The Human Brain Network - Biological Networks - page 204

connectome data sets using diffusion tensor imaging followed by the derivation
of average connection probabilities between 70-90 cortical and basal brain gray
matter areas. The obtained brain networks were then subjected to graph analysis.
All networks were found to have robust small-world attributes and “broad-scale”
degree distributions. An analysis of betweenness centrality in these networks
demonstrated high centrality for the precuneus, the insula, the superior parietal
and the superior frontal cortex. Motif analysis (Milo et al ., 2002) revealed that
significantly increased motifs were similar to those identified in connection
matrices derived by anatomical tract tracing in cat and macaque cortex (Sporns
and Kötter, 2004). A particularly innovative aspect of these studies is that all
network analyses were carried out while preserving the weighted connectivity
structure of the brain network. A separate study using DTI mapped a network of
anatomical connections between 78 cortical regions (Gong et al ., 2008). This
study also identified several hub regions in the human brain, including the
precuneus and the superior frontal gyrus.
These diffusion imaging studies were carried out at relatively low spatial
resolution (<100 brain nodes) at using DTI which is known to have difficulty
identifying connections in parts of the white matter where fiber bundles intersect.
Diffusion spectrum imaging can overcome the latter limitation (Wedeen et al .,
2005) and was applied in a series of studies by Hagmann et al . (2007, 2008).
The first study using DSI (Hagmann et al ., 2007) constructed a connection matrix
from fiber densities measured between homogeneously distributed and equal-
sized “regions of interest” (ROIs) numbering between 500 and 4000. A
quantitative analysis of connection matrices obtained for approximately 1000
ROIs and approximately 50,000 fiber pathways from two subjects demonstrated
an exponential (one-scale) degree distribution as well as robust small-world
attributes for the network.
A subsequent study (Hagmann et al ., 2008) allowed a detailed analysis of the
network structure of human cortical connectivity (Fig. 9.2), using a broad array
of network analysis methods including core decomposition, modularity analysis,
hub classification and centrality. The study presented evidence for the existence
of a structural core of highly and mutually interconnected brain regions, located
primarily in posterior medial and parietal cortex (Hagmann et al ., 2008). The
core is comprised of portions of the posterior cingulate cortex, the precuneus, the
cuneus, the paracentral lobule, the isthmus of the cingulate, the banks of the
superior temporal sulcus, and the inferior and superior parietal cortex, all located
in both cerebral hemispheres. In addition, the analysis detected several
additional modules of brain regions in temporal and frontal cortex that maintain