Understanding the Aging Mind
It is well known that the population of the United States is aging. As the proportion of older people in the population grows, it becomes increasingly important to understand age-related changes in cognitive functioning. For the many aging people in good physical condition, cognitive decline is the main threat to their ability to continue enjoying their favorite activities; for those whose physical activities are limited, cognitive decline is a major additional threat to quality of life.
The view that aging is synonymous with universal and rapid cognitive decline is giving way to a recognition that for some aging individuals, mental acuity continues well into advanced age. Moreover, recent scientific findings give growing reason to believe that it may be possible to help older people maintain more of their cognitive function into later years.
Research is showing that the adult brain has much greater capacity for plasticity than previously believed, growing new dendrites and perhaps even new neurons (Kolb and Whishaw, 1998; Buonomano and Merzenich, 1998; Gould et al., 1999b). In addition, adult brains respond positively to a variety of life experiences and to biochemical interventions. In animal studies, administration of nerve growth factor (NGF) has reversed deterioration in adult nerve cells (D.E. Smith et al., 1999). Antioxidants have improved cognitive performance and signal transduction in aged rats (Socci et al., 1995; Joseph et al., 1999). And behavioral interventions with animals, such as training, enrichment of the environment, increased social interaction, and simple physical exercise have increased neurogenesis in adult brains (Gould et al., 1999a; Kempermann et al., 1997, 1998; van Praag et al., 1999). In older people,
positive effects on cognitive function have been reported in response to antioxidants and to behavioral interventions, such as exercise and training (e.g., Jama et al., 1996; Paleologos et al., 1998; Albert et al., 1995; Kramer et al., 1999). Such findings are suggesting many new possibilities for effective intervention to improve cognitive function in older people.
Much remains unknown, however, about how great the potential is to avoid serious decline in cognitive performance as part of what has been called "usual aging" (Rowe and Kahn, 1998). Little is known about the mechanisms that explain these provocative findings or about how they might be turned into effective interventions to improve human lives.
Now is a time of great promise for learning more about the aging mind and for turning that knowledge to the advantage of aging individuals. Neuroscientists are making rapid progress in understanding the neural basis of sensation, memory, language, and other cognitive functions and are poised to understand, at the molecular and cellular levels, those neural changes that affect the life course of cognitive capabilities. The time is right for developing intervention strategies to maintain the integrity of neuronal function and to rescue and repair malfunctioning neurons. Behavioral researchers are making rapid progress in classifying types of cognitive functioning, measuring them, tracking changes in particular functions over the life cycle, and documenting declines, maintenance, and improvement in these functions over the life span. This research is making it possible to develop behavioral and technological interventions to maintain cognitive performance in older individuals. Researchers in cognitive science are developing detailed models and theories of cognitive processes that can help make sense of observed patterns of change in functioning and link them to observed changes in neural systems. Social scientists are demonstrating the significance of cultural supports and life experiences in shaping cognitive content and processes over the life span.
Much valuable and promising research is already going on in each of these fields of research related to the aging mind. However, the fields do not communicate with each other as much as is probably desirable. Neuroscientists may document a positive or negative neural change but often do not use behavioral tests to determine whether such change makes a difference to the behaving organism. Behavioral researchers may clarify changes in function but often do not investigate the biological basis for the changes. We believe that, given the current state of knowledge, much can be learned from studies with both humans and experimental animals that link recent advances in some of these fields to unresolved problems in other fields. This volume describes a set of highly promising research opportunities—areas in which research over the next several years is likely to yield major advances.
A CONCEPTUAL FRAMEWORK
Much progress is being made by behavioral science, cognitive science, and neuroscience researchers in understanding cognitive changes during the aging process. However, what is being learned from each research perspective has not fully penetrated the work of researchers proceeding from other perspectives. This realization drew the committee to consider the ways that each perspective might illuminate the others—to develop a conceptual framework that would facilitate making these connections.
Such a framework represents the performance of life's cognitive tasks by aging individuals as dependent on three interacting systems: cognitive structures and processes, neural health, and behavioral context, including task structure and social, cultural, and technological factors. The distinction between performance and cognitive structures and processes is analogous to that between phenotype and genotype: the latter represents a capability for the former that is actualized only under the proper somatic and environmental conditions.
