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Riddle DR, editor. Brain Aging: Models, Methods, and Mechanisms. Boca Raton (FL): CRC Press; 2007.

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Brain Aging: Models, Methods, and Mechanisms.

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Chapter 1Changes in Cognitive Function in Human Aging

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I INTRODUCTION: 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.

II BASIC COGNITIVE FUNCTIONS

A Attention

Attention is a basic but complex cognitive process that has multiple sub-processes specialized for different aspects of attentional processing. Some form of attention is involved in virtually all other cognitive domains, except when task performance has become habitual or automatic. Declines in attention can therefore have broad-reaching effects on one’s ability to function adequately and efficiently in everyday life. The construct of attention defies simple definition, however, and it has been partitioned in a variety of ways by different researchers and theorists. The divisions used here are those that have been investigated most extensively in normal aging (for a comprehensive review of attention and aging, see [3]).

1 Selective Attention

Selective attention refers to the ability to attend to some stimuli while disregarding others that are irrelevant to the task at hand. For example, in visual search tasks, people are asked to search a visual display for a target letter that is surrounded by other nontarget letters. The task can be made more difficult by increasing the similarity of targets and distractors (e.g., search for an F in a background of Es), or by increasing the number of relevant or irrelevant features that are part of the search criteria. In another task — the Stroop task — people are asked to name the color of ink in which an incongruent color word is printed, (e.g., the word “red” printed in green). Here, the word information tends to interfere with color naming, causing errors and an increase in response times. To perform well in these kinds of tasks, people have to select the relevant stimulus or dimensions for processing and ignore the irrelevant ones. Although findings are not entirely consistent across studies and may differ across tasks, in general older adults appear to be slower than younger adults in responding to the targets, but are not differentially affected by distraction [3, 4]. Thus, deficits found in many of these tasks can be largely attributed to a general slowing of information processing in older adults rather than to selective attention deficits per se.

2 Divided Attention and Attention Switching

Divided attention has usually been associated with significant age-related declines in performance, particularly when tasks are complex. Divided attention tasks require the processing of two or more sources of information or the performance of two or more tasks at the same time. For example, people may have to monitor stimuli at two different spatial locations, or they may be asked to make semantic judgments about visually presented words while simultaneously monitoring for the occurrence of an auditorily presented digit [5]. The cost of dividing attention is assessed by comparing performance under dual task conditions to performance when the tasks are performed separately. Results suggest that older adults are more affected by the division of attention than young adults, particularly when the attentional demands of the two tasks are high. In addition, older adults seem less able to allocate resources appropriately when instructions are given to vary task priority [6]. These findings cannot be completely accounted for by a general slowing of information processing, but instead are usually explained in terms of declining processing resources associated with normal aging. Such limited resources are over-extended in older adults when attention must be divided between two or more sources. Similarly, the performance of older adults is slowed to a greater degree than that of young adults when attention must be switched from one task to another, requiring a change of mental set [4].

There is evidence that age deficits in divided attention and attention switching can be reduced by practice or extended training [7] and by aerobic exercise [8]. The exact mechanism of such improvements, however, is unclear. In the case of task-specific training, it is possible that some aspects of the tasks become automatic with practice, thus requiring fewer attentional resources. Alternatively, participants may develop strategies with extensive training that reduce the attentional demands of the tasks. It has been hypothesized that cardiovascular fitness may improve the efficiency of neural processes or may provide increased metabolic resources for task performance. Interestingly, the enhancement effects of aerobic exercise appear to be greatest on tasks involving executive control of attention [9], which depend largely on prefrontal cortex.

3 Sustained Attention

Sustained attention refers to the ability to maintain concentration on a task over an extended period of time. Typically, vigilance tasks are used to measure sustained attention, in which people must monitor the environment for a relatively infrequent signal, such as a blip on a radar screen. In general, older adults are not impaired on vigilance tasks.

