MEMORY

Most people recognize that their memories are changing as they grow older. They have a harder time coming up with names; they have a harder time finding things they need; they have to rely more on external memory cues such as notes or calendars. In fact, research results support these perceptions. The bad news from this research is that memory declines are experienced throughout the adult life span and not just in the older ages. Forty-year-olds as a group are worse than twenty-year-olds, and fifty-year-olds are worse than thirty-year-olds. The good news is that research shows that, unlike the serious and ubiquitous memory declines associated with Alzheimer's disease and other dementias, memory changes associated with healthy aging are selective (e.g., Zack et al.). Some memory tasks show large and reliable adult age differences (e.g., working memory, episodic memory), while other memory tasks show little or no effects of age (e.g., semantic memory, implicit memory).

Figure 1 shows the results from study of 345 adults ranging in age from the twenties to the eighties (Park, Lautenschlager, et al.). Different types of memory were tested, including working memory (computation span), episodic memory (free recall of a word list), and semantic memory (defining words in a vocabulary test). The results were plotted in deviation units from the mean for all the participants on any test (z scores). As can be seen in the graph, the memory changes occur across the entire adult life span and are not limited to old age. Second, the graph shows that age has selective effects on memory. Working memory and episodic memory decline significantly across the life span, while semantic memory increases significantly.

Other theories suggest that age differences in memory depend on the extent of deliberate cognitive processing or cognitive resources required to perform the task (e.g., Park). According to this view, the size of age differences in different memory tasks is determined by the amount of cognitive resources needed to adequately remember in those tasks. Other theories suggest that age effects are limited to specific memory structures or types (e.g., Craik). Clearly, the explanation for the differential effects of age with different memory tasks depends on how one conceptualizes memory.

Memory stage theory

Memory stage theory separates memory into the temporal, sequential components that define any act of remembering. Information first has to be perceived or experienced (i.e., encoding). Then the information has to be maintained over a retention interval of some length of time (i.e., storage). Finally, the information has to be produced at the time memory is tested (i.e., retrieval). Early researchers believed that adult age differences in memory were located primarily at retrieval, the final of the three stages. Early laboratory research, for example, demonstrated that age differences were large when the recall of a word list was measured (with instructions such as "Write down all the words you can remember having seen on the list presented earlier."). Age differences, however, were greatly reduced or even eliminated when recognition memory was used to test memory at retrieval (with instructions such as "Select the words on this list that were presented on the list that you saw earlier."). Because the use of a recognition memory task is assumed to reduce the retrieval requirement of the memory task, it was then inferred that the locus of the age effect must be retrieval. Such findings were prevalent in the 1960s and 1970s.

More recent research, however, has clearly demonstrated that recognition memory is not totally insensitive to aging, and the stage theory lost its appeal because of the methodological difficulties in isolating one memory stage from another in different age groups. In order to isolate retrieval, for example, everything must be held constant until the time retrieval is tested. This is difficult to accomplish in aging research, however, because adults of different ages may process information differently at one of the earlier stages, thus violating the requirement that all be held constant until retrieval (Smith).

The major reason, however, for the loss of interest in identifying the stage at which aging had its effects is clear evidence that age has effects on all stages of memory: encoding, storage, and retrieval. For reasons not directly related to memory stage theory, however, memory research in the 1970s and early 1980s focused heavily on the encoding stage of memory. This focus on encoding was due to the development of a conceptual view of memory, the "levels of processing" framework, which proposed that the ability to remember was determined by the extent of semantic processing during the encoding of the to-be-remembered information (Craik and Lockhart). For this reason, much research on memory and aging during this period focused on the nature of encoding processes in different age groups. Even with the magnitude of the research effort, however, the relationship between aging and levels of processing is still unclear. The research did, however, suggest that encoding is especially affected by adult aging. For example, the memory performance of older adults, relative to younger adults, is more detrimentally affected by performing a divided attention task during the encoding stage of a memory task, but not if the divided attention task occurs at retrieval (Park, Smith, et al.).

