Efficiency, capacity, compensation, maintenance, plasticity: emerging concepts in cognitive reserve

Abstract

Cognitive reserve (CR) is a concept meant to account for the frequent discrepancy between an individual's measured level of brain pathology and her expected cognitive performance. It is particularly important within the context of aging and dementia, but has wider applicability to all forms of brain damage. As such, it has intimate links to related compensatory and neuroprotective concepts, as well as to the related notion of brain reserve. In this article, we introduce the concept of cognitive reserve and explicate its potential cognitive and neural implementation. We conclude that cognitive reserve is compatible and complementary with many related concepts, but that each much draw sharper conceptual boundaries in order to truly explain preserved cognitive function in the face of aging or brain damage.

Keywords: Alzheimer's disease; aging; cognitive reserve; epidemiology; imaging.

Figure 1

Representation of how cognitive reserve may mediate between AD pathology and its clinical expression based on epidemiological and imaging studies. The x-axis represents AD pathology, slowly increasing over time. The y-axis represents cognitive function. We assume that AD pathology increases over time at the same rate in two individuals with high and low reserve. The amount of pathology needed before cognitive function is affected is greater with higher CR, leading to a later change point [91, 92]. It follows that more pathology will be needed for the person with higher CR to meet clinical diagnostic criteria for AD, thus delaying the onset of the disease. Also, at any level of cognitive performance, AD pathology will be more severe in the individual with higher CR [27, 93]. Once cognitive decline begins, it is more rapid in the person with higher CR [92, 94].

Figure 2

Possible instantiation of CR via neural reserve and neural compensation. One study [82] using this model in the context of a working memory task observed decreasing network efficiency with volume loss within the network (path d). However, this decreasing efficiency was mitigated by higher measured CR (path f), consistent with the concept of neural reserve. Neural compensation was also observed via a second network expressed by older but not younger adults when efficiency was sufficiently impaired in the first network. More activation in the compensatory network was associated with diminished task performance (path e), which demonstrates that compensation need not always improve performance but may only maintain it, albeit at a lower level. Higher measured CR moderated the detrimental effect of the compensatory network on performance (path g), which suggests that individuals with higher CR may recruit additional networks not directly captured by task-related activation.