What atypical populations can reveal about language development: the contrast between deafness and Williams syndrome.

Two distinct lines of investigation are presented : the study of linguistic competence in the written language of deaf children and adults, and the study of linguistic development in children and adolescents with Williams syndrome (WS). Qualitative data focusing on spoken and written Italian and coming from cross-sectional and longitudinal studies conducted over the last ten years will be briefly reviewed and discussed.
Italian Deaf demonstrate selective difficulties with aspects of grammatical morphology playing a syntactic rather than a semantic function.
Italian WS subjects display a particular asymmetric fragmentation within linguistic abilities: a profile of strenght in phonological abilities but serious deficits in semantic and morphosyntactic aspects of language.
The case of these two very different population can offer us important cues for investigating which aspects of language and specifically of grammar are influenced by modality of perception