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out that in most cases these functional or effective patterns are not identical to the
patterns of the underlying structural connectivity from which they emerge. This
leads to the challenging theoretical issue of how structural, functional and
effective connectivity patterns are interrelated. Patterns of structural connectivity
are likely to play an important role in shaping emergent patterns of functional
and effective interactions between brain regions. Increasingly, these emergent
patterns of functional and effective connectivity are thought to be closely
associated with human cognition, and their disruption appears to be associated
with some forms of mental dysfunction and disease.
The following sections provide an overview of the structure of the human
brain network as revealed by network science. The brevity of this chapter and
the rapidity with which the field currently advances allow us only to provide a
snapshot of some of the current theoretical and computational issues and themes.
We will start by considering what we know about the global structural
(anatomical) organization of the human cerebral cortex.
9.2. Structural Connectivity of the Human Cerebral Cortex
The anatomical structure of the brain, in particular the human cerebral cortex, has
been a major focus of research since the beginnings of modern neuroscience.
The human brain network can be mapped and characterized at multiple spatial
scales, ranging from interconnections linking whole brain regions to patterns of
connections within a given brain region, for example those between cell
populations or even individual cortical neurons (Swanson, 2003). The complete
set of structural connections of a brain has been called the “connectome” (Sporns
et al
., 2005; Hagmann, 2005) which could be represented as a set of nodes and
their interconnections, or mathematically in the form of an adjacency or
connection matrix. Since there are at least three distinct organizational levels in
the human brain (individual neurons at a microscopic level, cell populations at a
mesoscopic levels, and brain regions at a macroscopic level) the connectome may
also be defined at these three levels of spatial resolution. For species with larger
brains, notably humans, a definition of the connectome at the microscopic level
currently presents formidable technological challenges. At cellular resolution the
connectome would comprise a map of roughly 10
11
nodes linked by 10
15
connections, a dataset many orders of magnitude larger than the map of the
human genome. While recent advances in cell labeling and optical microscopy
have opened up new avenues for tracing individual neurons and connections in
three dimensional blocks of neural tissue (Conchello and Lichtman, 2005; Livet
