of its interactions within a cluster, two nodes with highly correlated fluctuations $\xi(t)$ are likely to belong to the same community, and this can be enforced in the effective network by adding a connection between them. Further results for a variety of rich-club motifs are shown in the Supplementary Materials.

[1]{} ![[**Effective network of the cat cerebral cortex.**]{} We use the local dynamics as a spiking neuron coupled via electric synapses. (a) The cat cerebral cortex network with nodes colour coded according to the four functional modules. Rich-club members are indicated by red encircled nodes. (b) *The covariance matrix of the data* cannot detect communities. (c) *The covariance matrix of the fluctuations* can distinguish clusters of interconnected nodes. This matrix has entries color coded according to the key on the right with red entries corresponding to couple of nodes sharing a large numbers of nearest neighbours in the network, while blue nodes correspond to couple of nodes that share a small number of common neighbours. (d) A model in the cat cortex constructed via the effective network approach. From the matrix in (c) we can recover a representative effective network. The reconstructed network represents the real network in (a) with 