Evidence for remembering when events occurred in a rodent model of episodic memory.
ABSTRACT The content of episodic memory consists of representations of unique past events. Episodic memories are grounded in a temporal framework (i.e., we remember when an event occurred). It has recently been argued that episodic-like memory in rats is qualitatively different from human episodic memory because, rather than remembering when an earlier past event occurred, rats used the cue of how long ago it occurred. We asked, therefore, whether rats remember the time of day at which they encountered a distinctive event, in addition to what occurred and where it happened. Rats were tested in the morning and afternoon, on separate days. A distinctive flavor (chocolate) was replenished at a daily-unique location at only one of these times. The interval between first and second daily opportunities to eat (study and test, respectively) was constant. Rats adjusted their revisits to the chocolate location at different times of day by using time of day rather than the cue of how long ago an event occurred. Two lines of evidence suggest that rats remembered the time at which the distinctive event occurred. First, under conditions in which the time of test (but not time of study) was novel, rats immediately transferred their knowledge of the chocolate contingency to the new test time. Second, under conditions in which predictions for study and test times were put in conflict, rats again used study time. Our results suggest that, at the time of memory assessment, rats remember when a recent episode occurred, similar to human episodic memory.
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ABSTRACT: Source memory represents the origin (source) of information. Recently, we proposed that rats (Rattus norvegicus) remember the source of information. However, an alternative to source memory is the possibility that rats selectively encoded some, but not all, information rather than retrieving an episodic memory. We directly tested this 'encoding failure' hypothesis. Here, we show that rats remember the source of information, under conditions that cannot be attributed to encoding failure. Moreover, source memory lasted at least seven days but was no longer present 14 days after studying. Our findings suggest that long-lasting source memory may be modelled in non-humans. Our model should facilitate attempts to elucidate the biological underpinnings of source memory impairments in human memory disorders such as Alzheimer's disease.Biology letters 03/2014; 10(3):20140064. · 3.43 Impact Factor
Article: Episodic memory in nonhuman animals.[Show abstract] [Hide abstract]
ABSTRACT: Episodic memories differ from other types of memory because they represent aspects of the past not present in other memories, such as the time, place, or social context in which the memories were formed. Focus on phenomenal experience in human memory, such as the sense of 'having been there', has resulted in conceptualizations of episodic memory that are difficult or impossible to apply to nonhuman species. It is therefore a significant challenge for investigators to agree on objective behavioral criteria that can be applied in nonhuman animals and still capture features of memory thought to be critical in humans. Some investigators have attempted to use neurobiological parallels to bridge this gap; however, defining memory types on the basis of the brain structures involved rather than on identified cognitive mechanisms risks missing crucial functional aspects of episodic memory, which are ultimately behavioral. The most productive way forward is likely a combination of neurobiology and sophisticated cognitive testing that identifies the mental representations present in episodic memory. Investigators that have refined their approach from asking the naïve question "do nonhuman animals have episodic memory" to instead asking "what aspects of episodic memory are shared by humans and nonhumans" are making progress.Current biology: CB 09/2013; 23(17):R801-6. · 10.99 Impact Factor
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ABSTRACT: Evidence concerning the possibility of mental time travel into the future by animals was reviewed. Both experimental laboratory studies and field observations were considered. Paradigms for the study of future anticipation and planning included inhibition of consumption of current food contingent on future receipt of either a larger quantity or more preferred food, choice between quantities of food contingent on future pilfering or replenishment of food, carrying foods to different locations contingent on future access to those locations, and selection of tools for use to obtain food in the future. Studies of non-human primates, rats, black-capped chickadees, scrub-jays, and tayras were considered. It was concluded that current evidence favors future cognition in animals, and some theoretical issues concerning this ability were discussed.Learning and Motivation. 11/2012; 43(4):169–180.
