Traditionally, the structure of complex networks is studied through the statistical properties of the network. Here we present an alternative approach that uses the homological structure found via persistent homology. After introducing this methodology, we will define the homological backbones, two secondary networks designed to represent compactly the homological features of the original network under study and make them amenable to network theoretical tools. We apply these tools to compare the resting-state functional activity of volunteers after the infusion of > either placebo or psilocybin, a psychedelic compound found in magic mushrooms, finding that the mesoscopic network structures undergo a dramatic change showing greater integration in subjects in psychedelic state. Joint work with G.Petri (ISI Found.), P.Expert and F.Turkheimer (King's College London), R.Carhart-Harris (Imperial College London) Within EU Funded FET Open Project TOPDRIM