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Research

Changes in context-specificity during memory
reconsolidation: Selective effects of hippocampal
lesions
Gordon Winocur,1,2,3,4,10 Paul W. Frankland,5,6,7 Melanie Sekeres,5,6 Stuart Fogel,8and Morris Moscovitch1,4,9
1

Rotman Research Institute, Baycrest Centre, Toronto, Ontario M6A 2E1, Canada; 2Department of Psychology, Trent University,
Peterborough, Ontario K9J 7B8, Canada; 3Department of Psychology, University of Toronto, Toronto, Ontario M5T 1R8, Canada;
4
Department of Psychiatry, University of Toronto, Toronto, Ontario M5T 1R8, Canada; 5Programs in Neurosciences and Mental
Health, The Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada; 6Department of Physiology, University of Toronto,
Toronto, Ontario M5S 1A8, Canada; 7Institute of Medical Science, University of Toronto, Toronto, Ontario M5S 1A8, Canada;
8
Centre for Neuroscience Studies, Queen’s University, Kingston, Ontario K7L 3N6, Canada; 9Department of Psychology, Baycrest
Centre, Toronto, Ontario M6A 2E1, Canada
After acquisition, memories associated with contextual fear conditioning pass through a labile phase, in which they arevulnerable to hippocampal lesions, to a more stable state, via consolidation, in which they engage extrahippocampal
structures and are resistant to such disruption. The process is accompanied by changes in the form of the memory frombeing context-specific to context-general. However, when revived by a reminder, stable memories once again become
labile and susceptible to hippocampal disruption, and memory reconsolidation is needed to stabilize them. This study
addressed two questions with respect to this reconsolidation phenomenon: (1) How do reminders reinstate a hippocampally dependent memory trace? (2) As the memory changes from a stable to a labile state after a reminder, does its
form remain invariant, or does it also change? Using contextual manipulations at retrieval in a test of contextual fear
conditioning, we showed that when the fear-conditioning environment served as a reminder, the reinstated memory
regained its context specificity and, as a result, became vulnerable again to the effects of hippocampal lesions. By
comparison, exposure to a different environment during the reminder session reinstated a version of the original
memory that was dependent primarily on general features of the original context and, consequently, was less affected byhippocampal lesions. These findings, which relate loss of reactivated memories after hippocampal destruction (orinactivation) to changes in memory representation, are interpreted as consistent with the transformation hypothesis ofmemory processing.
½Supplemental material is available online at http://www.learnmem.org.

The traditional view of memory formation is that, initially, information is encoded and represented in the hippocampus. With
the passage of time, those memories become consolidated in
extrahippocampal (presumably neocortical) structures, where
they are stored and available for retrieval without hippocampal
involvement. An important element of this process, termed
systems consolidation, is that the long-term memory represented
in the neocortex is identical to that initially formed in the
hippocampus (Squire and Alvarez 1995; Dudai 2004).
An alternative view states that the shift in memory representation from hippocampal to extrahippocampal structures entails atransformation of the initial memory from one that is highly
detailed and context-specific to one that is more schematic, representing only general features of the environment (Moscovitch
et al. 2005; Winocur et al. 2007). The transformation view hasreceived support from studies involving contextual fear conditioning in which animals associate an aversive foot shock with thecontextual environment and exhibit freezing behavior whenplaced in that environment (Biedenkapp and Rudy 2007; Wiltgen

and Silva 2007a; Winocur et al. 2007). These investigators showedthat at relatively short delays following training, normal animals’
memory for the contextual fear response is specific to the training
context in which the response was learned. However, at longer
delays, the specificity declines and the freezing response generalizes to environments that only slightly resemble the original
training environment. The latter, which we refer to as contextgeneral memory, is considered comparable to semantic memoryin the human literature (Rosenbaum et al. 2001). The same effecthas been observed in an appetitive, socially acquired food preference task (Winocur et al. 2007). While some form of context
specificity may be retained over long intervals without the hippocampus, such memories are more fragile and differ in importantways from those that are hippocampally dependent (Wang et al.
2009). A critical element in this theoretical approach is that
context-specific and context-general memories, represented asthey are in different brain structures, can coexist, although one
or the other may dominate under different conditions (for review
of converging evidence in support of the transformation view, see
Moscovitch et al. 2006).
Issues related to memory representation are also relevant to
the reconsolidation phenomenon (Sara 1973; Nader 2003a; Morris
et al. 2006), which is the focus of this paper. In reconsolidation,