Evidence for remembering when events occurred
in a rodent model of episodic memory
Wenyi Zhou and Jonathon D. Crystal1
Department of Psychology, University of Georgia, Athens, GA 30602-3013
Communicated by Charles R. Gallistel, Rutgers, The State University of New Jersey, Piscataway, NJ, April 20, 2009 (received for review January 30, 2009)
The content of episodic memory consists of representations of
unique past events. Episodic memories are grounded in a temporal
framework (i.e., we remember when an event occurred). It has
recently been argued that episodic-like memory in rats is qualita-
tively different from human episodic memory because, rather than
remembering when an earlier past event occurred, rats used the
cue of how long ago it occurred. We asked, therefore, whether rats
remember the time of day at which they encountered a distinctive
event, in addition to what occurred and where it happened. Rats
were tested in the morning and afternoon, on separate days. A
distinctive flavor (chocolate) was replenished at a daily-unique
location at only one of these times. The interval between first and
second daily opportunities to eat (study and test, respectively) was
constant. Rats adjusted their revisits to the chocolate location at
different times of day by using time of day rather than the cue of
how long ago an event occurred. Two lines of evidence suggest
that rats remembered the time at which the distinctive event
occurred. First, under conditions in which the time of test (but not
edge of the chocolate contingency to the new test time. Second,
put in conflict, rats again used study time. Our results suggest that,
at the time of memory assessment, rats remember when a recent
episode occurred, similar to human episodic memory.
retrieval of episodic memories is analogous to traveling back in
time to experience specific events from one’s personal past
(2–4). An earlier definition focused on the content of episodic
memory, that is, answering 3 questions about a specific event:
what happened, where did it take place, and when did it transpire
(5)? We refer to this type of content as what-where-when memory.
Clayton and Dickinson (6) introduced the term episodic-like mem-
ory to emphasize that behavioral studies in animals evaluate the
content of episodic memory rather than subjective experiences.
Recent studies with nonhuman animals (6–18) suggest that
animals remember specific episodes from their past (i.e., what-
where-when memories). However, controversy has emerged
about the comparability of episodic-like memory in rodents and
episodic memory in humans (17). Roberts et al. (2, 17) suggested
that memory for when an event occurred suggests an ability akin
to mentally traveling in time to locate an event within a temporal
framework; such an ability would be similar to human episodic
memory, in which people reconstruct past experiences using an
absolute temporal dimension (1, 19, 20). By contrast, a judgment
of how long ago an event occurred is quite different from human
episodic memory. Such a judgment could also be solved by
simpler alternative mechanisms (e.g., timing an interval since a
distinctive event occurred or assessing relative familiarity of
temporally distant events). Remembering the time of day at
which an event occurred is different from an assessment of how
long ago it occurred (21). An animal’s sense of time of day
depends upon an internal circadian clock (19, 22). Because
animals have an internal circadian clock that provides informa-
tion about time of day, their knowledge about time of day does
not depend on external stimuli such as light onset or other
eople remember when a past event occurred within the time
frame of hours, days, or years (1). It has been argued that
environmental cues. Thus, we use the term time of day synony-
mously with the phase of a circadian oscillator (i.e., the propor-
tion within a 24-h cycle).
The objective of this research was to determine, at the time of
memory assessment, whether rats remember when (i.e., the time
of day at which) an earlier distinctive event occurred, in addition
to what occurred and where it happened. Rats can use how-
long-ago cues under conditions in which both when and how-
long-ago cues are available (17). Therefore, we sought behav-
ioral evidence for remembering when an event occurred by
eliminating the usefulness of how long ago an event occurred as
a temporal cue. This demonstration is important because devel-
opment of a rodent model of episodic memory may improve our
understanding of human disorders of memory (23).
Rats were placed in an 8-arm radial maze twice per day. Upon
first exposure, rats obtained their first opportunity to eat regular
rat chow and a preferred food type, chocolate. We refer to the
first feeding opportunity as first helpings. After a delay, rats were
returned to the maze. However, to obtain their second oppor-
tunity to eat chow (i.e., second helpings), they needed to avoid
revisiting locations where they obtained their first helpings
earlier that same day because the old locations no longer
provided chow. To obtain their second helpings of chocolate,
they had to revisit the same location that provided chocolate
earlier in the day, but the chocolate location replenished (or
failed to replenish) according to a temporal rule. Consequently,
obtaining chocolate at second helpings required the rat to
remember when and where they found the chocolate (what)
during their first helpings, documenting the use of what-where-
when memory. We refer to the first helpings of the day as a study
phase and the second helpings as a test phase; the first phase
second phase is a test of memories of first-helpings locations
because the rats obtain additional food by visiting the locations
that were not visited in the study phase. The delay between study
and test phases (i.e., first and second helpings) is referred to as
a retention interval.