4 Attention: Summary and Implications

Older adults show significant impairments on attentional tasks that require dividing or switching of attention among multiple inputs or tasks. They show relative preservation of performance on tasks that require selection of relevant stimuli; and although they are slower than young adults, they are not differentially impaired by distraction. They also are able to maintain concentration for an extended period of time. The tasks on which older adults show impairments tend to be those that require flexible control of attention, a cognitive function associated with the frontal lobes. Importantly, these types of tasks appear to be amenable to training and show benefits of cardiovascular fitness.

Attentional deficits can have a significant impact on an older person’s ability to function adequately and independently in everyday life. One important aspect of daily functioning affected by attentional problems is driving, an activity that, for many older people, is essential to independence. Driving requires a constant switching of attention in response to environmental contingencies. Attention must be divided between driving, monitoring the environment, and sorting out relevant from irrelevant stimuli in a cluttered visual array. Research has shown that divided attention impairments are significantly associated with increased automobile accidents in older adults [3, 10]. Given the previously noted findings of the effects of practice, extended training on driving simulators under divided attention conditions may be an important remedial activity for older people.

B Working Memory

Working memory is a multidimensional cognitive construct that has been hypothesized as the fundamental source of age-related deficits in a variety of cognitive tasks, including long-term memory, language, problem solving, and decision making. In fact, the majority of theories of cognitive aging seem to implicate working memory. Although there are several models of working memory, all agree that it is a limited capacity system that involves the active manipulation of information that is currently being maintained in focal attention (for reviews, see [11–13]). Short-term or primary memory, on the other hand, involves the simple maintenance of information over a short period of time. For example, one might maintain a phone number in short-term memory by simple rehearsal of the number. Older adults show minimal or no deficits in short-term memory and can typically hold about 7 ± 2 digits in mind as long as the digits are being rehearsed. Repeating the numbers backwards, however, requires an active reorganization or manipulation of the information held in short-term memory. This task thus requires working memory and shows impairments with age. In some sense, working memory is really a divided attention task — the contents of short-term memory must be maintained while simultaneously being manipulated or processed for some other purpose. Given the previously discussed findings of divided attention deficits with increased age, it is not surprising that older adults are impaired in working memory.

In the original working memory model of Baddeley and Hitch [14], the manipulation of information in short-term memory was handled by a central executive, and deficits in working memory were viewed as deficits in executive control, a function attributed primarily to prefrontal cortex. Recent neuroimaging research [15] has confirmed a role for dorsolateral prefrontal cortex (PFC) in the manipulation and updating of information in working memory, with left PFC involved more in verbal tasks and right PFC in visuospatial tasks. In recent years, however, the role of the central executive has been expanded to cover a range of executive control functions other than those associated strictly with working memory. These are elaborated in a later chapter section.

Although there is a general consensus that working memory is impaired in older adults, there is disagreement concerning the mechanisms involved, and much of the research has focused on testing a variety of theories. The next subsection outlines the main theories of working memory.

1 Theories of Working Memory

Three theories of cognitive aging have been articulated within the context of working memory deficits, although they may apply more broadly across other cognitive domains: (1) one theory proposes a reduction of attentional resources, (2) one focuses on reduced speed of information processing, and (3) one ascribes problems to a failure of inhibitory control (for review, see [16]).

a Attentional Resources

Theories of age-related decline in working memory generally assume some reduction in processing resources. Craik and colleagues [17, 18] have suggested that the resource limitation is attentional and reflects a reduction in mental energy. Tasks with high attentional demands show impairments, whereas tasks requiring little or no attention (i.e., that are relatively automatic) are largely intact. Working memory tasks by their very nature involve divided attention and are therefore more likely to strain the limited resources of older adults. This theory is intuitively appealing, but it seems more descriptive than explanatory. The construct of attentional resources is poorly defined; and although neurophysiological correlates such as arousal or neural efficiency have been suggested [3], they have not been demonstrated empirically.

