An Investigation of Phonological and Semantic Control Using TMS and fMRI

This thesis aimed to investigate the neural basis of linguistic and semantic control across brain networks, using fMRI and TMS. Firstly we assessed the contribution of premotor cortex (PMC) to speech perception: TMS disrupted phonological but not semantic judgments, indicating a constrained role for PMC in tasks involving explicit access to phonemic representations, but not in accessing meaning. Chapter 3 examined the role of brain regions within the language network to both domain (phonological/semantic) and executive control demands. The results suggest that there are specialisations at the extreme ends of LIFG for phonology (PMC/BA 44) and semantics (BA 47), as well as contributions from posterior temporal and parietal cortex to both domains. Furthermore, Chapter 4 investigated the role of these brain areas to amodal semantic cognition using picture and verbal semantic associations which varied in their difficulty (to manipulate semantic control demands). The findings suggest that while BA45/44 respond to control demands across modalities, there are specialisations within the semantic control network; for example, bilateral BA 47 showed a verbal semantic preference. Chapter 5 used TMS to further probe the role of BA 44, 47 and pMTG across domain (phonology/semantic) and modality (picture/verbal), confirming a preference for verbal semantic material in BA 44 and 47, and highlighting an amodal contribution of pMTG to associative semantic judgments. Lastly, Chapter 6 focussed on semantic control and manipulated retrieval and selection demands using a cyclical picture naming paradigm. We found that TMS to LIFG caused a specific disruption of naming at the point at which the demands on both selection and retrieval were maximal, while TMS to pMTG caused no disruption of retrieval or selection processes. The findings of this thesis shed additional light on the role of various areas throughout the language networks to domain, modality and control.