The purpose of this study was to explore the association of MDD patients with episodic depression, compard to HC, within the triple network and its relationship with rumination. The current resting state MRI found that, comparing with HC, MDD patients with depressive episode have significantly enhanced connectivity between the DMN and SN, notebly, each link showed a hyperconnectivity. In addition, the moderate correlation was shown between rumination scores of subjects and FC-values in almost each of brain regions with significant differences, in particular, the direction of correlation were all positive.
Patients with episodic depression mainly showed enhanced connection between DMN and SN, which to some extent copied the previous results[41]. Specifically, our results indicate that MDD patients found enhanced FC with mPFC in the DMN and ACC, SMG and bilateral AInsula in the SN, which is basically consistent with other studies[27]. The previous study has indicated the mPFC is related to self-reference thinking[42]. Among other regions in SN, AAC is found to function as broad emotional processing and also related to self-referential processing[43], as well as insula is implicated in self monitoring, saliency detection, and interoceptive awareness[25]. The hyperconnectivity, compared with HC, between mPFC and ACC is consist with results from Hong et. al, which were confirmed be correlative with impulsiveness, assessed by Behavioral Inhibition and Activation Scales[44]. Additionally, we have also found a significant correlation between these regions and mind wandering and attention in previous studies[45], which suggests that impaired attention may be the reason why rumination in MDD patients cannot be converted normally and in a timely manner. Another significant difference appears between PCC and AInsula. R as well as SMG. R in the SN. It is reported that activity in the rSMG has also been reported to inward-orientated self-awareness and an ability to overcome an emotional egocentricity bias[46, 47]. PCC mainly participates in memory[48]. The abnormal connectivity of this two links possibly indicates a dysfunction of MDD patients of coordinating internal and the memory influenced by patients’ egocentricity, that is, the content of rumination thinking.
As far as the function of the network is concerned, the function of DMN network is mainly focused on self referential thought[18], and the function of SN is more focused on attention allocation[15, 49]. It is known that depressed patients may be induced to the rumination state during highly self related negative emotional events concerning. Additionally, we also found a possible explanation in the Go/NoGo response inhibition task, in which the subjects had the related activation of Rostral ACC(rACC), from the SN, and mPFC, PCC, from the DMN during tasking state[50]. Intuitively, we speculate that, the enhanced connection, between DMN and SN, of MDD patients with high rumination characteristics may be due to the failure of their response inhibition to self related information at the beginning of rumination, which finally caused a vicious cycle. However, we also found that our results are contrary to a study on recurrent negative thinking(RNT) in a dynamic FC[30]. The probable reason is that, there still remains differences from their mechanism between induced RNT and pathological rumination thinking. Therefore it cannot directly connect the experimental results of the two.
Interestingly, there was no significant difference in the FC between DMN andFPN, SN and FPN. This may be related to the enrollment of MDD includes first-episode and recurrent-episode, despite all of them having a current depressive episode. Another reason is probably that the small sample size may raise risk of the result due to the representativeness of our sample.
Furthermore, the correlation analysis reported statistically significant results between the scores of the Rumination Self Reporting Scale and the vast majority of connectivity within the two network. In particular, the Symptom Rumination and Brooding sub-scales showed a moderate positive correlation with all FC with between-group differences[46], while Reflective Rumination sub-scale had the statistical relation with FC between mPFC, PCC and AInsula, ACC. Our results confirmed the abnormal cross-network interaction in the episodes of depression, this can be use as an effective biomarker for MDD clinical diagnosis and the potenbtial pathological mechanism in MDD rumination, which might be potential targets in future therapy for depression. Our self-report scale was used to reflect individual’s rumination trait measure. This measurement can absolutely represent the daily degree of MDD rumination and conduct resting-state neural research. As we covered above, mPFC as well as ACC and other regions are crucial to the pathophysiology of MDD[51, 52]. Our results basically consisted with the previous literature that mPFC and ACC are positively correlate with rumination and DMN FC in the MDD group[53].
Our study further elucidates the neural mechanisms behind rumination in patients with depression through three networks analysis, enhancing the persuasiveness of the functional network. The advantage lies in current depressive episode MDD and HC samples we recruited from outpatient clinics. Rumination was found to be associated with FC between DMN as well as SN. The results of previous functional network studies on MDD rumination above have focused mainly on interior of networks, there is relatively little evidence between triple network yet. The present findings strongly support past findings of abnormal communication between networks in depression. Of course, there are still problems in our research. For example, our equipment has an inherent disadvantage of magnet strength, as well as the number of both sample is a disturbing factor. Despite these limitations, we found some relatively credible results between the functional networks. This study contributes to the current literature on functional connections between DMN and SN regions, as they are associated with rumination and its subtypes in healthy and depressed patients. Future research may consider recruiting more samples of HC and MDD participants, and combining rumination induced task state MRI for inter network functional connectivity analysis. We hope the present study informs and provides some groundwork for the future.