David P. Corina and Laurel A. Lawyer
Current understanding of the brain systems involved in language has been largely derived through the study of spoken languages. However, as naturally occurring manual-visual sign languages used in Deaf communities attest, human languages are not limited to the oral-aural modality. The existence of sign languages used in Deaf communities provides a unique opportunity to test the generality of biological models of human language. The comparison of neural systems supporting spoken and signed language allows researchers to distinguish brain systems that are common across human languages from brain systems that reflect the modality of language expression (e.g., auditory perceptual versus visual perceptual processes). These comparisons make it possible to address long-standing issues regarding the expression of language in the brain. Neuroimaging and aphasia studies of deaf signers reveal great commonalties in the neural systems used for sign and speech and provide evidence for a core neurobiological substrate for human linguistic communication. Also observed are cases of modality-specific patterns of brain activation and modality-specific language impairments that speak to functional specialization based upon sensory and motor systems unique to speech and sign. As increasingly sophisticated neurobiological models of language processing emerge, researchers are poised to ask new questions about the biological substrates of human communication is all its various forms.
Richard P. Meier
This essay considers the acquisition of sign languages as first languages. Most deaf children are born to hearing parents, but a minority have deaf parents. Deaf children of deaf parents receive early access to a conventional sign language. The time course of acquisition in these children is compared to the developmental milestones in children learning spoken languages. The two language modalities—the oral-aural modality of speech and the visual-gestural modality of sign—place differing constraints on languages and offer differing resources to languages. Possible modality effects on first-language acquisition are considered. Historically, many deaf infants born to hearing parents have had little access to a conventional language. However, these children sometimes elaborate “home sign” systems. Lastly, the role of early experience in language acquisition is considered. Deaf children of hearing parents are immersed in a first language at varying ages, enabling a test of the critical-period hypothesis.