Cognitive Decline/Impairment

Cognitive decline is defined as a negative change in cognitive status over time that can be a function of normal ageing, brain injury, dementing brain pathology (e.g. Alzheimer's disease), or other mechanisms. There are two major areas in which assessment of cognitive decline is important: (1) research on the nature of ageing and how it affects cognitive processes and mechanisms, and (2) individual assessment, in which cognitive testing is used to determine whether a specific individual has undergone cognitive decline, possibly due to age-related disease processes, such as Alzheimer's disease or stroke. Assessment of normative age-related change often involves use of panel designs, in which a large sample of individuals are given a set of cognitive tests and tasks. Prototypic examples of this kind of research are the Seattle Longitudinal Study (Schaie, 1996), the Berlin Aging Study (Baltes & Mayer, 1999), and the Victoria Longitudinal Study (Hultsch, Hertzog, Dixon & Small, 1998). These kinds of studies focus on characterizing the normative patterns of cognitive decline, and also on assessing individual differences in rates of cognitive decline. In order to address the latter question, it is important that the same persons are followed over time in a longitudinal design, so that individual differences in rates of cognitive change can be estimated (Baltes, Reese & Nesselroade, 1988). When the goal is assessment of individuals with respect to cognitive decline, change is often assessed indirectly. That is, the typical neuropsychological assessment of decline is made through norm-referenced evaluation low performance relative to same-aged peers. However, it is also possible to follow individuals over time for purposes of assessment, as we discuss further.

One of the challenges with assessing cognitive decline in adulthood is that there are a large number of different types of cognitive abilities (Carroll, 1993). Research on cognitive decline must allow for the possibility that different abilities may be affected in different ways by the ageing process (Dixon & Hertzog, 1996). It is well known, for example, that tests of recognition vocabulary (knowledge of word meanings) show relatively little decline until late in the lifespan, whereas tests of inductive reasoning (the ability to observe patterns or regularities in phenomena) or deductive reasoning show earlier decline (Salthouse, 1991). Another challenge to assessing cognitive decline is that changes are often slow and gradual (occurring over years or decades, in the absence of significant pathology in the central nervous system). Hence researchers often use cross-sectional designs, in which persons of different ages are tested at a single point in time, and age differences within the sample are used to estimate magnitudes of average age-related declines. Although this approach probably provides accurate general information about [Page 220]abilities that are influenced by age, generational (or cohort) differences can lead to overestimation of the magnitude of decline. Moreover, cross-sectional data cannot be used to evaluate individual differences in rates of cognitive change. Cross-sectional studies allow quick assessment of possible cognitive decline, but the inferences in such cases should be validated with longitudinal data as rapidly as possible (Hertzog, 1996).

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