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Journal of Experimental Psychology:
Learning, Memory, and Cognition
2000, Vol. 26, No. 5, 1160-1169
Copyright 2000 by ihe American Psychological Association, Inc.
0278-7393/00/S5.00 DOI: 10.1037//U278-7393.26.5.1160
The Properties of Retention Intervals and
Their Affect on Retaining Prospective Memories
Jason L. Hicks
Richard L. Marsh and Edward J. Russell
Louisiana State University
University of Georgia
Five experiments were conducted to explore how the character of the retention interval affected
event-based prospective memory. According to the canons of retrospective memory, prospective performance should have been worse with increasing delays between intention formation and the time it was
appropriate to complete an action. That result did not occur. Rather, prospective memory was better with
increasing retention intervals in Experiments 1 A, IB, and 3. In manipulating the nature of the retention
interval, the authors found that there were independent contributions of retention interval length and the
number of intervening activities, with more activities leading to better prospective memory (Experiments 2 and 3). The identical retention intervals did not improve retrospective memory in Experiment 4.
Theoretical explanations for these dissociations between prospective and retrospective memory are
considered.
memory. Because human intentions vary widely in their nature and
in the means by which they can be accomplished, we are investigating just one type of intention, namely, event-based prospective
memories (e.g., Einstein & McDaniel, 1990). In event-based tasks,
an environmental cue triggers fulfillment of the intention, such as
seeing a pink telephone slip reminding one to return a call. Although other types of intentions have been identified (see Ellis,
1988; Kvavilashvili, 1992), the present study investigates eventbased prospective memory and how forgetting such intentions
might be affected by the nature of the retention interval.
This line of inquiry is not entirely new because the retention
interval has been directly manipulated in several previous studies
of prospective memory. Not all of them have been event-based
tasks, but for generality, we first summarize them to demonstrate
that no uniform agreement exists on the consequences of increasing retention intervals between intention formation and the time an
action is to be carried out. In the earliest study, Loftus (1971)
asked participants to write the state of their birth after completing
a questionnaire. She manipulated the length of the questionnaire
and found that prospective memory was worse after 15 questions
as compared with a shorter retention interval of only 5 intervening
questions. In this case, forgetting of prospective memories obeyed
the retrospective memory forgetting curve with worse performance
occurring at longer delays.
In another study, Meacham and Lciman (1982) had participants
mail postcards back to the experimenter after different delays. A
short delay was defined as 1 to 4 days after receiving the prospective instruction, whereas a long delay was defined as 5 to 8 days
later. The effect of retention interval in this study depended on
whether or not a reminder tag was attached to the participant's key
chain. Without the reminder, performance was better at shorter
delays. With the reminder, performance was better at longer delays. We have interpreted these data as indicating that prospective
performance might mimic retrospective forgetting when no reminders are used (i.e., better at a shorter retention interval and
In the classic studies performed by Hermann Ebbinghaus (1885/
1964), he demonstrated that memory declines with increasing
delays between study and test. By teaching himself lists of 13
syllables to a criterion of two correct recalls, he systematically
varied the retention interval from 20 min to 31 days. Ebbinghaus
discovered that the forgetting curve, as it is now known, was
approximated by a logarithmic function with a fast initial rate of
forgetting over the near term and a slower decline in forgetting as
retention intervals were increased. The forgetting curve appears to
be a very general property of retrospective memory because it is
true of foreign language learning (Bahrick & Phelps, 1987), typing
skills (Baddeley & Longman, 1978) and even cardiopulmonary
resuscitation skills (McKenna & Glendon, 1985). Although there
is no agreement on why this law of forgetting is so pervasive
across different materials, many memory theorists favor interference accounts of retrospective forgetting over decay theories that
depict memory traces as simply crumbling into parts over time
(e.g., Bower, 1967).
The present study investigates whether intentions to perform
activities might be forgotten in a manner similar to the classic
forgetting curve just described. Research concerning memory for
intentions is classified broadly under the rubric of prospective
Jason L. Hicks, Department of Psychology, Louisiana State University;
Richard L. Marsh and Edward i. Russell, Department of Psychology,
University of Georgia.
We thank Rebecca Winkler, Lorie Ritschel, Stewart Oliver, Drew
Shuler, Wendi Hood, and Eric Bryan for their dedicated help in collecting
the data. Experiment 3 served as Rebecca Winkler's honors thesis for an
undergraduate degree in psychology at the University of Georgia. We also
thank Thomas W. Hancock for helpful conversations on this project.
Correspondence concerning this article should be addressed to Richard
L. Marsh, Department of Psychology, University of Georgia, Athens,
Georgia 30602-3013. Electronic mail may be sent [email protected] or
[email protected].
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NATURE OF THE RETENTION INTERVAL
worse at a longer one). However, when one is provided, that
reminder may have a cumulative effect over time such that prospective memory performance is improved as the retention interval
grows longer. Brandimonte and Passolunghi (1994) also found that
forgetting in an event-based task declined rapidly over certain
types of 3-min intervals as compared with being tested immediately. Therefore, the data from three studies suggest that forgetting
of intentions follows the usual laws of retrospective forgetting in
which increasing retention intervals generally result in worse
performance.
However, in other work, the length of the retention interval has
been manipulated without any significant effect on prospective
performance. For example, Wilkins (1976) found no differences
when participants were asked to mail postcards back to the experimenter from 2 to 36 days later. That result contradicts Meacham
and Leiman's (1982) findings using a similar paradigm. Einstein,
Holland, McDaniel, and Guynn (1992) manipulated whether a
target occurred 15 or 30 min after intention formation in an
event-based prospective task. Although the results were suggestive
that performance was worse after a longer delay, the experimenters
found that the effect failed to reach statistical significance. More
recently, Guynn, McDaniel, and Einstein (1998) manipulated
whether an event-based intention had to be fulfilled 4 versus 20
min after the prospective instructions were delivered and again
found no difference. On balance, several studies have found that
forgetting an intention obeys standard forgetting functions,
whereas several others have found that forgetting an intention does
not conform to the traditional regularities of retrospective
forgetting.
