nodes appeared in a community together. Next we calculated the element-wise difference in association matrices, subtracting the mass excursion association matrix from the non-mass excursion matrix. The elements of the resulting ma- trix, Δ$_{ij}$, quantified how much more likely it was for two nodes to be assigned to the same community during non-mass excursions compared to mass excur- sions (Figure 6A). We performed this analysis for all participants and asked, for each node pair { i, j } whether Δ$_{ij}$ was consistently greater or less than zero across all subjects (mass-univariate t-test with statistical threshold of p < 0 . 05, FDR-corrected) (Figure 6B). Interestingly, the regions that consistently changed their community co- assignments were, disproportionately so, regions associated with default mode network (DMN). In particular, during non-mass excursions DMN regions are grouped with regions that form parts of the dorsal attention, ventral attention, somatomotor, visual, and control systems, whereas during mass excursions these same DMN regions dissociate from these systems. The regions that changed their community co-assignments most frequently were the bilateral posterior cingulate, dorsal and medial prefrontal cortex along with the inferior parietal lobule, all subcomponents of the DMN (Figure 6C). In order to identify whether excursions were driven by any particular func- tional connections or whether certain connections participated disproportion- ately in a greater number of excursions, we counted the number of times that a connection, { i, j } was stronger than expected, x + $_{ij}$. We did the same for weaker-than-expected connections, x − $_{ij}$. We found that, across all participants, all connections participated in at least one excursion. Interestingly, however, a small number of connections participated in disproportionately many (Fig- 18 Excursions occur most frequently for connections within visual and somatomotor networks