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Imaging the functional reorganization of the language network in recovery from aphasia

Friday, October 7, 8:00 - 10:00 am EDT, Regency Ballroom

Organizer: Stephen Wilson1; 1Vanderbilt University Medical Center
Presenters: Stephen Wilson, Anika Stockert, Fatemeh Geranmayeh, Julius Fridriksson, Cathy Price, Matt Lambon Ralph

Most individuals with aphasia after a stroke experience some degree of recovery of language function over time. Recovery from aphasia is thought to depend on neural plasticity, that is, functional reorganization of surviving brain regions such that they take on new or expanded roles in language processing. Using functional neuroimaging, we are beginning to make progress on understanding the mechanisms that underlie this process of functional reorganization. This symposium focuses on novel and innovative approaches to functional imaging of neuroplasticity in aphasia, highlighting important emerging themes including the dependence of reorganization patterns on lesion location, the importance of task design and patient performance considerations, consistency and variability of functional maps, the role of domain general networks in recovery, and the necessity of integrating behavioral and imaging findings in a theoretically grounded neurocomputational account.

Presentations

Imaging the language network in aphasia

Stephen Wilson1; 1Vanderbilt University Medical Center

As an introduction to the symposium, I will describe some of the challenges we need to overcome to make progress in characterizing the nature of neuroplasticity in recovery from post-stroke aphasia. First, we need valid, reliable, and practical methods for identifying and characterizing language regions of the brain in individuals with aphasia. Second, we need to control or account for performance differences between individuals, groups, and time points over the course of recovery, because task performance has a dramatic effect on brain activation patterns. Third, we need to work together to recruit large samples of individuals with aphasia so that we can perform statistically rigorous analyses and unravel the complex set of relationships between structural damage to different brain areas, functional changes in language regions and other regions, and behavioral outcomes. The remaining talks in this symposium describe five research programs that address these challenges in diverse and innovative ways.

Dynamics of language reorganization after temporo-parietal and frontal stroke

Anika Stockert1; 1University of Leipzig Medical Center

In my talk, I will present longitudinal fMRI data of patients with circumscribed lesions of either left temporo-parietal or frontal cortex. Lesion-dependent network reorganisation includes the following mechanisms: First, global network disturbance in the acute phase is characterised by reduced language activation including areas distant to the lesion (i.e., diaschisis) and subsequent subacute network reactivation (i.e., resolution of diaschisis). These phenomena are driven by temporo-parietal lesions. Second, a lesion-independent sequential activation pattern involves increased activity of perilesional cortex and bilateral domain-general networks in the subacute phase, followed by reorganisation of left temporal language areas in the chronic phase. Third, involvement of homotopic brain areas is observed only in patients with frontal lesions. Finally, irrespective of lesion location, language reorganisation predominantly occurs in pre-existing networks. These findings highlight that the dynamics of language reorganisation clearly depend on lesion location, creating new opportunities for neurobiologically motivated strategies for language rehabilitation.

Domain-general brain regions and language recovery after stroke

Fatemeh Geranmayeh1; 1Imperial College London

Post-stroke aphasia recovery does not occur in a vacuum. Rather recovery is likely to include a complex interaction between residual brain systems underpinning language function, brain systems involved in learning, and brain regions able to flexibly adapt to increasing task demand in the face of the newly imposed cognitive challenge. I will present evidence from fMRI studies that show that the activity of domain-general brain regions outside the classical language network, is related to better language outcomes after stroke. I will discuss the implications of these findings for future studies of aphasic recovery.

Brain systems that support treated recovery from aphasia

Julius Fridriksson1; 1University of South Carolina

Although it is now generally accepted that behavioral therapy can improve language processing in aphasia, the location of brain areas that support this recovery remains elusive. In a relatively large study (N=126) that included six weeks of aphasia therapy, we tested whether brain activation at baseline and changes in activation before and after therapy are associated with outcome. Brain activation was measured using fMRI and cortical regions of interest (ROIs) were grouped based on their location: left hemisphere, right hemisphere, domain-general regions, and peri-lesional regions. An initial prediction model that included biographical factors (aphasia severity, age, and diabetes) explained approximately 30% of the variance in therapy outcome. Adding fMRI data to the model accounted for as much as 50% of the total variance. Specific comparisons across the different groups of ROIs revealed insights into where brain changes occur that support the greatest improvements in language processing in aphasia.

Lesion site-dependence of functional reorganisation of speech production after stroke

Cathy Price1; 1University College London

We are using fMRI to investigate how functional reorganisation after stroke depends on lesion site. We have found that recovery of speech production increases activation in: (1) the right superior cerebellum in patients with focal left inferior frontal lobe damage compared to neurologically intact controls or patients with other left hemisphere damage; (2) the supplementary motor area in patients with focal right cerebellum damage compared to controls or patients with other focal cerebellar damage; and (3) the left posterior superior temporal cortex in patients with right hemisphere damage to motor regions (premotor and insula cortex) compared to controls or patients with other right hemisphere lesions. These results highlight the importance of considering lesion site in fMRI studies of post-stroke recovery and generate hypotheses for testing whether neural plasticity supporting recovery depends on the underlying anatomical/functional connectivity between different brain regions, and/or changes in cognitive strategies/demands.

Towards a neurocomputational account of partial recovery in post-stroke aphasia

Matt Lambon Ralph1; 1University of Cambridge

We developed a neurocomputational, bilateral pathway model of spoken language production, designed to provide a unified framework to assimilate data from neurologically intact individuals and individuals with aphasia. The model encapsulates various key computational principles including differential computational resources, emergent division of labour across pathways, and experience-dependent plasticity-related recovery. The model provides an explanation for the typical bilateral yet asymmetric lateralisation of language, chronic aphasia after left rather than right hemisphere lesions, and the basis of partial recovery of function in patients, which reflects a combination of retuning within the damage pathway and a changed division of labour across pathways. The model provides a formal basis for understanding the relationship between behavioural performance and brain activation. Overall, the unified model is consistent with the degeneracy and variable displacement theories of language recovery, and adds potential computational insights about the neural machinery underlying language processing and plasticity-related recovery following damage.

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