Biology-based distinctions between sign and speech 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. This article explores which aspects of language processing appear to be universal to all human languages and which are affected by the particular characteristics of audition versus vision, or by the differing constraints on manual versus oral articulation. Neither sign language nor spoken language comes pre-segmented into words and sentences for the...

Biology-based distinctions between sign and speech 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. This article explores which aspects of language processing appear to be universal to all human languages and which are affected by the particular characteristics of audition versus vision, or by the differing constraints on manual versus oral articulation. Neither sign language nor spoken language comes pre-segmented into words and sentences for the perceiver. In contrast to written language, sign and speech are both primary language systems, acquired during infancy and early childhood without formal instruction. This article discusses sign perception and visual processing, phonology in a language without sound, categorical perception in sign language, processing universals and modality effects in the mental lexicon, the time course of sign versus word recognition, tip-of-the-fingers, non-concatenative morphology, the unique role of space for signed languages, and speaking versus signing.