b Speed of Information Processing

Salthouse [19] has suggested that speed of processing might be considered a resource, and that age-related deficits in working memory and other cognitive tasks can be explained in terms of a general slowing of information processing. There is little disagreement that older adults are slower than younger adults and that slowing of fundamental cognitive processes may have detrimental effects on more complex tasks. Debate has focused, instead, on whether a generalized slowing can account for the bulk of the empirical findings or whether more process-specific components are also needed. Salthouse [20, 21] has demonstrated in numerous studies that slowing of information processing can account for a large proportion of the age-related variance in a variety of cognitive tasks, including working and long-term memory, and has argued that speed of processing is a cognitive primitive. Other investigators [22], however, have suggested that speed of processing and working memory provide independent contributions to higher-level cognition, and that working memory deficits must therefore be accounted for in terms of something other than speed. Finally, at some level, slowed processing, like attentional resources, is more a descriptor of aging cognition than an explanation for cognitive deficits and says nothing about what causes slowing with age. Here too, therefore, discovery of neurophysiological correlates may help to clarify mechanisms.

c Inhibitory Control

Hasher, Zacks, and May [23, 24] proposed that a lack of inhibitory control might account for cognitive deficits associated with aging. Specifically, failure to suppress irrelevant information in working memory may effectively reduce its capacity, denying access to relevant information. For example, working memory span tasks involve the successive presentation across trials of increasingly long strings of digits or words. Age deficits could be attributable to the failure to delete from working memory digits or words from prior trials, thus reducing the “working space” for new stimuli [25]. Although considerable data suggest that older adults experience more interference from irrelevant information under some conditions [26], findings are mixed and other data fail to support an inhibitory deficit account [3]. It may be that there are different kinds of inhibition or that age-related effects are task- or paradigm-specific.

2 Working Memory: Summary and Implications

Older adults exhibit significant deficits in tasks that involve active manipulation, reorganization, or integration of the contents of working memory. Although the mechanisms underlying these age-related deficits are as yet poorly understood, the effects of such deficits are very likely far-reaching. Many complex everyday tasks such as decision-making, problem-solving, and the planning of goal-directed behaviors require the integration and reorganization of information from a variety of sources. It seems likely that attention, speed of information processing, and the ability to inhibit irrelevant information are all important functions for effective performance of these higher-level cognitive tasks. Whether these functions might be subsumed under a domain-general executive controller that is impaired by normal aging —something akin to the central executive in Baddeley’s model of working memory — or whether there may be multiple control processes that are independently affected by aging, is currently an issue under investigation. The brain regions that are active during working memory tasks are also beginning to be identified in a variety of functional neuroimaging studies. Results suggest that different areas are activated in young and old adults, particularly within the prefrontal cortex, indicating that younger and older adults are performing these tasks differently [12]. An understanding of age-related neurophysiological changes may help to account for these differences.

C Long-Term Memory

The cognitive domain that has probably received the most attention in normal aging is memory (for reviews, see [13, 27]). Many older adults complain of increased memory lapses as they age, and a major focus of research has been to try to distinguish memory declines attributable to normal aging from those that are indicative of pathological aging, particularly Alzheimer’s disease. Like attention, memory is not a unitary construct; some kinds of memory remain relatively intact with age while others show significant declines. Long-term memory, unlike short-term and working memory, requires retrieval of information that is no longer present or being maintained in an active state. This information could have occurred a few minutes ago or been acquired many years ago. The next subsections review age-related changes in various kinds of long-term memory.

1 Episodic Memory

Episodic memory refers to memory for personally experienced events that occurred in a particular place and at a particular time. This kind of memory allows one to think back through subjective time — what Tulving calls mental time travel [28] —and it usually evokes an “I remember” response. Episodic memory may be distinctly human; it is the most advanced form of memory and is ontogenetically the latest to develop. It also seems the most susceptible to brain damage and the most affected by normal aging.

The episodic memory problems experienced by older adults may involve deficient encoding, storage, or retrieval processes. At the input stage, older adults may encode new information less meaningfully or with less elaboration, so that memory traces are less distinctive, more similar to others in the memory system, and thereby more difficult to retrieve [29]. Alternatively, older people may attend to focal or salient information but fail to take account of peripheral detail, or they may fail to integrate contextual aspects of an experience with central content — what is sometimes referred to as a source memory problem [30]. Many of the common everyday memory lapses reported by normal older adults, such as forgetting where they parked their cars, likely involve poor encoding. These kinds of memory failures have generally been attributed to reduced use of effortful encoding strategies, which depend particularly on prefrontal brain regions. Another possibility is that noticing and integrating the various aspects of an experience involve divided attention and require working memory.

