Frontiers in Psychology (May 2015)
Resting-state functional connectivity of orthographic networks in acquired dysgraphia
Abstract
Introduction. While task-based fMRI has been used extensively to study the functional neural networks that instantiate cognitive functions, there is increasing interest in analyzing the activity of the brain while it is not performing a task – in the resting state (RS). In RS-fMRI, participants lie in a relaxed awake state during fMRI scanning, and correlations of the activation time-series between their different brain areas are analyzed. These analyses have revealed many of the same networks identified in task-based fMRI, including the default-mode network (DMN), the dorsal attention network, the motor network, etc. Various network properties have been studied; for example, some networks have stronger correlations between the nodes of the same network (to which we refer as “within-network coherence”), than between nodes from different networks (“across-network coherence”), resulting in clustering of network nodes (e.g., Cole et al., 2014). There has been great interest in understanding how RS network connectivity may be affected by brain illness or injury (e.g., Finn et al., 2014, Park et al., 2011). However, one domain that has received scarce attention even in studies with neuro-typical individuals is orthographic processing (reading and spelling). We report on a study that examines the orthographic processing network (OPN) in neuro-typical individuals and in individuals with acquired dysgraphia subsequent to stroke, examining how this network can be affected by brain injury and how it responds to rehabilitation. Specifically, we examine the possibility that rehabilitation may lead to normalization of RS network connectivity such that the properties of the damaged network increasingly resemble those of intact networks. Methods. Participants are eight neuro-typical adults (NTA), and three adults with left-hemisphere stroke and acquired dysgraphia. NTAs had two RS-fMRI scans in the same session; dysgraphics had one scan, followed by 2-4 months of spelling rehabilitation, and two more scans. Regions of interest (ROIs) were identified using coordinates of the OPN (Turkeltaub et al., 2001) and the DMN (Laird et al., 2009). Within-network coherence corresponded to the average RS-fMRI correlations between every pair of ROIs within each network; across-network coherence was the average correlation between every pair of ROIs from the two different networks (Song et al., 2011). Results (Figure 1). (1) Each NTA showed stronger within-network coherence for the OPN than across-network coherence between OPN and DMN. (2) Before treatment, two of the dysgraphics showed weaker within-network coherence for the OPN than across-network coherence between OPN and DMN. Following treatment all three dysgraphics showed the same network coherence pattern as the NTAs. Discussion. The NTA findings indicate that the relationship between orthographic and default-mode networks is characterized by greater within- vs. across-network connectivity. Furthermore, we show for the first time a pattern of increasing within/across network “coherence normalization” following spelling rehabilitation. Additional dysgraphic participants and other networks (language, sensory-motor, etc.) will be analyzed to develop a better understanding of the RS orthographic network and its response to damage and recovery. Acknowledgements. The work is part of a multi-site, NIDCD-supported project examining language recovery neurobiology in aphasia (DC006740). We thank Melissa Greenberger and Xiao-Wei Song.
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