Poster E18, Thursday, August 22, 2019, 3:45 – 5:30 pm, Restaurant Hall
The effects of real and simulated lesions on the modular organization of the brain
Brenda Rapp1, Yuan Tao1;1Johns Hopkins University
Previous simulation studies directed at understanding the effects of lesions on functional organization have shown that damage to global hubs (nodes supporting cross-module integration) or local hubs (supporting within-module integration) have different effects on whole-brain modular organization (Sporns et al., 2007; Honey & Sporns, 2008). Specifically these studies found that greater damage to global hubs results increases modularity while greater local hub damage decreases it. However, the consequences of actual lesions have been scarcely studied. To better understand the basis of post-stroke functional re-organization, we examined the consequences of brain lesions in chronic stroke (n=18) and the impact of comparable pseudo-lesions in healthy individuals (n=23). For both participant groups, fMRI data were collected during performance of a spelling task and pairwise correlations of the residual time-courses between 235 nodes distributed throughout cortex (Power et al., 2013) were calculated. A reference modular structure was computed from the healthy control data and, on this basis, global (participation coefficient, or PC) and local (within-module degree, or WD) integration coefficients (Guimera & Amaral, 2005) were calculated for each node. For each lesion mask, overall PC and WD damage scores were computed by averaging the respective coefficients of the lesioned nodes. Pseudo-lesions were created by applying every lesion mask to each control participant’s dataset and modularity (Newman’s Q) was calculated for all data sets (lesioned and healthy participants). Finally, for both participant groups the lesion-mask PC and WD damage scores were correlated with the modularity values. We found that the comparable lesions across the two participant groups had very different consequences for modularity. Consistent with previous simulation studies, for the healthy participants with pseudo-lesions, greater WD (local hub) damage scores resulted in significantly lower overall modularity (r=0.38, p<0.05), although we did not find that the magnitude of PC (global hub) damage within the lesion mask was correlated with modularity values (r=0.08, n.s.). In contrast, for actual lesions, WD damage was uncorrelated with modularity (r=0.01, n.s) and, unlike previous simulation studies, greater PC damage resulted in significantly reduced modularity (r=-0.61, p<0.001). Moreover, the negative correlation between PC damage and modularity was present in both ipsi- and contra-lesional hemispheres (LH: r=-0.43, p<0.05; RH: r=-0.64, p<0.001). Also, the two groups also differed in that modularity values in the pseudo-lesions data sets were driven by lesion volume (r=0.78, p<0.001) whereas there was no such relationship in the real lesion data set (r=0.23, n.s.). Our findings from pseudo-lesions in healthy controls generally align with previous simulation results indicating that damage to global hubs have widespread consequences on global modularity. Most significantly, the discrepancies between pseudo- and real lesions indicate that lesion-driven functional re-organization cannot be explained as a simple subtraction of nodes from the healthy brain. In this way, the discrepancies in global modular organization between pseudo and real lesion conditions provide clear evidence of widespread and complex post-stroke functional reorganization that affects both the lesioned and unlesioned hemispheres.
Themes: Disorders: Acquired, Methods
Method: Functional Imaging