Semantic processes in implicit memory: Aging with meaning

Age Differences in Word and Language Processing Ph. Allen and Th.R. Bashore (Editors) 9 1995 Elsevier Science B.V. All rights reserved.

110

Semantic Processes in Implicit Memory: Aging with Meaning* David B. Mitchell Hebrew University and Southern Methodist University

Moses was 120 years when he died, but his eyes had not dimmed, and his natural powers had not left him_ (Deuteronomy 34:7) Unlike the Mosaic vitality, the experiential reality of shortcomings in our "natural powers" of memory is usually felt long before we near the maximum lifespan. At age 74, Donald Olding Hebb (1978) experienced "a dimini~qhing effective vocabulary" (p. 21), and at 78, Burrhus Frederick Skinner's (1983) response was that "in old age ... verbal behavior becomes less and less accessible" (p. 242). In spite of these testimonials by famous septuagenarian psychology professors, we shall see that widespread memorial decline is not inevitable in old age. This chapter will review one type of memory in particular-implicit memory--that may be invulnerable to the effects of aging. Indeed, a recent study in France (Kitchie, Ledsert, & Touchon, 1993) found evidence for robust memory performance through age 100! As shown in Figure 1, their findings contrast sharply with lifespan data fi'om Colorado (Davis, Cohen, Gandy, Colombo, Van Dusseldorp, Simolke, & Romano, 1990). I will argue in this chapter that the pattern of unswerving stability is the accurate portrayal of the relation between normal aging and implicit memory. I will also present evidence that the alternative standard pattern of decline is the product of a different memory system (not a different country). First, it will be necessary to define both "memory" and "aging." 1. AGING The operational definition for this term is necessarily arbitrary, as there is no consensus as to when aging begins (Hayflick, 1984). Similarly, "there is a surprising lack of agreement about ... the relation between age and cognitive functioning" (Salthouse, 1991, p. 32). Salthouse (1991) compiled a list of quotations regarding this relationship, and found opinions for cognitive decline starting in the early 20s, middle 20s, after 30, at 40, 50, 60, 65, and 70, in the 60s, and in "the post-retirement phase." However, the great majority of psychology studies tend to compare only two cross-sectional cohorts. In the gerontological research literature, and in this chapter as well, the standard "older" group is AUTHOR NOTES: Preparation of this chapter was facilitated by a leave granted by Dedman College, Southern Methodist University, a grant (AG07854) from the National Institute on Aging, and by Hebrew University, Jerusalem. Correspondence should be addressed to David Mitchell at the Psychology Department, S.M.U., Dallas, TX 75275, or via e-mail: [email protected].

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111

Implicit Memory: Two Aging Patterns Davis et al. Priming (%)

Ritchie et al. Priming (%)

60 50-

25

40

2o

\ \ \

30

\

~'~7 20-

10I"~ 0

....

Davis et al., 1990

I [

"~" Rilchie et al., 1993

I

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22

34

44

55

64

74

84

94

0

.

MIDPOINT OF AGE GROUP Figure 1. Two cross-sectional aging patterns of implicit memory performance from two different studies (Davis, Cohen, Gandy, Colombo, Van Dusseldorp, Simolke, & Romano, 1990, Exp. 2, Ritchie, Led~sert, & Touchon, 1993). Two data points were omitted for smoother interpolation (Davis et al.: age 55 = 39%, Ritchie et al.: age 84 = 38%).

made up of individuals over the age of 60 (i.e., the3A,e passed the midpoint of the maximum lifespan). The "young" group is most often made up of university students ranging from 18 to 25. (The astute reader has noticed that the data plotted in Figure 1 are already exceptions to this rule.) 2. M E M O R Y

Memory has been vigorously and rigorously researched since 1885, when Hermann Ebbinghaus published the first known scientific memory experiments, so there is more than one definition of the term PsycLit, a computerized database of journal articles, lists 1,016 entries for the keywords "aging AND memory" from 1984 though mid-1994. With "man's best friend" exhibiting age-related memory deficits paralleling his own, not even sleepy old dogs have been allowed to lie (Milgram, Head, Weiner, & Thomas, 1994). The memory phenomena discussed in this chapter can be circumscribed by excluding those types of memory that will not be dealt with. Thus, we are concerned with neither sensory memory nor working memory (e.g., Baddeley, 1992), but with long term memory. William James' (1890) definition is still apt, viz., "the knowledge of a former state of mind after it has already once dropped from consciousness" (p. 648). But there's more to it: Within long term memory, we can now distinguish three to five subsystems

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(Tulving, 1985, 1991; Tulving & Schacter, 1990). Although these theoretical distinctions are somewhat controversial and the terminology and classification schemes are still evolving (of. Roediger, 1990a; Schacter, 1992; Squire, 1992), Tulving's description of multiple memory systems and their monohierarchical arrangement provide--at the very least--a neat conceptual framework for organizing and understanding age-related memory patterns (cf. Mitchell, 1989, 1993). The major systems of interest here are episodic memory, semantic memory, and implicit memory. 3. EPISODIC M E M O R Y

Episodic memory is synonymous with autobiographical memories in space and time, memories that include conscious recollection (Tulving, 1985, 1991). Standard memory tasks such as free recall, cued recall, and recognition tap episodic memory. These tasks have received the most research to date, and with very few exceptions, there is a broad consensus that episodic memory declines systematically from young adulthood through old age (see Craik & Jennings, 1992; Howard, in press; Kausler, 1994; Light, 1991; Mitchell, 1993). 4. SEMANTIC M E M O R Y Semantic memory is also available to conscious introspection, but lacks the spatial, temporal, and autobiographical components of episodic memory (Tulving, 1985). Semantic memory represents general knowledge about the world. All of the following tasks involve semantic memory: naming pictures, defining words, general knowledge, judging the fame of a person's name or face, generating category exemplars, free associating to words, solving anagrams, solving word puzzles, completing a sentence, making lexical decisions, recalling names of U.S. presidents, geography, etc. Thus, whereas an episodic memory task requires one to recall a specific prior episode (e.g., what word from the list presented previously started with the letters ele?), a semantic memory task asks for retrieval without a specific temporal/spatial context (e.g., generate a word that starts with ele). The different age-related patterns in these two kinds of tasks has been noted for some time (Perlmutter, 1980, dubbed it "an apparent paradox"): While episodic memory tasks reveal age-related declines, semantic memory tasks generally reveal stability across adulthood (Kausler, 1994). Results from a number of studies suggest that aging does not adversely affect the organization of the lexicon (e.g., word association, Burke & Peters, 1986; category exemplar generation, Brown & Mitchell, 1991). Likewise, automatic activation seems to be unaltered by aging (e.g., flanker task, Shaw, 1991). When vocabulary or general knowledge tests are used as an index, older adults most often score higher than young adults (see Chapter 12 in Kausler, 1991). 4.1. Access

An exception to the stability of semantic memory functioning appears when specific words must be retrieved. Even though it appears that aging does not affect the activation of a lexical item or concept, activation per se does not guarantee successful

