Cognitive Changes Associated with Normal Aging

Julie L. Pickholtz

Barbara L. Malamut

With advances in health care in the United States and other industrialized countries, adults have been increasingly able to sustain their health status well into their 70s, 80s, and beyond. Associated with the improvement in health in older adults is a decrease in disease-related cognitive impairment. Thus, the significant mental decline that was once thought to be an inevitable part of the aging process is not considered characteristic of normal healthy aging. Adult offspring have been found to exhibit significantly less cognitive decline from their 60s to early 70s than their parents, providing evidence that cognitive aging has slowed (117). However, even though cognitive stability is greater than was once expected, specific age-related cognitive changes occur, even in healthy individuals. It is estimated that two of three intact elderly individuals exhibit some degree of performance decrement on neuropsychological testing (63). Research findings concerning the age at which decline begins and the trajectory of decline vary depending on the abilities studied and the study design (107). Some studies demonstrate that age-related cognitive changes decrease in a continuous fashion across the life span from as early as the 20s (87), while others indicate an acceleration of cognitive decline in later years (121).

There is an abundance of evidence in the aging literature that decline occurs in a number of areas of cognitive functioning in healthy, nondemented older adults. Although the focus of this chapter is on the neuropsychological, rather than biologic, aspects of aging, a short discussion of normal age-related changes in the brain is included because these neuropathologic events play a role in the cognitive changes.

NEUROPATHOLOGY

With aging, it is generally accepted that a number of changes in the brain occur universally, which are considered normal and are independent of disease processes (105). For example, there is evidence for age-related decreases in dopamine markers throughout the cortex and in the hippocampus, amygdala, and thalamus (7). Layer II of entorhinal cortex, an area necessary for forming new memories, shows pathology in virtually all humans over the age of 55 (78). Research also suggests that, as the nervous system ages, there are changes in the microvasculature in some regions of the brain (103).

Computed tomography (CT) and magnetic resonance imaging (MRI) studies have shown that aging is associated with enlargement of the cerebral ventricles and sulci and a decline in gray and white matter volumes (14,94,95). Gray matter loss is more pronounced than white matter loss in the overall brain. The age at which brain changes occur has been found to differ between men and women. A precipitous increase in ventricular volume begins in the fifth decade in men and the sixth decade in women, with an approximate 20% increase in ventricular volume per decade for both sexes (62).

Longitudinal MRI studies looking at regional differences indicate that age-specific volume changes do not appear to be uniform across brain areas. For both men and women, age differences are greatest in the parietal region compared with the frontal, temporal, and occipital areas. However, men show a greater loss of brain volume in all cortical regions (26,81,99). Loss of hippocampal volume has also been found to be greater for men than for women (26,99). In fact, volume decline in the hippocampus was found to begin as early as the third decade in men (92). With regard to gender-specific differences among brain regions, the greatest difference between the sexes appears to be in the volumes of the frontal and temporal regions.

The relationship between these age-related structural changes and functional decrements is less clear. Because normal aging affects some brain areas more than others, a decline would be expected in the cognitive functions supported by those regions. However, increase in ventricle volume independent of age did not explain normal age-related declines seen in intelligence scores (62). Similarly, when the effects of age were controlled in a different study, variations in total and limbic brain volumes were not predictive of memory test performance (129). In a cross-sectional MRI study of memory across the adult life span, changes in right-sided hippocampal volume with age were not related to
steeper rates of forgetting for visual spatial material. However, cortical volumes were also measured and found to be a better predictor of recognition memory than hippocampal volume (42). No age-related differences were found between the two structures in their predictive power of recall abilities.

Functional imaging studies have raised the issue that aging may involve a functional plasticity or reallocation of brain network operations when performing a cognitive task. For example, when old and young subjects were compared using positron emission tomography (PET) during a verbal memory task, hypometabolism in the frontal region and greater activation in the occipital region were associated with age-related decline (50). Similarly, results from another PET study comparing young and old subjects who were matched for level of performance suggested that a different hippocampal network was activated during a visual memory test in the older subjects (36). A general trend has emerged from many functional neuroimaging studies of aging that suggests that activation is more lateralized in younger individuals and bilateral and more diffuse in older adults when performing the same task (19). Overall, these studies provide evidence that, when older subjects perform the same task as younger individuals, they use different brain pathways, but it is not yet known whether older individuals go about the task using different strategies.

