How functional network connectivity changes as a result of lesion and recovery: An investigation of the network phenotype of stroke

Cortical and Subcortical Mechanisms of Orthographic Word-form Learning

We examined the initial stages of orthographic learning in real time as literate adults learned spellings for spoken pseudowords during fMRI scanning. Participants were required to learn and store orthographic word forms because the pseudoword spellings were not uniquely predictable from sound to letter mappings. With eight learning trials per word form, we observed changes in the brain's response as learning was taking place. Accuracy was evaluated during learning, immediately after scanning, and 1 week later. We found evidence of two distinct learning systems-hippocampal and neocortical-operating during orthographic learning, consistent with the predictions of dual systems theories of learning/memory such as the complementary learning systems framework [McClelland, J. L., McNaughton, B. L., & O'Reilly, R. C. Why there are complementary learning systems in the hippocampus and neocortex: Insights from the successes and failures of connectionist models of learning and memory. Psychological Review, 102, 419-457, 1995]. The bilateral hippocampus and the visual word form area (VWFA) showed significant BOLD response changes over learning, with the former exhibiting a rising pattern and the latter exhibiting a falling pattern. Moreover, greater BOLD signal increase in the hippocampus was associated with better postscan recall. In addition, we identified two distinct bilateral brain networks that mirrored the rising and falling patterns of the hippocampus and VWFA. Functional connectivity analysis revealed that regions within each network were internally synchronized. These novel findings highlight, for the first time, the relevance of multiple learning systems in orthographic learning and provide a paradigm that can be used to address critical gaps in our understanding of the neural bases of orthographic learning in general and orthographic word-form learning specifically.

Remapping and Reconnecting the Language Network after Stroke

Here, we review the literature on neurotypical individuals and individuals with post-stroke aphasia showing that right-hemisphere regions homologous to language network and other regions, like the right cerebellum, are activated in language tasks and support language even in healthy people. We propose that language recovery in post-stroke aphasia occurs largely by potentiating the right hemisphere network homologous to the language network and other networks that previously supported language to a lesser degree and by modulating connection strength between nodes of the right-hemisphere language network and undamaged nodes of the left-hemisphere language network. Based on this premise (supported by evidence we review), we propose that interventions should be aimed at potentiating the right-hemisphere language network through Hebbian learning or by augmenting connections between network nodes through neuroplasticity, such as non-invasive brain stimulation and perhaps modulation of neurotransmitters involved in neuroplasticity. We review aphasia treatment studies that have taken this approach. We conclude that further aphasia rehabilitation with this aim is justified.

Cognitive aging and the life course: A new look at the Scaffolding theory

Our understanding of human neurocognitive aging, its developmental roots, and life course influences has been transformed by brain imaging technologies, increasing availability of longitudinal data sets, and analytic advances. The Scaffolding Theory of Aging and Cognition is a life course model, proposed originally in 2009, featuring adaptivity and compensatory potential as lifelong mechanisms for meeting neurocognitive challenges posed by the environment and by developing or declining brain circuitry. Here, we review the scaffolding theory in relation to new evidence addressing when during the life course potentially enriching and depleting factors exert their effects on brain health and scaffolding, and we consider the implications for separable, and potentially reciprocal, influences on the level of cognitive function and the rate of decline in later life.

Switching between External and Internal Attention in Hippocampal Networks

Everyday experience requires processing external signals from the world around us and internal information retrieved from memory. To do both, the brain must fluctuate between states that are optimized for external versus internal attention. Here, we focus on the hippocampus as a region that may serve at the interface between these forms of attention and ask how it switches between prioritizing sensory signals from the external world versus internal signals related to memories and thoughts. Pharmacological, computational, and animal studies have identified input from the cholinergic basal forebrain as important for biasing the hippocampus toward processing external information, whereas complementary research suggests the dorsal attention network (DAN) may aid in allocating attentional resources toward accessing internal information. We therefore tested the hypothesis that the basal forebrain and DAN drive the hippocampus toward external and internal attention, respectively. We used data from 29 human participants (17 female) who completed two attention tasks during fMRI. One task (memory-guided) required proportionally more internal attention, and proportionally less external attention, than the other (explicitly instructed). We discovered that background functional connectivity between the basal forebrain and hippocampus was stronger during the explicitly instructed versus memory-guided task. In contrast, DAN-hippocampus background connectivity was stronger during the memory-guided versus explicitly instructed task. Finally, the strength of DAN-hippocampus background connectivity was correlated with performance on the memory-guided but not explicitly instructed task. Together, these results provide evidence that the basal forebrain and DAN may modulate the hippocampus to switch between external and internal attention.SIGNIFICANCE STATEMENT How does the brain balance the need to pay attention to internal thoughts and external sensations? We focused on the human hippocampus, a region that may serve at the interface between internal and external attention, and asked how its functional connectivity varies based on attentional states. The hippocampus was more strongly coupled with the cholinergic basal forebrain when attentional states were guided by the external world rather than retrieved memories. This pattern flipped for functional connectivity between the hippocampus and dorsal attention network, which was higher for attention tasks that were guided by memory rather than external cues. Together, these findings show that distinct networks in the brain may modulate the hippocampus to switch between external and internal attention.