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retrieval. In tasks that require highly specific retrieval, a cue is presented and the subject must deliver a particular target (e.g., fetch a word given a definition). People's names also fitthis category, as the cue is usually pictorial (i.e., a face or picture) or descriptive (e.g., the history teacher in 10th grade, the actor in King of Hearts, the Israeli professor at a Psychonomic Society conference). Thus, Bowles and Pooh (1985) gave their subjects only the definitions of low frequency words (e.g., unicorn) and found that older adults had a lower target retrieval success rate than younger adults. Heller and Dobbs (1993) found age-related decrements in word-finding, both in discourse (labeling characters and objects in a video) and in a category fluency task. Albert, Heller, and Millberg (1988) found a slight (7%) but statistically reliable drop in naming relatively low frequency pictures (e.g., trellis, Boston Naming Test). Thus, consistent with the introspective observations of Hebb (1978) and Skinner (1983), aging seems to be associated with increased access ditficulties. The most dramatic access failure experienced occasionally by everyone is the "Tipof-the-Tongue" state, when the inaccessibility of an known word can take on a dimension of frustration beyond mere cognitive disappointment. Studies have not found age differences in ToTs for objects (Maylor, 1990a; Mitchell, 1989), but have turned up agerelated increases for names of people, places, and movies (Burke, MacKay, Worthley, & Wade, 1991; Maylor, 1990b). The eventual resolution of a ToT (i.e., finding that word!) can and does happen in older folks (Finley & Sharp, 1989). As Skinner (1983) noted, "When I have time--and I mean something on the order of half an hour--I can almost always recall a name" (p. 240). 4.2. Beyond Access A number of investigators have been concerned with the issue of age-related "cognitive slowing," and whether the change in response speed is general or task-specific (see Schulz, 1994, and related articles). If the slowing is in fact cognitive (i.e., not just perceptual-motor speed) and global, then semantic retrieval efficiency would also be affected. In an ongoing mega-meta-analysis debate, one group of investigators concluded that semantic priming effects are qualitatively equivalent across age, differing only (and predictably) in slope and intercept (Myerson, Ferraro, Hale, & Lima, 1992), whereas another team concluded that there are process-specific age differences (Laver & Burke, 1993). If the slowing is task-specific, then we can ask whether the speed of access and/or the rate of spreading activation are slower in older adults. Balota and Duchek (1988) used a delayed pronunciation task: When a word appeared, the subject had to hold off pronouncing it until given a cue. With a short delay (150 msec) between the word and pronunciation cue (i.e., insufficient time for retrieval), the age difference was largest (about 200 msec); a longer delay (1200 msec) cut the age difference in half(about 105 msec). The latter difference was interpreted as "probably due to differences in output processes" (p. 91), whereas the initial (remaining) difference (i.e., 95 msec) was interpreted as an age difference in lexical access time. However, Cerella and Fozard (1984) found a larger age difference in vocalifing alone (94 msec) than in pronunciation (34 msec), leading them to conclude that lexical access speed was not impaired by aging. Using a similar procedure in picture naming, we (Mitchell & Schmitt,

114

D.B. Mitchell

1994) have also found a larger age difference for vocalization (96 msec) than for lexical retrieval (19 msec). As for the rate of the spread of activation (i.e, by manipulating the stimulus onset asynchrony), some studies have found age differences (Balota, Black, & Cheney, 1992) and others have not (Madden, Pierce, & Allen, 1993). While the issues of age differences in semantic access and semantic speed will have to be resolved ultimately, we won't examine them any further here. The larger debate on general age-related slowing is already 30 years old and still raging (of. Schulz, 1994; For a brief and excellent discussion of slowing, see the relevant section in the chapter by Ober & Shenaut, this volume). My view is that the notion of general slowing cannot account for the differential effects of aging on episodic, semantic, and implicit memory (for similar conclusions based on different data, see the chapters by Allen, Madden, & Slane and by Amrhein in this volume). 5. IMPLICIT M E M O R Y

Our major concern is with a third type of memory phenomenon. Once a concept has been accessed in semantic memory, can it be more readily or more quickly accessed again in the future? This facilitation is neither episodic--for conscious reconection is not required--nor is it solely semantic, for the "activation" ignited by a single retrieval has a very long lasting procedural impact. This indelible cognitive footprint is called priming. In the early 1980s, many reports of long-lasting priming emerged. For instance, Jacoby and Dallas (1981) gave subjects an identification task in which words were presented for 35 msec followed by a mask. Although familiar English words were very diflficult for subjects to perceive, a single exposure of a word was enough to make the word appear to "jump out" on its second presentation. They found that 1) once primed, low-frequency words could be identified just as well as unprimed high-frequency words, and 2) a primed word could be identified with the same facility 24 hours later as it was in an immediate test. Jacoby and Dallas called this phenomenon "perceptual learning." In a similar vein, Tulving, Schacter and Stark (1982) presented their subjects with a ditticult word fragment completion task (e.g., make a word by filling in the missing letters: o o ut, d _ f n str t on). Again, if the words had been presented in a prior list, subjects were much more likely to be able to complete them relative to unexposed words. Furthermore, the magnitude of the priming effect (calculated by the difference between the proportions of studied vs. unstudied word fragments completed) did not dissipate over a 1-week interval. This priming pattern was in stark contrast to episodic memory performance: When yes/no recognition decisions had to be made--with the words (defenestration and coconut) in full view--performance declined precipitously between the immediate and the 1-week tests. Years earlier, Warrington and Weiskrantz (1968) had reported equally dramatic dissociations in amnesic patients, and Kolers (1976) had reported 1-year priming in an inverted text reading paradigm But either the facts or their implications for memory theory were lost to mainstream cognitive psychology until the 1980s. Prior to this time, priming was regarded only as a temporary activation of a node (Collins & Loflus, 1975) or logogen (Morton, 1979) in semantic memory, lasting no more

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115

than a few hundred milliseconds or perhaps several seconds at the outside. Thus, Tulving et al. (1982) viewed the dissociation data as a "theoretically pregnant finding" and suggested that priming effects "may be mediated by a cognitive system other than episodic and semantic memory" (p. 336). Furthermore, priming was not only dissociated from episodic performance, but from semantic memory as well--even though the priming tasks themselves do involve semantic memory! This theoretical speculation was motivated by and supported by modality effects (e.g., auditory stimuli produce little or no priming to visual targets) and cross-format deficits (e.g., the usually robust picture superiority effect vanishes on a word t~agment completion task; cs Weldon & Roediger, 1987). Thus, we have at least two types of functional dissociations between episodic memory and priming: 1) long retention intervals produce decrements in episodic memory but not in priming (e.g., Mitchell & Brown, 1988); and 2) switching modality or format has little or no impact on episodic memory, but causes priming to drop off or disappear altogether (e.g., Rajaram & Roediger, 1993). These dissociations contradicted a concept of priming merely as "a modification o f ht e semantic memory system" (Tulving et al., 1982, p. 341). Dissociations between episodic and implicit memory are abundant, but what evidence is there for the distinction between semantic and implicit? One often overlooked dissociation reported by Dannenbring and Briand (1982) makes a compelling case for the difference between semantic priming and repetition priming. In semantic priming, one word is followed by a second related word (e.g., mouse-cheese). In repetition priming the same word occurs twice (e.g., cheese-cheese). To the extent that a lexical decision response to the second stimulus produces a faster reaction time, we have priming. Although both kinds of word sequences evince priming, Dannenbring and Briand varied the lag between the two trials, and found an interesting divergence between semantic priming and repetition priming. Their data are plotted in Figure 2, where it is clear that the magnitude and longevity of semantic priming is entirely different from that of repetition priming. Corroborating the behavioral priming data, Rugg (1987) found that event-related brain potentials (ERPs) were very different for semantic priming and word repetition. He concluded that "semantic priming and word repetition do not have their effects at a common locus (or loci) within the cognitive system" (p. 123). Similar electrophysiological repetition effects have been found across adulthood as well, whether measured by ERPs (Friedman, Hamberger, & Ritter, 1993) or SCRs (Skin Conductance Responses, Plottffe & Stelmack, 1984). Subsequent to these findings, the implicit~explicit terminology put forward by Graf and Schacter (1985) spread across the field like wildfire, and 1986 saw the first aging experiment on implicit memory published by Light, Singh, and Capps. Although Tulving used the term "procedural memory" in 1985, he later (1991)used this to refer exclusively to skills and simple conditioning phenomena. Tulving and Schacter (1990) used the term "priming," in particular reference to a Perceptual Representation System, which they proposed is responsible for the perceptual identification and priming of the structural components of objects and words. Squire (1992) used the term "nondeclarative (irnplicit)" to include skills and habits, priming, simple classical conditioning, and nonassociative learning. Throughout the rest of this chapter, I will use the term "implicit memory" as

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D.B. Mitchell

Repetition Priming vs. Semantic Printing (Dannenbring & Briand, 1982) 100

8O A

Ill O9

60 \

40

\

7

SAME WORD

k

20

RELATED WORD

tr

\

o %

-20 -40

9

I

I

I

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I

I

I

I

I

I

0

1

2

3

4

5

6

7

8

9

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!

