A protective role for the cerebellum in cognitive aging
BackgroundBrain reserve -- the brains resilience to age-related change or damage -- provides protection against cognitive decline. The cerebellum is relatively unstudied as a contributor to brain reserve. This study investigates cerebellar brain reserve in the largest cohort to date. MethodsWe used data from the Human Connectome Project (n=708, 36-100yrs), UK Biobank (n=45,013, 44-81yrs), and ADNI (n=1,423, 56-95yrs). ADNI participants were cognitively normal or had a diagnosis of mild cognitive impairment or Alzheimers disease (AD) dementia. We examined associations between cerebellar tissue volume, age, Montreal Cognitive Assessment (MoCA) scores, global PET amyloid burden, and APOE genotype. FindingsHCP-Aging data revealed heterogenous aging-associated changes in cerebellar volume, with the greatest effects in posterior hemispheric regions (crus I) (Bonferroni-corrected, p<0{middle dot}05). MoCA scores were associated with higher tissue density in the cerebellum (p<0{middle dot}0001) to the same extent as neocortex, and MoCA scores coupled most strongly with posterior cerebellar cortex. Strikingly, greater volume in MoCA visuospatial-related cerebellar cortex protected against aging-related cognitive decline (p=0{middle dot}0001). We replicated tissue aging results in the UK Biobank with the greatest aging-related effects in posterior cerebellum (p<0{middle dot}0001), and an association of greater cerebellar volumes with less cognitive decline (Trails Making-B: p<0{middle dot}00001; Digit Symbol Substitution: p=0{middle dot}034). AD patients with low amyloid-beta burden (A{beta}-) exhibited the strongest cerebellar association with MoCA (volume x group, A{beta}- AD: p=0{middle dot}0001). In A{beta}- individuals, APOE {varepsilon}4/{varepsilon}4 carriers showed the greatest effect with MoCA (volume x APOE, {varepsilon}4/{varepsilon}4: p=0{middle dot}017). InterpretationOur large-scale study demonstrates a potentially strong role for the cerebellum in mitigating cognitive decline. The persistence of this protection in APOE {varepsilon}4/{varepsilon}4 carriers reshapes our understanding of reserve and AD risk. Our findings open the cerebellum as a novel target for future clinical research on brain reserve in aging populations. FundingNational Science Foundation, National Academies of Sciences, Engineering and Medicine, National Institutes of Health. Research in ContextO_ST_ABSEvidence before this studyC_ST_ABSWe searched PubMed and GoogleScholar between August 12, 2023, and September 25, 2024 for articles irrespective of language or date of publication, relating to measures of cortical and cerebellar aging. Search terms included "cerebellum", "cognitive aging", "cerebellar reserve", "Alzheimers disease", "covariance", "retrogenesis", and "connectivity". Prior research demonstrated that the cerebellum may play a role in cognitive function. Studies have shown that cerebellar development is spatially heterogenous, with posterior regions undergoing the most protracted change in structure and function. Brain reserve has been predominantly studied in the neocortex, with some studies suggesting the cerebellum may contribute to cognitive changes in diseases like Alzheimers disease (AD) dementia. Cerebellar volume differences between young and older adults have been noted, as well as between healthy adults and neurodegenerative conditions like Parkinsons disease. However, the cerebellums contribution to cognitive reserve, particularly in healthy aging and individuals with a clinical diagnosis of mild cognitive impairment or AD dementia, had not been explored especially in large population-based datasets. Added value of this studyTo our knowledge, this study is the largest to date looking into how cerebellar structures contribute to cognitive outcomes in both healthy older adults, and those with mild cognitive impairment or AD dementia. We leverage three large neuroimaging datasets -- HCP-Aging, UK Biobank, and ADNI -- and demonstrate that cerebellar aging is spatially heterogeneous, with posterior regions showing the greatest age-related decline. We further establish a significant link between larger cerebellar volumes and better cognitive outcomes, suggesting that the cerebellum plays a role in cognitive resilience. Additionally, we identify how cerebellar structures predict cognitive performance and interact with amyloid-beta brain pathology and APOE genotype, particularly in those with low amyloid brain burden and those at greatest risk of AD such as in homozygous APOE e4 allele carriers. Implications of all the available evidenceThe aging of the global population raises the challenge of maintaining cognitive health in older age. Our research focuses on the cerebellum as a novel mediator of preserved cognitive function in old age and clinical dementia. Our findings have profound implications, given that individuals with robust cerebellar structures may be missed during cognitive and clinical screenings. Greater cerebellum volume seems to be most advantageous in those at higher risk for Alzheimers disease based on APOE4 status and those who have yet to accumulate substantial amyloid-beta brain pathology. Our study underscores the importance of the cerebellum as a novel brain reserve mechanism in aging populations.