Despite the apparent ease with which children learn language, there are striking individual differences intime course of growth of particular elements, including lexical and syntactic knowledge, use of gesture, andacquisition of reading. An important difference among children is the linguistic environment to which they areexposed, and this has a significant effect on the rate and timing of language growth.In our ongoing Longitudinal Language Study (LLS), we have shown that preschool skill levels areaffected by preschool input. The next step is to determine the relationship of early input and skill to laterachievement and to their neural mechanisms. In both oral language and reading, we focus on worddecoding and sentence comprehension at both the auditory and orthographic levels. We study thisdevelopmental biology both in typically developing children and those with early unilateral brain lesions.We organize our biological investigations around several putative processing pathways in the adultbrain, which develop over the course of childhood, and which are affected by the child's environment andexperience. This model divides both oral language and reading into two distinct components, word decodingand sentence comprehension. In the oral modality, word decoding involves two streams, a predominantlyleft lateralized 'dorsal stream' that maps sound onto articulatory representations and a bilaterallyrepresented 'ventral stream' that maps sound onto lexical meaning. In the written modality, single worddecoding also seems to involve two pathways, both relatively left lateralized, including a 'dorsal route' thatdecodes words by a decompositional (grapheme-phoneme conversion) mechanism and a 'ventral route'that uses direct visual (lexical/semantic) recognition. The mechanisms for sentence comprehension appearto be relatively modality independent, mediated by an anterior temporal and inferior frontal 'anterior stream',which maps words into sentence-level syntactic and semantic representations.Development of these decoding and comprehension systems requires input. We hypothesize thatphonological development, required for both auditory and written word decoding, involves development ofparietal and premotor circuits for observation and execution through imitation. Development of written worddecoding appears to require interaction between this phonological system and specialized visual processes.Finally, development of comprehension systems appears to rely on the complexity of caregiver speech, andthe early presence of complex speech and gesture.In summary, we aim to understand the patterns of neural activity associated with oral and writtenlanguage comprehension in school-age children, and to characterize the role of early preschool languageand gesture in determining the structure of this neural network architecture.
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