A principled analysis of the differences among these studies
proved difficult because one naturalistic task found an effect of
increased retention intervals (Meacham & Leiman, 1982), whereas
another did not (Wilkins, 1976). Likewise, one laboratory-based
task found an effect of increasing delay (Brandimonte & Passolunghi, 1994), whereas two others did not (Einstein et al., 1992;
Guynn et al., 1998). We took seriously Brandimonte and Passolunghi's suggestion that the majority of forgetting in prospective
memory tasks might occur shortly after intention formation (i.e.,
within several minutes). This hypothesis would be consistent with
the rapid forgetting that occurs in the early portion of retrospective
memory functions. If this were true, then perhaps manipulations
of 15 versus 30 min, or even of 4 versus 20 min, might not capture
a decline in laboratory-based prospective memory performance as
tested by Einstein, McDaniel, and their colleagues. By contrast,
Loftus's (1971) questionnaire was quite brief. If completing the
short, 5-question version took only 30 to 60 s, whereas completing
the 15-question version took no longer than 3 min, then perhaps
her study captured a critical window in which memory for intentions was most susceptible to forgetting. Very recent work corroborates significant decrements in prospective memory over the early
portion of retention intervals with losses in less than 1 min (Einstein, McDaniel, Manzi, Cochran, & Baker, in press).
Although there is no a priori reason to believe that prospective
versus retrospective forgetting should obey different psychological
functions, we believe that there are a number of variables that
might dissociate these two types of memories across different
retention intervals. For example, retrospective memories often lack
intentionality, and therefore, there may be no functional need to
refresh or to revisit those memories over a given retention interval.
By contrast, intentions may often need to be refreshed periodically
or reprioritized as decisions are required about fulfilling one intention rather than another (see Marsh, Hicks, & Landau, 1998,
who had participants count such naturalistic rehearsals). In fact,
aspects of Meacham and Leiman's (1982) study suggest that
longer retention intervals afford more thoughts or self-remindings
about prospective memories. As a consequence, revisiting such
intentions might preserve their status or otherwise partially inoculate them from more rapid forgetting.
This hypothesis could explain some of the failures to find
declines in prospective memory over longer retention intervals as
opposed to shorter ones. One could imagine that in the limit case,
as Meacham and Leiman (1982) found in their key tag study, there
might even be conditions in which prospective memory improves
over longer retention intervals. Consistent with such an interpretation, Einstein et al. (1992) found a slight improvement in eventbased performance for older adults when the prospective instructions preceded the event-based task by 30 min as compared with
only 15 min. Because older adults are generally more sensitive to
the fallibility of their memory, they may think more carefully
about the experimenter's instructions after 30 min than after 15
min. The consequence may be to increase their prospective memory performance over longer intervals as opposed to shorter ones.
We return to these ideas in more detail in the course of reporting
experiments that were designed to examine how changing the
character of retention intervals might affect event-based prospective remembering.
By way of overview, the standard Einstein-McDaniel eventbased paradigm was used, in which participants were engaged in
an ongoing cover task. For the present experiments, we used a
pleasantness rating task or a syllable counting task. The retention
intervals investigated were of intermediate length from 2.5 min
after intention formation to 15 min later before beginning the cover
task in which the target events were embedded. Experiments 1A
and IB constituted an initial investigation of whether prospective
memory was sensitive to changes in the nature of the retention
interval. The number of intervening tasks and the length of the
retention interval were confounded in these two experiments,
whereas in Experiment 2, the length of the interval was held
constant to manipulate the number of activities that occurred
during that interval. The design of Experiment 3 extended these
manipulations to crossing orthogonally the length of the retention
interval with the number of intervening tasks. Finally, we designed
Experiment 4 to ascertain whether the characteristics of the various
retention intervals manipulated in Experiments 1-3 would similarly affect two different forms of retrospective memory.
Experiments 1A and IB
The procedures in these two experiments were highly similar
and are reported together for brevity. However, the experiments
were originally conducted to compare performance in two conditions that we hypothesized might affect event-based memory (viz.,
how deeply targets were processed as they were encountered
during the cover task). Therefore, deeper pleasantness ratings and
shallower syllable ratings constituted the cover task. As fate would
have it, that manipulation did not work out as planned, and the
results of manipulating the retention interval became the focus of
our attention. To keep the experimental record complete, we have
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HICKS, MARSH, AND RUSSELL
nevertheless reported the null results of processing depth on the
event-based targets in these experiments.
In both experiments, four event-based prospective memory targets occurred in a series of 104 trials of the cover task. Participants
were told that in addition to the primary task, the experimenter was
interested in their memory to perform actions in the future. When
an animal word appeared, participants were asked to press a key on
the computer keyboard to indicate that they had remembered the
intention. Approximately half of the participants in each experiment started the pleasantness rating and syllable counting tasks 2,5
min after receiving this prospective instruction, and half started the
cover task after a 15-min delay. If event-based prospective memory had forgetting characteristics that are similar to standard retrospective memory, then performance should have been worse
after 15 min than after 2.5 min. However, if the Einstein et al.
(1992), Guynn et al. (1998), and Wilkins (1976) results captured
different forgetting characteristics of prospective memory, then
there ought to have been no significant memory loss after a longer
retention interval as compared with a shorter one.
Experiments 1A and IB differed only in whether the pleasantness rating and syllable counting tasks were administered within
subjects across the 104-trial sequence (Experiment 1A) or were
conducted between subjects (Experiment IB). In Experiment 1A,
randomly switching back and forth between making pleasantness
judgments and counting syllables across trials might have been
somewhat confusing and mentally taxing, thereby deflecting processing resources away from the prospective memory task. Such
task switching is known to involve central executive resources
(e.g., Rogers & Monsell, 1995), and recently. Marsh and Hicks
(1998) have claimed that event-based prospective memory requires some optimal level of such resources. Therefore, making the
pleasantness rating and syllable counting tasks a between-subjects
variable in Experiment IB could have improved overall performance, thereby allowing us to observe performance at a different
level in the response scale.
