26 To 'Proxy’ or Not to 'Proxy’: The Differential Effects of Cognitive Reserve on Late-Life Neuropsychological Functioning

Abstract

Objective:As the population continues to age at a rapid pace, an important question that has been raised by clinicians and researchers alike, is “how can we preserve our cognitive abilities as older adults?” Cognitive reserve (CR) is thought to account for inter-individual differences in the cognitive trajectories of older people experiencing similar levels of brain pathology. Being a hypothetical construct, CR cannot be measured directly and therefore, must be operationalized through a combination of demographic and lifestyle variables. While there is sufficient empirical evidence supporting the relationship between individual CR proxies and cognitive functioning, few studies to date have explored which CR proxy is most important in predicting aspects of late-life cognitive functioning and whether composite measure of CR accurately predict cognitive functioning, above and beyond variables traditionally associated with late-life cognitive ability (e.g., age, gender, cardiovascular risk, depression). The present cross-sectional study sought to examine the relationship between three well-established CR proxies—educational attainment, mental workplace demands, crystallized intelligence—and baseline neuropsychological functioning in a clinical sample of older adults without dementia.Participants and Methods:Using archival data from 248 older adult patients seen at geriatric specialist hospital in Ontario, Canada, we examined the cumulative and independent effects of educational attainment (years of formal education), mental workplace demands (mean analyst rating for 10 O*NET variables measuring cognitively complex work activities), and crystallized intelligence (score on WASI-I Vocabulary subtest) on performance across several neuropsychological tests assessing i.) verbal memory, ii.) visual memory, iii.) attention, concentration, and working memory, iv.) executive functioning, v.) visuospatial processing, and vi.) language. Composite scores for all six cognitive domains were calculated by averaging converted z-scores on domain-specific tests of neuropsychological functioning. A series of regression models were then constructed to evaluate the relationship between CR and late-life cognitive functioning. To determine the relative importance of each CR proxy, a follow-up relative weight analysis (RWA) was performed for each regression model.Results:After controlling for age, gender, cardiovascular risk, and depression, our composite measure of CR (average z-score of educational attainment, mental workplace demands, crystallized intelligence) proved to be a significant predictor across all domains of cognitive functioning. Of the six cognitive domains assessed, CR was the most important in predicting higher-order cognitive functions, such as working memory and executive functioning. Out of the three CR proxies, crystallized intelligence was the only CR proxy that significantly predicted performance across all six cognitive domains. RWA revealed that crystallized intelligence was the most important in predicting neuropsychological functioning, followed by educational attainment, and mental workplace demands.Conclusions:Despite being closely related, all three CR proxies demonstrated differential effects on late-life cognitive functioning. Overall, our findings suggest that the effects of CR are not generalizable across all cognitive domains and appears to be somewhat dependent on the CR proxy used. This study supports the existing literature in demonstrating the robustness of crystallized intelligence as a CR proxy and provides preliminary evidence supporting the idea that tasks that require higher levels of cognitive processing are the most influenced by CR.