Studies with deaf ASL signers with brain lesions suggest that the neural organization for language in deaf signers is similar to that found in hearing individuals at the gross hemispheric level. What is not known from the lesion studies is the extent to which there are subtle differences in the neural organization of language related systems within the left hemisphere of deaf individuals. The proposed experiments are designed to identify and characterize those aspects of the visual-gestural modality of sign language that may alter the neural basis of lexical retrieval and production. The experiments complement those proposed in Projects 1 and 3, such that direct comparisons can be made between the neural systems involved in retrieving spoken English words and in retrieving manual ASL signs. We will test four principal hypotheses in a series of [150]H2O PET experiments conducted in 60 normal deaf native ASL signers: (1) The retrieval of ASL signs denoting concrete entities belonging to distinct conceptual categories depends upon anatomically separable regions which roughly mirror those found in hearing English speakers. (2) The distribution of neural systems involved in mediation of lexical retrieval of signs for concrete entities can be partially altered when the form of a sign itself is conceptually-based (e.g., as in iconic vs. non-iconic ASL signs, but not when the difference between sign classes is linguistically- based, as in finger spelled vs. native signs). (3) The retrieval of signs for actions with distinct sensory-motor attributes depends upon partially segregated neural systems, and the motorically-based iconicity of ASL signs for actions with the hand/arm may alter this neural distribution. (4) Language modality may partially alter the neural systems involved in the retrieval of certain lexical expressions that refer to spatial relations; specifically, the retrieval of ASL classifier expressions which indicate spatial relations via a direct mapping from physical space to signing space will depend upon neural systems partially segregated from those involved in the retrieval of English and ASL prepositions denoting the same spatial relations. In addition to enhancing our understanding of the neurobiology of sign language (and thus of language itself), the results will be essential for improvements in diagnosis and for the development of rehabilitation strategies in deaf patients.
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