I

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I

I

10 11 12 13 14 15 16

LAG BETWEEN 1st and 2rid OCCURRENCE Figure 2. The different effects of lag interval on repetition priming and semantic priming. In repetition priming, the same word occurs on the first and second trials (e.g,, cheese-cheese); in semantic priming, the second word is related to the first word (e.g., mouse-cheese; data from Dannenbring & Briand, 1982).

defined by Schacter (1987): "previous experiences facilitate performance on a task that does not require conscious or intentional recollection of these experiences" (p. 501). 6. TYPES OF TASKS AND PROCESSING PROCEDURES Roediger and his colleagues have championed the concept of transfer-appropriate processing, especially as it applies to various implicit tasks. Transfer-appropriate processing means that "Tests of retention will benefit ... to the extent that the processing operations at test recapitulate or overlap those engaged during prior learning" (Roediger & Sriaivas, 1993, p. 21). Within this ~amework, a task is perceptual to the extent that priming is greatest when the surface form of a stimulus is changed the least from input to test (e.g., word identification: read a word at input, identify the same word presented tachistoscopically at test). At the other end of the continuum~ a task is conceptual when priming occurs with only semantic features overlapping between study and test stimuli (e.g., category instance generation: read a word at input, generate items from the word's taxonomic category at test). In order classify a given implicit memory task, we have to known both its input-output parameters (format, modality, test cue) as well as the amount of priming it produces. Thus, a task is perceptual if any of the following study-to-test changes reduce priming: 1) modality (e.g., auditory to visual); 2) format (e.g., word to

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picture); 3) perceptual details (e.g., typography, object orientation) (Roediger & McDermott, 1993; Roediger & Srinivas, 1993; Roediger et al., 1994). The "acid test" is to find a reverse generation effect, i.e., better priming following exposure to a word presented by the experimenter than to a word generated by the subject (Roediger, Weldon, & Challis, 1989). This is exactly what Jacoby (1983) found. He had subjects either read stimuli (e.g., xxx-COLD) or generate targets semantically (e.g., hot-????). Jacoby found better priming for read than for generated targets in a subsequent word identification task, but the opposite effect in a recognition memory task (i.e., better memory for generated items, the standard "generation effect"). As for conceptual tasks, they are so classified if priming is unaffected by any of the above physical manipulations, and if priming is enhanced by semantic processing (e.g., levels of processing, generation effect). Indeed, in a recent meta-analysis of 38 studies, we found a much larger levels of processing effect in conceptual (mean = .43) relative to perceptual (mean = .12) implicit tests (due to editorial prerogative, these magnitude means (deep minus shallow processing)/(deep + shallow means) were not published in Brown & Mitchell, 1994). Roediger and McDermott (1993) have classified nine verbal implicit memory tasks (see their Table 1) according to the criteria above, so their classification scheme was used to organize the aging data in the tables and figures that follow. Within perceptual tasks, one other distinction is important. When the study and test item share a similar morphology (e.g., elephant to ele , as in word stem completion), the task is said to involve direct priming. Priming can be enhanced even more, however, when both the display and the response are the same at study and test (e.g., name a picture, name a picture); this is called repetition priming (Roediger & McDermott, 1993; Roediger, personal communication, November 1994), which "may thus be viewed as a special case of direct priming" (Roediger, 1990b, p. 380). The difference between repetition priming and direct priming is illustrated most dramatically by a finding from word fragment completion. The standard method is to expose subjects to whole words (e.g., defenestration) and subsequently test them with fragments (e.g., d f n str ti n). Although this approach yields better priming relative to input-test changes in typography or modality, the greatest priming is achieved by presenting the identical fragment both times. Thus, d f n str ti on is presented along with a semantic clue ("the act of throwing a person out of a window") and then the same fragment is used at test. Gardiner, Dawson, and Sutton (1989) found that a fragment presented at input produced 41% priming compared to only 28% priming for a whole word presented at input. Variations on this paradigm are ripe for aging studies. 7. IMPLICIT MEMORY AND AGING 7.1. Issues

The remainder of this chapter will focus on the issue of age-related effects in implicit memory. This is controversial for two reasons, one theoretical and one empirical. The theoretical debates dispute whether different memory systems can be distinguished (e.g., episodic vs. semantic, semantic vs. implicit, three systems or more). For instance, in Kauslel~s (1994) chapter on "generic (semantic) memory" (parentheses his!), he expresses

1 18

D.B. Mitchell

his skepticism with a subhead in the form of a question: "Separate Memory Systems?" Others have also expressed reservations about memory systems as a vehicle for explaining age differences (Craik & Jennings, 1992; Salthouse, 1991). The major empirical argument focuses on a question articulated by Kausler (1994): "Is implicit memory truly age insensitive?" (p. 377). This question has come about because whereas most studies have found that age differences in implicit memory are not statistically simaificant, there is often a small priming advantage favoring the younger group. Salthouse (1991) speculated that "some of the failures to find significant age differences in memory when assessed by implicit procedures may be attributable to low statistical power" (p. 254). Hultsch, Masson, and Small (1991) argued even more passionately for statistical power failure, saying that "the failure of previous studies to find significant age effects on tasks similar to the present stem completion measure be may related to the power of the design rather than the absence of reliable age differences on such tasks" (p. P29). They took their argument to its logical extreme, and ran 544 subjects! 7.2. Review of the Data The implicit memory tasks to be reviewed in detail are limited to those that involve semantic memory at input and output. The focus is on priming in semantic memory because of our interest in the interface between these two systems. Therefore, the data review does not cover implicit memory tasks with no verbal component, such as serial pattern sequences (Howard & Howard, 1992), 2-dimensional representations of potential 3-dimensional objects (Schacter, Cooper, & Valdiserfi, 1992), or novel visual stimuli (e.g., Japanese ideograms presented to American subjects, Wiggs, 1993). Even though these three studies produced beautiful findings vis-a-vis implicit memory and aging (i.e., no age differences), there are not enough data yet on the question of priming of novel stimuli (see Howard, in press, for some detailed coverage). Along a similar line, Howard, Fry, and Bnme (1991) studied implicit memory for new associations (between familiar words) and found age differences when there were time limits at study; however, with more study time, age differences disappeared. But these and related findings are as yet inconclusive, and will not be discussed further. Pve also excluded studies where subjects were either unconscious (e.g., Brown, Best, Mitchell, & Haggard, 1992) or intentionally distracted by the experimenter during input (e.g., Howard & Pulido, 1994). In the studies below, encoding is assumed to have occurred under "normal" circumstances, i.e., subjects were paying some attention to the stimuli, usually without expecting any kind of memory test. The data from 36 studies are presented in three tables and one figure, organized according to repetition priming, direct priming, and indirect or conceptual or unclassified. These studies contained 48 separate experiments, which yielded 97 contrast pairs (i.e., young vs. old). Over 3,000 subjects are represented with overall mean ages of 24 and 71 in the young and older groups, respectively. Each table indicates the test used, its type (perceptual or conceptual direct or indirect), the encoding task, and other input or test conditions where relevant. The data include: the number of subjects, mean ages and mean priming for young and old, an Old:Young ratio, and Effect Size (ES). Priming was calculated as follows: When the measure was a proportion, the number of targets attained

