EUROPEAN JOURNAL OF COGNITIVE PSYCHOLOGY, 1996, 8 (3), 225± 234
E€ ects of Ageing on Di€ erent Explicit and Implicit
Memory Tasks
Marko Jelicic, Fergus I.M. Craik and Morris Moscovitch
Rotman Research Institute of Baycrest Centre, University of T oronto,
T oronto, Ontario, Canada
An experiment was conducted to investigate the e€ ect of ageing on two
implicit and two explicit memory tasks. Within each memory category
(implicit or explicit), the tasks di€ ered in their reliance on perceptual or
conceptual processing. Large age-related di€ erences were found on the two
explicit memory tasks, regardless of the perceptual± conceptual di€ erence.
Age-related di€ erences were much smaller on the implicit tasks; no di€ erences
were found on the perceptual version (word-fragment completion) but older
subjects showed less priming on the conceptual version (category production).
It is suggested that the dissociation between perceptual and conceptual
priming re¯ ects selective age-related impairments of di€ erent cortical regions.
INTRODUCTION
It is well documented that older adults exhibit poorer performance than
younger adults on most tests of recall and recognition (Craik & Jennings,
1992). Recall and recognition tests require deliberate, conscious recollection of a previous learning episode and have been labelled ``explicit
memory tasks’ ’ (Roediger, 1990; Schacter, 1987). Memory can also be
assessed with tests that do not involve any conscious recollection, socalled ``implicit memory tasks’’. Implicit memory is revealed by a facilitaRequests for reprints should be addressed to Marko Jelicic, Northern Centre for Health
Research, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The
Netherlands.
The work reported here was supported by a Baycrest Women’s Auxiliary Research Fellowship awarded to M.J. and by grants from the Natural Sciences and Engineering Research
Council of Canada to F.I.M.C. and M.M. We would like to thank Nachshon Meiran for
technical assistance and Lars Nyberg for helpful comments on an earlier draft of the manuscript. A preliminary report of the results presented here was given at the Second Annual
Meeting of the Cognitive Neuroscience Society in San Francisco, March 1995.
€ 1996 Psychology Press, an imprint of Erlbaum (UK) Taylor & Francis Ltd
226
JELICIC ET AL.
tion or improvement in performance on a task without the subject
realising a memory test is taking place. An example of an implicit task is
the word-fragment completion test (Tulving, Schacter, & Stark, 1982).
Subjects are presented with words (e.g. ELEPHANT) and are later asked
to complete fragments of words (e.g. _ L _ PH _ _ T). In the wordfragment completion test, priming is evidenced if fragments corresponding
to the study words are completed more often than fragments from words
that were not presented during the study phase. When younger and older
adults are tested with implicit measures of memory, age-related di€ erences
are often small or unreliable (see reviews by Graf, 1990; LaVoie & Light,
1994) .
A number of authors have pointed out that implicit memory tasks can
be broken down further into those relying primarily on perceptual information and those relying primarily on conceptual information for their
successful completion (Blaxton, 1989; Roediger, 1990; Tulving &
Schacter, 1990). Perceptual tasks (e.g. the word-fragment completion test)
challenge the perceptual system by presenting the test cue either rapidly
or in a fragmented form. These tasks are relatively insensitive to the
e€ ects of di€ erent orienting tasks at study, but are greatly a€ ected by the
perceptual similarity between study and test stimulus forms (Jacoby &
Dallas, 1981; Roediger & Blaxton, 1987). On the other hand, in conceptual tasks, the test cues are conceptually rather than physically similar to
the target items. An example of a conceptual implicit memory test is the
category production test. Subjects are presented with exemplars of
di€ erent categories (e.g. animals or ¯ owers) and are later asked to
generate examples of these categories. A match in modality between
study and test is less important for conceptual priming tasks than for
perceptual implicit tasks; however, deeper or more elaborate processing at
study enhances performance on such conceptually based tasks (Blaxton,
1989; Srivinas & Roediger, 1990). The perceptual/conceptual distinction
also applies to explicit memory tasks (Roediger, 1990). Graphemic-cued
recall is a€ ected more by study± test compatibility than by the level of
processing at study, whereas the reverse is the case for free recall. Hence,
graphemic-cued recall may be thought of as an example of a perceptual
explicit task and free recall as an example of a conceptual task.