Method
Participants. All participants were University of Georgia undergraduates who volunteered in exchange for partial credit toward a course
requirement. Each was tested individually. Of the 61 volunteers in Experiment 1A, 31 were assigned to the 15-min retention interval between the
prospective instruction and the commencement of the cover task,
whereas 30 participants served in the 2.5-min condition. In Experiment
IB, 48 new volunteers were recruited. Twelve participants were assigned
to the each of the four cells that resulted from orthogonally crossing
(between subjects) the two levels of the retention interval (2.5 vs. 15 min)
with the two types of processing task (pleasantness rating vs. syllable
counting).
Materials and design. One hundred and four concrete nouns of medium frequency were selected from the Kucera and Francis (1967) compendium. Four of these words represented animal names (e.g., lion) and
were randomly assigned anew for each participant to occur on trials
numbered 25, 50, 75, and 100. The remaining trials in the 104-trial
sequence were also randomly assigned anew for each participant. In
Experiment 1 A, participants made pleasantness judgments of the stimuli on
a 5-point Likert scale for half of the trials (i.e., 52), and for the remaining
half, they counted the number of syllables. The same processing dichotomy
was true of the four prospective memory trials. In Experiment IB, participants performed either the pleasantness rating or the syllable counting task
on all 104 trials depending on their assigned condition.
Procedure. Participants read the instructions for the cover tasks from
the computer screen. The experimenter reiterated these instructions and
informed the participants verbally of the prospective memory task. Participants were asked to press the / key when an animal name appeared in the
cover task to indicate that they had remembered the intention. After the
prospective instruction, participants in the 2.5-min retention interval condition rated how humorous Gary Larson's Far Side cartoons were on a
5-point Likert scale. The experimenter timed the interval with a handheld
stopwatch. In the 15-min retention interval condition, participants took the
Remote Associates Test (RAT) and solved the Raven Progressive Matrices
(a nonverbal task abbreviated as Ravens). The RAT was modified to
remove any questions containing animal words.1 Similar to Guynn et al.
(1998), manipulating the length of the retention interval confounded the
number and the type of intervening activities in Experiments IA and IB.
However, this issue is directly addressed by the manipulations reported in
subsequent experiments.
After the retention interval, participants were engaged in the cover task.
No mention was made of the prospective instruction that had been delivered earlier. On each trial, a warning tone and fixation point (•) appeared
for 500 ms. The query "SYLLABLES?" or "PLEASANTNESS?" replaced
the fixation point on the screen and remained there alone for 500 ms prior
to the actual stimulus word that appeared two lines beneath it. The two
queries were random across the trial sequence for the within-subjects
manipulation in Experiment 1 A, whereas in Experiment IB, one of the two
queries appeared consistently for a given participant. In both experiments,
the query and the stimulus word remained on the screen until the participant responded with a number from 1 to 5. For the prospective memory
trials, participants had been asked to press the / key prior to making their
judgment. The computer recorded successful prospective memory performance as well as the ratings and the counts entered on each trial. The
computer would have recorded late responses on subsequent trials if the
participant had remembered the prospective memory task during the 1-s
intertrial interval or on subsequent trials, which was the rationale for
having Trials 101 through 104 occur after Trial 100, on which the final
prospective response could have been made. In practice, such late responding never occurred.
Results and Discussion
Unless specified otherwise with a p value, all statistical tests are
significant at the conventional 5% alpha error rate. Standard errors
are provided in the tables of results. Table 1 summarizes the results
of Experiments 1A and IB as the average proportion of trials on
which participants remembered the prospective instruction. Each
experiment is considered in turn. For Experiment 1A, a 2 X 2
mixed-model analysis of variance (ANOVA) was conducted with
the processing task (syllable vs. pleasantness) as a within-subjects
variable and the length of the retention interval (2.5 vs. 15 min) as
a between-subjects variable. Performance differed as a function of
retention interval, F{\, 59) = 4.62, MSE = .18, but neither the
processing task nor the interaction was significant, F{\, 59) < 1.0
for both. The null effect of the depth of processing task is not
considered further.
As the reader must have noticed, the results from Experiment
1A confirmed neither of the a priori predictions concerning how
the length of the retention interval might affect prospective mem1
For consistency, at the end of the experiment we did administer the
Remote Associates Test and the Raven Progressive Matrices to participants
in the 2.5-min condition. We do not report the data from either test because
they did not ultimately make any interesting contribution (e.g., correlations
with performance) to the main questions addressed in this article.
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NATURE OF THE RETENTION INTERVAL
Table 1
Prospective Memory Performance Expressed as Proportions in
Experiments 1A and IB
Retention interval
Experiment and
processing task
Experiment 1A
Syllable
Pleasantness
Experiment IB
Syllable
Pleasantness
2.5 min
15 min
.20 (.06)
.18 (.06)
.39 (.07)
.32 (.07)
.44 (.11)
.38 (.07)
.60 (.10)
.63 (.09)
Note. Standard errors are in parentheses.
ory performance. On the basis of the retrospective memory literature, we believe that better performance would have been predicted with a shorter delay between establishing the intention and
the commencement of the cover task. Alternatively, if prospective
memories were more resistant to forgetting, then performance
might have been the same across the two different retention
intervals. In contrast to these two predictions, event-based prospective memory performance was found to be better after a longer
retention interval.
The results from Experiment IB constitute a conceptual replication and are summarized in the lower half of Table 1. In the
2 X 2 between-subjects ANOVA, the retention interval factor was
statistically significant, F(l, 44) = 5.40, MSE — .12, whereas
neither the main effect of processing task nor the interaction was
significant, F(l, 44) < 1.0 for both. As in Experiment 1A, eventbased prospective memory was better after a longer retention
interval than after a shorter delay. Thus, both experiments replicated one another in all of their essential properties. As we had
anticipated, making the processing cover tasks a between-subjects
variable in Experiment IB increased prospective memory performance as compared with Experiment 1A, F(l, 107) = 5.04,
MSE = .11. That finding is consistent with Marsh and Hicks's
(1998) claim that event-based memory can require some central
executive resources if randomly switching back and forth between
tasks places some demands on the central executive (e.g., Rogers
& Monsell, 1995). We corroborate this interpretation with the fact
that average reaction times to respond to the cover task components were longer in Experiment 1A (2,135 ms, SE — 86), which
required randomly switching between tasks, as compared with
Experiment IB (1,829 ms, SE = 83), which did not require this
switching, f(107) = 2.52, p < .05.