The right hemisphere's capacity for language: evidence from primary progressive aphasia

The role of the right hemisphere (RH) in core language processes is still a matter of intense debate. Most of the relevant evidence has come from studies of gray matter, with relatively little research on RH white matter (WM) connectivity. Using Diffusion Tensor Imaging-based tractography, the current work examined the role of the two hemispheres in language processing in 33 individuals with Primary Progressive Aphasia (PPA), aiming to better characterize the contribution of the RH to language processing in the context of left hemisphere (LH) damage. The findings confirm the impact of PPA on the integrity of the WM language tracts in the LH. Additionally, an examination of the relationship between tract integrity and language behaviors provides robust evidence of the involvement of the WM language tracts of both hemispheres in language processing in PPA. Importantly, this study provides novel evidence of a unique contribution of the RH to language processing (i.e. a contribution independent from that of the language-dominant LH). Finally, we provide evidence that the RH contribution is specific to language processing rather than being domain general. These findings allow us to better characterize the role of RH in language processing, particularly in the context of LH damage.

The analysis of brain functional connectivity of post-stroke cognitive impairment patients: an fNIRS study

Background

Post-stroke cognitive impairment (PSCI) is a considerable risk factor for developing dementia and reoccurrence of stroke. Understanding the neural mechanisms of cognitive impairment after stroke can facilitate early identification and intervention.

Objectives

Using functional near-infrared spectroscopy (fNRIS), the present study aimed to examine whether resting-state functional connectivity (FC) of brain networks differs in patients with PSCI, patients with Non-PSCI (NPSCI), and healthy controls (HCs), and whether these features could be used for clinical diagnosis of PSCI.

Methods

The present study recruited 16 HCs and 32 post-stroke patients. Based on the diagnostic criteria of PSCI, post-stroke patients were divided to the PSCI or NPSCI group. All participants underwent a 6-min resting-state fNRIS test to measure the hemodynamic responses from regions of interests (ROIs) that were primarily distributed in the prefrontal, somatosensory, and motor cortices.

Results

The results showed that, when compared to the HC group, the PSCI group exhibited significantly decreased interhemispheric FC and intra-right hemispheric FC. ROI analyses showed significantly decreased FC among the regions of somatosensory cortex, dorsolateral prefrontal cortex, and medial prefrontal cortex for the PSCI group than for the HC group. However, no significant difference was found in the FC between the PSCI and the NPSCI groups.

Conclusion

Our findings provide evidence for compromised interhemispheric and intra-right hemispheric functional connectivity in patients with PSCI, suggesting that fNIRS is a promising approach to investigate the effects of stroke on functional connectivity of brain networks.

Treatment-induced neural reorganization in aphasia is language-domain specific: Evidence from a large-scale fMRI study