10
Corresponding author.
E-mail gwinocur@rotman-baycrest.on.ca; fax (416) 785-2474.
Article is online at http://www.learnmem.org/cgi/doi/10.1101/lm.1447209.

16:722–729 Ó 2009 Cold Spring Harbor Laboratory Press
ISSN 1072-0502/09; www.learnmem.org

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Memory reconsolidation and context specificity

a stable memory trace, when revived by a reminder, once again
becomes labile and susceptible to amnestic agents. With respect tocontextual fear conditioning, Debiec et al. (2002) found that
reviving memories that have become independent of the hippocampus returns them to a state in which they are vulnerable tohippocampal disruption (for a consideration of conditions that
limit this effect, see Biedenkapp and Rudy [2004]). This phenomenon runs counter to systems consolidation theory, which stipulates that, once consolidated, a memory remains fixed and immutable in neocortex. Nor can this theory explain why a memory
should once again be disrupted by treatments that selectively
affect the hippocampus if the memory is no longer represented
there (Nader 2003a). To deal with these problems, Nader (2003a,b)
proposed a systems reconsolidation hypothesis whereby the reminder-induced reactivation process causes the trace again to
become hippocampus-dependent and susceptible to hippocampal
disruption.
Two important questions arise with respect to the memory
representation and reconsolidation: (1) How do reminders reinstate a hippocampus-dependent memory trace? and (2) as thememory changes from a stable to a labile state after a reminder,
does its form remain invariant, or does it also change? The
transformation hypothesis offers a perspective that addresses bothof these questions and shows how they are related. By this view,
reminding the animal with the conditioning environment directly
affects the representation of the memory such that the contextspecific, hippocampus-dependent memory dominates, whereas
before the reminder, the nonhippocampal, context-general memory prevailed. It follows that disrupting hippocampal function willinterfere with the expression of the context-specific memory. Thisaccount extends the systems reconsolidation hypothesis (Nader
2003a,b) by emphasizing a dynamic relationship between contextspecific and context-general memories across time.

In this study, we used contextual manipulation at retrieval totest the hypothesis that memories undergo transformation after
a reminder in a memory reconsolidation paradigm. Rats received
contextual fear conditioning in a distinctive context (CXT-A) and
28 d later were reminded of the training experience by being
placed briefly in that same context or in a different context (CXTB) that resembled CXT-A only in its general features. The animals
then received hippocampal lesions or control surgery and, uponrecovery, their memory for the contextual fear response was tested
either in CXT-A or CXT-B. Their performance was compared withthat of lesioned and control groups that were similarly tested after
a short or long delay, but without a reminder. Following thetransformation hypothesis, two critical predictions were made: (1)the memory reactivated by CXT-A, the context-specific reminder,
would engage the hippocampus and make the memory vulnerable
to hippocampal lesions; and (2) the memory reactivated by CXT-B,
the context-general reminder, would be nonspecific, independentof the hippocampus, and, consequently, unaffected by hippocampal lesions.

Results
Behavioral
The mean preoperative freezing times for all groups in the 60 sec
before and the 60 sec after fear conditioning groups and during thereminder sessions of the Reminder Condition are presented in
Supplemental Table 1. There were no differences between any of
the groups on these measures (ANOVA; P > 0.05).

No-Reminder Condition
Figure 1 presents the results for effects of delay and context change
on memory for a learned contextual fear response without reminders.