Older adults may also experience problems at the level of storage or consolidation. This aspect of episodic memory critically depends on medial temporal lobe structures, particularly the hippocampus. Consolidation is thought to involve the binding of various aspects of experience into a composite memory trace. What may be particularly critical for episodic memory and impaired in older adults is the extent to which an event is bound to its spatial and temporal context.

Finally, considerable evidence points to retrieval as a source of episodic memory problems in aging. Although it is clear that retrieval is at least partly dependent on encoding (i.e., well-encoded information is easier to retrieve), there are also effortful retrieval processes that appear to be impaired by aging. Older adults tend to show deficits on tests of free recall, to a somewhat lesser degree in cued recall, but minimally in recognition memory. Craik [18] has argued that the requirement to self-initiate strategic search processes in recall taxes the limited resources of older people. To the extent that environmental support can be provided at retrieval as well as at encoding (by providing good cues or using recognition tests, for example), the resource demands of encoding and retrieval are reduced and age differences are minimal. Similarly, Jennings and Jacoby [31] have demonstrated that recollection, which requires effortful retrieval of episodic detail, is impaired with age, whereas the more automatic judgments of familiarity are intact. Evidence from functional neuroimaging and neuropsychological studies suggests that these more strategic retrieval processes depend on the prefrontal cortex, as well as the hippocampus [32, 33].

2 Semantic Memory

Semantic memory refers to one’s store of general knowledge about the world, including factual information such as “George Washington was the first president of the United States” and knowledge of words and concepts. Such information is not tied to the space or time of learning, and its retrieval is generally prefaced with “I know.” Normally aging older adults do not have significant impairments in semantic memory. In fact, their knowledge of the world often exceeds that of young people. In addition, although access to information may be somewhat slower (particularly for words and names), the organization of the knowledge system seems unchanged with age (for review, see [34]). Semantic memories are believed to be stored in a variety of regions in posterior neocortex.

3 Autobiographical Memory

Autobiographical memory involves memory for one’s personal past and includes memories that are both episodic and semantic in nature. The bulk of the evidence suggests that recent memories are easiest to retrieve, those from early childhood are most difficult to retrieve, and there is a monotonic decrease in retention from the present to the most remote past, with one exception. Events that occurred between the ages of 15 and 25 are recalled at a higher rate — what is referred to as the reminiscence bump — a finding that has usually been attributed to the greater salience or emotionality of the memories during this time period. This general pattern holds across all ages, suggesting that autobiographical memory is largely preserved with age (for review, see [35]). More detailed analyses of the nature of the autobiographical information retrieved, however, has suggested that although memory for personal semantics is intact in old age, memory for specific episodic or contextual details about one’s personal past may be impaired. In a recent study, Levine et al. [36] observed that although older adults reported the gist of autobiographical event memories as well as young people, they reported fewer details. There may be exceptions to this finding, however. Recent studies of flashbulb memory have demonstrated that older adults remember as much as young adults about the details and circumstances surrounding highly emotional public events such as the death of Princess Diana or the 9/11 attack on the World Trade Center in New York City [37, 38].

4 Procedural Memory

Procedural memory refers to knowledge of skills and procedures such as riding a bicycle, playing the piano, or reading a book. These highly skilled activities are acquired more slowly than episodic memories through extensive practice. Once acquired, procedural memories are expressed rather automatically in performance and are not amenable to description (i.e., it is not easy to say “how” one reads). When talking about procedural memory or knowledge, one is likely to say, “I know how to.” In general, older adults show normal acquisition of procedural skills in both motor and cognitive domains and retain them across the lifespan. With high levels of expertise, there is often little slowing of skilled performance with age (at least until the very oldest ages), although some individual components of the skill may decline. So, for example, although the finger movements of a skilled typist slow down with age, overall typing speed is maintained because other aspects of the skill adjust (e.g., scanning further ahead in the text to be typed) [39]. Procedural memory depends on several brain regions, including the basal ganglia and the cerebellum.