The present study explored possible adult age-related di€ erences
between perceptually based and conceptually based tasks within the
categories of implicit and explicit memory tasks. There is some reason to
believe that normal ageing is associated with greater ine ciencies in
conceptual than in perceptual processing. This case was argued at the
behavioural level by Craik and Byrd (1982), although their conclusions
have been questioned by other researchers (e.g. Light, 1991). At the level
of neuropsychology , it has been shown that normal ageing is associated
AGEING AND MEMORY
227
with loss of neurons in the frontal and temporal neocortex and in the
hippocampus, with less atrophy in other brain areas (Coleman & Flood,
1987; Terry, DeTeresa, & Hansen, 1987). Given that perceptually based
tasks are probably mediated by the relevant perceptual area of the cortex
(e.g. visual tasks by the occipital cortex) and conceptually based tasks by
association areas in the temperoparietal association cortex (Butters,
Heindel, & Salmon, 1990; Gabrieli, 1991; Moscovitch, 1992; Tulving &
Schacter, 1990), it follows that age-related de® cits should be greater in
conceptual than in perceptual tasks. For example, performance on
visually mediated implicit tests should show negligible age di€ erences,
since they depend substantially on intact occipital lobe functions, whereas
performance on conceptually mediated memory tests (both implicit and
explicit) should show larger age-related de® cits, since they depend
substantially on impaired temporal and parietal lobe functions.
The present evidence for age-related di€ erences in perceptual and
conceptual implicit memory tasks is fragmentary and somewhat inconsistent. A recent meta-analysi s conducted by LaVoie and Light (1994) found
small but reliable age decrements in repetition priming, but no di€ erential
e€ ects of item and associative priming. Similarly, Light and Albertson
(1989) found slightly greater priming for younger than for older adults in
an exemplar generation task (the values were 0.18 and 0.13, respectively),
but this di€ erence was not signi® cant. On the other hand, the largest
e€ ect size in LaVoie and Light’s (1994) meta-analysis of implicit memory
tasks was for spelling bias (d+ = 0.54), and this task clearly involves
semantic processing. Also, Rabbitt (1982) found less facilitation in older
adults in a semantic classi® cation task when words from the same semantically related subclass recurred.
Surprisingly, the one study that directly compared age di€ erences in
perceptually based and conceptually based implicit memory tests (Small,
Hultsch, & Masson, 1995) found age-related di€ erences in the perceptual
test (stem completion) but not in the conceptual test (``fact completion’ ’,
in which subjects were asked general knowledge questions, the answers to
which had recently been presented as part of another task). This conclusion is directly contrary to our own expectations generated from the
preceding arguments and evidence. However, aspects of Small and coworkers’ (1995) study may limit the generality of their result. First, the
e€ ects obtained were statistically reliable but very small; the values of
priming on stem completion for young, middle-aged and older adults
were 0.15, 0.11 and 0.11 respectively (a reliable di€ erence given that
N = 417), and the priming values for the fact completion test were 0.05,
0.06 and 0.04 respectively, yielding a non-signi® cant age di€ erence.
Second, the target words were presented initially as part of a lexical
decision task. As the authors acknowledge, this means that the target
228
JELICIC ET AL.
words received relatively shallow processing at encoding. Di€ erent results
may be obtained with initial semantic processing, especially as Blaxton
(1989) has reported greater levels of priming on fact completion following
a more semantic study task.