Figure 1. No-Reminder Condition. (A) Schematic representations of timelines for experimental procedures. The numbers in parentheses represent the
number of rats in the various groups. (B,C) Contextual fear memory in control and hippocampal groups measured in terms of time spent freezing in thetest session after conditioning; surgery intervals of 24 h (B) and 28 d (C ) were used. In the 24-h test, controls displayed context specificity for the fear
memory, while hippocampal rats exhibited no memory of the fear response. At 28 d, both control and hippocampal groups exhibited strong contextual
fear memory in the training context (CXT-A) and in a different context (CXT-B) with no context specificity. Error bars represent 6 SEM.
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Memory reconsolidation and context specificity

When tested at the short delay (Fig. 1B), control rats froze much
more in the context in which they were trained (CXT-A) than ina different context (CXT-B), but at the long delay (Fig. 1C), theyfroze equally and extensively in both contexts. This was confirmed by a significant context 3 delay interaction, F(1,35) = 6.00,
P < 0.02, that was due to the reduced freezing by the control grouptested at the short delay in CXT-B.
When hippocampal lesions were produced within 24 h oftraining, rats exhibited little freezing when tested in either context(Fig. 1B). However, when hippocampal lesions were made 28 d
post-training, when performance was governed by general features, there was no effect and the lesioned rats exhibited considerable freezing in both contexts (Fig. 1C).

Reminder Condition (Fig. 1C). In the latter condition, control rats
froze equally in CXT-A and CXT-B (t < 1).
In absolute terms, control rats tested in CXT-B exhibitedcomparable levels of freezing with reminders (Fig. 2B,C) as
without reminders (Fig. 1B,C) regardless of delay. By comparison,controls tested in CXT-A after either a CXT-A or CXT-B reminder
froze approximately twice as much as controls tested in CXT-A
without a reminder, at both short and long delays.
In the reconsolidation test, hippocampal lesions after a CXTA reminder virtually eliminated freezing (Fig. 2B). After a CXT-B
reminder, when compared with controls that also received a CXTB reminder, rats with hippocampal lesions (HPC group) exhibited
reduced freezing when tested in CXT-A (t(22) = 3.57, P < 0.002; Fig.
2C) but not to the level seen after a CXT-A reminder (Fig. 2B). The
lesions, however, had no effect on freezing in CXT-B (t < 1) after
a CXT-B reminder (Fig. 2C). These results indicate that theeffectiveness of hippocampal lesions is related to the specificityof the memory that is reinstated by the reminder. These results are
consistent with the findings, in the No-Reminder Condition, thathippocampal lesions made 24 h after training, when memory wascontext specific, abolished freezing, but hippocampal lesions
made 28 d after training, when memory was context general,
had no effect (Fig. 1B,C).
Clearly, the two reminders in the reconsolidation tests
affected the HPC and control groups differently. The effects of
the CXT-B reminder are of interest because they were in opposite
directions. As noted in Figure 2C, after a CXT-B reminder, control
rats froze slightly more when tested in CXT-A than in CXT-B,
whereas the reverse was seen in the HPC group. This finding,which was confirmed by a significant group 3 test environment
interaction (F(1,43) = 4.54, P < 0.05), is discussed below.

Reminder Condition
In the Reminder Condition, reminders were provided 28 d after
fear conditioning. Providing a CXT-A reminder restored context
specificity in control animals, so that they froze much more whensubsequently tested in CXT-A than in CXT-B (Fig. 2A; t(18) = 3.42,
P < 0.005). This contrasts with the considerable freezing exhibitedby controls in both contexts when tested at the same long delay
without a reminder. ANOVA confirmed a significant reminder 3context interaction, F(1,35) = 4.02, P = 0.05, that was due to reduced
freezing by the control group that was reminded in CXT-A andtested in CXT-B. As indicated above, this was the same pattern
exhibited by control rats tested shortly after acquisition (Fig. 1A).
Providing a CXT-B reminder led to comparable freezing bycontrols in both contexts (Fig. 2C; t(28) = 1.91, P > 0.05), indicatingthat CXT-B was not as effective in restoring context specificity as
CXT-A. The latter results parallel the lack of context specificity
exhibited by control rats tested at the long delay in the No-