When we initially obtained these results, we found them to be
somewhat counterintuitive. One theoretical possibility is that participants were reminding themselves during the retention interval
about the prospective memory task. However, anyone who has
taken the RAT and the Ravens knows that they are both very
cognitively demanding tasks (at least as compared with cartoon
ratings). Thus, it is unclear how more engaging distractor activities
would result in increased self-reminding and better prospective
memory. Informal postexperimental questions were asked in Experiment IB concerning self-remindings that occurred during the
retention interval. Participants in both the 2.5- and 15-min conditions reported that they had thought about the intention, but the
data suggest that they did so with equal frequencies. Participants in
both conditions often reported that they had thought about Che
intention "once or twice." Although such observations are suggestive, they do not constitute definitive evidence of the frequency of
such thoughts.
A second possibility is that participants in the 15-min retention
interval condition engaged in self-reminding as they changed
distractor tasks from the RAT to the Ravens. Either the short break
might have afforded such a reminding, or the instructions for the
new distractor task might have served as an explicit reminder of
the prospective instruction. A third possibility is that after longer
delays of 15 min, participants reflected more deeply on the original
instructions and mentally refreshed them to a greater degree on
starting the cover task than did participants in the 2.5-min condition who might have felt that all of the instructions were still fresh
in their memories.
Although the data from Experiments 1A and IB do not disambiguate these alternatives, they do reveal the important confound
that participants assigned to the 15-min condition performed two
tasks (RAT and Ravens), whereas participants tested in the 2.5min condition performed only one (cartoon rating). Such a confound is not without precedent (e.g., Guynn et al., 1998). Moreover, Sellen, Louie, Harris, and Wilkins (1997) conducted a
naturalistic prospective memory experiment that is relevant to this
issue. In their study, office workers pressed electronic badges
when they thought about unfulfilled intentions or when they actually carried them out. One interesting outcome was that workers
more frequently remembered to fulfill intentions and to engage
themselves in self-remindings during downtimes such as when
they were in corridors or stairwells as opposed to when they were
occupied at their desks. Sellen et al. argued that people may
mentally consider unfulfilled intentions during natural breaks in
ongoing activities. If this theoretical position were true, then
prospective memory may have been better at longer retention
intervals in Experiments 1A and IB, because two intervening
activities constituted the 15-min delay rather than one activity in
the shorter, 2.5-min condition. We designed Experiment 2 to
explore this possibility by holding the length of the retention
interval constant and varying the number of distractor tasks that
occurred during that interval.
Experiment 2
The character of the retention interval was manipulated in this
experiment by having participants work on a single activity for 15
min or by having them work on five different tasks for 3 min each.
We also tested a third condition in which the retention interval was
filled with a single 15-min activity, but unfilled breaks were given
that identically corresponded to the position and duration of the
instructions in the condition with five different 3-min tasks.2
Comparisons among these three conditions discriminate among the
following possible alternative hypotheses. First, if the number of
breaks during the retention interval provides opportunities for
self-remindings about the prospective task, then event-based performance might be better with five segmentations of the retention
2
We thank Gil Einstein for suggesting that this condition be tested to
constrain the theoretical interpretations of the data.
1164
HICKS, MARSH, AND RUSSELL
interval as opposed to one. In this case, performance in the fivetask condition and in the single-task with unfilled breaks condition
would be equivalent, and both would be better than the single-task
condition.
Second, if prospective memory were equivalent in these two
conditions in which unfilled breaks occur and the distractor tasks
are changed, then this would suggest that participants do not
opportunistically reflect more often on the prospective task during
unfilled downtime. Alternatively, if prospective memory were
better with unfilled breaks as opposed to transitions that are filled
with instructions for the next distractor activity, then that result
would indicate that unengaged downtime is more beneficial to
prevent forgetting an intention than are filled transitionary periods.
Still another possible outcome is that performance might be better
in the five-task condition as compared with the single-task condition in which unfilled breaks occur, in which case some evidence
would be obtained that instructions for the new distractor tasks
may serve as a reminder of the prospective instruction.
Method
Participants. Seventy-five University of Georgia undergraduates volunteered in exchange for partial credit toward a course requirement. Each
participant was tested individually. Fifty participants were tested with a
retention interval composed of a single activity, and 25 participants were
tested with a retention interval composed of five different tasks. Participants assigned to the single activity were equally divided into two conditions. One group performed the single activity uninterrupted, whereas the
other group was given four unfilled breaks that were equivalent in position
and duration to delivering the task instructions in the five-task condition.
Thus, three experimental conditions were tested.
Materials. The same 104-trial sequence was used again, but participants made only pleasantness ratings throughout. Four event-based targets
were randomly assigned as they had been in Experiments 1A and IB. The
retention interval was always 15 min. In the two single-task conditions,
participants solved vocabulary problems consisting of picking synonyms
and antonyms of words from four possible alternatives (the Extended
Range Vocabulary Test, Version 3; French, Ekstrom, & Price, 1963). In the
five-task condition, we administered 3 min of the same vocabulary test just
prior to engaging in the cover task. In this fashion, all participants in all
conditions were solving vocabulary problems during the 3 min just prior to
engaging in the cover task in which the event-based targets were embedded. The four remaining tasks were paper-and-pencil measures. In this
five-task condition, all distractor tasks lasted for 3 min (for a total of 15
min), and they were administered in a fixed order.
The first distractor task was solving three-digit multiplication problems
by longhand. The second task was Ruffs 2 and 7 crossing-out task (a
sustained attention task). In this task, participants were presented with
blocks of random digits (with three rows of digits per block) and were
required to cross out all of the 2s and 7s, working from left to right and top
to bottom as quickly but as accurately as they could. The third task was
taken from Experiments 1A and IB and required participants to rate how
humorous cartoons were. The fourth task was an anagram solution task in
which participants identified jumbled English words and wrote the solution
next to the anagram. The fifth task was the vocabulary test.