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119

for unstudied items (i.e., baseline) was subtracted fi~om the number of studied targets completed. When the dependent variable was speed, response times for studied stimuli were subtracted t~om response times for stimuli not studied. The Old:Young ratio is an index of the degree of age-related deficit (originaUy devised by F.I.M. Craik): The mean score of the older group is divided by the mean of the younger group. Thus, a ratio near 1.00 indicates no age difference. As the ratio drops below 1, the age deficit increases, and as the ratio rises above 1, score one for gray panther memory power. The ES estimate is the value of r, calculated ~om t-test or F-test (ANOVA) values when available (using the program by Mullen & Rosenthal, 1985). Of 54 possible statistical age group main effect tests, only 31 values were reported explicitly. The majority of these occur in Table 2, dominated by the ubiquitous word stem completion task. Among the studies not reporting inferential statistics, the modal information was simply that the age difference was not statistically significant. The tasks listed in Tables 1-3 and Figure 3 are by now all fairly well known standard implicit memory tasks. Most of them are described and illustrated in Table 1 of Roediger and McDermott (1993). It is clear that word stem completion (see Table 2) is far and away the most popular task, showing up in eight studies. In the standard memory research literature (i.e., non-aging), the word t~agment completion task has been tested most extensively, so it's surprising that only one aging study (and the first one at that!) has employed this task. In the repetition priming tasks (Table 1), the same response was made at study and at test to the same stimulus (e.g., the word student had to be classified as animate or inanimate by pressing one of two buttons in the study by Hamberger & Friedman). In the direct priming tasks (Table 2), the same item was presented at study and test, but the test version of the stimulus was either modified (e.g., picture fragment, anagram) or made difficult to perceive via a brief presentation and/or by a mask (e.g., auditory or visual word identification). With the exception of the "free-association-tocategory-name" task--which is a bona fide conceptual priming task--the tasks in Table 3 are a motley collection reflecting the diversity in the field and the creativity of our cognitive colleagues. Finally, Figure 3 is dedicated exclusively to homophone priming. This special treatment was warranted because this task was not classified by Roediger and McDermott (1993) and because there are six experiments with 11 contrasts, second only to word stem completion (29 contrasts). In this task, subjects are first exposed to the lower frequency version of a homophone in a biasing context (e.g., "Joan of Arc was burned at the stake"). Later, the implicit task is to spell the homophone (presented aurally) out of context; priming occurs when subjects produce the biased spelling (e.g., stake not steak). Overall, the majority of the O:Y ratios fall below 1 (61 of 97 comparisons, or 63%). A box score, however, is insensitive to the magnitude of the age difference. Considering the size of the ratios, the overall mean = 1.053; SD = 0.84, median = .890. Tested against 1.0, the difference is not siLmificant, t (96) = 0.618, p = .538. However, some of the larger ratios (e.g., 6.00) are statistical outliers, both by z-scores and by the non-parametric definition of a ratio exceeding the upper hinge by 1.5 the hinge widths of a distribution. The ratios from some studies were also dropped as methodological outliers,

120

Table

D.B. Mitchell

I

Age Differences

in R e p e t i t i o n

Priming

Encoding Task

Mean Priminq Young Older

1 1

Lex/Dec--0 Lex/Dec--4

129ms 41ms

125ms 54ms

.97 1.32

1

Lex/Dec

63ms

53ms

.84

.ns

2 2 2a 2a

n a m e - - 5 0 % - - i rep. .05 name--50%--8 rep..I0 n a m e - - 3 7 % - - I rep. .-n a m e - - 3 7 % - - 8 rep. .--

.04 .13 .02 .I0

.80 1.30 .40 1.00

.09

1 1

typecase animate

12ms 29ms

02ms 30ms

.17 1.03

SPEEDED READING/P?/ H a s ~ t r o u d i et al., 1991 N=40; A g e s = 20, 70 la n=20; a g e = 69

1 la

inverted--450ms inverted--900ms

.i0 ---

.05 .07

M o s c o v i t c h et al., 1986 N=28; A g e s = 22, 68

1 1

transformed--2hr transformed--2wk

6.6s 2.8s

M o s c o v i t c h et al., N=24; A g e s = 23,

1986 71

2 2

d e g r a d e d sents. de9. w o r d p a i r s

M o s c o v i t c h et al., 1986 N=24; A g e s = 20, 71

3 3

Test/Type/Experiment

Exp

LEXICAL DECISION/P/ K a r a y a n i d i s et al., 1993 N=52; A g e s = 27, 67+ M o s c o v i t c h , 1982 N= 20; A g e s = young,

Effect Size

9 ns

70

DEGRADED WORD NAMING/P / H a s h t r o u d i et al., 1991 N = 60; A g e s = 19, 69 n= I0; A g e = 70

CLASSIFICATION/P? / H a m b e r g e r & Friedman, 1992; N=36; Ages=25,

lag lag

Old:Y Ratio

70+

.Ii

. ns

.50 .70

.28 .Ii

10.0s 3.6s

1.52 1.29

.ns .ns

160ms 090ms

470ms 470ms

2.94 5.22

.ns .ns

rand. w o r d p a i r s single words

220ms 070ms

430ms 260ms

1.95 3.71

.ns .ns

1 1 1

name--5 lag n a m e - - 2 5 lag n a m e - - 5 0 lag

178ms 176ms 151ms

155ms 158ms 136ms

.87 .89 .90

.09

1 1 1 1

name--immed. name--I day name--7 days n a m e - - i l days

134ms 079ms 072ms 055ms

104ms 059ms 055ms 062ms

.78 .75 .76 1.13

.08

plc~-~. N ~ I N Q / C ? / M i t c h e l l , 1989 N= 96; A g e s =

22,

70

M i t c h e l l et al., 1990 N= 96; A g e s = 20, 70

Notes. Test Type: P: Perceptual, C= Conceptual. Otd:Y r a t i o = old mean + young mean. E f f e c t size= s

Priming: ms: msec, s= sec, decimal= proportion. based o n s or ~; for f ~ l , assumed B:.50; .ns: Author(s)

reported only t h a t age d i f f e r e n c e was not s t a t i s t i c a l l y s i g n i f i c a n t . +Additional age groups were tested, but are not reported here. No age d i f f e r e n c e s were s t a t i s t i c a t t y r e l i a b l e in these studies.