Memory system theory

Another view of memory that has both behavioral and neurological support considers memory not as a unitary construct but as a collection of component systems. The general view is that memory consists of sensory memory, short-term memory, and long-term memory. Short-term memory is further divided into primary memory and working memory, and long-term memory is divided into episodic memory, semantic memory, and procedural memory. These memory systems differ in the nature of how memories are represented and in how these representations are maintained and retrieved. They also differ considerably in how they are affected by adult aging.

Sensory memory. After an event is experienced, it is first represented very briefly in the sensory system. Here the information is represented as it is processed and analyzed by the attentional and perceptual systems. There has been very little work on this type of memory and aging, but some research does suggest that older adults are less efficient in this early type of processing, especially in the visual system. It should be pointed out, however, that differences in sensory-perceptual processing would be an unlikely explanatory construct for memory differences found in later memory systems because later memory differences vary according to what system is examined (Craik). Adequate perceptual processing of the to-be-remembered stimulus would seem to be a requirement for all types of long-term memory, and the fact that some long-term memory systems are minimally affected by aging while others show large effects would not support an explanation that relied solely on faulty processing at the very early stage of processing.

Primary memory. Primary memory refers to the number of items that can be represented in the mind at one time. Primary memory is typically measured by digit- or word-span tasks. For example, when strings of numbers are presented one at a time at one-second intervals, primary memory would be the number of items that could be repeated back without errors. The digit-span test found on most intelligence tests is a measure of primary memory. When primary memory is tested by digit-span tests with individuals of different ages, no reliable age differences are found (Craik). Another measure of primary memory is recall of the last few items presented on a word list. Again, research finds no age differences in the ability to recall the most recently presented items in a word list.

Working memory. Age differences are found, however, on a measure of short-term memory called working memory, which takes into account both the ability to keep things in mind and, at the same time, the ability to process information. Working memory, unlike primary memory, requires individuals to keep information in mind while engaging in another processing task. One commonly used working memory task is reading span. Individuals read a series of short sentences (e.g., "The girl hit the ball.") and answer questions about the sentence (e.g., "Who hit?"). At the same time, individuals have to remember the last word in each sentence (e.g., "ball") and report the words when told to do so. The number of sentences presented varies, and reading span is the greatest number of words that can be remembered without making an error. Because working memory tasks require simultaneous storage (remembering the words) and processing (reading the sentences and answering the questions), it is a better simulation of everyday information processing.

Working memory is used to understand a conversation or to write an e-mail message on the computer. Other working memory tasks have been developed that involve different kinds of information processing, such as arithmetic calculations (computational span) and spatial manipulations (spatial working memory). Regardless of the type of information involved, however, large, reliable age differences are found on working memory tasks (Zacks et al.). Therefore, while the more passive primary memory tasks, such as simple digit span, do not show age differences, differences are found with working memory measures. As will be discussed later, working memory capacity is considered by many researchers to be a fundamental mechanism for more complex memory processing.

Long-term memories are not kept in conscious awareness, as short-term memories are, but instead have to be retrieved into consciousness when they are needed.

Episodic memory. Episodic memories are recollections that are actively retrieved as previous personal experiences. For this reason, episodic memory is sometimes called autobiographical memory. The memory is a reconstruction of an earlier experience very much like looking something up in an internal cognitive diary. For this reason, contextual information about when and how an event was originally experienced is often used to guide retrieval. "What did I have for dinner last night?" "Where did I park my car?" "Did I take my medicine this morning?" "Did you see Joan at the party last night? To answer each question, one attempts to reconstruct the original event.

Older adults do worse on most episodic memory tasks than do younger adults (see Figure 1). In the laboratory, typically a list of words or some other to-be-remembered information is presented to individuals, and later, after a retention interval, memory is tested. Because the items presented are commonly used words, they are already known to the individuals. The memory task, therefore, is to remember the words in the particular context of the original list. Episodic memory requires one to put what one is trying to remember in a specific context. Even though the magnitude of age differences varies considerably among different memory tasks using different materials and types of tests, older adults tend to have greater problems with episodic remembering than do younger adults.

As will be discussed later, the variable that seems important in determining the magnitude of age differences is the degree to which the memory task involves deliberate processing by the individual. At both encoding and retrieval, the more intentional processing required to perform the task, the larger the age differences that will be found with that task. For example, free recall of a word list requires more deliberate processing than recognition tasks, and age differences are larger on free recall (Craik). Instead of trying to actively recall the items, individuals in a recognition task only have to pick out the words they saw earlier on a longer list of words.