Studies investigating the effects of language intervention on the re-organization of language networks in chronic aphasia have resulted in mixed findings, likely related to-among other factors-the language function targeted during treatment. The present study investigated the effects of the type of treatment provided on neural reorganization. Seventy individuals with chronic stroke-induced aphasia, recruited from three research laboratories and meeting criteria for agrammatism, anomia or dysgraphia were assigned to either treatment (N = 51) or control (N = 19) groups. Participants in the treatment group received 12-weeks of language intervention targeting sentence comprehension/production, naming, or spelling. At baseline and post-testing, all participants performed an fMRI story comprehension task, with blocks of auditorily-presented stories alternated with blocks of reversed speech. Participants in the treatment, but not control, group significantly improved in the treated language domain. FMRI region-of-interest (ROI) analyses, conducted within regions that were either active (or homologous to active) regions in a group of 22 healthy participants on the story comprehension task, revealed a significant increase in activation from pre-to post-treatment in right-hemisphere homologues of these regions for participants in the sentence and spelling, but not naming, treatment groups, not predicted by left-hemisphere lesion size. For the sentence (but not the spelling) treatment group, activation changes within right-hemisphere homologues of language regions were positively associated with changes in measures of verb and sentence comprehension. These findings support previous research pointing to recruitment of right hemisphere tissue as a viable route for language recovery and suggest that sentence-level treatment may promote greater neuroplasticity on naturalistic, language comprehension tasks, compared to word-level treatment.

Improvement of cognition across a decade after stroke correlates with the integrity of functional brain networks

BACKGROUND AND OBJECTIVE

We recently reported improvements of working memory across 10 years post stroke among middle-aged individuals. However, the mechanisms underlying working-memory recovery are largely unknown. This study investigated the associations between long-term improvement of working memory and resting-state functional connectivity in two frontoparietal networks: the frontoparietal network and the dorsal attention network.

METHODS

Working memory was repeatedly assessed by the Digit Span Backwards task in 21 persons, within 1 year after stroke onset and again 10 years post stroke onset. Brain functional connectivity was examined by resting state functional magnetic resonance imaging at the 10-year follow-up.

RESULTS

A significant improvement of working memory was found among 21 persons after stroke (median age = 64) at the 10-year follow-up compared to the within-one-year assessment. The magnitude of performance improvement on the Digit Span Backwards task was significantly positively correlated with stronger brain connectivity in the frontoparietal network (r = 0.51, p = 0.018) measured at the 10-year follow-up only. A similar association was observed in the dorsal attention network (r = 0.43, p = 0.052) but not in a visual network (r = -0.17, p = 0.46) that served as a control network. The association between functional connectivity within the above-mentioned networks and Digit Span Backwards scores at 10-year after stroke was in the same direction but did not reach significance.

CONCLUSIONS

The present work relate stronger long-term performance improvement on the Digit Span Backwards task with higher integrity of frontoparietal network connectivity.

Motor network reorganization after motor imagery training in stroke patients with moderate to severe upper limb impairment

BACKGROUND

Motor imagery training (MIT) has been widely used to improve hemiplegic upper limb function in stroke rehabilitation. The effectiveness of MIT is associated with the functional neuroplasticity of the motor network. Currently, brain activation and connectivity changes related to the motor recovery process after MIT are not well understood.

AIM

We aimed to investigate the neural mechanisms of MIT in stroke rehabilitation through a longitudinal intervention study design with task-based functional magnetic resonance imaging (fMRI) analysis.

METHODS

We recruited 39 stroke patients with moderate to severe upper limb motor impairment and randomly assigned them to either the MIT or control groups. Patients in the MIT group received 4 weeks of MIT therapy plus conventional rehabilitation, while the control group only received conventional rehabilitation. The assessment of Fugl-Meyer Upper Limb Scale (FM-UL) and Barthel Index (BI), and fMRI scanning using a passive hand movement task were conducted on all patients before and after treatment. The changes in brain activation and functional connectivity (FC) were analyzed. Pearson's correlation analysis was conducted to evaluate the association between neural functional changes and motor improvement.

RESULTS

The MIT group achieved higher improvements in FM-UL and BI relative to the control group after the treatment. Passive movement of the affected hand evoked an abnormal bilateral activation pattern in both groups before intervention. A significant Group × Time interaction was found in the contralesional S1 and ipsilesional M1, showing a decrease of activation after intervention specifically in the MIT group, which was negatively correlated with the FM-UL improvement. FC analysis of the ipsilesional M1 displayed the motor network reorganization within the ipsilesional hemisphere, which correlated with the motor score changes.

CONCLUSIONS

MIT could help decrease the compensatory activation at both hemispheres and reshape the FC within the ipsilesional hemisphere along with functional recovery in stroke patients.