121

Semantic processes in implicit memory

Table

2

Age Differences

in D i r e c t

TEST/Type /Experiment

Priming

Exp

Mean Primina Young Older

Encoding Task

Old:Y Ratio

Effect Size

.71 .89

.15

.50

.35*

9

WORD FRAGMENT COMPLETION/P / L i g h t et al., 1986 1 N= 64; A g e s = 23, 69 1

immediate 7 days

.17 .09

1 1 1 1 1 1

vowel--Omin vowel--13min vowel--46min pleasant--Omin pleasant--13min pleasant--46min

.52 .28 .21 .47 .34 .36

.23 .38 .29 .28

.64 i. I0 .81 .85 .78

D a v i s et al., 1990 N=147; A g e s = 2 2 , 74+

2

likability

.51

.26

.51

D i c k et al., 1989 N=48; A g e s = 21,

1 1 1 1 1 1

read/trial 1 generate/trial read/trial 2 generate/trial read/trial 3 generate/trial

.21 .24 .43 .40 .60 .54

.26 .22 .39 .37 .47 .45

1.24 .92 .91 .92 .78 .83

2 2 2 2

l e t t e r - - s a m e type letter--diff.type syllable--s.type syllable--d.type

.16 .15 .25 .12

.12 .I0 .Ii

.75 .67 .72 .92

H u l t s c h et al., 1991 N=544; A g e s = 24, 74+

1

lexical d e c i s i o n

.16

.Ii

.69

.16"

J a v a & G a r d i n e r , 1991 N=32; A g e s = 22, 73 N=32; A g e s = 21, 70

1 1 2 2

letter free-ass read generate

.21 .19 .30 .22

.14 .18 .27 .15

.67 .95 .90 .68

.12

L i g h t & Singh, 1987 N=64; A g e s = 24, 68 N=64; A g e s = 23, 68

1 1 2

vowel pleasantness pleasantness

.24 .29 .28

.17 .24 .20

.71 .83 .71

.16

P a r k & Shaw, 1992 N=287; A g e s = 19,

1 1 1 1

e-a: 3 l e t t e r s e-a: 4 l e t t e r s pleasant: 3 let. pleasant: 4 let.

.07 .ii .08 .12

.05 .09 .07 .12

.71 .82 .88 1.00

WORD STEM COMPLETION/P/ C h i a r e l l o & Hoyer, 1988 N=144; A g e s = f 9 , 68

G i b s o n et al., N= 88; A g e s =

73

1993 young,

70

68

1 2 3

.12 .08

.26 .18

.18

.54*

- .12

.ns

.12

.26 .04

122

Table

D.B. Mitchell

2 continued

Age Differences

in D i r e c t

Study

Priming

Exp

Encoding Task

M e a n Primina Young Older

Old:Y Ratio

Effect Size

ANAGRAM SOLUTION/P? / Java, 1992 N=32; A g e s = 19, 70

1 1

letters free-ass

.14 .05

.07 .Ii

.50 2.20

.12

DEGRADED WORD.NAMING*/P/ L i g h t & Singh, 1987 N=64; A g e s = 21, 69

3 3

vowel (80 %) pleasant (80%)

.ii .16

.09 .12

.82 .75

.20

1

read name

.27

.27

1.00

-.05

.14 .19

.07 .12

.50

syllable/visual syllable/aud. pleasant/visual pleasant/aud.

.18 .08 .23 .12

.15 .09 .16 .08

.83 1.12 .70 .67

.16

AUDITORY WORD IDENTIFICATION/P/ L i g h t et al., 1992 2 pleasant/aud. N= 64; A g e s = 24, 70 2 pleasant/visual

.I0 .06

.12 .06

1.20 1.00

.09

R i t c h i e et al., 1993 N=575; A g e s = 65, 95+

V I S U A L W O R D IDENTIFICATION~P~ 2 read/high freq. A b b e n h u i s et al., 1990 2 read/low freq. N= 22; A g e s = 23, 73 L i g h t et al., 1992 N= 64; A g e s = 22,

72

1 1 1 1

PICTURE

FRAGMENT. I D E N T I F I C A T I O N / P /

Heindel N=24;

et al., 1990 A g e s = 55, 72

R u s s o & Parkin, 1993 N= 96; A g e s = 26, 74 HQMOpHONE See F i g u r e

.58"

.63

1

name picture

.16

.16

i. 00

.ns

1

name picture

.17

.16

.94

.05

SPELLING/P? / 3

Notes. Test Type: P= Perceptual, C= Conceptual. Priming: decimal= proportion. Old:Y ratio= old mean + young mean. Effect size= s based on F or ~; for ~<1, assumed g=.50; .ns= Author(s) reported only that age difference was not s t a t i s t i c a l l y s i g n i f i c a n t . *Author(s) reported age difference was s t a t i s t i c a l l y s i g n i f i c a n t at 9<.05 or tower. +Additional age groups were tested, but are not reported here. AThis degraded naming task d i f f e r e d from the task by the same name in Table 1 only v i s - a - v i s direct vs. r e p e t i t i o n priming; Hashtroudi et at. (1991) started off the f i r s t t r i a l with a degraded stimulus, whereas Light & Singh (1987) and Ritchie e t a [ . (1993) presented intact words i n i t i a l l y , and tested with degraded words.

123

Semantic processes in implicit memory

Table

3

Age D i f f e r e n c e s

in I n d i r e c t / C o n c e p t u a l

Exp

TEST/Type/Experimen t

Encoding

Priming

and Other Unclassified

Task

Mean Priminu Young Older

Tasks

Old:Y Ratio

Effect Size

,,

FREE ASSOCIATION

TO C A T E G O R Y

Light & Albertson, N= 64; A g e s = 22, G r o b e r et al., 9 N= 68; A g e s =

1989 70

1992 30, 78

NAME/I/C/

1

pleasantness

.18

.13

.74

.17

1

name p i c t u r e

.46

.31

.67

.ns

.01 .12

.06 .06

6.00 .50

.08

S~.~rrv.Nc~. com~T,~.Tios/i/c/ H a r t m a n & H a s h e r , 1991 N= 68; A g e s = 20, 66

- read + read

1

generate

1

generate

1 2

association association

.II .13

.I0 .19

.91 1.46

.12 -.12

1 2

intentional intentional

.22 .39

.37 .42

1.68 1.08

.nr .nr

D y w a n & J a c o b y , 1990 N= 48; A g e s = 20, 71

1

name

-. Ii*

.06

I. +?

.nr

B a r t l e t t et al., 1991 N= 39; A g e s = 28, 72

2

face fame

-.07

.07

I.+?

.nr

priming -.06 f a m i l i a r i t y .31 priming .51 priming .23 f a m i l i a r i t y .62 familiar. .62

.04 .38 .53 .37 .58 .66

i.+? 1.23 I. 04 1.61 .94 1 .'06

.nr .nr .nr .nr .22

KNOW J o ' o ~ r r s / o / c / M A n t y l ~ , 1993 I. N=32; A g e s = 2. N=32; A g e s =

27, 26,

Parkin & Walter, i. N=40; A g e s = 2. N=90; A g e s =

1992 34, 80 22, 68+

72 70

.FALSE FAM]E/D/C?/

PROCESS

DISSOCIATION

PROCEDURE/D/P?/

J e n n i n g s & Jacoby, 1993 I. N=72; A g e s = 20, 74 2. N=32; A g e s = 19, 70

Notes.

fame

1 1 2 2 2 2

read: read: read: solve: read: solve:

Test Type: X= I n d i r e c t , D= D i r e c t ; P= Perceptual, C= Conceptual.

Exp= Experiment number; Priming

means are p r o p o r t i o n s . Otd:Y r a t i o = otd mean § young mean. Effect size= s based on f or ~; f o r f ~ l , assumed ~=.50; .ns= Author(s) reported onty that age d i f f e r e n c e was not s t a t i s t i c a t t y s i g n i f i c a n t ; .nr= i n f e r e n t i a t s t a t i s t i c s not reported; *dacoby et at. (1989) obtained priming of .08 when young subjecs were tested o v e r n i g h t . +Additionat age groups were tested, but are not reported here. No age e f f e c t s were s t a t i s t i c a t t y r e l i a b t e in these studies.