Because episodic memory is so dependent on contextual information, one of the reasons for older adults' poorer performance on these tasks is probably their inability to encode and remember contextual information easily. Older adults, for example, do not do as well as younger adults in identifying the way in which information was presented to them (i.e., source memory). They do worse when asked to remember whether a word was presented in uppercase or lowercase, spoken by a male or a female voice, in one color versus another, or in the upper part of a computer screen or the lower part (Zacks et al.). These tasks require individuals to remember contextual detail. Because older adults encode less contextual detail, they do not do as well on tasks in which contextual detail provides the cues for retrieval. In fact, because older adults encode less context, they have problems distinguishing events they actually experience from those they have only thought about, a phenomenon called "reality monitoring." In a reality-monitoring task, individuals either read words at encoding or generate words in response to some cue. Older individuals have greater problems in determining whether remembered events were the ones read or the ones imagined in response to the cue (Norman and Schacter).

In addition to the problems associated with remembering context itself, older adults have problems with binding the context with the to-be-remembered information. In one study, for example, younger and older adults were presented with pairs of words and asked to generate a sentence that included both words. There were no age differences in the nature of sentences generated, but older adults did have more problems generating sentences, especially for unrelated word pairs that required them to generate the binding sentence through deliberate processing (Smith et al.). Older adults were better able to generate sentences when the two words were related to one other. They were also better able to recall one word from the word pair when given the other word as a cue. By having related word pairs, there was less need to bind the two words together because their relationship provided an existing bond. Again, because less deliberate processing was required in both the encoding and the retrieval conditions when related pairs were used, age differences were smaller. In fact, research has shown that age differences in the ability to recall a target picture when another picture is given as a cue depends on the relationship of the cue to the target. Age differences are large when the two pictures are unrelated, but smaller when the two pictures are either semantically related or presented as perceptually interacting (Park, Smith, et al. 1990).

One interesting type of episodic memory, "prospective memory," does not involve remembering something from the past, but instead involves intending to do something in the future. "Stop by the store when you leave work, and bring home some milk." "When you see Wanda, tell her to look at my new Web page." "Take two of these pills every other day after lunch." These are examples of prospective memory tasks. In the laboratory, prospective memory tasks simulate these real-world examples (e.g., "Press the key when you see a word with an 'R' in it" or "Press the key every ten minutes"). The prospective task is combined with some other cognitive task, such as trying to study a word list for a later memory test. With simple laboratory tasks, however, such as pressing a key when a certain letter is found in a word, age differences in prospective memory are often not found. Again, the determinant of whether age differences are found seems to be the degree of deliberate recollection required to perform the task, either for the prospective task or for the background task. As the difficulty of either task is increased, requiring more deliberate processing, age differences are increased.

Age differences are often larger in time-based tasks (e.g., individuals are asked to press a computer key every ten minutes while performing another computer cognitive task) than in event-based tasks (e.g., individuals are asked to press the key when a certain cue word appears). Because event-based prospective remembering involves less deliberate processing, given the external cue, age differences sometimes were not found. It is also clear that prospective memory errors in older adults increase when the background task they have to perform becomes more demanding in terms of processing resources (Einstein et al.).

Semantic memory. Not all remembering requires one to reconstruct the experience of encoding. There are many examples of remembering without reference to how or when what one is trying to remember was originally learned. There is access to a great deal of knowledge that has lost all connection to the context of its original episodic learning. "What is the capital of North Dakota?" "What bug has eight legs and weaves webs?" "Where were you born?" Answering these questions requires semantic memory. Semantic memories are retrieved conceptually rather than contextually, and represent accumulated knowledge. Instead of using a cognitive diary, semantic memory is like looking something up in an internal cognitive encyclopedia. Of course, the information is not alphabetically organized, but instead organized conceptually or semantically.

As mentioned earlier, tests of semantic memory typically show either no age differences or improvement over the adult life span (see Figure 1). Vocabulary tests and tests of general knowledge (such as found in Trivial Pursuit games) show either no age differences or increases for older adults up until very late in life (eighties or nineties). In Figure 1, vocabulary knowledge increased steadily through the seventies and showed only a slight decline in the eighty-year-old group.

