Conceptual knowledge (or semantic knowledge) refers to our shared knowledge for words, objects, people and emotions. The anterior temporal lobes (ATLs) have been identified as a critical region for the representation of conceptual knowledge through convergent evidence from fMRI in healthy participants, cortical electrode implantation and damage-deficit correlations. With the involvement of the ATLs established, recent research has begun to focus on the functions of subregions of the ATLs - with particular interest surrounding the functions of the left and right ATLs. This thesis investigated three main research questions: (1) What are the functions of the left and right ATLs in semantic representation? (2) How does unilateral damage affect the semantic system and what mechanisms underlie the robustness of the system? (3) Do functional gradations exist within the ATLs? These questions were addressed using convergent methodologies including functional neuroimaging and transcranial magnetic stimulation (TMS) in healthy participants and behavioural and neuroimaging investigations in patients who have undergone unilateral ATL resection. To address the question of left vs. right ATL function, this thesis began by directly comparing the predictions of the different accounts of ATL function in a large-scale meta-analysis of the existing neuroimaging literature (Chapter 2) and in a large sample of patients who had undergone unilateral left or right ATL resection (Chapter 3). The overarching finding was that conceptual knowledge is underpinned by a primarily bilateral ATL system, whereby both the left and right ATLs are critical for normal semantic processing. Secondary to this bilateral representation, relative functional gradations were observed both between and within the ATLs. To address the second research question, Chapter 4 investigated the robustness of the semantic system to unilateral damage, specifically regions involved in the maintenance of conceptual knowledge were localised. Results showed that upregulation occurred within regions previously associated with semantic knowledge. The upregulation of activation after unilateral resection also mimicked the upregulation in control participants during more challenging semantic processing. Chapter 5 examined the behavioural relevance of upregulation in the contralateral ATL after unilateral perturbation using a novel TMS protocol in healthy participants. The findings observed here suggest that the bilateral ATL system is resistant to a degree of unilateral damage/perturbation because semantic representations are distributed between the hemispheres. Therefore, unilateral damage/disruption only results in a mild semantic impairment, as the undamaged/unperturbed hemisphere is available to compensate. Finally, Chapter 6 explored functional gradations within the ATLs by comparing responses in the ventral ATL to different conceptual categories, presented as visual and auditory inputs. The functional gradations observed here are proposed to emerge via differential structural and functional connectivity between the ATLs and sensory-motor and limbic cortices.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:764532 |
| Date | January 2017 |
| Creators | Rice, Grace |
| Contributors | Lambon Ralph, Matthew |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/using-neuroimaging-and-transcranial-magnetic-stimulation-to-probe-conceptual-knowledge-in-the-left-and-right-anterior-temporal-lobes(b834299d-8650-4c7e-89f9-0c503ce64868).html |
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