*4.3. rs-fMRI studies associated with the ECN in AD and MCI*

Additionally, in AD and MCI, rs-fMRI studies using the ICA analysis identified a significant difference in ECN connectivity across AD and MCI patients and normal controls [197]. In the case of intraconnectivity of the ECN, results seem inconclusive, with some studies reporting no changes in AD patients and others reporting increased connectivity [121,127,198]. However, studies using seed-based analysis consistently reported abnormal FCs between the hippocampus and nodes of the ECN. Previous studies have demonstrated that functional brain activity within portions of the ECN was abnormal in patients with MCI and AD [182,199]. Specifically, the directed FCs from the left hippocampus to the right superior frontal gyrus (SFG) and left medial frontal gyrus (MFG) to the right hippocampus were significantly decreased in MCI or AD patients. The SFG [175] and the MFG [200,201] are essential components of the dlPFC that play essential roles in the ECN.

Moreover, Cai et al. (2017) reported different effective connectivity patterns for the ECN in normal controls and three subgroups of MCI: (1) MCI-R—MCI reverted to the normal functioning state and stabilized to the normal state in 24 months; (2) MCI-S— MCI patients who remained in a stable disease state for 24 months; (3) MCI-P—MCI that progressed to AD and stabilized to AD in 24 months. In this study, the effective connectivity patterns in the ECN were less disrupted and less obvious among MCI-R and MCI-S to MCI-P. In addition, ECN connectivity strengths were not changed in MCI-R patients and normal controls compared to MCI-S and MCI-P patients [181]. These results suggest the importance of the ECN in dementia progression from MCI to AD. We presented key findings associated with AD and MCI in Table 4 and characteristics of main rs-fMRI studies in Supplementary Table S5.

**Table 4.** Summary of key findings of rs-fMRI studies associated with the executive control network (ECN) in Alzheimer's disease (AD) and mild cognitive impairment (MCI) patients included in the review.


Abbreviations: ECN, executive control network; AD, Alzheimer's disease.