There have also been attempts to examine the architecture of semantic memory (i.e., the way semantic organization is conceptually associated). Free association tests are one way to look at semantic organization. Individuals are given a word and asked to generate another word, the first word that comes to mind when thinking about the word given. A category name is given in another type of test (e.g., vehicle), and individuals are asked to generate the first five vehicles they can think of. If semantic information in memory is organized in different ways by different age groups, then there should be qualitative differences in the nature of the responses given on free association tests or category generation tests. If older adults' semantic memories are organized differently, then the organization should produce differences in the strength of associations between different concepts. Norms of free associations, however, as well as norms of generating instances in categories, show no differences between age groups (Smith and Earles).

One aspect of semantic memory that does seem to decline with aging is the ability to find a word, given its definition. This phenomenon extends to finding proper names and to the "tip-of-the-tongue" phenomenon (Craik). A tip-of-the-tongue state is created when a person knows that he or she knows something but cannot think of it at the moment. Often one can generate information about the answer that is correct but cannot think of the answer itself. Some of this effect (word finding, name finding, tip-of-the-tongue), however, is associated with older adults simply being slower to respond. Some research suggests equivalent word finding in different age groups with difficult words (Craik). Other research suggests that older adults eventually can resolve tip-of-the-tongue states when given enough time (MacKay and Abrams).

Procedural memory. Several times in this entry it has been stated that age differences in memory seem to increase when the degree of deliberate processing required to remember increases. Procedural memories are assumed not to require any deliberate, intentional processing at all. They instead involve only automatic processing and, in fact, do not even require conscious awareness of the effects.

Procedural memory tasks, sometimes called implicit memory tasks, often use repetition priming as a measure. For example, individuals first examine a list of words (stand, chair, . . . radio), not aware that a later memory test is involved. Rather, they are told to make some judgment about the words, such as to rate the pleasantness of each word on a five-point scale. Then several other word tasks are performed. Finally, a series of word stems is presented (fe___, ra___, . . . bl___) and the individuals are asked to complete the stem with the first word that comes to mind. Some of the word stems could be completed with a word presented earlier on the list. Even though they are not aware that the word stem list contains stems for words seen earlier, individuals will use the presented words to complete the stems at a level greater than chance. This increase in "remembering" previously presented items represents implicit or procedural memory.

As might be expected, given the lack of deliberate recollection, age differences are typically not seen on implicit memory tasks (Zacks et al.). If they are found, they are very small, especially when compared with the large effects seen in explicit episodic recall.

In summary, the memory systems approach has been very useful in describing when significant age differences in memory are found. Whether or not memory systems eventually are supported by neuroscientific data, they have provided a useful conceptual framework for organizing memory phenomena and for showing dissociations with age.

Cognitive resources

Another view of memory is more closely based on the observation that age differences in memory tasks seem to be determined by the degree of deliberate processing required to perform the task. Deliberate processing requires processing resources, and if processing resources are diminished in older adults, then the ability to engage in deliberate processing will be reduced. According to this view, age differences in memory are assumed to be caused by an age-related reduction in cognitive resources available to perform memory tasks as well as other cognitive tasks. Support for this theoretical position comes from studies which show that individual differences in measures of cognitive resources can account for age-related differences in memory performance. This research approach is represented in Figure 2. The circles in the figure represent individual differences in age, memory, and cognitive resources. Overlap in the circles represents shared variance among the variables. The age-related variance in memory is reflected by the overlap in the age and memory circles (a + b). The degree to which processing resources mediate the relationship between age and memory is shown by b.

Four mechanisms have been suggested as estimates of cognitive resources: perceptual speed, working memory, inhibitory function, and sensory function (Park).

Perceptual speed. Older adults are slower at performing simple perceptual/motor operations. In one test, for example, individuals are asked to look at two strings of letters (e.g., xpltvg — xpltvg) and indicate in the space between them whether the strings are the same or different. The number of letter comparisons that can be completed in ninety seconds declines significantly across the life span. Perceptual speed is assumed to be an estimate of the efficiency of neural functioning, and thus to be a possible mechanism to account for age differences on memory and other cognitive tasks (Salthouse). Timothy Salthouse has produced an impressive amount of evidence that individual differences on simple measures of perceptual speed can account for most of the age-related variance in complex cognitive tasks, even those that do not require completion in a given time (i.e., speeded).