We use the term "the aging mind" to refer to change in cognitive structures, processes, and content. We have adopted this old-fashioned term to enlarge the conception of cognitive aging to include not only changes that can be directly observed in the brain or by standard laboratory tests of cognitive function, but also cognitive aspects of the self, personality, and interactions with other people. This sort of broadening of the concept of cognition is evident in ideas of "situated cognition" and "interactive minds'' (e.g., Baltes and Staudinger, 1996) and is consistent with recent work in developmental science showing how minds—in evolution and ontogeny—are adapted and tuned to their particular environments (Gigerenzer et al., 1999). This work also includes a strong focus on the knowledge-based content of the aging mind (Baltes et al., 1999).
Several features of the conceptual framework deserve elaboration.
The framework is intended for understanding the performance of life activities. We presume that research on the aging mind is ultimately directed toward understanding how individuals perform activities of living. Research on the aging mind is motivated not only by curiosity about cognitive functioning, but also by a search for practical ways to maintain and improve the cognitive performance of aging individuals.
We thus conceive of "mind" broadly. It includes the so-called higher-level functions of language, thought, judgment, attention, learning, memory, and decision making, as well as cognitive functions involved in less intellectual activities that depend on neocortex, such as locomotion, perception, and driving a car. It also includes the content and structure of knowledge and other aspects of cognition necessary for functioning in society.
The aging mind is shaped by a conjunction of factors. These include direct changes in the brain, variations in behavioral context (for example, task structure, motives, cultural meanings, social and technological supports), and somatic events (e.g., nonneurological disease, sensory-motor changes). Each of these is driven in part by factors that change over the life course and each may affect cognitive functioning in real-life contexts. Thus, although healthy neurons and the integrity of neural systems are necessary for adequate cognitive functioning, other factors, which affect the brain only indirectly, are important as well. For instance, cultural and social supports are necessary for adequate cognitive functioning in advanced years.
Interactions and mutual causation among neural, behavioral, and somatic phenomena are important topics for study. Discoveries about neural plasticity have put an end to the notion that the brain functions only as an independent variable in brain-behavior relationships. Experience shapes the brain and thus influences cognitive functioning across the entire life span, although these effects may be particularly strong early in life. Certain somatic processes, such as cardiovascular disease and sensory-motor changes, may affect cognitive function indirectly through their direct effects on brain functioning. Systematic differences in life experiences between cultural groups may also affect cognitive aging by altering the brain. Thus, research on the aging mind includes studies to determine the health of aging neurons; the ways in which social, behavioral, and somatic variables affect neural health and cognitive structure, content, and process; and the effects of molecular, cellular, and behavioral interventions on neural health and cognitive functioning.
Adaptive processes are central to understanding the aging mind. A prevalent model has been that of more-or-less inexorable cognitive decline. Normal aging was presumed to inevitably involve loss of neural capabilities, which in turn led automatically to loss of function. The evidence indicates, however, that different cognitive functions have different life courses (Schaie, 1994, 1996; Baltes, 1997; Baltes et al., 1999). Many people retain many cognitive capabilities into very advanced years, indicating that interindividual variation exists in rates of change with aging (Hultsch et al., 1998; Schaie, 1994, 1996; Willis, 1991), although the importance of these variations is still a matter of controversy. Three explanations of the variations can be considered. Neural changes may not be as uniform or as profound as once believed; various adaptive processes at the neural, behavioral, and social levels may mitigate the behavioral effects of the neural changes that do occur; and finally, the cultural and social environments may offer opportunities for adaptation and new growth.
The role of adaptations is particularly important. Older people adapt to
changes in their nervous systems and their environments and, at the same time, both types of changes affect their ability to perform cognitive tasks. To separate the various causes of cognitive change, it is necessary to examine inter-and intraindividual differences in cognitive function both cross-sectionally and over time to identify patterns. Such examinations should highlight the roles of dynamic adaptive processes, including changes in neuronal structure and function and in behavioral and social factors (e.g., social opportunity structures, the individual's routines and physical environment, the individual's goals, and the use of social and technological supports) that codetermine an individual's ability to function effectively.
IDENTIFYING RESEARCH OPPORTUNITIES
Our task has been to identify areas of opportunity in which additional research support from the National Institute on Aging (NIA) would substantially improve understanding of cognitive functioning in aging by drawing on recent developments in behavioral science, cognitive science, and neuroscience that are not yet fully applied to this subject area. As already noted, we have adopted a broad definition of cognitive function.
