| Summary: | This thesis presents an investigation of brain networks mediating both language-specific and domain-general control of propositional speech production in health and following left hemisphere stroke. The research used univariate and multivariate analyses of functional magnetic resonance imaging data as participants produced sentential speech in response to stimuli (Speech). The activity of specific brain networks during Speech was compared against different baseline conditions. Two studies on healthy participants confirmed a language-specific role for a left fronto-temporo-parietal (LFTP) network. Propositional speech was accompanied by a relative suppression of activity in domain-general networks including a right fronto-temporo-parietal (RFTP) network, default mode network (DMN), and cingulo-opercular (CingOper) network. A separate study investigated alterations in the activity of these networks following a chronic left hemisphere stroke. The network measures in patients were related to those from the healthy participants. There was an up-regulation of the CingOper network during Speech, a task the patients found difficult. Although the activity within individual networks was not predictive of speech production in patients, the relative activity between networks was a predictor of both within-scanner and out-of-scanner performance, over and above that predicted from lesion volume and various demographic factors. Specifically, the robust functional imaging predictors were the differential activity between the DMN and both the left and right fronto-temporo-parietal networks. The patients also demonstrated an altered speech-related between-networks functional connectivity. A further study of cerebrovascular reactivity measurements suggested that signal changes in the healthy tissue remote from the infarct were unlikely to be confounded by cerebrovascular reactivity changes following stroke. The results of the thesis indicate that recovery of speech production after stroke, is not adequately captured by notions of brain regions 'taking over' lost domain-specific functions. Rather it is best considered as the interaction between what remains of domain-specific networks and the domain-general systems that regulate behaviour.
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