Neural basis of language and motor behaviour: Perspectives from American Sign Language

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Abstract
Since sign languages utilize a transmission modality different from that of spoken languages, the study of sign languages offers a unique opportunity for insight into the nature of neural mechanisms for language. To understand the extent to which neural mechanisms for language are modality independent, and the extent to which they are modality-bound, we naturally turn to sign languages because through their study we can begm to solve the problem of brain organization for language in general. American Sign Language (ASL) displays both complex linguistic structures and spatial relations, thereby exhibiting properties for which each of the hemispheres of hearing people shows a differing specialization. Because of these dual attributes, the study of ASL offers a valuable opportunity for refining our concept of cerebral specialization for language. We review some recent findings, and present new ones, bearing on the hnctional organization of the brain for sign language. We focus on studies of sign language that bring to bear special properties of the visual-gestural modality in investigating brain and language relationships. One such property is the interplay between linguistic and spatial relations. Another is the interplay between control of arm movements and linguistic expression. Since movements of the articulators in sign, unlike the case in speech, are directly observable, we can investigate signing not only as linguistic behaviour, but also as motor behaviour, and directly contrast linguistic- representational and motor-execution disorders of signing. Furthermore, our techniques of three-dimensional motion analysis open up a wide range of new research opportunities in the investigation of neural control of language and movement. We begin with a dlscussion of the nature of language in the visual-gestural modahty.