Figure 2. Reminder Condition. (A) Schematic representations of timelines for experimental procedures. The numbers in parentheses represent the
number of rats in the various groups. (B,C) Contextual fear memory in control and hippocampal groups when either CXT-A (B) or CXT-B (C) was provided
as a reminder 28 d after fear conditioning in CXT-A and 24 h before surgery. CXT-A, as a reminder, restored context specificity of the contextual fear
memory and, consequently, controls froze only when tested in CXT-A. Hippocampal lesions eliminated the context-specific memory when it wasreactivated by CXT-A. CXT-B was not an effective reminder for context specificity and resulted in generalized freezing in control and HPC groups. Error
bars represent 6 SEM.
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Memory reconsolidation and context specificity

Finally, the time spent freezing in each minute of the 8-mintest periods was examined to determine whether HPC and control
groups exhibited different patterns of discrimination or generalization over the test periods. In the No-Reminder Condition, thedata were analyzed by comparing the time spent freezing by HPC
and control groups in the first and second 4-min time periods
when tested in CXT-A or CXT-B at the short and long delays. In
general, freezing times were distributed equally over the two time
periods, and ANOVA revealed no significant interactions or main
effects involving time period (all Ps >0.05). Similar comparisons,
performed on the data of the Reminder Condition, yielded noeffect of time period (all Ps > 0.05). Thus, the evidence indicatesthat the relative influence of context-specific and context-generalmemories was fairly consistent over the entire test period in thevarious conditions. (Time spent freezing on a minute-by-minutebasis is provided for all conditions in Supplemental Table 1.)11

Histological evaluation
Figure 3A provides photomicrographs of coronal sections ofa representative hippocampal lesion (left) in relation to a normal
brain (right) and Figure 3B shows schematic drawings of minimal(gray) and maximal (black) extents of lesions meeting our histological criteria.
The nature and extent of lesions were similar to thosereported in recent studies (Winocur et al. 2005, 2007). In all therats with hippocampal lesions, damage extended bilaterally todorsal and ventral regions of the structure. Half the lesioned ratssustained damage to 50%–80% of the hippocampus proper.
Approximately 25% had very large lesions that affected 80%–
100% of the hippocampus, including extensive damage to all thesubfields (CA1–CA3, dentate gyrus). Five rats with lesions that
affected 30%–40% of the hippocampus were included because
their performance fell within the range of their respective group.
Overall, the median value for hippocampal destruction was 72%,
with the extent and pattern of damage to dorsal and ventral
regions similar in the No-Reminder and Reminder Conditions. In
all cases, extrahippocampal damage was minor or nonexistent.
ANOVA, performed on the measures of total hippocampal
lesion, confirmed that there was no difference in the averagevolume of hippocampal damage across the eight independent
HPC groups in this study (F < 1). As well, there was no evidence
that lesion size was related to the amount of freezing in the NoReminder (r = 0.23, P < 0.32) and Reminder (r = 0.18, P < 0.74)
Conditions.

Figure 3. Lesion representations. (Top) Photomicrographs of representative hippocampal lesion (left) in relation to a normal brain (right).
(Bottom) Schematic drawings of minimal (gray) and maximal (black)extents of hippocampal lesions meeting the histological criterion included
in our experiment.

