Neuroplasticity mechanisms in post-stroke aphasia: investigating the differential role of the domain-general multiple demand and language networks
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Abstract
Language recovery trajectories of people with post-stroke aphasia (PWA) vary widely and while some people reach complete recovery, others have to live with this chronic disability for years. Multiple neuroplasticity mechanisms have been proposed to explain inter-individual differences in language recovery after stroke. Previous literature suggests that spared parts of the language network reorganize over time and regain their language processing function. Furthermore, evidence from other studies indicates that domain-general cognitive control networks, particularly the multiple demand network (MD), could assist the language network in processing language post-stroke. This dissertation first introduces a new theoretical framework to investigate the different neuroplasticity mechanisms supporting language recovery after a stroke accounting for homeostatic plasticity, Hebbian learning plasticity and cognitive control mechanisms. Second, this dissertation examines 1) how the MD network is engaged during language processing in PWA using fMRI precision mapping and 2) how the engagement of the MD network during language processing relates to post-stroke language abilities and stroke damage.
The first study used subject-specific precision mapping to identify the language-specific and MD networks within each PWA and examined differences with age-matched controls in activity and connectivity patterns within and between these two networks during language processing. People with aphasia recruited mainly the spared parts of the language network during language comprehension tasks. Despite a widespread disruption of within-network and between-network connectivity, the language-specific and the MD networks remained functionally dissociated in PWA and the engagement of the MD network in PWA was minimal during language comprehension, similar to controls.
The second study investigated how activity and connectivity within and between the language and MD networks relate to linguistic and nonlinguistic cognitive abilities after a stroke causing aphasia. The findings indicated that the extent of recovered general linguistic abilities was mainly explained by the level of language activity in the language network, whereas verbal expression abilities were better explained by the degree of connectivity between the language and the MD networks. The study further revealed that nonlinguistic cognitive abilities in PWA were related to the strength of the MD network connectivity.
In the third study, we examined the influence of the extent and location of stroke damage in language regions on the compensatory potential of the MD network during language processing. Results suggest that the engagement of the MD network during language processing and its integration with the language network may be beneficial for individuals with extensive damage in the language regions, in particular in temporal regions, and detrimental for individuals with minimal damage in language regions.
The results of these studies suggest that the MD network maintains its domain-general support role in PWA, as it does in controls, when general language processing becomes difficult and neural language resources are reduced. Future studies should further investigate the engagement and interaction of the language and MD networks during language production and at various stages of recovery.
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Attribution-NonCommercial-NoDerivatives 4.0 International