We investigated whether rats remembered the time of day at
which they had recently encountered a distinctive food type
(chocolate) using a group of rats that served in 4 experiments in
sequence. Rats were trained in the morning and afternoon, on
separate days, but chocolate replenished at a daily-unique
location at only 1 of these times (counterbalanced across rats).
The rats searched for food (i.e., their first helpings of the day)
in an initial study phase. Subsequently, the rats searched again
for food in a test phase (i.e., their second helpings of the day) on
the same day; the delay between study and test phases was 2 min.
Chocolate was found in the maze at a randomly selected location
in their first helpings each day; for rats to obtain chocolate at
Author contributions: J.D.C. designed research; W.Z. performed research; W.Z. analyzed
data; and J.D.C. and W.Z. wrote the paper.
The authors declare no conflict of interest.
1To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/cgi/content/full/
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vol. 106 ?
no. 23 ?
they encountered the chocolate during their first helpings (i.e.,
chocolate was only available at the same location that provided
chocolate in their first helpings). We made the delay between
first and second helpings irrelevant to finding chocolate by using
a constant 2-min retention interval in both morning and after-
noon sessions. Rats were given only one opportunity to obtain
first and second helpings per day, and both of these feedings
1. Availability of chocolate during the second helpings on any
one day depended on whether rats obtained their first-helpings
(study phase) in the morning or in the afternoon on that day. By
contrast, locations baited with chow during first helpings never
replenished at second helpings. If rats use what-where-when
memories, then they should be able to revisit the chocolate
location at second helpings at the correct time of day (and limit
are implicated, in this case, because the rat would need to
remember when and where they found the chocolate (what)
during their first helpings (study phase).
(where) at the times of day (when) at which these locations were
scheduled to replenish chocolate (what; Experiment 1), consis-
tent with the use of what-where-when memory. Next, we used a
phase shift of light onset in the colony (24, 25) to determine
whether the rats used time of day (i.e., circadian phase) or an
interval cue (i.e., the interval since light onset) to find their
second helpings (Experiment 2). Under conditions in which
predictions for circadian time of day and an interval cue were
dissociated, we observed revisits to the chocolate location based
on circadian time of day. Finally, we used 2 techniques to
determine whether, at the time of second helpings, the rats
remembered when the first helpings occurred, rather than
discriminating the time of day at second helpings. First, we
introduced 7-h retention intervals that maintained the familiar
helpings (Experiment 3). The rats immediately transferred (i.e.,
without feedback) their chocolate revisit strategy to the novel
situation. Thus, the rats used the familiar time of day at their first
helpings to guide chocolate revisits. Second, we conducted a
conflict test. Because early and late sessions overlapped in time
in Experiment 3, we were able to begin a session with a late
opportunity for first helpings and end with an early opportunity
for second helpings in the conflict test (Experiment 4). Under
these conditions, predictions for using the time of day at first and
second helpings were dissociated. In both Experiments 3 and 4,
revisits to the chocolate location were guided by the time of day
at first helpings. Our results suggest that, at the time of memory
assessment, rats remember when an earlier episode occurred (in
addition to what and where information). These findings repre-
sent a qualitative similarity to human episodic memory.
Experiment 1. The design of Experiment 1 is shown in Fig. 1A.
Morning or afternoon was randomly selected for presentation of
first and second helpings, separated by a 2-min delay. Chocolate
was available at a randomly selected location during first help-
ings and sometimes replenished at that location during second
in the morning, and non-replenishment of chocolate occurred in
the afternoon; this arrangement was reversed for the remaining
rats. To obtain chocolate in second helpings (test phase), the rats
needed to remember where they found it during first helpings
(study phase) of that same day. Thus, availability of chocolate in
their second helpings on any given day depended on whether
their first helpings occurred at 7:00 a.m. or 1:00 p.m. on that day.
Rats preferentially revisited the chocolate location when it was
about to replenish. The probability of revisiting the chocolate
location in the first 4 arm entries during second helpings was
higher at the replenishment time of day than at the non-
replenishment time of day [t (15) ? 4.3, P ? 0.001; see Fig. 2A].
The probability of revisiting chocolate was above the probability
expected by chance in both replenishment [t (15) ? 10.2, P ?
