Various theories of developmental language impairments have sought to explain these impairments in modality-specific ways – for example, that the language deficits in SLI or Down syndrome arise from impairments in auditory processing. Studies of signers with language impairments, especially those who are bilingual in a spoken language as well as a sign language, provide a unique opportunity to contrast abilities across language in two modalities (cross-modal bilingualism). The aim of the article is to examine what developmental sign language impairments can tell us about the relationship between language impairments and modality. A series of individual and small group studies are presented here illustrating language impairments in sign language users and cross-modal bilinguals, comprising Landau-Kleffner syndrome, Williams syndrome, Down syndrome, Autism and SLI. We conclude by suggesting how studies of sign language impairments can assist researchers to explore how different language impairments originate from different parts of the cognitive, linguistic and perceptual systems.
