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Title: Temporal and spatial dynamics of the semantic network : explorations using Transcranial Magnetic Stimulation (TMS) and fMRI
Author: Jackson, Rebecca
ISNI:       0000 0004 5363 9828
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2014
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Convergent findings have elucidated the regions involved in semantic cognition. The anterior temporal lobes (ATL) act as a hub for multimodal semantic processing alongside modality-specific ‘spoke’ regions. In addition, areas of inferior parietal, posterior temporal and frontal cortex are necessary for semantic cognition. However, many questions remain. Little is known about the timing of the ATL or how distributed regions interact in order to perform semantic processing. In order to gain knowledge of the precise spatial and temporal dynamics of the ATL and semantic cognition network, a series of studies was performed. Chapter 3 investigated the time at which the ATL is necessary for a semantic judgement using chronometric TMS. The ATL was found to be necessary for semantic cognition from 400ms post-stimuli presentation. This is known to be a critical time for semantic processing. Processing of items presented in different modalities converges around this time. This supports the role of the ATL in multimodal semantic cognition. Chapter 4 used offline repetitive TMS to investigate the role of ATL subregions and posterior temporal cortex in semantic and phonological processing. However, no significant TMS effects were demonstrated. Chapter 5 employed dual echo fMRI to assess how different types of semantic relationships are instantiated within the brain. Association (spatially and temporally co-occurring concepts) and conceptual similarity (concepts sharing features) were shown to rely on the same cortical regions. This provides evidence against theories suggesting separate representational hubs for these different relationship types. Instead it supports the reliance of both relationship types on the ATL hub. These two kinds of relationship may be more similar than previously thought, with the hub-and-spoke model able to explain both. The semantic network identified here included ATL, posterior temporal, frontal and ventral parietal cortex. This network of semantic regions was shown to be interconnected in Chapter 6 during a semantic task (using a psychophysiological interaction analysis) and during rest (using a seed-based functional connectivity analysis). Differential connectivity was identified between the ventral ATL (to multimodal semantic regions) and the aSTG (to language-related regions). The semantic network overlapped with the default mode network (DMN) and involved regions previously found to constitute the frontoparietal network (FPN).Emergent questions related to the overlap between previously identified network and the semantic network were addressed with preliminary independent component analyses in Chapter 7. This showed the dynamic connectivity of the ATL in task and rest. The semantic network was found to be distinct from but overlapping with the DMN and FPN. The role of this network in semantic cognition was confirmed, whereas the DMN was not found to relate to semantic processing. The anterior DMN component appeared semantic based on activity alone, suggesting prior results relating the DMN to semantic cognition fail to take the dynamic connectivity of the regions in to account. The left FPN overlapped with semantic control regions but appeared to relate to more general control processes. When assessed with dual echo fMRI, the ATL appears to be highly connected in a dynamic fashion and may be an important region currently under-represented within studies of the connectome. Overall, these studies add to the hub-and-spoke model of semantic cognition, elucidating the types of relationship involved, how regions interact and the precise temporal and spatial dynamics of these areas.
Supervisor: Not available Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Conceptual processing ; fMRI ; TMS ; Semantics ; Resting state ; Language ; Functional connectivity