5 Implicit Memory

Implicit memory refers to a change in behavior that occurs as a result of prior experience, although one has no conscious or explicit recollection of that prior experience. For example, laboratory experiments have shown that it is easier to identify a degraded stimulus (e.g., from a brief exposure or partial information) if the stimulus was seen previously, even if one does not remember the prior occurrence. This “priming” probably occurs ubiquitously in everyday life and appears relatively intact in normal aging, although there are some inconsistencies in the literature (for review, see [40]). The most extensively studied form of implicit memory is perceptual priming, which occurs in response to a perceptual cue. Perceptual priming is modality specific and depends on sensory processing areas of the brain (e.g., in the visual domain, priming involves extra-striate regions of the visual cortex). Conceptual priming, which requires semantic processing and is observed in response to a conceptual cue, is also preserved in many older adults, and has been associated with left frontal and left temporal cortical regions.

6 Prospective Memory

Much of what we have to remember in everyday life involves prospective memory — remembering to do things in the future, such as keep appointments, return a book to the library, or pay bills on time (for review, see [41]). Older adults do quite well on these daily tasks, using a variety of external aids such as calendars and appointment books to remind themselves of these activities. Certain habitual tasks such as taking medications at the appropriate times each day, however, may create difficulties for older people. For these tasks, there often are no salient reminders or cues in the environment, and so the tasks require the kinds of self-initiated activities that seem to be particularly problematic for older adults. Prospective memory may also rely on some aspect of working memory to maintain future intentions over time and likely also involves divided attention, both functions that show age-related deficits. Prospective memory and episodic memory tend not to be correlated and probably depend on different regions of the prefrontal cortex.

7 Long-Term Memory: Summary and Implications

Aging principally affects episodic memory, namely memory for specific events or experiences that occurred in the past. Although many older adults believe that their memories for remote events are better than their memories for recent events, it is likely that older memories have become more semantic or gistlike, retaining the general core information but lacking details, particularly spatial and temporal context. These older memories have often joined the realm of things that we now “know.” More problematic for older adults is remembering context or source information: where or when something was heard or read, or even whether something actually happened or was just thought about, what has been called “reality monitoring” [42]. Encoding and retrieval of these kinds of specific or peripheral details about a prior event may be particularly demanding of attentional resources, and good cues for the retrieval of such information may often be lacking. Although semantic memory is largely preserved in old age, the fact that what is retrieved from semantic memory is general knowledge, not specific detail, may contribute to the absence of age differences. The exception to this pattern might be the retrieval of a person’s name or a specific word for a specific context, both of which show deficits in normal aging. The specificity of the information to be retrieved may therefore be a critical determinant of age differences [43]. There is some suggestion that age-related deficits in memory may be reduced for emotionally arousing events or materials [38], and so emotional or personal investment in an experience may be an important variable in episodic memory in older adults. High levels of emotion or stress, however, generally have negative effects on memory.

D Perception

Most people view perception as a set of processes that occurs prior to cognition. However, the boundaries between perception and cognition are unclear, and much evidence suggests that these domains are interactive with top-down cognitive processes affecting perception and perceptual processing having a clear impact on cognition. Evidence indicates that perceptual function is reduced in most older adults and is not always correctable by external aids (for review, see [44]). This suggests, at the very least, that researchers should pay careful attention to and control for sensory and perceptual deficits when conducting cognitive experiments. Evidence from a range of large-scale aging studies has demonstrated that a significant proportion of the age-related variance in several cognitive tasks can be accounted for by hearing and vision loss and that once these sensory differences are statistically controlled, there are no longer age differences in cognitive functioning [45]. Baltes and Lindenberger [45] proposed that overall neural degeneration may account for both sensory and cognitive deficits — what has been called the common cause hypothesis. Alternative explanations have also been proposed, however. For example, Schneider and Pichora-Fuller [44] suggested that perception and cognition are part of a highly integrated system and draw on a common pool of attentional resources. When parts of this system are stressed, such as when auditory or visual acuity are compromised and are essential to a task, other parts of the system will be negatively affected.