We also obtained further evidence for the contextual specificity ofrecently acquired fear responses in the No-Reminder Condition(Biedenkapp and Rudy 2007; Wiltgen and Silva 2007a,b; Winocur
et al. 2007). When control animals were tested one day after
training, they showed freezing only in the context in which they
were trained, but when tested at 28 d, the response generalized toother contexts. Taken together, these findings indicate thathippocampal lesions selectively affect context-specific, but notcontext-general, memories.
With this as background, we now can interpret the major
new findings in this study, which address questions concerningchanges in memory representation in reconsolidation. Debiec
et al. (2002) reported that long-term memories that are resistantto the effects of hippocampal inactivation or lesions become
vulnerable once again after a reminder (see also reviews by Nader
2003a; Sara 2007; Nader and Hardt 2009). In this study, we showthat this effect is found only for reminders that restore thecontextual specificity of the memory. When the original context(CXT-A) serves as a reminder of a learned contextual fear response,
a normal rat will show extensive freezing in that context but not ina different context (CXT-B). By comparison, a new context (CXTB) reminder, while capable of activating a fear response to thegeneral context, is not as effective at reinstating context specificity. As can be seen in Figure 2C, although control rats froze in both
environments, they froze only slightly more in the original
training environment than in the new one.
A second major finding is that the context-specificity of thememory determines its susceptibility to hippocampal lesions. Thiswas seen both in the No-Reminder Condition, after initial acquisition (see Fig. 1B), and in the Reminder Condition, when rats
were tested following a reminder with the conditioning context

Discussion
These results confirm a number of established findings in theliterature. For example, we found temporally graded retrograde
amnesia for contextual fear conditioning following hippocampal
lesioning, with recent memories being far more impaired thanremote memories, which were relatively preserved (Kim andFanselow 1992; Anagnostaras et al. 1999; Winocur et al. 2007;
but see Lehmann et al. 2007; Sutherland et al. 2008 for reports ofconditions in which extensive retrograde amnesia of a contextual
fear response after hippocampal lesioning can occur). Although
we had to allow seven days’ recovery time after hippocampal
surgery, this result is consistent with those obtained in studies
using protein synthesis inhibitors and shorter delays between
reactivation and testing (e.g., Milekic and Alberini 2002; for otherstudies, see the review by Nader and Hardt 2009). This indicatesthat the time of surgery, and not the necessary delay imposed byrecovery from surgery, was the determining factor in our results.

11

An anonymous reviewer suggested this analysis.