Working memory. Many researchers believe that working memory is a good measure of processing resources. Like perceptual speed, individual differences in working memory can account for much of the age-related differences on long-term memory tasks. Denise Park and her colleagues conducted a study with over 300 adults of different ages that measured perceptual speed, working memory, and different measures of long-term episodic memory. The long-term memory measures included free recall of a word list (looking at a word list and then recalling as many words as possible), cued recall (presenting word pairs at encoding and then presenting one word from each pair as a cue for the other word at retrieval), and spatial recall (remembering what quadrant of the computer screen words had been presented in earlier) (Park, Smith, et al. 1996). This study showed that when perceptual speed as a construct was included in a model of memory performance, it accounted for essentially all of the age-related variance in memory performance. Measures of working memory, however, were also important in the model with the more effortful measures of episodic memory, free and cued recall. Speed alone accounted for the age differences in the less effortful spatial recall task, but both speed and working memory were necessary to account for the age differences in free and cued recall, tasks that are assumed to require more processing resources. These results suggest that no single mechanism may be adequate and that multiple measures may be necessary to account for age differences in memory.

Inhibitory function. Another construct that has been suggested as the mechanism for cognitive resources is inhibition (Hasher and Zacks). Inhibitory function allows the individual to focus on information relevant to the task and suppress information that may be activated but is irrelevant to the task. Lynn Hasher and Rose Zacks suggest that older adults are less able to inhibit irrelevant information, and thus cannot focus as well on the information needed to perform the task. According to this view, it is not working memory capacity that is limited in older adults, but the inability to inhibit irrelevant information from cluttering up the content of working memory. The result of this "mental clutter" is that older adults experience more interference at both encoding and retrieval. There is a great deal of evidence that older adults are in fact deficient in inhibiting irrelevant information when performing memory tasks (Zacks et al.).

Sensory function. Paul Baltes and Ulman Lindenberger, in their large-scale Berlin Aging Study, found that auditory and visual acuity can account for much of the age-related variance on a variety of cognitive tasks, including associative memory. In one study, they found that sensory function accounted for over 90 percent of the age-related variance in cognition. Other research, however, has shown that this relationship is not simply due to the fact that poorer vision and hearing cause a decline in cognition. Instead, Baltes and Lindenberger suggest that the sensory measures are one of a large number of physical and cognitive variables that reflect the efficient functioning of the nervous system. According to their view, there is a "common cause" related to biological aging that affects a variety of abilities, both cognitive and somatic, are affected by aging.

Deliberate processing

In summary, different theoretical mechanisms have been proposed to account for age differences in memory performance. Each mechanism has been shown to be important, and research is needed to better understand the relationship among the different constructs. It is becoming clear that no single mechanism can account for all age-related variance in memory performance, and future research will address the relationship among the mechanisms when predicting performance on different types of memory tasks. All of the theoretical mechanisms, however, assume that older adults have more limited processing resources.

One theme that has emerged from the discussion is that age differences in memory are determined by the degree of deliberate processing. Fergus Craik has suggested that memory performance is determined by an interaction between internal (self-initiated processing) and external (environmental support) factors (Zacks et al.). The amount of deliberate processing required in a task decreases as the task itself becomes more supportive. As mentioned earlier, a great deal more processing resources are needed to remember the words in a free recall task than in a recognition task. In a recognition task, the words themselves serve as retrieval cues and the processing required to recognize is minimal. No explicit cues, however, are provided in a free recall task, and the individual must engage in a great deal of self-initiated processing in order to retrieve the words. Age differences in recall therefore are much greater than age differences in recognition (Craik). As the amount of deliberate processing increases, age differences should increase; as the environmental support provided by the task increases, age differences should decrease.

There have been some research attempts to determine the extent to which a task requires deliberate processing versus the extent to which it relies on automatic processing. One such attempt is known as the "remember-know" procedure. After individuals correctly identify words in a recognition memory experiment, they are asked to estimate whether the word was deliberately recollected ("remember") or whether the recognition was based on familiarity, with no specific recollection of encoding the word ("know"). Several different experiments have found that older adults produce a smaller proportion of "remember" judgments for the words they recognize, and a greater proportion of "know" judgments (Zacks et al.). This finding implies a reduced ability to deliberately recollect the items at the time of test and a greater reliance on familiarity.

