  The analysis of the activity of neuronal cultures is considered to be a good
proxy of the functional connectivity of in vivo neuronal tissues. Thus, the
functional complex network inferred from activity patterns is a promising way
to unravel the interplay between structure and functionality of neuronal
systems. Here, we monitor the spontaneous self-sustained dynamics in neuronal
cultures formed by interconnected aggregates of neurons (clusters). Dynamics is
characterized by the fast activation of groups of clusters in sequences termed
bursts. The analysis of the time delays between clusters' activations within
the bursts allows the reconstruction of the directed functional connectivity of
the network. We propose a method to statistically infer this connectivity and
analyze the resulting properties of the associated complex networks.
Surprisingly enough, in contrast to what has been reported for many biological
networks, the clustered neuronal cultures present assortative mixing
connectivity values, as well as a rich--club core, meaning that there is a
preference for clusters to link to other clusters that share similar functional
connectivity, which shapes a `connectivity backbone' in the network. These
results point out that the grouping of neurons and the assortative connectivity
between clusters are intrinsic survival mechanisms of the culture.
