Amnestic mild cognitive impairment: topological reorganization of the default-mode network.

PURPOSE: To investigate the topologic reorganization of the default-mode network (DMN) in patients with mild cognitive impairment (MCI) and whether, relative to healthy control subjects, patients with MCI would be more likely to show disrupted functional connectivity and altered topological configuration of the DMN during the memory task compared with that observed during the resting state.

MATERIALS AND METHODS: This study was approved by the institutional review board of Beijing Normal University Imaging Center for Brain Research. Written informed consent was obtained from each participant. Healthy control subjects (n = 26) and patients with amnestic MCI (aMCI) (n = 25) performed an episodic memory task and also rested while undergoing functional magnetic resonance imaging. Task-induced deactivations were identified and parcellated into different regions associated with the DMN. Functional connectivity across all pairs of regions was computed to construct the DMN architecture. Graph theoretical approaches were used to characterize topological properties of this network.

RESULTS: Patients with aMCI showed similar deactivation in the DMN to that observed in healthy control subjects (P > .05) but showed significantly decreased anterior-to-posterior functional connectivity only during the task (P < .05). Significant increases in local efficiency (P < .05), but not in global efficiency (P > .05), were observed in aMCI only during the task. Decreased functional connectivity was predictive of increased local efficiency (r = -0.35, P = .015). Significant correlations between these network measures and cognitive performance (P < .05) indicated their potential use as early markers to assess the risk of Alzheimer disease (AD).

CONCLUSION: This study suggests the early onset functional reorganization of the DMN toward a nonoptimized regularity configuration in aMCI and expands the understanding of dynamic functional reorganization in brain networks along the continuum from normal aging to AD dementia.