Our focus is on cognitive function in aging people who are not suffering from a dementia-causing disease—that is, on "normal" rather than "pathological" aging. The boundary between normal and pathological aging is hard to define. For one thing, some individuals who are aging "normally" at one point in time may later develop dementia, suggesting that their earlier ''normal" functioning was in fact compromised by preclinical signs of that dementia. For another, conditions associated with aging that are not themselves considered as cognitive pathology, such as hypertension, sensory decline, and certain cardiovascular events, may have cognitive effects, as discussed in Chapter 4. People experiencing such conditions who do not suffer from one of the dementias must be considered as aging "normally" for the purposes of research in the near term; however, research may eventually discover that for some of them, a pathological process was directly affecting their cognition. Thus, a population of "normally" aging individuals defined at one time may be determined on the basis of later research or later life events to have included some individuals whose cognition was compromised by a pathological process.
A major difficulty for our task is that there are many promising research directions from which to choose. The aging mind is a topic at the intersection of several active fields of behavioral science, cognitive science, and neuroscience within which research is continually opening new vistas. New research opportunities arise often in these fields, even without special efforts to find them. We have therefore made a special effort to identify opportunities that might not automatically flow from current lines of research. For ex-
ample, we looked for opportunities that might arise by applying recently developed concepts, methods, or insights in one field to problems in other fields of research on the aging mind in which their implications have not yet been much explored.
We sought particularly to identify research opportunities that would link behavioral science, cognitive science, and neuroscience approaches to cognition and aging in new ways. Thus, in identifying areas of possible research opportunity, we considered the following issues:
Does the research directly address or have clear potential applications to cognitive function in aging?
Can the research improve understanding of cognition and aging by drawing on recent developments in behavioral science, cognitive science, and neuroscience that are not yet fully applied in this area?
Will the research open new possibilities to link these different approaches to cognition and aging?
Will the research lead to fundamentally new and promising directions for investigation, as well as adding to existing knowledge?
Does research progress depend on developing infrastructure, such as new technology or datasets?
Needless to say, the research opportunities we identify in this report are not the only promising ones. Other research directions might also meet the above tests, and there are other possible ways of formulating a research agenda. However, the research opportunities identified in this report appear to us to be particularly compelling. In our view, their vigorous pursuit will lead to significant advances in knowledge.
ABOUT THIS BOOK
Our report consists of the five chapters of this volume. Background papers that provide supportive detail are presented in appendixes. This chapter explains the committee's conceptual framework and the way we went about identifying research opportunities. Chapters 2, 3, and 4 focus on three substantive areas in which we propose major research initiatives, and Chapter 5 addresses implementation issues in pursuing these initiatives. All four of these chapters contain recommendations for action.
Chapter 2 focuses on neural health. It discusses emerging developments in the study of processes in the brain that provide a substrate for age-related change in cognitive functioning. It recommends a major research initiative aimed at understanding processes at the molecular and cellular levels that affect neural health, intervening in these processes to improve cognitive func-
tioning, and relating neural changes to changes in cognitive functioning and behavior.
Chapter 3 focuses on cognition in context. It discusses recent developments in understanding how behavioral, social, cultural, and technological contexts shape the content, structure, and process of cognition throughout the life cycle, including cultural-experiential influences on brain development. It recommends a major research initiative to understand the aging mind in relation to the neurobiological effects of life experience and the effects of cultural difference and cultural and behavioral supports on the aging mind. The initiative would also investigate possibilities for maintaining cognitive functioning by changing life experience or providing supportive technologies.
Chapter 4 is concerned with the structure of the aging mind. It discusses the major interindividual differences in rates and patterns of change in cognitive content, structure, and process during aging and recommends a major research initiative to specify and explain these patterns in relation to age-related changes in the brain, sensory and motor systems, nonneurological diseases, and life experiences that may predispose toward or protect against cognitive decline. The chapter discusses the value of brain-imaging technology for contributing to the needed understanding; the need to develop behavioral indicators that are closely associated with the action of particular neural circuits; and the potential for applying concepts and methods from other areas of cognitive research, particularly cognitive development in early life, to studying the dynamics of cognitive aging.
Chapter 5 discusses implementation of the research initiatives. It addresses the investments needed to build the interdisciplinary literacy and collaborations needed to advance knowledge at the conjunction of established fields and research traditions; to improve the available stock of longitudinal data on cognitive aging; and to develop other kinds of infrastructure that are of broad use to the health sciences as well as essential for understanding the aging mind.
The appendixes include signed papers commissioned by the committee to provide more detailed accounts of the current state of research in fields that we see as leading to exciting advances and to speculate about possible directions in these fields. Although these papers are not the work of the committee, we consider them to be useful aids to thinking about how to move cognitive aging research in several of the directions we recommend.