AREAS OF COGNITIVE DECLINE IN NORMAL AGING

INTELLECTUAL ABILITIES

Intelligence can be conceptualized as consisting of two broad factors: fluid abilities, which reflect neurobiologic processes, and crystallized abilities, which depend more on experience and sociocultural influences (21,55). The basic trend in general intellectual abilities over time is for slight improvement in these abilities in early adulthood, stability in the middle years, and decline in later years (69). However, the decline is not uniform.

In general, fluid intelligence is vulnerable to the effects of aging, whereas crystallized intelligence is relatively spared. Fluid abilities such as processing speed, memory, and fluid reasoning are particularly vulnerable to the effects of aging, whereas crystallized abilities such as vocabulary and general knowledge remain stable (29,121). Many of these age-related changes are reflected on standardized intellectual testing. On the Wechsler Adult Intelligence Scale (WAIS), decline on Performance scales occurs at about the age of 60, whereas decline on the Verbal scales occurs at around age 80 (1,137). Research investigating differential lifespan trajectories indicates that fluid intelligence reaches a peak in the mid-20s, with decline starting by the mid-30s and continuing into old age (70). In contrast, crystallized intelligence does not peak until the 40s and remains stable for several decades, with some studies suggesting decline as early as the early 70s (70) and others not until the 90s (121).

PERFORMANCE SPEED

Aging is associated with slowing of many motor, perceptual, and cognitive behaviors and is most readily seen on timed tests. Age-related changes in motor and cognitive speed play a role in lower scores on the performance subtests of the WAIS because many of the subtests are timed (63). It has been found that performance on simple reaction time tests starts to decline at approximately age 50 (37). Speed of motor performance is inversely related to age on measures of walking speed and finger-tapping (109). However, performance decrements noted on many timed measures are not caused simply by motor slowing, but involve slowing of higher level cognitive processes as well (37,45). Thus, any reaction-time task that involves initiation, redirection, or decision making may be slowed because of slowed mental processing. An even more direct measure of cognitive slowing that assesses central processing time (i.e., event-related P300 and other evoked potentials) demonstrates a positive relationship between increasing age and longer latency intervals. It has been suggested that slowing of information processing is ubiquitous among people over 60 years old (9,117), whereas Salthouse (112) has found that processing speed declines from the 20s through old age. In adults over the age of 70, perceptual speed declines at increasingly rapid rates as times goes on (121). Processing of emotional stimuli, such as identification of facial expression, is also slower in older adults (64).

MEMORY

The most common cognitive complaint among elderly people is a change in memory. However, complaints about declining memory do not always correlate with poor performance on tests of memory (96,141,144). As seen in Table 6-1 and discussed in the following sections, memory can be divided into several components (i.e., immediate, declarative, nondeclarative, and remote), and studies have shown that different aspects of memory are sensitive to aging in various degrees.

Immediate Memory Span

Immediate memory is the ability to retain small amounts of information that remain untransformed for a very short period of time. Immediate memory span is usually measured with the number of digits one can repeat in correct sequential order immediately after presentation (i.e., digit span). Older adults perform
more poorly on tests of immediate memory than younger adults (147), although recent studies have suggested that the reduction of performance on traditional immediate memory tests may be related to factors other than memory. For example, some evidence indicates that the slower articulation rate of older adults contributes to their reduced span measures, and, when the rate of presentation is slowed, older adults perform equivalent to younger adults (80). Furthermore, items from early trials involving digit span have been found to interfere with performance on later trials. Because older adults are more susceptible to interference, they are at a disadvantage on later trials, which are used to measure higher levels of memory span (73).

Table 6-1. Changes in Memory Functioning with Normal Aging

Memory Type			Change
Immediate memory			Unchanged
Short-term memory			Reduced
Long-term memory			Variable
	Declarative
		Semantic	Unchanged up to age 70 years
	Episodic		Reduced
		Free recall		Reduced
		Cued recall		Large benefit
		Recognition		Unchanged
		Acquisition		Reduced
		Retrieval		Reduced
		Percent retention		Unchanged
		Source memory		Reduced
	Nondeclarative		Mild decline
Remote memory			Unchanged

Short-Term Memory

Short-term memory refers to the ability to recall larger amounts of new information very shortly after it is presented. Studies have shown significant age-related declines in the immediate recall of stories, word lists, and designs. This is due to difficulties in accessing stored information (31). As with immediate memory, age-related effects can be attenuated when information is presented at a slower pace, cuing is provided, and rehearsal is allowed (80).

Long-Term Memory

Long-term memory refers to the ability to learn and retain large amounts of newly learned information over a relatively long period of time. It involves encoding, consolidating, and retrieving the material. Long-term memory can be subdivided into declarative and nondeclarative memory.

