Age-Related Changes in Brain Connectivity: Alterations of the Default Mode Network

Abstract

The default mode network (DMN) is a system of brain regions observed on functional magnetic resonance imaging (fMRI) when an individual is resting and deactivated during performance of goal-directed cognitive tasks, and is thought to be involved in self-related information processing. While differences with age have been observed in anatomical and functional connectivity, resting activity, and task-related deactivation of the DMN, age-related differences in the interaction between resting connectivity and active processing in the DMN are not well understood. In this study, the relation between functional connectivity and cognitive activation during performance of a task known to involve key DMN regions (i.e., posterior cingulate, medial frontal, medial temporal, and parietal regions) was investigated. Statistical Parametric Mapping (SPM) was performed on fMRI scans in healthy young (n=11) and older (n=19) adults to assess functional connectivity of the DMN at rest, and activation during a self-related source memory task. Older adults were then divided based on task performance into high- and low-performing groups to assess individual differences in connectivity and activation. Though both young and older adults showed robust connectivity among DMN regions, older adults showed greater connectivity between the DMN and other areas, particularly in frontal regions; this expansion was especially evident in low performers. Activation of the DMN during encoding and retrieval of self-related versus other-related information was greater in young adults than older adults. While low-performing older adults showed no differences between self- and other-related activation at retrieval, high performers engaged regions outside the DMN during other-related retrieval. These results suggest that older adults whose self-related source memory performance is similar to young adults exhibit preservation of DMN connectivity, self-related activation in the DMN which more closely resembles that of young adults, and additional recruitment of non-DMN networks to achieve higher memory performance. Aging in low performers is associated with dedifferentiation of DMN connectivity with expansion particularly into frontal regions, and reduced ability to engage the DMN or other networks in discriminating self- from non-self-related information. Further, preservation of DMN-specific functional connectivity is directly related to greater activation differences during retrieval of self-related versus non-self-related information in older adults.