Procedure. Participants read instructions for the pleasantness rating
task from the computer screen. The experimenter reiterated them and
delivered the prospective instruction concerning remembering to press a
key when an animal occurred during the rating task. In the five-task
condition, the experimenter explained each activity and timed the 3-min
interval for each task with a handheld stopwatch. The instructions took
only about 15 s for each task. In the two single-task conditions, the
vocabulary test was administered for 15 min (timed by stopwatch). One
group was required to work on the vocabulary problems for 15 min without
interruption. The other group was stopped every 3 min and asked to take
a short break, which the experimenter timed at 15 s. Functionally, the
15-min interval in the five-task condition and in the single-task condition
with unfilled breaks was slightly longer by approximately 1 min. All other
procedural aspects were identical to those reported before.
Results and Discussion
We did not score performance on the distractor tasks because we
could not see how they would be directly related to event-based
performance in any theoretically important way.3 Although the
retention interval was constant at 15 min in all three conditions, the
nature of the distracting activities affected prospective memory,
F(2< 72) = 8.60, MSE = .09. Performance was worst in the
single-task condition (0.27 ms, SE = 0.06), better with five intervening activities (0.48 ms, SE = 0.07), and best with a single
distractor task that was segmented with unfilled breaks (0.62 ms,
SE — 0.05). Simple contrasts between pairs of conditions verified
that prospective performance was worse in the uninterrupted
single-task condition as compared with the two remaining conditions, smaller f(48) = 2.18. Obviously, prospective memory was
better either when tasks were switched during the retention interval
or when participants were given unfilled breaks during that period.
Simple comparison of the five-task condition to the single-task
condition with unfilled breaks did not reach conventional significance. Nevertheless, the 14% difference in performance strongly
suggests that the number of intervening activities, per se, may not
be the variable critical to improving prospective performance.
Rather, the downtime between these multiple tasks (when new
instructions are delivered for the next activity) may afford participants with the opportunity to engage in some psychological process that heightens or refreshes intentions about the upcoming
cover task and the embedded prospective memory task. The fact
that performance was numerically (but not statistically) better in
the single-task condition with unfilled breaks as compared with
delivering instructions in the five-task condition could suggest that
this putative refreshing or reminding process benefits from greater
attentional resources being available to it. However, the data
clearly contravene the hypothesis that instructions for the multiple distractor tasks reminded participants of the prospective
instruction.
There are a number of different ways to conceive of the results
from this experiment. Perhaps the most straightforward interpre3
Jason L. Hicks did examine distractor task performance to ensure that
all participants performed the tasks as they had been instructed. Every
participant was in complete compliance with instructions, perhaps as a
consequence of sitting next to the experimenter, who was watching them.
Nonetheless, some readers may believe that the activities in the five-task
condition were not as difficult as the vocabulary task used in the single-task
condition or perhaps that these other, multiple tasks were more interesting
and engaging. It is not clear to us that longhand multiplication, a crossingout task, or solving anagrams are subject to either criticism. Our goal was
simply to use multiple tasks that were less homogeneous than those used by
Guynn et al. (1998) in their four-task condition. We do acknowledge that
a confounding variable might have been introduced by testing multiple
versus single tasks over identical retention intervals. However, to observe
the effects of single versus multiple tasks on prospective memory performance, it is impossible to avoid this confound.
NATURE OF THE RETENTION INTERVAL
tation is to assume that participants who were switched between
distractor tasks (and perhaps who were given unfilled breaks) had
time to engage in self-remindings about the cover task and the
prospective instructions. If such self-remindings occur consciously, then this position would be consistent with SelLen et al.'s
(1997) theory that people think about intentions more often during
downtimes in which one task has been completed and another one
has yet to be started. Although it is entirely unclear whether such
self-remindings do occur consciously, the fact that unfilled breaks
during a single distractor activity resulted in numerically better
performance as compared with filled breaks between multiple
activities suggests that these self-remindings may reach conscious
awareness. This speculation rests entirely on the notion that more
resources would be available to conscious processes when attention was not divided (i.e., unfilled breaks) as compared with when
attention was more occupied (i.e., with new task instructions).
Together, the results of Experiments 1A-2 suggest that prospective memory performance is better after a longer retention interval
if and only if that longer interval is punctuated with changes in
distractor activities or with unfilled rest breaks. Unfortunately, the
findings thus far stand in stark contrast to previous investigations
that have found either a decrement to prospective memory performance with longer retention intervals or no effect at all from
manipulating delay. Clearly, a single experiment is needed that
examines the conjoint effect of the number of intervening activities
and the length of the retention interval. Experiment 3 was designed
to tease apart these two factors.
Experiment 3
In Experiment 3, the length of the retention interval was crossed
orthogonally with the number of intervening activities constituting
the delay. Previously in Experiments 1A and IB, the retention
interval was either 2.5 or 15 min. In this experiment, those two
retention intervals were augmented with a third intermediate one
of 5-min duration. We repeated the manipulation used in Experiment 2 of having participants perform either one distractor task
(the vocabulary test) or five different activities (as described
earlier). With three different retention intervals (2.5, 5, and 15
min) and two different numbers of tasks (one vs. five), this
experiment required that six groups of participants be tested. On
the one hand, if the number of intervening activities (or the
transitions between them) were the critical variable leading to
better prospective memory, then performance in the five-task
conditions should have exceeded that in the single-task conditions.
Such a result would suggest that punctuation or transition periods
during the retention interval benefit prospective memory. On the
other hand, if downtime caused by transitions were not the only
causal variable affecting prospective memory, then performance
may have increased as retention interval was lengthened regardless
of whether one versus five activities filled that interval.
Method
Participants. One hundred and forty-four undergraduates enrolled at
the University of Georgia volunteered in exchange for partial course credit.
Participants were added to the six experimental cells by arbitrarily numbering the cells 1 through 6 and adding each successive participant to
successive cells repeatedly until 24 participants were tested in each treatment combination. Because of the large size of this experiment, we used
1165
this procedure to minimize any differences in the testing population either
across or within the academic terms in which it was conducted.
