The psycholinguistics of signed and spoken languages: how biology affects processing

inguistic research over the last few decades has revealed substantial similarities between the structure of signed and spoken languages (for reviews see Emmorey, 2002; Sandler and Lillo-Martin, 2006). These similarities provide a strong basis for cross-modality comparisons, and also bring to light linguistic universals that hold for all human languages. In addition, however, biology-based distinctions between sign and speech are important, and can be exploited to discover how the input–output systems of language impact online language processing and affect the neurocognitive underpinnings of language comprehension and production. For example, do the distinct perceptual and productive systems of signed and spoken languages exert differing constraints on the nature of linguistic processing? Recent investigations have suggested that the modality in which a language is expressed can impact the psychological mechanisms required to decode and produce the linguistic signal. This chapter explores what aspects of language processing appear to be universal to all human languages and what aspects are affected by the particular characteristics of audition vs. vision, or by the differing constraints on manual versus oral articulation. Sign language processing is appropriately compared to speech processing, rather than to reading, because unlike written text, which can be characterized as “visual language,” sign language consists of dynamic and constantly changing forms rather than static symbols. Further, neither sign language nor spoken language comes pre-segmented into words and sentences for the perceiver. The production of writing, although performed by the hand, differs substantially from sign language production because writing derives its structure from a separate system (the orthography of a spoken language). In contrast to written language, sign and speech are both primary language systems, acquired during infancy and early childhood without formal instruction.