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Memory reconsolidation and context specificity(CXT-A) 28 d after training (see Fig. 2B). By comparison, CXT-B, asa reminder, effectively reinstated the freezing response in rats withhippocampal lesions. As can be seen in Figure 2C, the HPC groups
froze substantially more when tested in CXT-A and CXT-B,
following a CXT-B reminder, than they did following a CXT-A
reminder (Fig. 2B). Interestingly, whereas control rats froze slightly
more in CXT-A than in CXT-B, the HPC groups showed the
opposite pattern. This result suggests that the CXT-B reminder
revived some components of the context-specific memory but, as
would be expected, to a greater degree in control rats.
Taken together the results reveal a feature of the dynamics ofmemory representation that has interesting theoretical implications. According to the transformation hypothesis, in systems
consolidation, a memory undergoes qualitative changes fromcontext-specific to context-general as it becomes represented ina distributed extrahippocampal network. It is now clear from thefindings of the Reminder Condition that a similar process occurs inreverse in reconsolidation following a context-specific reminder.
Previous studies examined the effect of intrahippocampal
infusion of the protein synthesis inhibitor, anisomycin, ona reactivated contextual fear memory at short and long delays
after fear conditioning (Debiec et al. 2002; Frankland et al. 2006).
In all cases, the fear conditioning trial, the reminder experience,and testing were conducted in the same context. In line with our
results, the consistent finding was that hippocampal disruption
abolished retrieval of the contextual fear response when the delay
between conditioning and reactivation was relatively short. At
long delays, comparable to ours, Debiec et al. (2002) found thathippocampal disruption was equally effective in disrupting reactivated recent and remote contextual fear memories. In contrast,
at long delays, Milekic and Alberini (2002), Suzuki et al. (2004),
and Frankland et al. (2006) found that anisomycin, infused
directly into the hippocampus or injected systemically, had noeffect, as animals displayed the typical freezing response whenreplaced in the conditioning chamber. Apart from the fact thatthese studies used anisomycin to disrupt hippocampal function
and we made lesions, there are other procedural differences aswell. For example, Frankland et al. (2006) and Suzuki et al. (2004)
tested mice in their research, while we, Milekic and Alberini(2002), and Debiec et al. (2002) tested rats. As well, Milekic andAlberini (2002) used an inhibitory avoidance task rather than the
more typical contextual fear conditioning. It is difficult to say how
such differences affected the results, but if they contributed todifferences in the strength of the original memory trace that may
be a potential factor (Alberini 2007). It is significant that Frankland
et al. (2006) found that systemic infusions of anisomycin, which
likely affect both the hippocampus and neocortex, disrupted contextual fear memory at long delays. This suggests, in line with our
hypothesis, that when the memory is distributed across bothhippocampus and cortex, damage to both regions, but not to either alone, is necessary to disrupt the memory.
Our results go further and suggest that the effect of the CXT-A
reminder was to revive the context-specific memory and toreassert its dominance. Because the context-specific memory ishippocampus-dependent, hippocampal lesions destroyed thatmemory and eliminated freezing. In contrast, the CXT-B reminder
reasserted the context-general memory, which was expressed byfreezing in both contexts and, consequently, was relatively invulnerable to hippocampal lesions. These findings extend thesystems reconsolidation hypothesis (Nader 2003a,b; Nader andHardt 2009) by relating loss of reactivated memories following
hippocampal inactivation or destruction to changes in memory
representation; only those reminders that reinstate context specificity are susceptible to such loss. (In a study involving secondorder fear conditioning, Debiec et al. [2006] also considered theeffect of reminders on different memories that bear a relationshipwww.learnmem.org