Another method for examining deliberate and automatic remembering has been developed by Larry Jacoby and his colleagues. The "process dissociation" procedure actually provides quantitative estimates of the deliberate and automatic processing requirements of different memory tasks. As an example of process dissociation, Jennings and Jacoby looked at age differences in recognition memory. Younger and older participants first looked at a list containing words that they simply read. Then they listened to a second list and were told they would be tested on the second list later. Following the two lists, they were given two different recognition tests. In both tests they were given pairs of words, one of which they saw earlier, either as a word they read in list 1 or as a word they heard in list 2. The second word in the pair was a new word they had not seen or heard previously.

On the first memory test, they were misinformed that one word in each pair had been presented auditorially in list 2, and the other word was either new or one that they read in list 1. They were to pick the word that they had heard in list two (exclusion test). If they picked a word that was presented in list 1, they could do that only through familiarity, because if they had recollected the word, they would have correctly rejected it because it was a list 1 word. In the second memory test, they were told that one word from each pair was a new item, and they were to pick the one they had either seen or heard before. In this case, their judgments could be based on either familiarity or recollection (inclusion test). By subtracting the estimate of recognition due to familiarity derived from the first test (exclusion) from the scores on the second test (inclusion), an estimate of recall based on recollection alone could be derived. The results showed that the age effects were limited to the deliberate recollection component of recognition memory. Estimates of familiarity showed no age effects. This analytical procedure provides further support for the conclusion that age effects are determined by the extent of deliberate processing required in a task.

Dementia: age-related memory pathologies

So far, the discussion has been limited to healthy older adults. For most of this research, good health is a requirement for participation in the research. A small percentage of older adults, however, develop dementias that have a primary symptom of memory loss. There are many types of dementia, the most common one being Alzheimer's disease, which accounts for over two-thirds of all cases.

Because there are memory changes associated with normal, healthy aging, it is very difficult to diagnose Alzheimer's disease and other dementias early. There are many neuropsychological memory tests that can determine the progression of the disease once it has been established, but it is much more difficult to determine the early signs of dementia that distinguish Alzheimer's disease from normal memory change and that could be used as a diagnostic test. Unfortunately, the types of memory that are associated with very early dementia (episodic memory and working memory) are the very ones most affected in normal aging (Hodges). This means that the boundary between healthy memory change and unhealthy memory change is often not clear. One possible early difference is in the ability to remember things after retention intervals (delayed recall). One of the earliest symptom of Alzheimer's disease seems to be the loss of newly learned information after delay intervals (Albert and Killian). Forgetting rates often are the same in healthy adults if information is learned to the same criterion of performance. Alzheimer's patients, on the other hand, show greater delayed recall and more forgetting over the retention interval.

Very accurate cognitive diagnosis, however, remains difficult until the patient reaches the mild to moderate level, when other memory changes occur that are not typically associated with normal aging except for the very old (e.g., semantic memory and visuospatial memory). Category fluency (generating instances of categories) and providing verbal definitions seem to show the greatest sensitivity to early Alzheimer's disease (Hodges).

Anderson D. Smith

See also Alzheimer's Disease; Brain; Dementia; Memory Dysfunction, Drug Treatment; Memory, Everyday; Memory Training; Metamemory; Neuropsychology.

BIBLIOGRAPHY

Albert, M. S. and Killiany, R. J. "Age-Related Cognitive Change and Brain-Behavior Relationships." In Handbook of the Psychology of Aging, 5th ed. Edited by J. E. Birren and K. W. Schaie. San Diego: Academic Press, 2001. Pages 161–185.

Baltes, P. B., and L indenberger, U. "Emergence of a Powerful Connection between Sensory and Cognitive Functions across the Adult Life Span: A New Window to the Study of Cognitive Aging?" Psychology and Aging 12 (1997): 12–21.

Craik, F. I. M. "Age-Related Changes in Human Memory." In Cognitive Aging: A Primer. Edited by D. C. Park and N. Schwarz. Philadelphia: Psychology Press, 2000. Pages 75–92.

