White matter fiber density for vocabulary better maintained than fluid abilities in aging

Abstract

AbstractBackgroundAging research has repeatedly shown that crystallized abilities, cognitive processes reliant on learned knowledge, are well preserved well into old age. On the other hand, fluid abilities, processes that are not knowledge‐dependent, show decline as early as the third decade of life. It is still unclear what mechanism enables the two starkly contrasting aging trajectories to exist all within the same brains.MethodFrom the Reference Ability Neural Network (RANN) study, 181 participants (age=51.2±17.2, 101females) with both baseline and 5‐year follow‐up diffusion weighted imaging data were included in the current analysis. Based on fMRI activation data, the set of brain regions were derived that are associated with performance of each of four cognitive abilities: vocabulary, processing speed, episodic memory, and reasoning. Using these regions as seeds, white matter tracts connecting the brain regions for each ability were extracted and used to calculate the mean fiber density associated with each ability. Three models, linear, partial linear, and quadratic were performed to model the longitudinal changes in fiber density for each ability. Mixed effect regression was used to test whether vocabulary, the only crystallized ability, shows more gradual decline than white matter for the other three abilities, which were all fluid abilities. The non‐linear trend of age was modeled using B‐splines with the knot point selection based on Bayesian information criteria (BIC).ResultConsistent with cross‐sectional results reported in Gazes et al. 2020, mean fiber density for white matter tracts associated with the vocabulary ability was shown to follow a slower decline with age than that for the three fluid abilities – processing speed [F(2,1251)=57.5, p<.001], episodic memory [F(2,1251)=5.32, p<.001), and reasoning [F(2,1251)=4.05, p<.001]. Figure 1 shows the age‐related trajectory modeled with quadratic trend, which was the best fitting model based on BIC values. Vocabulary, in purple, clearly demonstrates a shallower decline than the other three abilities for fiber density.ConclusionThis study demonstrated that longitudinal changes in the microstructural integrity of white matter tracts are better preserved for vocabulary, a crystallized ability, than that for the three fluid abilities, suggesting that preservation of white matter contributes to the maintenance of crystallized abilities with age.