (word order among them), but little progress in acquiring other components of language, including morphology (Curtiss, 1977; Goldin-Meadow, 1978).

This developmental pattern again suggests that certain components of language may be resilient—here in the face of variations in the timing of acquisition—while other components may be relatively fragile. The ability to learn the fragile components of language does not drop off precipitously. Rather, there appears to be a decline after age 6 or 7—a decline that begins to plateau and become less steep in late adolescence. Importantly, unlike early learners who tend to follow the same developmental trajectory (that is, there is strikingly little variability across them), late learners vary quite a bit. Some achieve native-like competence even on the fragile properties of language, while others do not. This research is providing a much more refined understanding of the ways in which early language experience provides a foundation for later language facility.

Studies of brain activity patterns (using event-related brain potentials, called ERPs, which measure electrical activity recorded at the scalp) provide further evidence that language is not a single entity and that developmental mechanisms may differ for different properties of language (see Neville and Mills, 1997). For example, Neville and her colleagues have found, in normal, right-handed, monolingual adults, that nouns and verbs (words that provide semantic information—that is, about meaning) elicit a markedly different pattern of brain activity than do prepositions and conjunctions (functional words that provide grammatical information). These findings suggest that different neural systems mediate the processing of semantic and grammatical information in adults (in particular, a greater role for more posterior temporal-parietal systems in semantic processing and for anterior temporal systems within the left hemisphere in grammatical processing). Impressively, these findings are robust across languages, including sign languages (although there appears to be more right-hemisphere involvement in processing a sign language like American Sign Language than in processing a spoken language like English).

The work of Neville and her colleagues also bears on issues of the timing of environmental inputs. In studies of cerebral organization in individuals who learned English at different times in the life span, Neville and colleagues have found that aspects of semantic and grammatical processing differ markedly in the degree to which they depend on the timing of lan-

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