THE BIMODAL BILINGUAL BRAIN : FMRI INVESTIGATIONS CONCERNING THE CORTICAL DISTRIBUTION AND DIFFERENTIATION OF SIGNED LANGUAGE AND SPEECHREADING

Abstract

Many users of signed languages also have access to a spoken language. They are bilin- gual in two modalities : spoken language and signed language. Here we consider some fmri findings relevant to bimodal bilingualism. We explored comprehension of signs and of seen spoken words in bimodal bilinguals - native signers of British Sign Language (bsl) who are proficient speechreaders of English. Both deaf and hearing bimodal bilinguals were tested. Seen words and signs activated different regions of the temporal lobes bilaterally. Signs activated more posterior and inferior regions, whereas seen speech activated middle and superior posterior temporal regions to a greater extent. We also observed characteristic dissociations within BsL in the bimodal bilingual participants depen- dent on hearing status. In deaf respondents, manual signs with 'mouthings' (oral speechlike actions) and manual signs with 'mouth gestures' (oral non-speechlike actions) showed distinctive patterns that resembled those where speech and sign were contrasted directly. The dissociated pattern was only partly replicated in hearing bimodal bilinguals. That is, hearing status can moderate cortical activation related to oral and manual actions in sl processing. A further analysis identified amodal language regions in the deaf bimodal brain. Superior temporal regions that were activated for both si- gns and seen speech in deaf bilinguals were only activated by seen speech in hearing monolinguals.