In summary, the present study was designed to examine possible agerelated di€ erences on four di€ erent memory tests: a perceptual implicit
task (word-fragment completion), a conceptual implicit task (category
exemplar production), a perceptual explicit task (stem-cued recall) and a
conceptual explicit task (free recall). It is known that word-fragment
completion and stem-cued recall are quite sensitive to modality shifts
between study and test (Craik, Moscovitch, & McDowd, 1994), whereas
category production and free recall are sensitive to depth of processing
manipulations at study (Craik & Tulving, 1975; Srivinas & Roediger,
1990). Based on the results of previous studies (Graf, 1990; LaVoie &
Light, 1994), we expected the major age-related di€ erences to be between
the implicit and explicit tasks, but our reading of the literature led us to
predict larger age di€ erences on the conceptually based than on the
perceptually based implicit task. Such an outcome would contradict the
results of Small and colleagues (1995) but, in contrast to these
researchers, we employed a semantic encoding task in the present study.
METHOD
Subjects
The younger group consisted of 24 subjects (19 females, 5 males) aged
21± 37 years (x = 26 years), 9 of whom were volunteers from the subject
pool of the Rotman Research Institute; the others were undergraduat e
students from the University of Toronto. The older group comprised 24
volunteers (17 females, 7 males) aged 62± 81 years (x = 71 years) from
the subject pool of the Rotman Research Institute. All the subjects lived
in the community and most had English as their ® rst language. None of
the subjects had a history of neurological or psychiatric disorders, head
injury and/or alcoholism. The younger subjects had received 14± 17 years
(x = 15.0 years) of formal education, the older subjects 10± 17 years
(x = 13.2 years). The age di€ erence in formal education was statistically
signi® cant [t(46) = 3.30, P < 0.01]. The Vocabulary Subtest of the
Wechsler Adult Intelligence ScaleÐ Revised (WAIS-R) (Wechsler, 1981;
maximum score = 70) was administered to assess verbal ability. Mean
scores on this subtest were similar for each group, 54.0 for the younger
adults (range 42± 56) and 55.8 for the elderly adults (range 39± 69)
[t(46) = 0.86, NS]. All subjects in the older group reported being in good
health.
AGEING AND MEMORY
229
Materials
Sixty-four stimulus words with a relatively low frequency were selected
for each of the four memory tests. Two sets of 32 words were created for
the word-fragment completion test, the category production test and the
stem-cued recall test. Four lists of 16 words were used for the free recall
test. The mean KucÏ era± Francis (1967) frequency was 20.2 for the target
words of the fragment completion test, 16.9 for the target words of the
category production test, 25.7 for the target words of the cued-recall test
and 25.5 for the target words of the free recall test. The words and test
cues of the fragment completion test, the cued recall test and the free
recall test were chosen from a pool of items developed for use in other
experiments (Rajaram & Roediger, 1993; Roediger, Weldon, Stadler, &
Riegler, 1992). Some examples of the words and fragments of the
fragment completion test are as follows: arrow, _ r _ _ w; diamond,
_ i a _ _ n d; violin, _ i o _ _ n. Most fragments had only one solution.
Examples of the words and stems of the cued recall test are: antenna,
ant___; climate, cli___; spatula, spa___. Each word-stem had at least ® ve
di€ erent completions. The two lists for the category production task
consisted of four members of eight taxonomic categories, with di€ erent
categories represented on each list. Category members were selected from
the less frequent instances generated by younger and elderly adults in
Howard’s (1979) category norming study. Most of the items in the
category production task were the 11th, 12th, 13th, 14th or 15th most
commonly generated exemplars. Examples of the category members are:
gira€ e, goat, mouse, monkey (animals); fraud, kidnapping, mugging,
forgery (crimes).
Procedure
The subjects were told that, although their memory for some items would
be tested, they were taking part in a study carried out to obtain norms
for future research. This was done to encourage the subjects to view the
two implicit memory tests as unrelated to the presentation of the stimulus
words.