Procedure. The event-based cover task with 104 trials, in which participants made pleasantness ratings throughout, was used again. After the
instructions were read from the computer screen, the experimenter reiterated them and verbally delivered the prospective instruction. In the three
single-task conditions, participants were given the vocabulary distractor
activity consisting of synonym and antonym problems. The experimenter
timed the retention intervals with a stopwatch and ended them after 2.5, 5,
or 15 min according to the assigned condition. In each of the five-task
conditions, participants performed the various activities described in Experiment 2. In the 2.5-min five-task condition, participants performed each
task for 30 s. In the 5-min condition they performed each distractor activity
for 1 min each. Participants in the 15-min condition performed each task
for 3 min apiece. Although the instructions for each task were trivially easy
to explain, we do acknowledge that functionally these retention intervals in
the five-task conditions were not precisely 2.5, 5, and 15 min, respectively,
and were probably just a bit longer as we acknowledged in the previous
experiment. We do not believe that this slight inaccuracy in timing is a
theoretically important variable. After the retention interval, all participants
engaged in the pleasantness rating cover task without any mention of the
prospective memory task.
Results and Discussion
The average proportion of event-based prospective responses is
summarized in Table 2. A 2 (number of intervening activities) X 3
(length of the retention interval) between-subjects ANOVA was
conducted on the data. As can be seen in Table 2, prospective
memory improved as the length of the retention interval increased,
F(2, 138) = 3.82, MSE — .09. Event-based performance was also
better in the five-task conditions than it was in the single-task
conditions, F(l, 138) = 4.75, MSE = .09. These main effects were
not qualified by an interaction between the two variables, F(2,
138) < l.O.4 The purpose for conducting this experiment was to
untangle the independent contributions of the length of the retention interval from the number of intervening tasks. As in the
previous experiments, retention intervals filled with more activities
resulted in better prospective memory performance. Had number
of tasks been the only variable tested in this experiment, one might
have concluded that prospective memory uniquely benefited from
the punctuated downtime that occurred as a result of changing
activities during the retention intervals. However, in each of the
single-task and five-task conditions, prospective performance improved with longer retention intervals. That outcome suggests that
when the number of breaks is held constant over retention intervals, independently increasing the delay can still improve prospective memory performance.
The effects of both lengthening retention intervals and increasing the number of activities could be explained theoretically by
increased opportunities to engage in self-remindings. One problem
with verifying this theory is that it is impractical to observe and to
record self-remindings directly. We have attempted to do so pre4
Some readers may note that performance was better in this experiment
than in the previous one. Because the same experimenter conducted both
experiments, we have no ready explanation for this difference. However,
great care was taken to assign participants to cells in this large-scale
experiment. The important point is that regardless of overall performance
differences across experiments, the relative differences between conditions
within Experiments 2 and 3 conceptually replicated one another.
1166
HICKS, MARSH, AND RUSSELL
Table 2
Prospective Memory Performance Expressed as Proportions in
Experiment 3
Retention interval
Condition
2.5 min
5 min
15 min
Single-task
Five-task
.52 (.08)
.59 (.00)
.57 (.05)
.69 (.00)
.66 (.06)
.79 (.00)
Note. Standard errors are in parentheses.
viously in a naturalistic paradigm but admittedly were open to the
criticism that recording self-reminders is itself a prospective memory task (see Marsh, Hicks, & Landau, 1998). Sellen et al.'s (1997)
study is subject to the same criticism. An alternative explanation is
that filled retention intervals with multiple events are perceived as
psychologically longer in duration than intervals with fewer activities or stimulus events (for a review, see Friedman, 1993). By this
view, transitions between changing activities functionally segment
the interval, increasing the perceived duration of the passage of
time. We did conduct an experiment in which we measured selfremindings and also obtained estimates of the perceived duration
of the retention interval. As we had feared, having participants
record their self-remindings fundamentally changed their prospective memory performance (by equating responding in each of the
three retention interval conditions tested in Experiment 2). However, consistent with the hypothesis that multiple tasks increase the
perceived duration of the retention interval, the 15-min five-task
condition was perceived as longer (14.6 min) than the 15-min
single-task condition (11.8 min). Given these difficulties of measuring self-remindings, in this next experiment, we addressed the
question of whether the characteristics of retention intervals that
have been shown to affect prospective memory might similarly
affect retrospective memory.
Experiment 4
The results obtained thus far suggest that the retention function
for prospective memories might be different from retrospective
memories. One principal difference between prospective and retrospective memory tasks is that in the former, participants know
that they will have to recall or otherwise use information at some
later point in time. The same is not necessarily true of retrospective
memory. If self-remindings contribute to the retention functions
observed in Experiments 1A-3, then similar processes might be
operative over a retention interval if participants are informed
specifically about what aspects of their retrospective memories
will be tested later. The objective of Experiment 4 was to test this
idea.5
More specifically, we designed this next experiment to test the
hypothesis that retrospective memory could also improve as a
function of the character of the retention interval if participants
knew exactly what aspect of their memories would be important in
the later test. Theoretically, participants could use the same opportunities for rehearsal or self-reminding to preserve material that
had already been learned (and perhaps improve memory as well).
To replicate the most essential properties of the prospective memory conditions in Experiments 1A-3, we had the participants learn
a list of words in which four animals were embedded. Immediately
after learning, they were informed that we were particularly interested in their ability to recall the animals later. Retention intervals
of 5 min and 15 min were implemented and compared with an
immediate assessment of retrospective memory. At the 15-min
retention interval, two conditions were tested during which participants either performed a single task or five 3-min tasks (cf.
Experiments 2 and 3). Because event-based retrieval of intentions
is probably a blend of free recall processes and cued retrieval, we
tested separately both free recall and cued recall of the study list.
Testing cued recall necessitated that the study list be categorized in
some manner. If self-remindings or rehearsal occurs over the
retention interval, then retrospective memory may not decline and
may be improved with increasing retention interval and also as a
function of the number of intervening activities.
