Abstract
Recently, increasing attention has been focused on the investigation of the human brain connectome that describes the patterns of structural and functional connectivity networks of the human brain. Many studies of the human connectome have demonstrated that the brain network follows a small-world topology with an intrinsically cohesive modular structure and includes several network hubs in the medial parietal regions. However, most of these studies have only focused on undirected connections between regions in which the directions of information flow are not taken into account. How the brain regions causally influence each other and how the directed network of human brain is topologically organized remain largely unknown. Here, we applied linear multivariate Granger causality analysis (GCA) and graph theoretical approaches to a resting-state functional MRI dataset with a large cohort of young healthy participants (n = 86) to explore connectivity patterns of the population-based whole-brain functional directed network. This directed brain network exhibited prominent small-world properties, which obviously improved previous results of functional MRI studies showing weak small-world properties in the directed brain networks in terms of a kernel-based GCA and individual analysis. This brain network also showed significant modular structures associated with 5 well known subsystems: fronto-parietal, visual, paralimbic/limbic, subcortical and primary systems. Importantly, we identified several driving hubs predominantly located in the components of the attentional network (e.g., the inferior frontal gyrus, supplementary motor area, insula and fusiform gyrus) and several driven hubs predominantly located in the components of the default mode network (e.g., the precuneus, posterior cingulate gyrus, medial prefrontal cortex and inferior parietal lobule). Further split-half analyses indicated that our results were highly reproducible between two independent subgroups. The current study demonstrated the directions of spontaneous information flow and causal influences in the directed brain networks, thus providing new insights into our understanding of human brain functional connectome.
Highlights
Increasing attention has been focused on the investigation of the human brain connectome that describes the patterns of structural and functional connectivity networks of the human brain [1,2]
Considering that the different thresholds have effects on the number of edges of the resulting brain networks and thereby influence the topological properties, we evaluated the topological stability of the brain functional networks over a wide sparsity ranging from 5% to 50%
The short-range/local edges may be associated with the short fibers that constitute the local circuitry, but longrange edges may be associated with the commissural fibers and long intra-hemispheric association fibers [56]
Summary
Increasing attention has been focused on the investigation of the human brain connectome that describes the patterns of structural and functional connectivity networks of the human brain [1,2]. Many studies have demonstrated that the human brain network follows a small-world topology (i.e., high clustering and short path lengths linking different nodes) [3,4,5,6,7,8] and has an intrinsically cohesive modular structure [9,10,11]. Several studies in cats and monkeys have utilized anterograde and retrograde tracing techniques to investigate directed brain networks [18,19,20] These techniques can be used to identify the information flow between brain regions, they cannot be applied to human beings in vivo because of their invasiveness
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