Figure 2.

Two regimes of dFC. A, Given two nodes n and m the edge CA signal E_nm(t) (orange box) is defined as the product of the z-scored BOLD signal B_n(t) and B_m(t). Averaging the BOLD-CA matrix over time we obtain the Pearson correlation across each pair of brain regions n and m (in black box, right), defining the static FC. Each column of the BOLD-CA matrix represents an istantaneous realization of the FC (iFC). B, The elements (t_i, t_j) of the dFC_e matrix are defined as the Pearson correlation between iFC(t_i) and iFC(t_j). Note in panel A the presence of transient bouts of strong BOLD-CA (e.g., in the blue boxes). During these events, the iFC remains relatively correlated for few consecutive time points, which gives rise to diagonal (yellow) blocks in the dFC_e matrix. The same CA burst (e.g., at t_i) can re-occur in time after long periods of time (e.g., at t_j), which gives rise to an off-diagonal dFC_e block (e.g., at the crossing of the dashed lines in panel B).