Method
Participants. One hundred and sixty-six volunteers from the University of Georgia received course credit for participating. Eighty-three were
tested with free recall, and the remaining 83 were tested with cued recall.
All participants arrived at die laboratory in small groups of two to eight
people. Within each type of test, we tested four retention functions:
immediate, 5-min single-task, 15-min single task, and 15-min five-task
conditions. The group testing procedure did not make it entirely feasible to
equate cell sizes in die eight conditions, and the number of participants
assigned to each cell is reported in Table 3.
Materials. A list of 40 words was prepared that was composed of 4
words from each of 10 categories. The 4 animal words used in Experiments
1A-3 constituted the primary categoiy of animals. Nine other categories
were chosen from the Battig and Montague (1969) norms, and further, 4
words were selected from the eight most frequent associates in each of
these categories. The 40 words were randomly mixed by a computer
program into a single order and transferred to an audiotape by a female
speaker. Distractor materials for the retention interval manipulations were
identical to those used previously. Free recall was collected on a sheet of
paper with instructions at the top to write down as many words as could be
remembered. The same instructions were printed at the top of the cued
recall sheet, but in addition, the names of the 10 categories appeared in two
columns.
Procedure. Preliminary instructions were given to each group that
their memory for a list of words would be tested. Therefore, learning was
intentional. Participants listened to the study list played from a tape deck
at the front of die room. The words were spoken at a 5-s rate, and a short
tone preceded each word. After list presentation, the experimenter announced that we were particularly interested in people's memory for the
animal words that had been spoken during the list. Participants were also
informed that their memory for these animal words would be tested later.
If participants were going to engage in processes that benefited retrospective memory for the animal words, men these instructions should have
promoted the use of those processes. Participants who had been assigned to
the immediate retention interval conditions were then handed a cued recall
or a free recall sheet. All participants in a given testing session performed
the same task. The participants in the 5-min and 15-min single-task
conditions were handed a packet of vocabulary problems, whereas the
participants in the 15-min five-task condition were handed a packet containing the tasks described earlier. Participants worked on their assigned
tasks with the experimenter timing activities with a stopwatch and changing the task as needed in the 15-min five-task condition.
5
We thank Gus Craik and Thomas Goschke for highlighting the importance of testing such conditions.
1167
NATURE OF THE RETENTION INTERVAL
Results and Discussion
The data were scored for any evidence of interesting intrusions
and repetitions, but such errors were too negligible to analyze
meaningfully. The results are summarized in Table 3 as the proportion of nonanimal words retrieved (out of 36) and animal words
recollected (out of 4). For free recall in the upper half of Table 3,
we found that recall of nonanimal words declined with increasing
retention interval, F(3, 82) = 3.53, MSE = .02, as did the number
of animal words, F(3, 82) = 4.71, MSE = .06. Average performance indicates that the immediate and the 5-min retention interval conditions were similar to one another, whereas the two 15-min
conditions were similar. This outcome was true for animal words
that participants had been warned would be tested as well as for the
remainder of the study list. These observations were confirmed by
Student-Newman-Keuls post hoc tests. It appeared that loss of
animal words from memory was greater in the two 15-min intervals (about 20%) than was the loss of nonanimal words (about
10%). However, in a 2 (type of word) X 4 (retention interval
condition) mixed-model ANOVA, the interaction failed to support
this observation, FQ, 79) = 1.55, ns. Obviously, animal words
were better recalled overall as compared with nonanimal words as
a consequence of the instruction to remember them, H I ,
79) = 27.96, MSE - .03. Therefore, with a free recall measure of
retrospective memory, the critical outcome is that animal words
were more poorly recollected with longer retention intervals, and
in addition, the number of intervening tasks performed during that
interval did not have any effects as it did with prospective memories in the previous experiments.
Cued recall performance is summarized in the lower half of
Table 3. As can be seen there, no differences were observed as a
function of the type of word (animal vs. nonanimal) and importantly, as a function of the length or character of the retention
interval (all F values were less than 1.0). The fact that category
cuing attenuated loss from memory is hardly surprising, but the
null differences observed here with retrospective memory do not
parallel the improvements in prospective memory observed with
increasing intervals or the number of intervening tasks. With cued
recall, the retention functions for nonanimal words were identical
Table 3
Proportion of Nonanimal Versus Animal Words Recollected as a
Function of the Type of Test and Retention Interval in
Experiment 4
Type of word
Test type and condition
Free recall
Immediate
5-min single-task
15-min single-task
15-min five-task
Cued recall
Immediate
5-min single-task
15-min single-task
15-min five-task
n
Nonanimal
18
22
24
19
.56 (.04)
.54 (.02)
.46 (.03)
.44 (.05)
.74
.75
.52
.57
(.04)
(.03)
(.06)
(.07)
24
22
17
20
.69 (.02)
.70 (.02)
.67 (.02)
.70 (.02)
.69
.69
.72
.76
(.04)
(.04)
(.04)
(.04)
Note. Standard errors are in parentheses.
Animal
to animal words, which participants had been warned were of
particular interest and would be tested. Therefore, regardless of
whether one wants to argue that the properties of an event-based
prospective memory task are more similar to cued versus free
recall (see Einstein & McDaniel, 1996), the retention function for
retrospective versus prospective memories appears to be dissociated by the manipulations investigated in this article. We turn now
to considering what the results of these five experiments might
indicate more broadly about event-based prospective memory.
General Discussion
The results from Experiments 1A and IB suggest that eventbased prospective memory is better following a longer rather than
a shorter retention interval. By serendipity rather than by design,
the longer delay was confounded with two intervening activities,
whereas the shorter delay had only one (cf. Guynn et al., 1998). In
Experiment 2, the retention interval was held constant at 15 min,
and performance was better with five intervening activities rather
than one. In that same experiment, prospective memory was also
numerically (but not statistically) better with unfilled breaks during the interval as compared with transitions filled with instructions. Because the retention interval was fixed, both numbers of
activities or transitions between them were shown to be an important determinant of prospective performance. In Experiment 3,
increasing the number of distractor activities and increasing the
duration of the retention intervals both independently improved
event-based prospective remembering. Identical manipulations of
delay did not stem the losses of retrospective memories in Experiment 4 and certainly did not improve performance as a function of
increasing the delay or the number of intervening activities. Therefore, these data converge on the notion that retention functions for
prospective and retrospective memories can be different at least
with the procedures used here.