0.0001; chance ? 0.41] and non-replenishment [t (15) ? 5.7, P ?
0.0001] conditions. Because the delay between first and second
helpings was constant for both morning and afternoon sessions,
the constant delay provided no information as to the replenish-
ment of chocolate. Thus, the cue of how long ago first helpings
rates of revisiting chocolate-flavored locations were accomplished
while rats accurately avoided revisits to depleted chow-flavored
locations [see supporting information (SI) Table S1].
Experiment 2. Experiment 1 suggests that the adjustment of
revisits to the location recently baited with chocolate in Fig. 2A
may be based on time of day (i.e., morning vs. afternoon). As
noted above, the delay between first and second helpings in
Experiment 1 did not provide information about chocolate
replenishment. However, these data do not tell us whether the
rats used a remaining interval to solve this task. Rats could have
timed the interval between light onset in the colony and the daily
session and used this cue to adjust revisit rates. As noted in Fig.
1A, light onset occurred at 6:00 a.m. in Experiment 1. Because
morning sessions occurred 1 h after light onset and afternoon
sessions occurred 7 h after light onset, an alternative explanation
for the chocolate-revisit data shown in Fig. 2A is that rats used
these intervals to guide revisits to the chocolate location. In
Experiment 2, we put predictions for circadian time-of-day and
interval hypotheses in conflict by shifting the light onset by 6 h
(to 12:00 a.m.), using the same rats. The first helpings occurred
at 7:00 a.m. in this experiment, which was 7 h after 12:00 a.m.,
as shown in Fig. 1B. A circadian oscillator is unaffected by a
single manipulation of light onset, whereas an interval would be
affected in this case (21, 22, 26). According to the time-of-day
hypothesis, if rats used circadian time of day, then they should
session. Alternatively, according to the interval hypothesis, if the
rats timed 7 h from light onset, then they should revisit chocolate
at the same rate that usually occurs in an afternoon session.
Rats adjusted revisit rates on the basis of the circadian time of
day at which the session occurred rather than using the interval
between light onset and the session. Fig. 2B shows data from this
experiment relative to baseline data from Experiment 1 accord-
ing to interval and time-of-day hypotheses; the baseline data
come from Fig. 2A. Observed revisit rates were significantly
different from the baseline for the interval hypothesis [t (15) ?
2.3, P ? 0.04], suggesting that the rats did not time the interval
between light onset and study-test sessions. The observed data
were not reliably different from the time-of-day hypothesis [t
(15) ? ?0.03, P ? 0.8], consistent with the proposal that the rats
adjusted their revisit rates to chocolate on the basis of the time
of day at which sessions occurred.
Experiment 3. The phase-shift manipulation of light onset in
Experiment 2 suggests that the rats were using circadian time of
day to adjust revisit rates to the chocolate location in morning
and afternoon sessions. However, these data do not tell us
whether the rats were using the time of day at their first helpings
or the time of day at their second helpings to produce this
adjustment in revisits. It is important to note that the 2-min
separation between the time of day at first and second helpings
is too small for the rats to discriminate on the basis of circadian
time of day (24, 27). Yet, documenting remembering is critically
important to establish the existence of episodic-like memory.
Remembering would be implicated if the rats used the features
(what-where-when) of their first helpings to adjust revisits; by
contrast, remembering would not be implicated if they used the
www.pnas.org?cgi?doi?10.1073?pnas.0904360106Zhou and Crystal
features of their second helpings. Thus, it is important to
determine whether the rats were, at the time of their second
helpings, remembering the time of day at which their first
helpings occurred. An alternative hypothesis is that at the time
of their second helpings the rats may discriminate circadian time
of day and adjust revisit rates on the basis of this information
without remembering the time of day at their first helpings.
Consequently, in Experiment 3 we substituted a 7-h delay for the
2-min delay, using the same rats. The time of day at which the
rats obtained their first helpings was the same as in Experiment
1 (i.e., 7:00 a.m. for early and 1:00 p.m. for late first helpings).
However, the introduction of the 7-h delay between first and
times (2:00 p.m. for early and 8:00 p.m. for late sessions; Fig. 1C).
