Elaborating a Hypothetical Concept: Comments on the Special Series on Cognitive Reserve

The four studies in this symposium emerged from an exciting 2009 Friday Harbor workshop that focused on using the most advanced psychometric approaches in the study of cognitive reserve (CR). The workshop participants had the pleasure of working with three outstanding data sets, all from the Rush group in Chicago, directed by David Bennett. The data sets were based on cohorts of extremely well characterized, prospectively followed elders. The autopsy rate was outstanding, as was the quality of the postmortem data. In addition, the study investigators have had a long-standing interest in issues surrounding reserve and have thoughtfully assembled a rich array of relevant data about features of the life course that might be associated with increased reserve. What made the workshop especially fun was the presence of psychometricians/biomedical statisticians who were up to the task of working with this rich data set and giving it its due. The cognitive reserve hypothesis emerged from the observation that there is not a direct relationship between the amount of brain changes or pathology and the cognitive or functional consequences of those brain changes (Stern, 2002, 2009). Instead, there are a great deal of individual differences in people’s susceptibility to pathology. The cognitive reserve hypothesis posits that these individual differences may stem from differential ability to cope with or compensate for pathology, which in turn is driven by individual differences in how tasks are processed in the brain. The epidemiologic evidence suggests that different exposures during the life course may contribute to this reserve (e.g., Scarmeas, Levy, Tang, Manly, & Stern, 2001; Stern et al., 1994). There is also more direct evidence that specific life exposures might moderate the relationship between measured brain changes and cognitive performance (Bennett et al., 2003; Stern, Alexander, Prohovnik, & Mayeux, 1992). Starting with this background, each of the studies in this series addresses some important piece of the puzzle in trying to understand how reserve may be imparted and implemented. The study by Fyffe et al. examines the difficult issue of ethnic differences in test performance. The authors grappled with two issues. First, using differential item functioning (DIF) methodology they measured equivalence of their memory measures across whites and African Americans. Importantly, they found minimal impact of DIF, indicating that observed ethnic differences in performance, as well as the potential influence of other covariates, are not due to measurement bias. The authors then went on to investigate whether various lifetime exposures that have been associated with cognitive reserve might account for performance differences across the two groups. These variables included personal and parental education, childhood cognitive activity frequency, income at age 40, and education quality as measured by reading level. These variables were chosen because they have been used as proxies of cognitive reserve in previous studies. The authors found that the observed memory test performance differences between the two groups were best accounted for by educational quality, as assessed with reading level. This is an interesting finding, which replicates