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Auteur: Karen EMMOREY

The Neurobiology of Language - Perspectives from Sign Language

Abstract/Résumé: The study of sign languages provides a powerful tool for investigating the neurobiology of human language. Signed languages differ dramatically from spoken languages with respect to linguistic articulators and perceptual systems required for comprehension. I explore whether key brain areas identified for processing spoken language are also critical for the comprehension and production of sign languages. Specifically, the brain region historically known as Wernicke’s area is proximal to auditory cortex, but this area nonetheless supports language comprehension for visual-manual languages. In addition, Broca’s area is involved in modality-neutral processing during language production, while left parietal cortex appears to be uniquely engaged during sign language production. These findings indicate neural plasticity within left hemisphere language circuits and suggest that the human language faculty adapts to biological differences in the perceptual and motor systems exploited by signed and spoken languages. I also explore domains where language interfaces with other cognitive systems, specifically motion processing and action generation. I report two fMRI experiments that showed motion-related semantics modulates neural activity in motion-sensitive visual areas (MT+) for both spoken English and for ASL. These results indicate that a) neurons in modality-specific cortices respond to higher-level linguistic semantic features and b) linguistic biology does not alter the interface between linguistic and sensory representations. Within the domain of human action, the primary articulators for sign language (the hands) are also used for meaningful, but non-linguistic expressions, e.g., pantomime production. In contrast, the use of the speech articulators to express meaningful non-linguistic information is relatively limited. Furthermore, ASL handling classifier verbs, such as HAMMER and STIR, resemble the actions they denote. I report the results of a PET study that investigated the neural bases for the production of pantomimes, ASL handling verbs, and non-iconic ASL verbs. The results revealed that production of both types of ASL verbs engaged left inferior frontal cortex (including Broca’s area), which subserves lexical retrieval and selection functions. In contrast, pantomime production engaged bilateral superior parietal cortex, which is implicated in action planning and movement execution. In sum, the study of sign languages provides a unique window into the factors that do and do not influence the neural organization for language. As we begin to uncover the new biology of language, moving beyond the classic brain regions of Broca and Wernicke, investigations of sign language will help characterize and identify the neural architecture that supports the human language faculty.