Thus, the time of first helpings was at a familiar time of day (i.e.,
from Experiment 1), and the time of second helpings was at a
novel time of day. If rats adjusted their revisit rates on the basis
of the time of day at which their first helpings occurred, then they
should continue to differentially revisit chocolate locations more
in the replenishment condition than in the non-replenishment
condition. Alternatively, if the rats were adjusting revisit rates on
the basis of the time of day at which their second helpings were
available, then there is no basis for the rats to know that a higher
revisit rate should occur at the novel times of 2:00 or 8:00 p.m.
It is important to note that the strongest version of these
predictions comes from the very first transfer test to novel early
and late test times (i.e., before receiving feedback from exposure
to the new test times). Consequently, the critical data for
evaluating the above hypotheses come from the very first early
and late sessions.
The rats were more likely to revisit the chocolate location in
the replenishment condition than in the non-replenishment
condition, consistent with memory of the time of day at first
helpings; thus, rats remembered their first-helpings episode (i.e.,
what, where, and when memories). Importantly, this difference
was observed on the very first replenish and non-replenish trials
[i.e., before feedback with the new test times; t (15) ? 2.24, P ?
0.04; see Fig. 2C]. Thus, at the time of second helpings, the rats
remembered the time of day at which their first helpings (study
phase) occurred and adjusted chocolate revisit rates accordingly.
First and Second Helpings
Light OnsetLight offset
Experiment 1 Experiment 2
after a 2-min retention interval. Chocolate- or chow-flavored pellets were available at 4 randomly selected arms in the study phase; access to the other 4 arms
was prevented by closed doors. After a 2-min delay, chow-flavored pellets were available at previously inaccessible locations in the test phase. In the
condition, chocolate did not replenish at the other time of day (shown in the afternoon session). Chocolate replenished at second helpings in the test in the
morning (7:00 a.m.) session but not in the afternoon (1:00 p.m.) session for half of the rats; these contingencies were reversed (not shown) for the remaining
rats. For each rat, 1 session (i.e., first and second helpings) was conducted per day. The same arms were used to illustrate morning and afternoon sessions in the
morning session (i.e., starting at 7:00 a.m.). Note that 7 h elapsed between light onset and the study-test sequence (solid horizontal line), which is comparable
for time-of-day and how-long-ago cues in conflict. Thus, a rat would be expected to behave as in its morning baseline (on the basis of time of day) or as in its
afternoon baseline (on the basis of how long ago). (C) Transfer-test design of Experiment 3. The time of day at which first helpings occurred was the same as
at novel times of day (2:00 p.m. in early and 8:00 p.m. in late sessions). Early and late sessions had study times (but not test times) that corresponded to those
was randomly selected. Differential revisits to the chocolate location are expected if the rats were adjusting revisit rates on the basis of the time of day at which
the study episode occurred; revisit rates are expected to be equal in early and late sessions if the rats used time of day at which the test phase occurred. Study
and test phases were as in Experiment 1, except that they were separated by 7-h delays (shown by horizontal brackets). (D) Conflict-test design of Experiment
and early-session second helpings occurred in Experiment 3. The design of the experiment put predictions for time of day at study and time of day at test in
conflict. Thus, a rat would be expected to behave as in its early-session, second-helpings baseline (on the basis of test time of day) or as in its late-session,
second-helpings baseline (on the basis of study time of day).
Experimental design. (A) Design of Experiment 1. The morning or afternoon was randomly selected for presentation of first helpings (study phase) and
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The probability of revisiting chocolate was above the probability
expected by chance [replenish: all rats revisited, SEM ? 0;
non-replenish: t (15) ? 3.0, P ? 0.01]. As expected, with
additional training the rate of revisiting chocolate continued to
be higher in the replenishment condition compared with the
non-replenishment condition [t (15) ? 6.8, P ? 0.0001; see Fig.
2D]. The probability of revisiting chocolate was above the
probability expected by chance [replenish: t (15) ? 19.2, P ?
0.0001; non-replenish: t (15) ? 2.3, P ? 0.05].
Experiment 4. To provide a converging line of evidence for
memory of the time at which the first helpings occurred, the
same rats were subjected to a novel conflict test that dissociated
predictions based on time of day at first and second helpings.
