  Small-World Networks (SWNs) represent a fundamental model for the
comprehension of many complex man-made and biological networks. In the central
nervous system, SWN models have been shown to fit well both anatomical and
functional maps at the macroscopic level. However the functional microscopic
level, where the nodes of a network are composed of single neurons, is still
poorly understood. At this level, although recent evidences suggest that
functional connectivity maps exhibit small-world organization, it is not known
whether and how these maps, distributed in multiple brain regions, change
across different conditions. We addressed these questions by simultaneous
multi-array extracellular recordings in three brain regions diversely involved
in somatosensory information processing: the ventropostero-lateral thalamic
nuclei (VPL), the primary somatosensory cortex (S1) and the centro-median
thalamic nuclei (CM). From both spike and Local Field Potential (LFP)
recordings, we estimated the functional connectivity maps by using the
Normalized Compression Similarity (spikes) and the Phase Synchrony (LFPs).
Then, by using graph-theoretical statistics, we characterized the functional
map topology both during spontaneous activity and sensory stimulation. Our main
results show that: (i) spikes and LFPs show SWN organization during spontaneous
activity; (ii) After stimulation onset, while substantial functional map
reconfigurations occur both in spike and LFPs, small-worldness is nonetheless
preserved (iii) The stimulus triggers a significant increase of inter-area LFP
connections without modifying the topology of intra-area functional
connections; (iv) Through computer simulations of the fundamental concept of
cell assemblies, transient groups of activating neurons can be described by
small-world networks.
