Mobilization and redistribution of default mode network from resting state to task state in amnestic mild cognitive impairment.

Brain task-negative networks (default mode network, DMN) and task-positive networks appear to operate largely in opposition, such that task-negative networks show activation during resting states, whilst task-positive networks are deactivated with the reverse being true during goal-oriented behavior. Altered DMN and task-positive network activity has been observed in amnestic mild cognitive impairment (aMCI) subjects. However, no study has directly linked the patterns of between-state differences in the same aMCI cohort regarding these two types of functional networks. The spatial and temporal characteristics of intrinsic, low frequency BOLD signal fluctuations both during resting state and episodic memory fMRI task were assessed in 28 aMCI subjects and 23 matched healthy controls, using a posterior cingulate cortex- based temporal correlation analysis. aMCI subjects showed impaired attenuation in the DMN between rest and task state, and greater cognitive impairment was associated with decreased ability to attenuate DMN during task engagement. Moreover, more redistributed resource from DMN appeared to be required in aMCI to maintain the similar task performance possibly to offset their inability to engage task-positive networks. In order to complete a given task, mobilized and redistributed resources of DMN appeared to replace task-positive network function to some degrees in aMCI subjects. This may represent an inability to control the switching of functional modes between these types of network.