3 (see Fig. 1C), we were able to begin a session in Experiment
4 with a late opportunity for first helpings (1:00 p.m.) and
end with an early opportunity for second helpings (2:00 p.m.;
Fig. 1D). Consequently, we could determine whether the rats
at first helpings or the time of day at second helpings. According
to the study-time hypothesis, if rats remember when their first
helpings occurred (i.e., study phase) at their second helpings,
then they should revisit chocolate at the same rate that usually
occurs at a late second helping (i.e., 8:00 p.m., which is the usual
time of day at second helpings after their 1:00 p.m. first
helpings). Alternatively, according to the test-time hypothesis, if
rats use time of day at their second helpings (i.e., test phase) to
adjust revisit rates, then they should revisit chocolate at the same
rate that usually occurs at an early second helping (i.e., 2:00 p.m.,
a.m. first helpings). It is important to note that rats had never
received this sequence of 1:00 p.m. and 2:00 p.m. and that the
data were collected before any feedback occurred with respect
to replenishment or non-replenishment of chocolate.
The rats adjusted chocolate revisit rates on the basis of the
time of day at which their first helpings occurred rather than
using the time of day at which their second helpings occurred
(consistent with remembering their first-helping episodes). Fig.
2E shows data from this experiment relative to baseline data
according to the study-time and test-time hypotheses; baseline
data come from Fig. 2D. The observed data were significantly
different from the baseline for the test-time hypothesis [t (15) ?
4.2, P ? 0.001], suggesting that the rats did not use the time of
day at second helpings (test phase) to adjust chocolate revisit
rates. The observed data were not reliably different from the
study-time hypothesis [t (15) ? ?0.7, P ? 0.5]. Thus, at the time
of second helpings, the rats remembered the time at which the
first helpings occurred.
In a series of manipulations, we documented that the rats
remembered when the study episode occurred. First, we ruled
out timing the interval between light onset and the feeding
opportunities by shifting light onset to an earlier time that put
interval and time-of-day predictions in conflict. Presumably, the
estimate of time of day comes from an endogenous circadian
oscillator. A characteristic feature of such a system is that
adjustment to phase shifts of the light cycle is gradual (22, 27);
thus, the representation of time of day would be unaffected by
a single manipulation of light onset. The rats did not use the
interval between light onset and the session, which suggests that
they used time of day. Second, we sought evidence that the rats
remembered the time of day at first helpings, rather than using
the time of day at second helpings, by introducing a 7-h delay
between first and second helpings. The time of day at first, but
not at second, helpings was familiar to the rats from the earlier
experiment. The rats continued to revisit at a higher rate in the
replenishment condition compared with the non-replenishment
condition; thus, we infer that the rats used time of first helpings
rather than time of second helpings to adjust revisit rates. Third,
we provided a novel test for the rats by beginning a session with
a late opportunity for first helpings and ending the session with
an early opportunity for second helpings. Thus, we could deter-
mine whether the rats adjusted revisits at second helpings on the
basis of remembering the time of day at first helpings or by using
the time of day at second helpings. At the time of memory
assessment, the rats remembered the time of day from the study
to replenish in Experiment 1. The probability of a revisit to the chocolate
location in the first 4 choices of a test phase is shown for replenishment (Repl)
and non-replenishment (Non-repl) conditions; replenish and non-replenish
sessions were presented in random order. *P ? 0.001 difference between
conditions. (B) Rats used time of day, rather than an interval, to adjust revisit
rates in Experiment 2. Rats treated the study-test sequence as a morning
session, suggesting the use of time of day rather than an interval-timing
mechanism. The figure plots the difference between observed and baseline
revisit rates. For the bar labeled interval, the baseline was the probability of
revisiting chocolate in the afternoon; thus, the significant elevation above
baseline shown in the figure suggests that the rats did not use an interval
mechanism. For the bar labeled time of day, the baseline was the probability
of revisiting chocolate in the morning; thus, the absence of a significant
elevation above baseline is consistent with the use of time of day. The
horizontal line corresponds to the baseline revisit rate to the chocolate
against the hypothesis indicated on the horizontal axis. *P ? 0.04 different
from baseline. (C and D) Rats preferentially revisited the chocolate location
when it was about to replenish when the study, but not the test, time of day
was familiar in Experiment 3. The probability of a revisit to the chocolate
location in the first 4 choices of a test phase is shown for first replenishment
and first non-replenishment conditions (c; initial) and for subsequent sessions
(D; terminal). *P ? 0.04 and **P ? 0.0001 difference between conditions. (E)
Rats remembered the time of day at which the study episode (i.e., first
as a late-session test phase, suggesting memory of the time of day at study
rather than discriminating time of day at test. The figure plots the difference
baseline was the probability of revisiting chocolate in the second helpings of
the early session (test phase) in Experiment 3; thus, the significant elevation
above baseline suggests that the rats did not use the time of day at test to
adjust revisit rates. For the bar labeled study time, the baseline was the
probability of revisiting chocolate in the second helpings of the late session
baseline is consistent with memory of the time of day at study. The horizontal
line corresponds to the baseline revisit rate to the chocolate location from
Experiment 3 (terminal). Positive difference scores correspond to evidence
against the hypothesis indicated on the horizontal axis. *P ? 0.001 different
from baseline. (A–E) Error bars indicate SEM. (A, C, and D) The probability
expected by chance is 0.41.