D.B. Mitchell

124

Homophone Priming Studies ES Rose et al., 1986

I

Davis et al., 1990 Howard, 1988, Exp. 1 Howard, Exp. 2, lmm. Howard, "

"2-Day

Howard, Exp. 3, Imm. Howard, "

" 2-Day

M & B, 1990, Imm.

M & B, 1990, 1-Day

1

M & B, 1990, 7-Day

YOUNGER OLDER

M & B, 1990, 21-Day J

-10

0

10

20 30 PRIMING (%)

J

40

50

Figure 3. Mean priming for young and older adults in homophone spelling or usage tasks from six studies. ES = effect size (r); *statistically significant age difference; .nr = Inferential statistic not reported; .ns = author reported only that age difference was not significant. Retention intervals: Imm = immediate; I, 2, 7, and 21 days. M&B= Mitchell & Brown (1990). The data from Davis et al. (1990) are from their Experiment 1.

when priming was not observed in one of the age groups.* These ratios are detailed in Table 4, where the means and medians for each of the four groupings are summarized Across the entire sample of 97 contrasts, a total of nine ratios were dropped. Table 1 lost three ratios as extreme statistical oufliers (2.94 and up). Table 3 lost one point as a statistical outlier (6.00) and three other points because the experiments did not produce the phenomenon in the younger adults. These were process dissociation procedures, in which remembering an item eliminates the false fame or priming for that item (Bartlett et al., 1991; Dywan & Jacoby, 1990; Jennings & Jacoby, 1993, Exp. 1). Because younger subjects' episodic memory was working too well, the phenomenon did not occur in these conditions, and thus an age comparison is not possible. (In order to get the false fame effect in young adults, Jacoby, Kelley, Brown, & Jasechko, 1989, delayed the test for 24 hours, when episodic memory was waning.) Two data points were dropped from the homophone studies (Figure 3) because the older subjects did not produce priming. The failure to get priming in these studies (Davis et al., 1990, Exp. 1; Rose et al., 1986) suggests a methodological flaw, since recognition memory in the older groups was above chance. Davis et al. used Rose et al.'s identical materials, and both studies actually observed negative priming in the older group (i.e., studied homophones were less likely to spelled in the lower frequency meaning than unstudied homophones. I am not aware of this problem in any other published study.). Davis et al. (1990) went so far as to conclude that "the homophone test appears to be poorly suited for assessment of priming in the elderly" (p. 292). Howard (1988, Exp. 1), however, reported too much test awareness with the spelling technique, and instead had subjects generate sentences at test in

125

Semantic processes in implicit memory

before and after outliers are removed. Only the data in Table 2 reveal an age-related deficit reliably different from 1.0. Indeed, only four studies reported statistically si~ificant main effects for age. All of these are in Table 2, and three of them are based on word stem completion priming. (Three other si~ificant age main effects occur in homophone priming, Figure 3, but two of the studies (Davis et al., 1990; Rose et aL, 1986) were unable to get any priming at all in the older adults, and the other, Howard, 1988, Exp. 1, uncovered bona fide episodic contamination. Thus, these are treated separately.) Table 4.

Old:Young Before

Ratios

and After

from Tables Removing

Before N

Mean

1-3 a n d F i g u r e

3,

Outliers

After

Removal Median

N

Removal

Mean

Median

t

Table

1

24

1.323

.935

21

.946

.890

0.63

Table

2

46

.845

.820

46

.845

.820

3.92*

Table

3

16

1.370

1.020

12

1.077

1.050

0.72

II

.875

.690

9

1.070

1.080

0.31

tested

against

Figure

Notes.

3

*~<.01,

1.0.

order to determine which homophonemeaning a subject had in mind. We (Mitchell & Brown, 1990)also utilized this "usage" procedure, which may account for the difference between the positive and negative findings.

126

D.B. Mitchell

In the sections that follow, we shall see that the data with the smallest O:Y ratios (i.e., that suggest the largest age deficit in priming) all come from studies where there is clear evidence of methodological shortcomings. The shortcomings involved either the susceptibility of the implicit tasks to contamination by conscious recollection, the number of stimuli (too few), or the number of repeated trials (too many). These problems are directly responsible for producing a spurious age difference in priming. Once the problems are eliminated, the data unequivocally support age invariance in priming. In Table 1, three-quarters of the data are priming based on response speed measures. The lowest O:Y ratio (.17) was not an outlier, but the three largest values (2.94 and up) were. Before removing the outliers, a liberal interpretation is that older adults show the best priming on implicit tasks that involve identical repetition and/or response speed measures. However, note that the largest O:Y ratio actually comes from Table 3 (Hartman & Hasher's unique sentence completion task), whether before or after removal of outliers (see Table 4). Table 3 contains the greatest number of as yet unclassified tasks, so we can't make any conclusions here about specific tasks or types of tasks. The worst age deficit occurs in Table 2, but we will see below that this is not due the use of direct or perceptual tasks per se. 8. EPISODIC CONTAMINATION IN IMPLICIT M E M O R Y TASKS One explanation that can account for age differences in implicit memory performance when they do appear is episodic contamination (cf Howard, in press; Schacter, Bowers, & Booker, 1989). That is, although subjects are not instructed to use their recollection as a tool for solving the word or picture puzzle in front of them, they do so anyway. For example, when we give a subject a word stem, INT , the implicit instructions only ask that the subject complete the stem with the first word that comes to mind. If the subject has total anterograde amnesia, then we can be confident that he will follow the instructions. Otherwise, there is usually no guarantee that the subject will not notice the relation between the test stems and the words previously seen or heard on the experimenter's list. When subjects do notice, and then go on to utilize their memory to fill in the missing letters, then implicit memory is no longer being measured properly, and the performance is "contaminated" by episodic memory. If this phenomenon would occur with equal frequency across age groups, and if its occurrence would affect priming scores equally in different age groups, then contamination would be no more than a nagging nuisance, and--at least for the purpose of comparing age differences--we could ignore it. Salthouse (1991) has raised a potential problem with this line of thinking: "While the suggestion that many i~licit assessments are contaminated by explicit remembering or episodic processes preserves the original hypothesis, it does so only at the risk of introducing an uncomfortable circularity. That is, age differences are presumed to be present because the tasks include conscious recollection or the episodic system, but the tasks are apparently inferred to include conscious recollection of the episodic system because age differences are present. As when any

Semantic processes in implicit memory

127

hypothetical construct is postulated, some independent means of establishing its existence seems necessary to minimize reliance upon assumptions whose validity cannot be verified." (pp. 254-256) Those independent means are now available, and are reviewed below. We shall see that 1) younger subjects are more likely to become aware that the implicit task is a veiled memory test, and 2) younger adults are better at exploiting the study item/test cue relations, thereby (artificially) boosting their implicit memory performance. Akhough the explanation for why the young adults do both things is that their episodic memory is working better, note that the evidence for episodic contamination is n o t based on the finding of an age difference in priming. We will first review direct evidence for both points, and then see additional ramifications of this phenomenon reflected in certain data patterns.