Craik, F. I. M., and Lockhart, R. S. "Levels of Processing: A Framework for Memory Research." Journal of Verbal Learning and Verbal Behavior 11 (1972): 671–684.

Einstein, G. O.; Smith, R. E.; McDaniel, M. A.; and Shaw, P. "Aging and Prospective Memory: The Influences of Increased Task Demands at Encoding and Retrieval." Psychology and Aging 12 (1997): 479–488.

Hasher, L., and Zacks, R. T. "Working Memory, Comprehension, and Aging: A Review and a New View." In The Psychology of Learning and Motivation, vol. 22. Edited by G. H. Bower. San Diego: Academic Press, 1988. Pages 193–225.

Hodges, J. R. "Memory in the Dementias." In The Oxford Handbook of Memory. Edited by E. Tulving and F. I. M. Craik. Oxford: Oxford University Press, 2000. Pages 441–459.

Jennings, J. M. and Jacoby, L. L. "Automatic Versus Intentional Uses of Memory: Aging, Attention, and Control." Psychology and Aging 8 (1992): 283–293.

MacKay, D. G., and Abrams, L. "Language, Memory, and Aging: Distributed Deficits and the Structure of New-Versus-Old Connections." In Handbook on the Psychology of Aging, 4th ed. Edited by J. E. Birren and K. W. Schaie. San Diego: Academic Press, 1996. Pages 251–265.

Norman, K. A., and Schacter, D. L. "False Recognition in Younger and Older Adults: Exploring the Characteristics of Illusory Memories." Memory & Cognition 25 (1997): 838–848.

Park, D. C. "The Basic Mechanisms Accounting for Age-Related Decline in Cognitive Function." In Cognitive Aging: A Primer. Edited by D. C. Park and N. Schwarz. Philadelphia: Psychology Press, 2000. Pages 3–21.

Park, D. C.; Lautenschlager, G.; Hedden, T.; Davidson, N.; Smith, A. D.; and Smith, P. K. "Models of Visuospatial and Verbal Memory Across the Adult Life Span." Psychology and Aging 17 (2002).

Park, D. C.; Smith, A. D.; Dudley, W. N.; and Lafronza, V. N. "Effects of Age and a Divided Attention Task Presented during Encoding and Retrieval on Memory." Journal of Experimental Psychology: Learning, Memory, and Cognition 15 (1989): 1185–1191.

Park, D. C.; Smith, A. D.; Lautenschlager, G.; Earles, J. L. K.; Frieske, D.; Zwahr, M.; and Gaines, C. "Mediators of Long-Term Memory Performance across the Life-Span." Psychology and Aging 11 (1996): 621–637.

Park, D. C.; Smith, A. D.; Morrell, R. W.; Puglisi, J. T.; and Dudley, W. N. "Effects of Contextual Integration on Recall of Pictures by Older Adults." Journal of Gerontology: Psychological Science 45 (1990): P52–P57.

Salthouse, T. A. "The Processing-Speed Theory of Adult Age Differences in Cognition." Psychological Review 103 (1996): 403–428.

Smith, A. D. "Age Differences in Encoding, Storage, and Retrieval." In New Directions in Memory and Aging. Edited by L. Poon, J. Fozard, L. Cermak, D. Arenberg, and Larry Thompson. Hillsdale, N.J.: Lawrence Erlbaum Associates, 1980. Pages 23–46.

Smith, A. D., and Earles, J. L. K. "Memory Changes in Normal Aging." In Cognitive Changes in Adulthood and Aging. Edited by F. Blanchard-Fields and T. Hess. New York: McGraw-Hill, 1996. Pages 192–220.

Smith, A. D.; Park, D. C.; Earles, J. L. K.; Shaw, R. B. and Whiting, W. W. "Age Differences in Context Integration in Memory." Psychology and Aging 13 (1998): 21–28.

Zacks, R. T.; Hasher, L.; and Li, K. Z. H. "Human Memory." In The Handbook of Aging and Cognition, 2d ed. Edited by F. I. M. Craik and T. A. Salthouse. Mahwah, N.J.: Lawrence Erlbaum Associates, 2000. Pages 293–358.