to each other. Although their study differed from the present one
in several ways [e.g., task, amnestic agent, brain region of interest:
amygdala], an important similarity is that the ability of anamnestic treatment to disrupt expression of a reactivated memory
was related to the nature of the memory and its representation.)
A central assumption of the transformation hypothesis is thata memory undergoes qualitative changes from context-specific tocontext-general, as the representation shifts from the hippocampus to a distributed extrahippocampal network in systems consolidation (Winocur et al. 2007; see also Moscovitch et al. 2005;
Wiltgen and Silva 2007a,b). As we noted above, the reverse process
occurs in reconsolidation following a reminder that is closely
associated with the environment in which original learning
occurred. Importantly, a reminder that is associated with generalfeatures of the conditioning environment is also capable ofreactivating the learned response, although that memory lacks
the context specificity of a newly formed memory.
While the hippocampus clearly mediates context-specificmemory, more research is needed to determine which extrahippocampal structures and pathways are implicated in contextgeneral memory. Converging evidence from studies using immediate early gene mapping (Frankland et al. 2004; Maviel et al. 2004;
Ross and Eichenbaum 2006), protein synthesis inhibitors (Naderet al. 2000; Morris et al. 2006), temporary region-specific neuronal
inactivation (Frankland et al. 2004; Maviel et al. 2004), and lesions(Remondes and Schuman 2004) have identified a number ofstructures that are involved in the process of long-term memory
systems consolidation, with the anterior cingulate cortex as themost prominent candidate (Bontempi et al. 1999; Frankland et al.
2004; Maviel et al. 2004; Frankland and Bontempi 2005). According to our view, these structures would be implicated only incontext-general memory, but, to our knowledge, no publishedstudy has tested this prediction.
Having established that two types of memory must be taken
into account in studies of memory reconsolidation, new questions
are raised about the relationship between context-specific andcontext-general memories and the structures that mediate them.
For example, if as shown, the context-general memory is represented in extrahippocampal structures after a long delay (Fig. 1B),
why, in the Reminder Condition, did the CXT-A reminder activate
the context-specific memory and render it vulnerable to hippocampal lesions, and why was the context-general memory not
expressed in the HPC group that had received the CXT-A reminder? These are important questions, and gene expression andimaging techniques should prove useful in finally resolving them(Frankland et al. 2004). One possibility, of course, is that bothforms of the memory were eliminated by hippocampal lesions. An
alternative suggestion, consistent with our hypothesis that both
memories can coexist, is that, over the 24-h period between thereminder and surgery, during which the context-specific, hippocampus-dependent memory becomes dominant once again, neocortically mediated context-general memories are inhibited or’’overshadowed’’ (Frankland et al. 1998; Anagnostaras et al.
1999; Eisenberg et al. 2003). This inhibitory process may beconsidered analogous to the supremacy of hippocampal memories
vis-à-vis memories mediated by other neural systems (e.g., Packardand McGaugh 1992; Sutherland et al. 2006).
In considering the mechanism of such an inhibitory effect,
one possibility is that the specific reminder of the hippocampal
memory (i.e., re-exposure to CXT-A) initiates a retrieval process
that, once enacted, blocks access to the general memory fora period of time, even if the specific memory is no longer available.
Although the inhibitory or retrieval effects outlast the removal of
the hippocampus, it is entirely possible that they are not permanent and that the context-general memory may recover with time.
The evidence for recovery of contextual fear conditioning is mixed
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Memory reconsolidation and context specificity(see Nader and Hardt 2009). One prediction that emerges from ourdata, but which has not been tested, is that if memory does
recover, it should be context general.
Another possible mechanism is suggested by the distinctionthat can be made between a potential memory and an actualized
one. The actualization of a memory depends on the presence ofappropriate cues that enable the reconstruction and ultimate
retrieval of that memory. In our study, following the CXT-A
reminder and the actualization of the context-specific memory,the context-general memory, while available in principle, was notaccessed directly, did not become actualized, and remained a potential memory. The effect of removing the hippocampus after the
CXT-A reminder was to eliminate the only memory that was
actualized to that point, leaving the other memory in a dormant
state. This interpretation can account for the poor memory
exhibited by rats with hippocampal lesions in both contexts (seeFig. 2B). When a CXT-B reminder was given prior to hippocampal
destruction, the general memory did become actualized and the
hippocampal lesion was much less effective in eliminating it (seeFig. 2C). This interpretation differs from the previous inhibitory
interpretation in that no active mechanism is needed to suppress
the context-general memory. It merely remains dormant, never
having been activated. These ideas are being evaluated in current
experiments designed to test specific predictions that follow fromthe two interpretations.
By focusing on the interplay between memory and its reminders, the transformation hypothesis reinterprets reconsolidation as an instance of memory retrieval. According to this interpretation, the reminders serve as cues to retrieve a previously
forgotten or inaccessible context-specific memory. Once retrieved,
this memory is again vulnerable to hippocampal disruption.
Within this framework, the reminder (reconsolidation) paradigm
affords the opportunity to study selectively the neurobiologicalmechanisms underlying retrieval, in contrast to consolidation
paradigms that emphasize memory formation and retention(Dudai 2006). Viewed in this way, the study of consolidation
and reconsolidation processes has much to contribute to ourunderstanding of the relation between encoding and retrieval inlearning and memory.

