Age differences in the functional organization of the prefrontal cortex: analyses of competing hypotheses.

We employed a mixed design task for block and event-related functional magnetic resonance imaging with manipulations of levels of abstraction and duration in task-relevant cues and probes. Age-related differences between younger and older adults in task-related functional brain activity patterns of the prefrontal cortex (PFC) were reported. The results showed that (1) the low episodic condition evoked more activity in the more anterior PFC than the high episodic control condition for both age groups; (2) the low abstraction condition evoked more activity in the more anterior PFC than the high abstraction condition for both age groups; and (3) the signal change did not vary as a function of activity dynamics (transient and sustained responses) and maintenance duration (single-trial and multiple-trial). The findings showed that baseline conditions evoked more activity in the more anterior PFC for the older group than the younger group across most task contrasts and conditions, where these additional activities in the brain regions overlapped within the default mode network (DMN). We tentatively concluded that deficiency in the anterior DMN deactivation during externally driven tasks might be attributed to less efficiency in modulating local connectivity propagate to surrounding tissue, which may paradoxically increase brain activity.