Declining sensory and perceptual abilities have important implications for the everyday lives of older adults. Hearing loss can isolate older people, preventing them from engaging in conversation and other social interactions. Visual impairments can limit mobility and interact with attentional deficits to make driving a particularly hazardous activity. As older people develop strategies to compensate for declining sensory abilities, the ways in which they perform other cognitive tasks may also be altered and may be less efficient. Retraining and practice on these tasks may help the adjustment and improve performance.

III HIGHER-LEVEL COGNITIVE FUNCTIONS

A Speech and Language

Speech and language processing are largely intact in older adults under normal conditions, although processing time may be somewhat slower than in young adults. In fact, there is evidence that discourse skills actually improve with age. Older people often tell well-structured elaborate narratives that are judged by others to be more interesting than those told by young [46]. They usually have more extensive vocabularies; and although they exhibit the occasional word-finding difficulty, older adults are easily able to provide circumlocutions to mask the problem. They are skilled conversationalists and appear to have few difficulties in processing ongoing speech. As noted above, however, some older adults have hearing loss and so, in conversational settings, may be required to interpret a weak or distorted acoustic signal. Even under these conditions, older people seem able to maintain good levels of comprehension by effectively using context to interpret the message [47]. Nevertheless, this compensatory top-down processing may have negative consequences for other cognitive operations and may be at least partly responsible for reducing the functional capacity of working memory. The converse relation has also been proposed, however, namely that the well-documented reduced working memory capacity in older adults limits the comprehension of syntactically complex text. The fact that comprehension of text is often measured by recall, a cognitive function known to be impaired in aging, complicates still further the interpretation of comprehension deficits. Older adults also experience problems with comprehension when individual words are presented at a very rapid rate, but they show sharply reduced impairments when such words form meaningful sentences. Here also, older people seem able to engage intact top-down processes to bolster deficiencies in bottom-up processing. They thus appear to retain good language skills well into older age. Deficits that occur under difficult processing conditions seem primarily attributable to sensory loss or working memory limitations, not to impairments in basic language capacities per se (for a comprehensive review, see [48]).

B Decision Making

Relatively little research has been done on the effects of aging on decision-making. Most of the work has highlighted the potential impact of attentional and working memory limitations on the ability to make decisions, but also has incorporated ideas involving motivation, relevance, emotional investment, and prior knowledge as important moderators of those effects, particularly in real-life contexts. Decision-making seems to be a domain that makes clear demands on processing resources, but in everyday life those demands may be reduced by life-relevant knowledge or expertise in the problem-solving domain. For example, research has shown that when making decisions about healthcare alternatives, buying a car, or buying insurance, older adults often come to the same kinds of decisions as younger adults but reach their conclusions in a different way. They tend to rely more on prior knowledge about the problem domain and less on new information, whereas young people, who likely have less knowledge about these issues, tend to sample and evaluate more current information and consider more alternatives before making their decisions (for review, see [49]).

Older adults, again possibly because of working memory limitations, tend to rely on expert opinion to a greater degree than young adults. Although this strategy may work reasonably well when the expert is well-qualified (e.g., a physician for medical decisions), it may leave older people susceptible to things such as investment scams. Poor decision-making may also be a result of episodic memory decline, particularly the loss of memory for details or source. For example, remembering that “Stock ABC is a good investment,” without remembering where one heard such information, could lead to a bad decision.

