Despite the apparent ease with which children learn language, there are striking individual differences in time course of growth of particular elements, including lexical and syntactic knowledge, use of gesture, and acquisition of reading. An important difference among children is the linguistic environment to which they are exposed, 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 are affected by preschool input. The next step is to determine the relationship of early input and skill to later achievement and to their neural mechanisms. In both oral language and reading, we focus on word decoding and sentence comprehension at both the auditory and orthographic levels. We study this developmental 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 adult brain, which develop over the course of childhood, and which are affected by the child's environment and experience. This model divides both oral language and reading into two distinct components, word decoding and sentence comprehension. In the oral modality, word decoding involves two streams, a predominantly left lateralized """"""""dorsal stream"""""""" that maps sound onto articulatory representations and a bilaterally represented """"""""ventral stream"""""""" that maps sound onto lexical meaning. In the written modality, single word decoding also seems to involve two pathways, both relatively left lateralized, including a """"""""dorsal route"""""""" that decodes words by a decompositional (grapheme-phoneme conversion) mechanism and a """"""""ventral route"""""""" that uses direct visual (lexical/semantic) recognition. The mechanisms for sentence comprehension appear to 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 that phonological development, required for both auditory and written word decoding, involves development of parietal and premotor circuits for observation and execution through imitation. Development of written word decoding 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, and the early presence of complex speech and gesture. In summary, we aim to understand the patterns of neural activity associated with oral and written language comprehension in school-age children, and to characterize the role of early preschool language and gesture in determining the structure of this neural network architecture.
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