Neural mechanisms underlying word learning in post-stroke aphasia and healthy ageing

Abstract

The overarching aim of this thesis was to explore the neural underpinnings of word learning in neurotypical populations and post-stroke aphasia. Word learning has been previously proposed to fit within a powerful framework, the Complementary Learning Systems (CLS) model. This model considers the acquisition, consolidation, and generalisation of new knowledge, whereby a hippocampal system complements neocortical memory systems in a computational trade-off between learning episode specifics and regularities across episodes. In this thesis, this proposed neural division of labour was tested in adults through an investigation of the consolidation and long-term retention of newly learned, native vocabulary and neuroimaging. Consistent with CLS theory, naming of newly learned items was functionally supported by a combination of regions associated with episodic memory and the language-semantic areas that supported established vocabulary (Chapter 1). Additionally, the results suggest that interactivity between the complementary episodic and domain-specific neocortical memory systems supporting new knowledge is time-limited (Chapter 2). The structural correlates of word learning were also explored. Associations between right-sided structural intensities and better behavioural performance were moderated by structural integrity of dominant left hemispheric language regions. These results are considered within a set of ideas termed variable neuro-displacement, a principle that may be applicable across several different populations (Chapter 3). In post-stroke aphasia, the functional correlates of word re-learning were largely normal up to a critical point of damage to dominant language processing regions (Chapter 4). Similarly, novel, native word learning in aphasia largely mirrored that of healthy controls. Although the functional correlates were largely normal, structural abnormalities in the dorsal and ventral white matter pathways were associated with the longevity of learning retention (Chapter 5). Overall, the results of the thesis provide evidence of a general framework of word learning in healthy adults and post-stroke aphasia, which fits within the CLS model of knowledge acquisition. This framework held up until a critical point of damage in post-stroke aphasia. The theoretical and clinical implications of these results are discussed.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Matthew Lambon Ralph (Supervisor) & Anna Woollams (Supervisor)