Signed and spoken language: a unique underlying system?

Sign language has only recently become a topic of investigation in cognitive neuroscience and psycholinguistics. In this paper, we review research from these two fields; in particular, we compare spoken and signed language by looking at data concerning either cortical representations or early acquisition. As to cognitive neuroscience, we show that clinical neuropsychological data regarding sign language is partially inconsistent with imaging data. Indeed, whereas both clinical neuropsychology and imagery show the involvement of the left hemisphere in sign language processing, only the latter highlights the importance of the right hemisphere. We discuss several possible interpretations of these contrasting findings. As to psycholinguistics, we survey research on the earliest stages of the acquisition of spoken language, and consider these stages in the acquisition of sign language. We conjecture that under favorable circumstances, deaf children exploit sign input to gain entry into the language system with the same facility as hearing children do with spoken input. More data, however, are needed in order to gain a fuller understanding of the relation of different kinds of natural languages to both the underlying anatomical representations and their early acquisition.