Plasticity in the Language System after Altered Language Experience

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1. Delayed Exposure to Language

How is language learning affected by delayed exposure to a language? Many investigators have studied this question by comparing individuals who learned a second language at different times in development. English-Korean speakers who came to the United States and were immersed in English at varying ages have been studied. Their data clearly establish a steady decrease in performance as a function of age at first

Figure 5 Current source density (CSD) analyses of neural activity in response to closed-class words at 200 msec. The CSDs illustrate sinks, i.e., activity flowing into the head (purple), and sources, i.e., activity flowing out of the head (orange), at three age groups. Top: At 20 months the CSD shows sinks over both the left and right hemispheres. Middle: At 28-30 months the CSD show sinks that are bilateral but slightly more prominent over the right than the left hemisphere. Bottom: At 36-42 months the CSD shows a sink over left anterior regions. Reprinted from Neville and Bavelier (1999), In "The New Cognitive Neurosciences,'' 2nded., M.S. Gazzaniga, (Ed.), pp. 83-98, with permission of MIT Press.

Figure 5 Current source density (CSD) analyses of neural activity in response to closed-class words at 200 msec. The CSDs illustrate sinks, i.e., activity flowing into the head (purple), and sources, i.e., activity flowing out of the head (orange), at three age groups. Top: At 20 months the CSD shows sinks over both the left and right hemispheres. Middle: At 28-30 months the CSD show sinks that are bilateral but slightly more prominent over the right than the left hemisphere. Bottom: At 36-42 months the CSD shows a sink over left anterior regions. Reprinted from Neville and Bavelier (1999), In "The New Cognitive Neurosciences,'' 2nded., M.S. Gazzaniga, (Ed.), pp. 83-98, with permission of MIT Press.

exposure (Fig. 6). The effect of age of exposure was especially striking for those aspects of language that rely on grammatical processing. Interestingly, duration of exposure cannot compensate for this effect; thus, a 14-year-old who has been immersed in English for only 4 years is more likely to show a good command of English intricacies than a 50-year-old who has been immersed in English for the last 25 years. It has been argued that the difference in late learners of a second language is due to competition from their extensive knowledge of a first language rather than their late start in learning. The observation of parallel results in deaf adults exposed at varying ages to their first and unique language (American Sign Language) suggests, however, that age of exposure is the key factor in this pattern of results. As in second language learners, late learners of American Sign Language (ASL) had particular problems with the ASL equivalent of morphemes and syntactically complex sentences.

The available results suggest that aspects of semantic and grammatical processing differ markedly in the degree to which they depend upon language input. Specifically, grammatical processing appears to be more vulnerable to delays in language experience. In Chinese-English bilingual speakers, delays of as long as 16 years in exposure to English had very little effect on the organization of the brain systems active in lexical-semantic processing. In contrast, delays of only 4 years had significant effects on those aspects of brain organization linked to grammatical processing. Further evidence on this point was provided by ERP

Figure 6 Total score on a test of English grammar in relation to age of arrival in the United States. These data show that the participants' performance declines as the age of exposure to the language is delayed. Reprinted from Johnson and Newport (1989), Cog. Psych. 21, 60-99, with permission of Academic Press and Erlbaum Inc.

Figure 6 Total score on a test of English grammar in relation to age of arrival in the United States. These data show that the participants' performance declines as the age of exposure to the language is delayed. Reprinted from Johnson and Newport (1989), Cog. Psych. 21, 60-99, with permission of Academic Press and Erlbaum Inc.

studies of English sentence processing by congenitally deaf individuals who learned English late and as a second language (ASL was the first language of these subjects). Deaf subjects displayed ERP responses to nouns and semantically anomalous sentences in written English that were indistinguishable from those of normal hearing subjects who learned English as a first language. These data are consistent with the hypothesis that some aspects of lexical-semantic processing are largely unaffected by the many differences in language experience between normally hearing and congenitally deaf individuals. By contrast, deaf subjects displayed aberrant ERP responses to grammatical information like that presented in function words in English. Specifically, they did not display the specialization of the anterior regions of the left hemisphere characteristic of native, hearing-speaking learners (Fig. 7). These data suggest that the systems that mediate the processing of grammatical information are more modifiable and more vulnerable in response to altered language experience than are those associated with lexical-semantic processing.

2. Visuospatial Languages

We have employed ERP and fMRI techniques to further pursue the preceding hypothesis and also to obtain evidence on the question of whether the strongly biased role of the left hemisphere in language occurs independently of the structure and modality of the language acquired first. ERPs recorded from native signers in response to open- and closed-class signs in ASL sentences displayed timing and anterior-posterior distributions similar to those observed in native speakers processing English. However, whereas in native speakers of English responses to closed-class English words were greatest over anterior regions of the left hemisphere, in native signers closed-class ASL signs elicited activity that was bilateral and that extended posteriorly to include parietal regions of both the left and right hemispheres. These results imply that the acquisition of a language that relies on spatial contrasts and the perception of motion may result in increased recruitment of right hemisphere regions into the language system. Both hearing and deaf native signers displayed this effect. However, hearing people who acquired ASL in their late teens did not show this effect, suggesting there may be a limited time (sensitive) period when this type of organization for grammatical processing can develop. By contrast the response to semantic information was not affected by age of acquisition of ASL, in keeping with the results from studies of English that suggest these different subsystems within language display different degrees of sensitivity to altered experience.

In an fMRI study comparing sentence processing in English and ASL, researchers observed evidence for biological constraints and effects of experience on the

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