Ruminating on Rumination

Abstract

H i H amilton et al. (1) reported some very interesting and important findings in their article “Default-Mode and TaskPositive Network Activity in Major Depressive Disorder: Implications for Adaptive and Maladaptive Rumination.” In the article, the authors explored dominance of the default-mode network (DMN) over the task-positive network (TPN) and how that dominance might be related to depression and rumination. Dominance of the DMN over the TPN was defined for time points where the DMN blood oxygen level dependent signal was greater than the TPN blood oxygen level dependent signal. This calculation was performed across the whole brain where the DMN and the TPN were defined from seed-based approaches of Fox et al. (2) during eyes-shut resting state scans. The authors found that subjective reflection scores from the Rumination Response Scale (RRS) of Treynor et al. (3) correlated negatively with DMN dominance. That is, for participants with major depressive disorder (MDD), the more they reflected, the less dominance there was of the DMN over the TPN. By contrast, depression sub-scales of the RRS correlated positively with DMN dominance for participants with MDD. The authors did not find reliable correlations between DMN dominance and RRS measures for the healthy control (HC) group. Therefore, DMN dominance might be a reliable measure of depression severity. With a spatial frequency analysis, the authors also found that a region of the right frontal insular cortex (RFIC) was more likely included in the TPN for MDD subjects, relative to the HC subjects. Interestingly, the RFIC has been implicated in switching between DMN dominance and TPN dominance (4). In a separate regionof-interest analysis on the RFIC, the authors found voxels that interacted by group and network. These voxels in RFIC increased in activation during DMN dominance (i.e., DMN peaks) for MDD subjects, but they activated in the opposite condition for HC subjects during TPN dominance (i.e., TPN peaks). Although the authors could not examine what role the RFIC might be playing psychologically, they proposed a potential mechanism. If one role of the RFIC might be to monitor for the presence of undesired bodily states (5), then when MDD subjects are in a ruminative state represented by DMN dominance, the RFIC might jumpstart the TPN-based affect regulation mechanisms (1). By contrast, for HC subjects, activation of the RFIC might get participants out of analytical and cold cognitive processing represented by TPN dominance and into more creative processes such as mind wandering and daydreaming (1,6,7), which might be represented by DMN dominance. These findings are novel and important in that they point to a potentially important neural marker of psychological dysfunction and rumination. Surprisingly, no overlap in voxels was found in RFIC between this analysis and the spatial frequency analysis (an issue that merits further attention). These results are related to recent work exploring the relationship of default-network connectivity to rumination in de-