  Analyzing the brain in terms of organizational structures at intermediate
scales provides an approach to negotiate the complexity arising from
interactions between its large number of components. Focusing on a wiring
diagram that spans the cortex, basal ganglia and thalamus of the Macaque brain,
we provide a mesoscopic-level description of the topological architecture of
one of the most well-studied mammalian connectomes. The robust modules we
identify each comprise densely inter-connected cortical and sub-cortical areas
that play complementary roles in executing specific cognitive functions. We
find that physical proximity between areas is insufficient to explain the
modular organization, as similar mesoscopic structures can be obtained even
after factoring out the effect of distance constraints on the connectivity. We
observe that the distribution profile of brain areas, classified in terms of
their intra- and inter-modular connectivity, is conserved across the principal
cortical subdivisions, as well as, sub-cortical structures. In particular
provincial hubs, which have significantly higher number of connections with
members of their module, but relatively less well-connected to other modules,
are the only class that exhibits homophily, i.e., a discernible preference to
connect to each other. By considering a process of diffusive propagation we
demonstrate that this architecture, instead of localizing the activity,
facilitates rapid communication across the connectome. By supplementing the
topological information about the Macaque connectome with physical locations,
volumes and functions of the constituent areas and analyzing this augmented
dataset, we reveal a counter-intuitive role played by the modular architecture
of the brain in promoting global interaction.
