Shedding light on gray(ing) areas: Connectivity and task switching dynamics in aging

Control-demanding tasks rely on communication among regions of the frontoparietal network, areas that undergo significant age-related decline. Here, we integrate data from brain anatomy, electrophysiology (ERPs), and optical imaging (event-related optical signals, EROS) to characterize the spatial and temporal dynamics of preparatory control processes in middle to old age. Older adults participated in an experiment that required switching between a position and a meaning task (spatial Stroop), a paradigm that has been shown to primarily recruit prefrontal cortex in opposite hemispheres and is thought to involve the corpus callosum (CC). In middle-aged participants, switch trials resulted in greater negativity over F3 early in the preparatory period. Across the whole older adult sample, this switch-related frontal negativity was correlated with reaction time (RT) switch costs and EROS switch-related upregulation in the left middle frontal gyrus (MFG). Anterior CC volume was not directly correlated with switch costs, although CC volume predicted task-dependent coupling of left MFG and right MFG. Crucially, left MFG-seeded lagged cross-correlations revealed task-dependent connectivity; in the right-hemisphere-dependent position task, the timing and strength of switch-related coupling between left MFG and right MFG significantly predicted RT switch costs, even after controlling for age, CC volume, and the F3 switch effect. Together, these results suggest that a strong functional connectivity, likely hinged on the integrity of the underlying structural connections, is critical to being able to meet the demands of shifting processing across hemispheres, and that difficulty engaging such control dynamics leads to suboptimal performance.