Declarative Memory

Declarative memory, which is also known as “explicit memory,” refers to conscious learning and remembering of events and facts and is tested by measures of recall and recognition. Many studies have shown that declarative or explicit memory declines with age, but the degree of change depends on the testing method. In general, older people perform significantly worse on free recall than recognition tests. Furthermore, age-related decline is not uniform across all of its components (63). Declarative memory can be further subdivided into semantic and episodic memory.

Semantic memory, which refers to memory for factual information or general word knowledge, is relatively resilient to the effects of aging. Research demonstrates an increase in general knowledge until early to middle old age and thereafter a modest decrease (83,87,107).

Episodic memory, which involves memories for personally experienced events that occur at a specific place and time (130), has been shown to be more age sensitive than semantic memory (83) and declines more rapidly (107). Such decline has been reported in both the verbal and visuospatial modalities (87). The ability to acquire episodic memory appears to decline progressively after the age of 70 years (22,122). The age-related changes are not limited to performance on standard memory tests, but also occur on batteries of tasks that are designed to emulate memory in everyday life such as prospective remembering of tasks (41,93). Aging is also associated with a decline in source memory (i.e., remembering specific information about the circumstances under which an event is encountered) (139). It has been found that older adults have more difficulty than younger adults in remembering the time, place, and other contextual features of events (39).

Although a reduction in the acquisition of new information can occur, healthy aging individuals do
not tend to lose stores of information that have been learned. However, difficulty in retrieving that information has been observed. There is considerable evidence that the ability for strategic and effortful retrieval of newly learned information appears to be at least partially responsible for the decline in episodic memory. First, older adults have a lowered tendency for spontaneously using an organizational strategy when learning new information (126). Second, recall performance of older adults improves when retrieval cues are provided (89). Third, older adults perform more favorably on tests of recognition than on tests of free recall (32,83), which require more strategic and effortful functions. Hedden et al. (53) found evidence that working memory, which plays a role in strategic processing, has stronger relationships to free recall tasks and weaker relationships to cued recall and recognition tasks. Since working memory declines significantly with age, performance decline is less notable on tasks that provide an increasing level of environmental support (i.e., cued recall and recognition tasks).

In clinical settings, it is often difficult to detect declines in memory because elderly individuals are sometimes able to provide elaborate autobiographic information about their remote history. Evidence suggests that, in old age, personal experiences from early in life are remembered more easily than experiences from later in life. In contrast, younger adults show the reverse pattern, remembering recent events more easily than more remote events (93).

Nondeclarative Memory

Nondeclarative memory, also known as “implicit memory,” refers to learning as a result of prior experience without conscious reference to that experience. It has been noted that implicit memory declines less dramatically with age than explicit memory (63,93). One measure of nondeclarative memory is skill learning, in which improvement in speed or accuracy on a challenging task is measured. Some of the limited studies that have been performed with respect to aging have generally suggested that, relative to younger adults, the rate of skill learning in older adults is slower (145).

To summarize, aging is associated with a decline in declarative short-term and long-term memory, particularly for new episodic information. This memory decline is most pronounced when strategies are required for new learning. Aged individuals perform most favorably when asked to recognize rather than recall information. Furthermore, semantic information is remembered more easily than episodic information. Some evidence also suggests that certain types of nondeclarative memory decline with aging, although to a lesser extent than declarative memory.

ATTENTION

Attention is composed of a complex set of functions responsible for maintaining focus and selecting and processing certain aspects of experience. As is the case with memory, different aspects of attention are affected in aging to varying degrees.

Selective Attention

The ability to attend selectively to specific aspects of the environment is important for everyday behaviors that require an individual to ignore irrelevant information. Selective attention is important for such tasks as driving an automobile and conversing in noisy environments. Studies have demonstrated that older adults are more negatively affected by the presence of distracting information when engaged in a task (20,40). Relative to younger adults, they tend to have greater difficulty ignoring irrelevant information (73) and are more distracted by both visual (136) and auditory information (4).

Divided Attention

It has been noted that older subjects are more penalized than younger subjects when they must divide their attention between two sources (30). Although experimental findings have varied, task difficulty and task novelty have been found to mediate the degree to which age differences in divided attention are demonstrated (114,131). Thus, declines in divided attention are most apparent in aging adults when the task is novel or at a high level of difficulty.

Sustained Attention

Sustained attention, also known as “vigilance,” refers to the ability to maintain concentration or focus over time. The studies that have investigated age differences in vigilance task performance have generally demonstrated little evidence for age-related declines (73). However, some evidence indicates that, under conditions when task difficulty is increased, sustained attention performance declines with age (79).

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