Effects of task complexity and age-differences on task-related functional connectivity of attentional networks

Studies investigating the strength and membership of regions within multiple functional networks primarily focus on either resting state or single cognitive tasks. The goals of the current study were to investigate whether task-related functional connectivity changes with task complexity, and whether this connectivity-complexity relationship is age-sensitive. We assessed seed-to-voxel functional connectivity for the default mode network (DMN) and two attentional networks [cingulo-opercular (CO), fronto-parietal (FP)] in three cognitive control tasks of increasing complexity (Single task, Dual task, and Memory Updating), across younger and older adults (N = 52; N_Young = 23; N_Old = 29). The three tasks systematically varied in cognitive control demands due to differing maintenance, switching, and updating requirements. Functional connectivity for all networks, resulting from task > rest contrasts, increased with greater task complexity, irrespective of age and gray matter volume. Moreover, between-network connectivity for DMN, CO, and FP regions was greatest for working memory updating, the most complex task. Regarding age-related differences in accuracy, none were observed for Single or Dual tasks, but older adults had poorer accuracy in Memory Updating. More anterior frontal clusters of functional connectivity were observed for older, compared to younger, adults; these were limited to seeds of the two attentional networks. Importantly, increased connectivity in these additional frontal regions in older adults were non-compensatory, because they were associated with detrimental task performance, especially Memory Updating. For the Memory Updating > Rest, the younger > older contrast resulted in greater DMN seed connectivity to regions in the other two attentional networks, implicating increased reliance on between-network connectivity for the DMN seeds during complex cognitive tasks. Our results also implicate functional connectivity between attentional networks and the cerebellum during cognitive control. Reliability of multiple seeds in the seed-to-voxel connectivity is also discussed.