Half of the subjects were ® rst given the word lists for fragment completion, category production and cued recall. For these three di€ erent acquisition lists, the subjects were presented with either one of the two sets of
32 words. The other set was used during the test phase to estimate
baseline performance. Each set of words was used equally often as target
words. The two di€ erent versions of the three word lists were counterbalanced across subjects. The items on the three acquisition lists, a total of
111 words (96 targets preceded by 10 ® llers to prevent a primacy e€ ect
230
JELICIC ET AL.
and followed by 5 ® llers to prevent a recency e€ ect), were completely
intermixed and presented as one long list on a computer screen at a rate
of one item every 5 sec. To ensure a good level of memory performance,
all subjects performed a semantic task to maximise encoding of the
stimulus words. They were asked to look at each word carefully and rate
the pleasantness of the word’ s meaning on a 5-point scale (1 = ``very
pleasant’’ , 5 = ``very unpleasant’’). The subjects were requested to give a
verbal response so that the experimenter could record their responses on
an answer sheet. After a 5± 7 min ® ller task (the Vocabulary Subtest of
the WAIS-R), they were given, in a counterbalanced order, the wordfragment completion test, the category production test and the stem-cued
recall test.
The word-fragment completion test consisted of 64 fragments printed
on a sheet of paper. Half of the fragments belonged to study words, the
others corresponded to non-studied words. The subjects were asked to
complete each fragment by producing the ® rst English word that ® tted
the fragment within 15 sec. They were not permitted to give plurals or
proper names. For the category production test, the subjects were
presented with an answer sheet listing 16 taxonomic categories, and
requested to write down the ® rst eight examples of each category that
came to mind. Eight categories related to words presented at study, the
other eight did not relate to the study words. The subjects were allowed
16 min for this test. The stem-cued recall test consisted of 64 three-letter
word-stems printed on a sheet of paper. Half of the word-stems corresponded to target words, the others did not. The subjects were informed
that some of the stems belonged to words that were presented during
the study phase, and were asked to use the stems as cues to recall the
study words. They were told to complete the word-stem only if they
were sure that the stem corresponded to a target word. For each stem,
the subjects were allowed 15 sec to recall the corresponding word from
the study list.
After completing the fragment completion test, the category production
test and the cued recall test, the subjects received a list of 16 words for
the ® rst free recall test. Again they were told that their memory would be
tested for some items, but that their primary task was to judge words on
their pleasantness. After a distraction task (an arithmetic task) of 45 sec
duration to prevent a recency e€ ect, the participants were asked to recall
the items from the list within 90 sec. Next, they were presented with 16
new words for the second free recall test (with the same instructions).
After a second arithmetic task, the subjects were requested to recall the
words from the second list. Again they were given 90 sec for this task.
The four sets of words for the free recall tests were counterbalanced
across subjects.
AGEING AND MEMORY
231
The other half of the subjects ® rst received the two free recall tests
followed by the fragment completion test, the category production test
and the cued recall test. After completion of all the tests, the subjects
were asked whether or not they had been aware that some of the test
cues of the word-fragment completion test and the category production
test related to words from the study list. Finally, the subjects were
debriefed and thanked for their participation.
RESULTS AND DISCUSSION
The scores on the four memory tests for the two groups are provided in
Table 1. Note that the score on the free recall test reported in the table is
the average for the two free recall tests. Both groups demonstrated
reliable priming in the two implicit memory tasks. On the fragment
completion test, fragments belonging to target words were more likely to
be completed than fragments corresponding to baseline items
[t(23) = 7.84, P < 0.001 for the younger adults; t(23) = 8.61, P < 0.001
for the older adults]. On the category production test, target words were
generated more often than baseline items [t(23) = 10.15, P < 0.001 for the
younger adults; t(23) = 7.00, P < 0.001 for the older adults]. Table 1
TABLE 1
Mean (± SD) Proportions Correct on Word-fragment Completion, Category Generation,
Stem-cued Recall and Free Recall in Young and Older Adults
T ask and Measure
Y oung Adults
Older Adults
t(46)
P
Fragment completion
Target
Baseline
Priming
0.47 ± 0.11
0.32 ± 0.12
0.15 ± 0.09
0.39 ± 0.14
0.25 ± 0.11
0.14 ± 0.08
0.47
0.64
Category generation
Target
Baseline
Priming
0.39 ± 0.11
0.16 ± 0.06
0.23 ± 0.11
0.32 ± 0.08
0.15 ± 0.13
0.17 ± 0.11
2.06
< 0.05
Stem-cued recall
Target
Baseline
0.47 ± 0.13
0.00
0.29 ± 0.11
0.00
5.34
< 0.001
Free recall
Target
0.41 ± 0.11
0.25 ± 0.13
4.74
< 0.001
Note: The t-values and associated P-values are for age di€ erences in all cases.