Originally, we had not anticipated that prospective memory
would improve as retention intervals grew longer. After all, consistent with standard canons of retrospective memory, several
studies have reported that prospective memory is worse after
longer delays. By contrast, the present results, in conjunction with
studies that have not found any decrement with increasing retention intervals, could suggest that intentions are stored in memory
with different characteristics than are retrospective memories. For
example, Goschke and Kuhl (1993) argued that the representation
of intentions in memory may be privileged in terms of their natural
level of activation. In their paradigm, participants learned pairs of
short scripts. After learning, participants were told which script of
the pair they would have to perform later, thereby creating an
intention to perform those actions. Words from the to-beperformed script were responded to more quickly as compared
with words from the neutral script about which there was no
intentionality (see also Marsh, Hicks, & Bink, 1998; Marsh, Hicks,
& Bryan, 1999). These results suggest that intentions may reside in
memory with heightened activation or that they can be revived
faster from their baseline resting state. Extending the logic from
those earlier reports, we contend that if prospective memories were
to have a privileged status in memory prior to their completion,
then perhaps they would be somewhat more immune to forgetting
and therefore might be more resistant to the effects of lengthening
a retention interval.
1168
HICKS, MARSH, AND RUSSELL
The possibility that intentions have privileged status in memory
is a thesis that is consistent with the notion that these memories
could be more available to the cognitive processes that are responsible for self-remindings. Concerning such processes, we have
argued that there could be at least three distinct cognitive mechanisms contributing to increases in prospective memory performance associated with changes in the retention interval. First,
lengthening the retention interval might increase the probability
that the intention comes to mind independent of the number of
intervening activities. If increasing the length of the interval has a
cumulative effect, then performance might improve over longer
delays. Second, and related, as the number of intervening activities
is increased, thoughts concerning the prospective task may occur
with greater frequency during breaks, transitions, and other downtimes (Sellen et al., 1997). In support of these first two mechanisms, we found independent contributions of the length of the
retention interval and the number of intervening distractor activities in Experiment 3. Third, filled or segmented retention intervals
are often perceived as psychologically longer than intervals filled
with fewer or less densely occurring events (Friedman, 1993).
Such differences in perceived durations could make the cover task
and prospective instructions feel older and therefore encourage
people to reflect more deeply on the original instructions as retention intervals grow longer and as more intervening activities occur
since intention formation.
These are several of the most likely mechanisms by which
event-based memory could be affected by the character of the
retention interval. In emphasizing these possibilities, we have
largely appealed to how manipulations of the retention interval
might change the frequency of self-remindings. We have done so
because across all of the tasks and conditions reported here, the
prospective instruction and the cover task itself were identical
(except in Experiments IA and IB in which performance was not
affected). The only thing that differed was the character of the
retention interval. Therefore, the differences in prospective memory must be theoretically linked to the changes associated with the
cognitive processing that is occurring during the retention interval.
Appealing to self-remindings may be premature (or even incorrect)
because very little is known about the cognitive processing associated with them. Marsh, Hicks, and Landau (1998) did have
participants record the number of times per day (over a 3-day
period) that people thought about their natural intentions or made
decisions about completing one intention rather than another. In
that study, cognitive processes associated with prospective memory occurred quite frequently and, in some conditions, averaged
over 18 to 20 times per day per person. Thus, those results are
entirely consistent with the notion that intentions as memories
might receive more attention, or at least come to mind more
frequently or readily, than do other retrospective memories.
As we have acknowledged, measuring self-reminders or
thoughts pertaining to prospective memories is a somewhat tricky
business. An investigator can never be certain that the rememberer
will record each self-initiated reminding because doing so is itself
a prospective memory task. To complicate matters, only consciously occurring reminders can be recorded. Because many
cognitive processes occur outside the realm of conscious thought,
the variables found to affect prospective memory in these experiments could be exerting their influence at that level as well. For
example, transitions and breaks between tasks may heighten the
availability of intentions without these memories becoming available to conscious reflection. Unfortunately, the current laboratory
techniques for exploring such hypotheses about unconscious influences have lagged woefully behind such theorizing. As further
research will dictate, the variables tested here ultimately may be
shown to constitute several pieces of a more complex psychological function, only part of which is consciously driven, with the
remainder being relatively automatic or otherwise out of the realm
of conscious processing. Equally as important, event-based prospective memory ultimately may be shown to be consciously
driven in some contexts and much more automatic in others.
We began this article with the interference theory of forgetting.
That theory and the forgetting curve accurately describe losses
from retrospective memory. A great temptation exists to apply that
theory to the present experiments. Doing so would imply that
shorter retention intervals or ones filled with less interfering material should lead to better prospective performance. Contrary to
that theory, we found that event-based performance was generally
better when either the information load or the delay was increased
between establishing an intention and the time it was to be performed. The problem with concluding from the present data that
retrospective and prospective memories have different retention
functions is that the fate of the two types of memories may be
identical when the same variables operate on each type of memory.
For example, retrospective memory may not decline over retention
intervals if remindings or rehearsals occur to preserve them (although this did not happen in Experiment 4). As such, the difference between prospective and retrospective memory may not be in
how they are affected by the variables that have been examined
here but rather may lie in the differences of how often various
cognitive processes naturally apply these variables to the two
different types of memories. In other words, memories qua memories may not differ as a function of whether they refer to events
in the past or future, but the particular cognitive processing that is
likely to operate on these memories might differ. If this account is
true, then it would contravene the hypothesis that prospective
memories exist in some privileged state. Although the tasks and
conditions tested here cannot immediately resolve all of these
issues, the present results appear to represent an important interim
step in the discovery of how prospective and retrospective memories may or may not be dissociable from one another.
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Received June 25, 1998
Revision received January 18, 2000
Accepted February 28, 2000