(A) Rats preferentially revisit the chocolate location when it is about
www.pnas.org?cgi?doi?10.1073?pnas.0904360106 Zhou and Crystal
episode (i.e., time of day at which first helpings occurred). These
conditions in which how-long-ago cues were made irrelevant
to performance. Importantly, the relative strength of memories
that decay over time cannot explain our results because the
delay between first and second helpings was constant in each
It is unlikely that the rats solved the task at first helpings by
prospectively forming an action plan to visit (or refrain from
visiting) the chocolate location. First, an action plan to refrain
from visiting the chocolate location predicts below-chance levels
of revisiting the chocolate location in the non-replenishment
condition. However, rats revisited the chocolate location in the
non-replenishment condition at rates reliably above chance
(Figs. 2 A, C, and D). Thus, this hypothesis is rejected. Second,
an action plan based on the failure to encode the chocolate
location at first helpings predicts chance levels of revisiting the
chocolate location in the non-replenishment condition. This
hypothesis is also rejected by the observation that chocolate
revisit rates were above chance in the non-replenishment con-
dition. Third, we and others (13–16) have changed the value of
a distinctive flavor after the completion of first helpings. In each
of these tests of what-where-when memory, the rats flexibly
adjusted their revisit rates to take into account the current
the execution of an action plan that was formed at first helpings.
distinctive flavor could not be predicted at first helpings, yet the
rats discriminated what, where, and when.
In a series of elegant experiments, Roberts et al. (17) uncon-
founded time of day at study and the cue of how long ago the
study episode occurred. The data of Roberts et al. suggest that
how-long-ago dominates when multiple temporal cues are avail-
able, which may be a form of overshadowing under conditions of
cue competition (28). Thus, we sought to evaluate what-where-
when memories under conditions in which how-long-ago cues
were irrelevant to predicting replenishment. When how-long-
ago was rendered irrelevant in the present studies, we found
evidence for remembering when an earlier study episode oc-
curred, in addition to what and where knowledge about the
People can describe when earlier events occurred using cal-
endar-date-time systems (i.e., a representational system that
retains the time of occurrence of earlier events) (19). Our data
suggest that, at the time of memory assessment, rats remember
when specific events occurred in time. Moreover, these experi-
ments provide insight into the type of temporal representational
systems that rats may use to support episodic-like memory,
namely a timing system that retains the time of occurrence of
General Methods. Sixteen male Long-Evans rats encountered chocolate, at a
daily unique location (study), which sometimes replenished later (test). The
replenishment depended on the time at which chocolate had been initially
encountered. Optimal performance is to revisit the chocolate location at the
test when replenishment is imminent but to reduce this tendency when the
chocolate replenishment is not forthcoming. Chow-flavored locations from
first helpings never replenished at second helpings. Thus, solving this task
requires knowledge about what and where events occurred in addition to
information about when the critical events occurred because chocolate was
available at a daily-unique location. Because the interval between study and
test was constant within each experiment, how long ago a chocolate encoun-
ter occurred could not be used to predict replenishment. Detailed methods
appear in SI Methods.
earlier version of the manuscript. This work was supported by National
Institute of Mental Health Grant R01MH080052 (to J.D.C.).
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Zhou and Crystal PNAS ?
June 9, 2009 ?
vol. 106 ?
no. 23 ?