232
JELICIC ET AL.
shows that there was no di€ erence between the younger and older adults
in respect to the amount of priming on the word-fragment completion
test. However, the younger subjects demonstrated signi® cantly more
priming in the category generation test than the elderly subjects. Eight
subjects (two old and six young participants) indicated that they had
been aware that some of the words from the study list corresponded to
the categories from the category generation test. They denied deliberate
production of list members, however. Note that the di€ erence in performance between the two groups on the conceptual implicit task remains
when the aware subjects are not included in the analysis [t(38) = 2.20,
P < 0.05]. As expected, age-related di€ erences on the two explicit tests
were much larger than on either of the implicit tests. However, there were
no obvious di€ erences in the patterns of age-related decrements between
the perceptual and conceptual tasks.
The ® nding that elderly subjects showed large decrements on the two
explicit memory tests is consistent with previous research (Craik &
Jennings, 1992; LaVoie & Light, 1994). Given the hypothesised agerelated ine ciency in conceptual processing, it was expected that older
people would show a greater de® cit in free recall than in stem-cued recall,
but such a result was not found. With respect to the implicit tests, no age
di€ erences were observed in word-fragment completion, in line with many
previous studies (LaVoie & Light, 1994; Light, Singh, & Capps, 1986). A
reliable age di€ erence was found, however, on the conceptually based
category generation task. This di€ erence remained signi® cant when
``aware’ ’ subjects were excluded from the analysis, so the age-related
di€ erence does not appear to be a function of greater ``explicit memory
leakage’ ’ in this case (cf. Light & Albertson, 1989).
The present ® nding of adult age di€ erences in conceptually based but
not perceptually based implicit tests of memory, is in direct contradiction
to the results of Small and colleagues (1995). A comparison of the young
and elderly groups in the two studies shows that priming values on the
perceptual task were 0.15 and 0.11 respectively in Small and co-workers’
study, and 0.15 and 0.14 respectively in the present study. For the
conceptual task, the corresponding values were 0.05 and 0.04 in Small
and co-workers’ study and 0.23 and 0.17 in the present study. Clearly,
the di€ erence between the studies lies in the much higher priming values
for the conceptual task in the present experiment, possibly due to our use
of a deeper encoding task. A two-way analysis of variance was carried
out on the two age groups and two implicit memory tasks. The interaction between groups and tasks was not statistically reliable
[F(1,46) = 2.38, MSe = 0.02], so the di€ erential age e€ ect should be
treated somewhat cautiously. Nonetheless, the present result contradicts
the previous report of Small et al. (1995), who did not report the age ´
AGEING AND MEMORY
233
task interaction. More importantly, we argue that the present result is
more in line with what is known from behavioural and neuropsychological studies; that is, age-related decrements are more likely to be found
on tasks involving deeper, conceptual processing than on those involving
shallow, perceptual processing.
In summary, the contribution of the present study has been to show
that normal ageing yields a pattern of ® ndings similar to (although less
extreme than) that shown in Alzheimer’s disease (Gabrieli, 1991; Keane et
al., 1991) and in patients with temporal-lobe damage (Blaxton, 1992;
Moscovitch & Winocur, 1992). That is, the results show no age di€ erences in a perceptual implicit task, slight but reliable age decrements in a
conceptual implicit task, and large age-related decrements in explicit
memory tasks that are insensitive to the perceptual/conceptual issue. In
this sense, the results contribute to the growing body of evidence linking
behavioural results on memory and other cognitive tasks with their
neurological substrates.
Manuscript received July 1995
Revised manuscript received January 1996
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