Changes in Cognitive Function in Human Aging

As people age, they change in a myriad of ways — both biological and psychological. Some of these changes may be for the better, and others are not. This book primarily concerns the normally aging brain, the neuroanatomical and neurophysiological changes that occur with age, and the mechanisms that account for them. It is not primarily about the behavioral or cognitive concomitants of those changes. Nevertheless, there is ample evidence that alterations in brain structure and function are intimately tied to alterations in cognitive function. The complexity of both the neural and cognitive functions, however, makes exact mapping between brain and behavior extraordinarily difficult, and so these relations remain largely speculative, although ultimately testable. Establishing such links between brain and cognition is the principal goal of cognitive neuroscience. The purpose of this chapter is to outline the changes in cognition that occur in normal human aging, in an effort to provide a backdrop against which neural changes can be interpreted (for review, see [1]). Although the relationship between brain and cognition is a dynamic one and may change across the lifespan, changes in these two domains will ultimately be related, and mechanisms underlying the changes will be discovered. Understanding age-related cognitive change will help focus and constrain neurobiological theories of aging in much the same way as theories of cognitive aging will be adapted to take account of new findings about the aging brain. Just as age-related changes in brain structure and function are not uniform across the whole brain or across individuals, age-related changes in cognition are not uniform across all cognitive domains or across all older individuals. The basic cognitive functions most affected by age are attention and memory. Neither of these are unitary functions, however, and evidence suggests that some aspects of attention and memory hold up well with age while others show significant declines. Perception (although considered by many to be a precognitive function) also shows significant age-related declines attributable mainly to declining sensory capacities. Deficits at these early processing stages could affect cognitive functions later in the processing stream. Higher-level cognitive functions such as language processing and decision making may also be affected by age. These tasks naturally rely on more basic cognitive functions and will generally show deficits to the extent that those fundamental processes are impaired. Moreover, complex cognitive tasks may also depend on a set of executive functions, which manage and coordinate the various components of the tasks. Considerable evidence points to impairment of executive function as a key contributor to age-related declines in a range of cognitive tasks. Finally, although these cognitive functions will be reviewed separately below, it is abundantly clear that they overlap and interact in interesting and complex ways. Although the overall picture might seem to be one of cognitive decline, enormous variability exists across individuals. Many older people out-perform young people, at least on some cognitive tasks, and others of the same age do at least as well as the young [2]. A question of great interest to aging researchers is what accounts for this variability. This chapter highlights the cognitive domains that show the greatest declines with age and are also the most variable. Areas of cognitive strength in normal aging are also discussed, because these may be recruited to compensate for areas of weakness. Theories of cognitive aging that have developed within each cognitive domain are outlined and brain regions hypothesized to underlie these functions are noted. The next chapter section reviews some of the evidence for age-related impairments in basic cognitive functions, focusing primarily on attention and memory, and also discusses briefly the attentional and memory processes that show relative preservation with age.