IQ moderation of cognitive decline supports cognitive reserve and not brain maintenance

Age-related cognitive changes can be influenced by both brain maintenance (BM), which refers to the relative absence over time of changes in neural resources or neuropathologic changes, and cognitive reserve (CR), which encompasses brain processes that allow for better-than-expected behavioral performance given the degree of life-course related brain changes. This study evaluated the effects of age, BM, and CR on longitudinal changes over two visits, 5 years apart, in four reference cognitive abilities that capture most of age-related variability. Participants included 254 healthy adults aged 20-80 years at recruitment. Potential BM was estimated using whole brain cortical thickness and white matter mean diffusivity at both visits. Education and IQ (estimated with AMNART) were tested as moderating factors for cognitive changes in the four reference abilities. Consistent with BM, after accounting for age, sex, and baseline performance, individual differences in the preservation of mean diffusivity were associated with relative preservation in the four abilities; differential preservation of cortical thickness was associated with preservation of reasoning, processing speed, and memory. Consistent with CR, after accounting for structural brain changes, higher IQ, but not education, was associated with reduced 5-year decline in reasoning. A similar marginal association was seen for memory but not for processing speed. These results demonstrate that both CR and BM can moderate cognitive changes in healthy aging and that the two mechanisms can make differential contributions to preserved cognition. SignificanceThe theoretical constructs of brain maintenance (BM) and cognitive reserve (CR) are postulated to account for individual differences in age-related cognitive decline. To further understand BM and CR mechanisms, this study used a lifespan sample to examine 5-year changes in four cognitive abilities that comprehensively capture cognitive aging. Education has been the most used life exposure when studying CR, but recent studies suggest that education does not moderate age-related changes in brain and cognition in healthy aging so IQ was also considered. The results demonstrated that BM and CR can both exert unique influences on the same cognitive ability and that the two mechanisms differentially moderate change in the four reference abilities.