C Executive Control

In the past decade, there has been an increasing focus on executive control as a primary contributor to cognitive decline with age. Executive control is a multi-component construct that consists of a range of different processes that are involved in the planning, organization, coordination, implementation, and evaluation of many of our nonroutine activities. This so-called central executive [14, 50] plays a key role in virtually all aspects of cognition, allocating attentional resources among stimuli or tasks, inhibiting distracting or irrelevant information in working memory, formulating strategies for encoding and retrieval, and directing all manner of problem-solving, decision-making, and other goal-directed activities. Executive control is particularly important for novel tasks for which a set of habitual processes is not readily available. Executive function depends critically on prefrontal cortex, which exerts its broad-reaching controlling influence via extensive reciprocal connections with posterior cortical regions. A parsimonious explanation of cognitive aging ascribes a causal role to executive control deficits — what has been called the frontal lobe hypothesis of aging [51]. In support of this hypothesis, both structural and functional neuroimaging studies have revealed a preferential decline in older adults in volume and function of prefrontal brain regions [52].

IV INTER-INDIVIDUAL VARIABILITY IN COGNITIVE FUNCTION

Although there are clear generalities and common principles that can be demonstrated in cognitive aging, what is perhaps most compelling about age-related cognitive change is its variability. Cognitive decline is not inevitable. Some older adults retain excellent cognitive function well into their 70s and 80s and perform as well or better than younger adults. Others, although within the normal range, show signs of decline by age 60. In addition, decline is not uniform across cognitive domains. For example, some older adults have excellent episodic memory function but impaired executive function, and vice versa [53]. So, although there are clear interactions among cognitive domains, it seems evident that they also have some degree of independence and may be more or less susceptible to aging in different individuals. What accounts for this variability is of considerable interest to researchers and to the increasing numbers of older people who want to ensure that their cognitive functioning remains intact well into their later years.

Inter-individual variability is likely attributable to a range of factors and mechanisms — biological, psychological, health-related, environmental, and lifestyle. One possibility is that variability is related to differential internal compensatory mechanisms. A number of recent functional neuroimaging studies have found different patterns of brain activation in older and younger adults while performing identical memory or working memory tasks. One such pattern involves greater bilateral activation in older adults for tasks that activate only unilateral brain regions in young adults [54, 55]. This increased activation has been observed particularly in a sub-group of high-functioning older people [56], and has been interpreted by many as compensatory activity, representing perhaps some reorganization of the aging brain. Others have suggested, however, that bilateral activation represents inefficient or less selective cognitive processing in older adults [57]. Another possibility is that such changes relate to declining sensory and perceptual abilities [44], which older people compensate for in a variety of different ways (for discussion, see [58]).

Lifestyle variables have also been the focus of much recent research on factors related to differential cognitive aging. Active lifestyles are generally associated with better outcomes, and aerobic exercise in particular has been shown to produce substantial benefits to cognitive function, particularly on those tasks requiring executive control [9]. Performance on these same kinds of non-automatic tasks is also particularly sensitive to circadian rhythms. For example, older people perform better at their peak time of day, usually in the morning, on tasks requiring inhibitory control [24]. Interestingly, stimulants such as caffeine have been found to reduce the time-of-day effects on strategic memory tasks, by enhancing performance during non-peak times of day [59].

V SUMMARY

Age-related changes in cognitive function vary considerably across individuals and across cognitive domains, with some cognitive functions appearing more susceptible than others to the effects of aging. Much of the basic research in cognitive aging has focused on attention and memory, and indeed it may be that deficits in these fundamental processes can account for much of the variance observed in higher-level cognitive processes. The mapping of cognitive processes onto neural structures constitutes a relatively recent research enterprise driven largely by advances in neuroimaging technology (see Chapter 12, this volume). Early work in this area focused on establishing brain regions associated with different kinds of cognitive performance and revealed that normally aging older adults often appear to activate different brain structures than young people when performing cognitive tasks. The reasons for these differences are a matter of considerable debate. Ultimately, the understanding of age-related changes in cognition will require a parallel understanding of the age-related changes in the brain and the underlying mechanisms responsible for those changes. This volume explores the current state of research on the aging brain, providing some initial hypotheses concerning how changes in the nervous system may be related to the kinds of age-related cognitive changes that are outlined in this chapter.

ACKNOWLEDGMENTS

Preparation of this chapter was supported by The National Institute on Aging, Grant No. R01AG14792.

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