Category Exemplar Priming (Light & Albertson, 1989) 25 AGE GROUP 20

-

l

"t'otrNc

~/A OLDER

A

15

r

7

1o rr n 5

UNAWARE

AWARE & TRIED

SUBJECTS' AWARENESS OF MEMORY TEST Figure 4. Priming from category exemplar generation in young and older adults, as a function of whether subjects were aware of the relation between the test and the previously studied exemplars (Light & Albertson, 1989). Subjects on the left side of the figure reported being unaware of the relation, whereas those on the fight side reported both awareness and intentional attempts to generate studied target words from each category. When experimenters have asked their subjects whether they noticed a relationship between the implicit task and previous stimuli, younger subjects are the best detectives. Light and Albertson (1989) asked their subjects "whether they noticed that they were generating previously seen list members and whether they deh'berately tried to do so" (p. 489). Although 34% of the young and 12% of the old reported intentionally trying to

D.B. Mitchell

128

produce kems from the study list, fully 87% of the young indicated awareness relative to 54% of the elderly. In Hartman and Hashel~s (1991) study, 54% of their young adults "became aware of the relationship between the study sentences and the sentence completion test" (pp. 591-592), compared to only 13% of the older subjects. However, among their young subjects, awareness was not correlated with priming (sentencecompletion task). Park and Shaw (1992) found awareness in only 10% of their large sample, about two thirds of which were young. Within their young subjects, awareness was not correlated with priming. However, when aware subjects were excluded, the analysis revealed identical priming (.08 each) for young and older adults (cs the means in Table 2). When Light and Albertson compared the priming performance as a function of episodic awareness, the age differences varied dramatically. These differences are illustrated in Figure 4, where it is clear that pure implicit memory performance was age invariant, in stark contrast to an enormous advantage for the younger subjects who noticed the relationship between test cues and list items, tried to use that information explicitly, and succeeded.

Explicit Contamination" Picture Fragment (Russo & Parkin, 1993) 2.0 EPISODIC STATUS I

NOT RECALLED

~

RECALLED

A

r 1.5 _.1 tlJ > iii _.! 1.0

o z

tw 12. 0.5

0.0 OLDER ADULTS

YOUNGER ADULTS

Figure 5. The relationship between picture fragment identification priming and recall of specific items in young and older subjects (Russo & Parkin, 1993). Positive relationships between test awareness and priming have also been reported by Howard (1988), Grober et aL (1992), Park and Shaw (1992), and Russo and Parkin (1993). Grober et al. found a statistically nonsi~ificant yet substantial age difference in a free association task to category names following picture naming (O:Y ratio = .67).

129

Semantic processes in implicit memory

However, in a second experiment, Grober et al. demonstrated 1) that 58% of their young subjects were test-aware, and 2) those that were aware produced si~ificantly more targets than test unaware subjects (48% vs. 15%). These contamination effects are not limited to conceptual priming tasks. Russo and Parkin (1993) found that even in picture fragment identification--arguably a strongly perceptual task (cs Roediger and Srinivas, 1993) potentially impermeable to episodic contamination-item specific recall was utilized advantageously by younger adults but not by the older ones. These data are displayed in Figure 5. (Note that these groups are different from the subjects listed in Table 2; the latter were n o t given a recall test prior to picture fragment identification.)

Word Fragment Priming (Light, Singh, & Capps, 1986) 20 YOUNGER OLDER

IMMED.

7 DAYS

RETENTION INTERVAL Figure 6. The effect of retention interval on word fragment completion priming in young and older adults (Light, Singh, & Capps, 1986). Since episodic recognition dropped precipitously (from 70% to 24%) across the same interval, there was much less opportunity for episodic contamination after 7 days.

Confider another angle. In the studies above, subjects were asked. But we can see evidence without asking, as follows. If episodic contamination is indeed a major factor accounting for the ubiquitous small but statistically unreliable age differences in implicit memory, then the age difference should get smaller as the opportunity for contamination diminishes. Experiments with multiple retention intervals give us the proper conditions to test this hypothesis. We know that explicit memory performance declines systematically over time, for both young and old alike. Therefore, if the contamination hypothesis is correct, we would expect any age differences in implicit memory to actually get smaller, even when the explicit memory age differences become

130

D.B. Mitchell

greater. This is exactly the case in studies where both explicit and implicit memory were tested over long retention intervals. At short intervals, where there was greatest oppommity for contamination, age differences in implicit performance were largest. At long intervals, where explicit performance dropped off equally at best or even more for the older group--implicit performance did not follow this pattern. Thus, Light et at (1986) reported a much smaller age difference in priming after 1 week than after 1 hour (see Figure 6). Second, Chiarello and Hoyer (1988) found the largest age difference in wordstem completion priming on an immediate test, and the smallest difference after a 46-rain interval, in spite of parallel drops in explicit memory. Finally, Mitchell et al. (1990) found the smallest age difference-actually favoring the elderly--at a 3-week interval, when the age difference in explicit memory was greatest. This phenomenon is plotted in Figures 7 and 8 using the O:Y ratio as the index. As can be seen, the ratio exceeded .90 in Chiarello and Hoyer and flipped above 1.0 in Mitchell et at

Old:Young Memory Ratios (Chiarello & Hoyer, 1988) 1.0

0.9

0.8

0.7 "

0.6-

x

,a~

IMPI.ICIT

"-El-

EXPLICIT

%%

0.5-

% E3

0.4-

0.3

I

0

I

I

I

I

I

I

I

I

I

I

!

I

1

I

!

I

!

I

I

I

!

!

I

I

I

I

I

!

I

I

!

13

I

I

I

I

I

!

!

I

I

I

!

I

I

!

46.

TEST DELAY (minutes)

Figure 7. The effect of short retention intervals (minutes) on word stem completion priming, with O:Y priming ratios as the dependent variable (calculated from Chiarello & Hoyer, 1988). As argued in Figure 6, there was less episodic involvement at longer delays, diminishing the younger adults' advantage.

The flip side of this can be seen when performance is elevated by repeated trials. Dick et al. (1989) found that word stem priming improved across repeated trials, but at a higher rate for young subjects than for older subjects. The greater benefit of increases in conscious recollection for young adults' priming is easy to see in Figure 9. Taken together

Semantic processes in implicit memory

131

with the evidence presented earlier, word stem comletion as a task seems to be particularly susceptible to contamination. Further evidence for contamination in word stem priming can be seen in the correlation between the magnitude of priming and the O:Y ratio. (The contamination hypothesis predicts that age deficits are inflated as priming increases; that is, priming is abnormally large because of a boost from episodic participation.) Considering word stem priming alone, the young adults' mean = .235, SD = .123. Chiarello and Hoyer (1988) and Davis et al. (1990, Exp. 2) each had a priming cell mean of .52 and .51, respectively. These are statistical outliers, but more to the point, were associated with two of the lowest O:Y ratios (.50 and .51; see Table 2). With these numbers in, r = -.39, but the correlation between priming and O:Y drops to .08 without them One other finding not listed on Table 2 was reported by Hultsch, Hertzog, Small, McDonald-Miszczalg & Dixon (1992). Even though Hultsch et al. (1991) found a statistically significant age difference in word stem priming (see Table 2), their 1992 study

Old:Young Memory Ratios (Mitchell, Brown, & Murphy, 1990) RATIO 1.2 1.1 1.0 0.9 0.8 0.7 "V 0.6 0.5

- I --~- EXPLICIT I

I

0

1

I

I

I

~ I

I

IMPLICIT I

I

I

[ I

I

I

I

I

I

I.

