![]() A systematic search was conducted in two databases via Ovid, MEDLINE and EMBASE, from inception until 6 November 2019 ( Supplementary Table 1). A cognitive trajectory was defined as assessing cognitive function using three or more waves of data. Briefly, any studies investigating longitudinal trajectories of cognitive function in a general adult population (18 + years) were eligible. Further details regarding the methodology are described in the Supplementary Material. This systematic review was registered (PROSPERO CRD42020156754) and conducted in line with PRISMA guidelines. The aim of this review is to systematically summarize the research findings from the studies which identified multiple latent classes of cognitive trajectories. This data-driven and hypothesis-free approach take both the intra-individual change and inter-individual difference into account, which more comprehensively delineates the natural process of cognitive aging. These studies further identified latent classes of cognitive trajectories by classifying and clustering the participants with similar cognitive trajectories. In the recent decade, some studies of cognitive trajectories have emerged that also considered population heterogeneity. Either design might have underestimated the population heterogeneity. Also, some other studies compared the trajectories between subsamples classified based on a pre-specified factor (i.e., male versus female), with a hypothesis that this factor contributes to differential trajectories. For example, some studies analyzed the average patterns of the cognitive trajectories based on a single curve, assuming that the sample was from a homogenous population. However, most of these studies did not consider the inter-individual difference. This approach presents the cognitive assessments longitudinally as a curve and facilitates a more detailed description of the natural evolution of cognitive aging as well as its intra-individual change. Using longitudinally collected cognitive data from at least three timepoints provides opportunities to determine the ‘patterns’ or ‘trajectories’ of cognitive function, highlighting the dynamic nature of cognitive aging and reflecting its continuous change over time. Importantly, there also appears to be a group of ‘successful cognitive agers’ who maintain their cognitive function at high levels, even with advancing age. It is likely that a fraction of individuals might experience gradual age-related cognitive loss to varying degrees as they age. The group of individuals who do not experience severe cognitive impairment might also be highly heterogeneous. A lack of longitudinal repeated cognitive assessments might not sufficiently capture the likely substantial intra-individual heterogeneity and the natural process of cognitive aging over time. However, most have primarily compared two groups, those with severe decline or dementia compared to all other individuals, and often only across two assessments. There is extensive research literature focused on the identification of risk and protective factors of severe cognitive impairment, or incident dementia. When severe, these changes can be considered as symptoms of pathological conditions. Since cognitive function is one of the most important capacities which significantly influence the well-being of older adults, maintenance of optimal cognitive function is essential for healthy aging.Ĭognitive aging refers to the dynamic and variable longitudinal changes in cognitive function that inherently occur throughout the aging process. It highlights the absence of age-related decline in intrinsic capacities during the aging process. Healthy aging has been defined as the process of developing and maintaining the functional ability that enables well-being in older age.
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