Do age and episodic memory task performance differentially relate to tract-specific white matter microstructure? Cross-sectional and longitudinal analyses in a healthy adult sample

Advancing age is typically associated with declines in both episodic memory function and white matter microstructure (WMM), although there is substantial variability in these trajectories. Relatively little work has attempted to isolate the associations between age, episodic memory function, and WMM in tracts associated with episodic memory, namely the fornix, superior longitudinal fasciculus, the uncinate fasciculus, and the cingulum bundle. Given the limited research modeling these variables together, we investigated their interrelations in cognitively normal adults drawn from the Nathan Kline Institute Rockland Sample by examining, both cross-sectionally (n = 301; age M = 52.4; Range = 19-85) and longitudinally (n = 147; baseline age M = 56.36; Range = 38-71), whether individual differences in WMM modify the typical age-related trajectory of episodic memory decline as indexed by delayed free recall. Cross-sectional results reveal that both delayed free recall and WMM decreased significantly with age. When modeled together, however, age predicted delayed free recall, whereas WMM was associated with delayed recall performance only in the fornix above and beyond variance explained by age. Longitudinal analyses reveal that changes in WMM did not significantly predict change in delayed free recall performance over time, and advancing age was also not a strong predictor of changes in WMM. Together, findings from our healthy sample across the adult lifespan suggest that episodic memory measured by delayed free recall may have more subtle associations with WMM maintenance than it does with chronological age.