7

I

I

I

I

I

21

RETENTION INTERVAL (Days) Figure 8. The effect of long retention intervals (days and weeks) on picture naming priming again with O:Y priming ratios as in Figure 7 (calculated from Mitchell, Brown, & Murphy, 1990). The same rationale mentionedunder Figures 6 and 7 applies here, demonstratingthat with less episodic support, the age deficit in priming can be eradicated. did not find an age difference in the same task (ages 65 to 68 and 75 to 78, either crosssectionally or longitudinally), and their sample was large (N = 328). Besides the word stem completion studies, only one other study in Table 2 produced a large and statistically reliable age difference in priming. Although the authors

132

D.B. Mitchell

did not consider contamination, a careful examination reveals some problems. Abbenhuis, Raajimakers, Raajimakers, and van Woerden (1990, Exp. 2) used individually set thresholds for tachistoscopic exposures in a word identification study. Three groups of 14 words were presented once, twice, or three times. The tachistoscopic exposure time was calibrated individually for each subject, starting at 100 msec for young, 200 msec for older adults, and then working up or down by 20-40 msec increments until a criterion near 40% correct identifications was reached. The older adults' threshold was much higher than the young adults' (132 msec and 27 msec, respectively). Even with the longer exposures, the older group had reliably lower priming performance (9.6% vs. 16.6%; their recognition memory deficit was also substantial 63% vs. 88%). However, Abbenhuis et aL reported a hardware problem that may have disproportionately aided the young adults: Their CRT had a ret~esh cycle of 20 msec, so that words were sometimes exposed for 40 msec, and thus "some [young] subjects could read the words too well" (p. 580). The same 20-msec variability around the older adults' mean threshold (132 msec) would not have given them

Word-Stem Priming: Repeated Trials (Dick, Kean, & Sands, 1989) 60

50

40

30

/ f YOUNG

OLDER 20

I

!

I

1

2

3

TRIAL Figure 9. The effect of repeated trials on word stem priming in young and older subjects (Dick, Kean, & Sands, 1989). Here it is argued that while repeated exposure enhanced both implicit and episodic memory, it was the latter that gave the younger subjects the greater boost in priming (cf. Tulving's, 1991, concept of co-determination). any comparable advantage. In addition, there was a substantial educational gap between the two age groups. The young group was composed of university students, with an average of 16.6 years of education. In contrast, the older adults had less than a high school education on the average (mean = 11.2 years). Since other studies have found

Semantic processes in implicit memory

133

correlations between education and cognitive functioning, it would not be surprising if this confound accounted for more of the age difference in priming than aging did. Finally, the fact that a third of the target items were presented three times each (and then combined with twice-presented targets for the analysis, unfommately) may have given the young subjects an edge: That is, three exposures (cs Figure 9, Dick et aL, 1989) may have pushed many of the targets over into a level of awareness that promoted episodic contamination in the young adults. Indeed, their recognition performance for this group of words was near ceiling (mean-92%). 9. AGE INVARIANCE ACROSS THE BOARD? In spite of the compelling data reviewed above, perhaps not all implicit tasks will be so robust with respect to age-invariance. Consider an interesting case: When format is switched between input and test, priming usually suffers. One exception we have found is

Picture Priming (Steen-Patterson, Jones, Brown, & Mitchell, 1993} MSEC 150

120

9O

'UT PRIME II PICTURE ~] WORD

30

YOUNG ADULTS

OLDER ADULTS

Figure 10. The effect of changing format (word to picture) in picture naming priming in young and old adults (Steen-Patterson, Jones, Brown, & Mitchell, 1993). Subjectsread either words or pictures at input, and always named pictures at test. The words matched the pictures' names. that words can prime picture naming just as well as pictures do, which we termed "transfer inappropriate processing" (Brown, Neblett, Jones, & Mitchell, 1991). However, when we test this paradigm with older adults, there is clear evidence of transfer appropriate processing. That is, they show less priming from words than from pictures (see Figure 10, Steen-Patterson, Jones, Brown, & Mitchell, 1993). It's not that words can't prime picture

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naming in older adults. Across a very brief interval (1.2 sec), words primed picture naming m o r e in older adults than in younger adults (see Figure 11, Thomas, Fozard, & Waugh, 1977). The important thing to note, however, is that even w h e n older adults show more priming than younger adults, they still show less cross-format priming. (This is not true, however, for cross-modality priming, as Light et aL, 1992, found similar patterns across age groups; see Table 2. For some explanations of why older adults sometimes show greater priming, see Laver & Burke, 1993, and the chapter by Ober and Shenaut, this volume). Could the age difference in cross-format priming implicate a perceptual breakdown? That is, do older adults have greater difficulty perceiving the object, as opposed to needing more time to retrieve the lexical label? I have argued (Mitchell, 1993) that picture naming is a conceptual task, but aging data may necessitate a revision of that claim

Picture Priming (Thomas, Fozard, & Waugh, 1977) MSEC 300 270 240

1

210 180

INPUT PRIME I

150

PICTURE

WORD

120 90 60 30 0

YOUNG ADULTS

OLDER ADULTS

Figure 11. The effect of changing format (word to picture) in immediate sequential trials in young; and older adults (Thomas, Fozard, & Waugh, 1977). In Figure 10 (Steen et al., 1993), the study and test trials were in blocks separated by a few minutes.

10. CONCLUSIONS We have reviewed data from a slew of studies to answer two questions: 1) Is implicit memory age-invariant? and 2) Can implicit memory be considered a memory system separate from semantic memory? The answer to both questions is affirmative, albeit with different levels of certainty.

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Regarding the first question, we examined the claim that age differences in implicit memory are real but subtle, simply requiring more statistical power for detection. A casual glance at Tables 1-3 and Figure 3 would appear to support this point of view: After all, the majority of the studies report slightly better priming for younger subjects, and a few studies even obtained statistically reliable age differences. But careful scrutiny revealed that every single one of the studies with a statistically reliable age difference could be discounted by methodological problems. The most parsimonious explanation for siLmificant age differences in three word stem completion priming studies (Table 2) was episodic contamination, and additional procedural problems plagued one word identification study (Table 2). Two homophone priming studies that found an "age difference" turned out to have no priming performance at all in the older group. But what of the majority of studies that were successful in producing viable priming in older subjects, but still obtained slightly lower magnitudes? Even excluding the problematic studies, 55 ofthe remaining 88 contrasts (62.5%) have a ratio below 1.0. My sense is that many of those still have contamination effects. For instance, word stem completion dominates the sample, where the proportion of ratios falling below 1.0 is 93%! Clearly, we need a wider sample of implicit tasks in aging research, with greater care taken to avoid tasks and/or procedures that are likely to be affected by episodic contamination. On the second question, we do not yet have enough information on the memory systems. It seems clear, however, that most implicit memory is not really affected adversely by normal aging, and that in combination with findings from other areas of cognitive research, the evidence will clearly indicate the presence of at least one memory system dissociated t~om both semantic and episodic memory. Furthermore, the differential aging effects in implicit memory are also consistent with the view of process-specific age differences (el Allen, Madden, and Slane, this volume). Thus, the notion of general slowing is inadequate to account for the implicit memory phenomena reviewed in this chapter. 11. SUMMARY A large of number of studies of implicit memory in normally aging adults was reviewed. The outcomes of 36 studies produced 97 contrasts between young and older adults, with 18 implicit memory tasks represented. Although some studies obtained statistically significant age differences in implicit memory tasks, these differences were virtually eliminated when episodic contamination was taken into account. That is, in a number of tasks intended to test only implicit memory, in fact subjects used episodic memory to artificially boost their performance. In every case, the majority of these subjects were young adults, thus producing a spurious age difference in implicit priming scores. I concluded that implicit memory performance is not affected by normal aging, and that implicit memory phenomena can be best understood in the context of multiple memory systems. This modular approach allows for selective effects of aging on some systems and processes, leaving other cognitive components intact.

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