Compensatory Language Reorganization in Two Distinct Stroke Populations

Post by Giulia Baracchini 

What's the science?

Our brain is organized in a number of large-scale networks, so lesions in particular brain areas due to stroke will affect functionally connected regions as well. Post-stroke recovery mechanisms (i.e. brain plasticity) will similarly depend on lesion site and network architecture. How this framework applies to stroke in the context of language impairment (i.e., aphasia), how this framework varies based on lesion site and how it differently impacts acute (1 week after stroke onset), subacute (1-2 weeks) and chronic (> 6 months) post-stroke recovery phases remains unknown. This week in Brain, Stockert and colleagues used functional magnetic resonance imaging (fMRI) to characterize and directly compare language plasticity mechanisms from acute to chronic post-stroke phases in a longitudinal cohort of left temporo-parietal and frontal stroke patients. 

How did they do it?

Thirty-four first ischaemic stroke patients were included in the study, of which 17 showed a left frontal lesion and 17 a left temporo-parietal lesion. Seventeen healthy age-matched controls were also enrolled in the study. All participants underwent fMRI. While in the scanner, participants completed an auditory comprehension task, in which they were asked to actively listen to short German sentences played both in regular order and in reverse. The authors compared activity in language-related brain networks between the frontal lesion stroke patients, the temporo-parietal lesion stroke patients, and controls. Language brain networks were defined as comprising (i) language-relevant areas, (ii) perilesional regions (i.e., 3-15 mm around the lesioned tissue) and (iii) lesion-homolog areas (contralateral areas to the lesioned ones). Importantly, the two patient groups were scanned on three separate occasions, during acute (1 week after stroke onset), subacute (1-2 weeks) and chronic (> 6 months) post-stroke phases. This allowed the authors to investigate language reorganization mechanisms as a function of time post-stroke onset. 

In addition to fMRI, both patient groups completed — for each of the three post-stroke phases — the Aachen Aphasia Test. This test provides a behavioural measure of language production and comprehension performance and quantifies post-stroke language improvement. The authors measured the link between these behavioural scores and brain activation (via fMRI) within and across the two patient groups.

What did they find?

When compared to healthy controls, temporo-parietal patients demonstrated an initial decrease in activity over a vast network including temporo-parietal regions and perilesional frontal areas. This initial global network dysregulation disappeared during the subacute phase and an increase in activity in bilateral frontal areas became apparent, suggesting the restoration of functional connections between temporo-parietal and frontal areas. In contrast, frontal patients in the acute phase reported local dysregulation effects within frontal areas and otherwise preserved perilesional (i.e. left temporo-parietal) and lesion-homolog areas (i.e. right frontal). These findings highlight the importance of considering the lesion site when evaluating early post-stroke mechanisms of language network plasticity. Similarly, when all post-stroke stages were considered together, only frontal patients showed recruitment of lesion-homolog regions, providing further evidence for lesion-dependent reorganization effects.

Lesion-independent reorganization mechanisms were also present during later post-stroke phases. Both temporo-parietal and frontal patients showed increased recruitment of perilesional areas and non-language-specific cognitive control networks (i.e. domain-general networks: cingulo-opercular and fronto-parietal networks) in subacute stages, followed by the recruitment of left language-specific temporal areas (anterior and posterior temporal areas) in chronic phases. These findings suggest that post-stroke functional reorganization occurs in pre-existing functionally connected networks and varies across time after stroke onset. Finally, an overall improvement in behavioural performance for both patient groups was seen, and this improvement was associated with activation in language-specific networks (in both groups) and domain-general networks (in temporo-parietal patients). This finding reflects compensatory mechanisms of language-related and domain-general networks during post-stroke recovery. 

What's the impact?

This study is the first to longitudinally assess post-stroke recovery mechanisms in two distinct stroke populations — temporo-parietal and frontal stroke patients — and directly compare their brain activation patterns. The results speak in favor of a network model for language reorganization that depends on lesion-site and varies based on time post-stroke onset. These novel findings will help to guide post-stroke aphasia rehabilitation strategies, such as non-invasive brain stimulation.

Stockert et al. Dynamics of language reorganization after left temporo-parietal and frontal stroke (2020). Access the original scientific publication here.