The role of motor systems on the functional neural organisation of language processing: Cross-linguistic evidence from sign and speech

Background: Theories of speech processing suggest different features as the crucial determinants of language processing. Generative grammar suggests that grammatical information drives speech perception [1] whereas the motor theory of speech perception proposes that language perception involves access to the speech production network. Close links between brain areas involved in motor production and the perception of meaningful actions, including speech, have been clearly established.
For speech perception, both grammar and the potential connection between perception and production are confounded - as speech conveys grammatical information serially via a single articulator (mouth). However, signed languages (SLs), are produced by multiple articulators -- hands, mouth, head, which are under independent cortical control.
Objectives: This project will use an integrative neuroscience approach combining neuroimaging (functional magnetic resonance imaging, fMRI) and neurostimulation (motor evoked potentials, MEP, transcranial magnetic stimulation, TMS) to systematically explore the articulatory effects on the functional organisation of language processing in deaf native signers of British Sign Language (BSL). We will conduct parallel studies in hearing speakers of English where the questions of interest are permissible within the language structure, in order to gain a cross-linguistic approach to the motor systems in language perception.
Methods: FMRI will be used to explore the effects of the perceived language articulator on lexical and grammatical processing by examining the patterns of activation in signers as they process different signed syntactic structures such as negation, which can be realised solely by non-manual markers (headshake), or by coextensive non-manual and manual markers. A second study will focus on hearing non-signers processing audio-visual (AV) English to contrast AV sentences with non-obligatory facial actions. The findings will provide information about how the motor and grammatical systems may interact.
Neurostimulation will be used to examine the relationship between the relevant cortical areas by recording MEPs in response to single pulses to the hand and mouth regions of premotor cortex in deaf signers viewing language produced using different articulators and to establish the causal link between motor production and language perception using TMS. Including non-linguistic mouth and hand actions will allow us to assess the domain specificity of these effects. A parallel experiment in hearing non-signers viewing the same materials will allow us to separate the effects due to size and trajectory of articulatory movement (hand > mouth) and language knowledge (sign and speech reading in deaf participants, speech reading-only in hearing participants). Concurrent behavioural measures may offer insight into individual variability in these neuroimaging and TMS data.
Schedule: Year1 -fMRI-BSL study and BSL Level 1, Year2 -fMRI-AV & MEP-BSL studies and BSL Level 2, Year3 -MEP-AV & TMS-BSL studies , Year4 TMS-AV study
Impact: This research will provide a major contribution to our understanding of the cortical organisation of language processing as well ultimately informing the development of therapeutic interventions for language impairment (e.g., aphasia). In addition, working on the project will develop the student's academic career through gaining expertise in fMRI, MEP and TMS techniques, and will establish (or continue) a strong record of high quality papers published in high impact, peer-reviewed journals.