Executive functions in primary progressive aphasia: A meta-analysis

Executive functions (EFs) refer to a set of cognitive processes, specifically shifting, inhibition, updating of working memory, and are involved in the cognitive control of behavior. Conflicting results have been reported regarding impairments of EFs in Primary Progressive Aphasia (PPA). We performed a multi-level meta-analysis to confirm whether deficits of EFs exist in this population, focusing on a common EFs composite, and the components shifting, inhibition and updating separately. We included 141 studies that report on 294 EFs tasks. The overall mean weighted effect size was large (d = -1,28), indicating poorer EFs in PPA as compared to age-matched cognitively healthy controls. Differences between effect sizes of the EFs components were not significant, indicating all components are affected similarly. Overall, moderator analysis revealed that PPA variant and disease duration were significant moderators of performance, while task modality and years of education were not. The non-fluent/agrammatic PPA and the logopenic PPA variants were similarly affected, but the semantic variant was affected to a lesser extent. We discuss implications for clinical and research settings, and future research.

Inter-hemispheric synchronicity and symmetry: The functional connectivity consequences of stroke and neurodegenerative disease

Stroke and neurodegenerative diseases differ along several dimensions, including their temporal trajectories -abrupt onset versus slow disease progression. Despite these differences, they can give rise to very similar cognitive impairments, such as specific forms of aphasia. What has been scarcely investigated, however, is the extent to which the underlying functional neuroplastic consequences are similar or different for these diseases. Here, for the first time, we directly compare changes in the brain's functional network connectivity, measured with resting-state fMRI, in stroke and progressive neurological disease. Specifically, we examined two groups of individuals with chronic post-stroke aphasia or non-fluent primary progressive aphasia, matched for their behavioral profiles and distribution of left-hemisphere damage. Using previous proposals regarding the neural functional connectivity (FC) phenotype of stroke as a starting point, we compared the two diseases in terms of homotopic FC, intra-hemispheric FC changes and also the symmetry of the FC patterns between the two hemispheres. We found, first, that progressive disease showed significantly higher levels of homotopic connectivity than neurotypical controls and, further, that stroke showed the reverse pattern. For both groups these effects were found to be behaviorally relevant. In addition, within the directly impacted left hemisphere, FC changes for the two diseases were significantly correlated. In contrast, in the right hemisphere, the FC changes differed markedly between the two groups, with the progressive disease group exhibiting rather symmetrical FC changes across the hemispheres whereas the post-stroke group showed asymmetrical FC changes across the hemispheres. These findings constitute novel evidence that the functional connectivity consequences of stroke and neurodegenerative disease can be very different despite similar behavioral outcomes and damage foci. Specifically, stroke may lead to greater independence of hemispheric responses, while neurodegenerative disease may produce more symmetrical changes across the hemispheres and more synchronized activity between the two hemispheres.

The role of disrupted functional connectivity in aphasia

Language is one of the most complex and specialized higher cognitive processes. Brain damage to the distributed, primarily left-lateralized language network can result in aphasia, a neurologic disorder characterized by receptive and/or expressive deficits in spoken and/or written language. Most often, aphasia is the consequence of stroke-termed poststroke aphasia (PSA)-yet, aphasia can also manifest due to neurodegenerative disease, specifically, a disorder called primary progressive aphasia (PPA). In recent years, functional connectivity neuroimaging studies have provided emerging evidence supporting theories regarding the relationships between language impairments, structural brain damage, and functional network properties in these two disorders. This chapter reviews the current evidence for the "network phenotype of stroke injury" hypothesis (Siegel et al., 2016) as it pertains to PSA and the "network degeneration hypothesis" (Seeley et al., 2009) as it pertains to PPA. Methodologic considerations for functional connectivity studies, limitations of the current functional connectivity literature in aphasia, and future directions are also discussed.

Abnormally weak functional connections get stronger in chronic stroke patients who benefit from naming therapy

Language recovery in aphasia is likely supported by a network of brain regions, but few studies have investigated treatment-related changes in functional connectivity while controlling for the absence of treatment. We examined functional connectivity in a 38-region picture-naming network in 30 patients with chronic aphasia who did or did not receive naming therapy. Compared to healthy controls, patients had abnormally low connectivity in a subset of connections from the naming network. Linear mixed models showed that the connectivity of abnormal connections increased significantly in patients who benefited from therapy, but not in those who did not benefit from or receive therapy. Changes in responders were specific to abnormal connections and did not extend to the larger network. Thus, successful naming therapy was associated with increased connectivity in connections that were abnormal prior to treatment. The potential to strengthen such connections may be a prerequisite for a successful treatment response.