We studied the interactions between different temporal scales of diffusion processes on complex networks and found them to be stronger in scale-free (SF) than in Erdos-Renyi (ER) networks, especially for the case of phase-amplitude coupling (PAC)-the phenomenon where the phase of an oscillatory mode modulates the amplitude of another oscillation. We found that SF networks facilitate PAC between slow and fast frequency components of the diffusion process, whereas ER networks enable PAC between slow-frequency components. Nodes contributing the most to the generation of PAC in SF networks were non-hubs that connected with high probability to hubs.Additionally, brain networks from healthy controls (HC) and Alzheimer's disease (AD) patients presented a weaker PAC between slow and fast frequencies than SF, but higher than ER. We found that PAC decreased in AD compared to HC and was more strongly correlated to the scores of two different cognitive tests than what the strength of functional connectivity was, suggesting a link between cognitive impairment and multi-scale information flow in the brain.interactions correspond to the phenomenon known as cross-frequency coupling (CFC) 13 . We focus on three types of CFC: phase-amplitude coupling (PAC), the phenomenon where the instantaneous phase of a low frequency oscillation modulates the instantaneous amplitude of a higher frequency oscillation 14 15 ; amplitude-amplitude coupling (AAC), which measures the co-modulation of the instantaneous amplitudes of two oscillations 16 ; and phase-phase coupling (PPC), which corresponds to the synchronization between two instantaneous phases 17 .
Results
Diffusion of simulated ER and SF networksWe start by considering an unweighted network consisting of nodes. We place a large number ( ≫ ) of random walkers onto this network. At each time step, the walkers move randomly between the nodes that are directly linked to each other. We allow the walkers to perform time steps. As a walker visits a node, we record the fraction of walkers present at it, which we term node activity. Thus, after time steps, we obtain time series reflecting different realizations of the flow of information in the network.Two types of simulated complex networks are considered here, ER and SF networks. An ER network is a random graph where each possible edge has the same probability of existing. The degree of a node i ( ) is defined as the number of connections it has to other nodes. The degree distribution ( ) of an ER network is a binomial distribution, which decays exponentially for large degrees , allowing only very small degree fluctuations 18 . On the other hand, SF networks