Hand and mouth: cortical correlates of lexical processing in British Sign Language and speechreading English

Spoken languages use one set of articulators -- the vocal tract, whereas signed languages use multiple articulators, including both manual and facial actions. How sensitive are the cortical circuits for language processing to the particular articulators that are observed? This question can only be addressed with participants who use both speech and a signed language. In this study, we used functional magnetic resonance imaging to compare the processing of speechreading and sign processing in deaf native signers of British Sign Language (BSL) who were also proficient speechreaders. The following questions were addressed: To what extent do these different language types rely on a common brain network? To what extent do the patterns of activation differ? How are these networks affected by the articulators that languages use? Common peri-sylvian regions were activated both for speechreading English words and for BSL signs. Distinctive activation was also observed reflecting the language form. Speechreading elicited greater activation in the left mid-superior temporal cortex than BSL, whereas BSL processing generated greater activation at the temporo-parieto-occipital junction in both hemispheres. We probed this distinction further within BSL, where manual signs can be accompanied by different types of mouth action. BSL signs with speech-like mouth actions showed greater superior temporal activation, whereas signs made with non-speech-like mouth actions showed more activation in posterior and inferior temporal regions. Distinct regions within the temporal cortex are not only differentially sensitive to perception of the distinctive articulators for speech and for sign but also show sensitivity to the different articulators within the (signed) language.