laboratory room (6.3 3 6.1 m). The room contained standard
furniture (e.g., desk, table, bookshelf along one wall, etc.), as wellas pictures, light fixtures, etc., on the walls. Illumination wasprovided by overhead fluorescent lights under rheostatic control.
The environment in which rats received fear conditioning isreferred to as Context A (CXT-A).
In both the Reminder and No-Reminder Conditions, approximately half the rats were tested in a different environment (CXTB). The test chamber used in CXT-B was smaller (40 3 30 3 18 cm)
than the CXT-A chamber. It was also made of Plexiglas but withsemi-transparent tinted walls. The roof was clear Plexiglas withventilation holes, and the floor consisted of metal rods, spaced
1.3 cm apart. The CXT-B chamber was placed in a smaller room
(3.4 3 2.1 m) on a table that was situated against a wall. Care wastaken to ensure that the configuration of furniture, pictures,lighting conditions, etc., were different from that of the room inwhich fear conditioning took place. Thus, CXT-B resembled CXT-A
only in terms of the latter’s general features—that is, a Plexiglas
test chamber placed in a room other than its living environment.

Fear conditioning procedure
This procedure, which was the same for all conditions, wasadapted from Anagnostaras et al. (1999) and identical to thatroutinely followed in our laboratory (e.g., Moses et al. 2007;
Winocur et al. 2007). Each rat received one fear conditioning trialthat began with the rat being placed in the chamber in CXT-A andallowed to explore freely for 5 min. The rat then received 10 toneshock pairings at variable intervals. Beginning 30 sec after the lastshock and over a 64-sec period, freezing behavior was recorded
every 8 sec (eight observations). Following Anagnostaras et al.
(1999), freezing was defined by an immobilized crouching response in which the only detectable movement was the rat’s
breathing. The rat was then removed from the box and returned toits home cage. The test session was video recorded, and the timespent freezing was manually recorded using a stopwatch.

Testing procedures
Testing procedures were identical, regardless of whether rats were
tested in CXT-A or CXT-B, and whether or not they had received
a pretest reminder session. Testing consisted of a single trial inwhich the rat was placed in the appropriate chamber for 8 min
and, in the absence of the tone, the amount of time spent freezing
was recorded. Immediately after testing, the rat was removed fromthe chamber and returned to its home cage. As in the conditioning
session, the test session was video recorded and the time spent
freezing was recorded.
In repeated experiments, we found these procedures to behighly effective in producing a strong contextual fear response innormal rats (Winocur et al. 2006, 2007; Moses et al. 2007). During
testing, we and others (e.g., Phillips and Ledoux 1994) consistently find that rats with hippocampal lesions and control rats
establish a conditioned freezing response to the tone equally well.
When tested subsequently in the same context, but without thetone, the equally consistent finding is that only the control rats
continue to freeze. In the study by Phillips and Ledoux (1994), theeffects of pairing a tone with foot shock on contextual fear
conditioning was compared directly with the effects of presenting
the foot shock alone. The results showed that lesions to the
hippocampus were far more effective in abolishing the contextual
fear response in the paired than in the unpaired condition. Since
freezing in response to the tone, was not of interest in this study, it
was not measured.

Materials and Methods
Subjects
Male, adult Long–Evans rats, ;5 mo old, were obtained from theCharles River Laboratories (Saint-Constant, Québec, Canada) andserved as subjects in both experiments. The rats were housed
individually in shoebox cages with unlimited access to standard
laboratory chow and water and maintained on a reversed 12-h
light/dark cycle (lights on at 1800 h and off at 0600 h). All testing
took place during the high activity, dark phase of the cycle.
The experimental protocol and all handling procedures
conformed to guidelines set out by the Canadian Council on
Animal Care and were approved by the Trent University Animal
Care Committee. Throughout the research, the rats were examined regularly by a veterinarian.

Apparatus
All rats received fear conditioning in a chamber (50 3 40 3 18 cm)
that consisted of four walls made of clear Plexiglas, a hinged clear
Plexiglas roof with holes to allow ventilation, and a floor thatconsisted of metal rods, spaced 1.3 cm apart. As part of the fear
conditioning procedure, a tone, presented through a centrally
mounted speaker attached to the roof of the box, was paired witha 1.5-mA foot shock that was delivered by a TechServe Model 452A
shock generator.
The fear-conditioning chamber was positioned on a table,
1.3 m above the floor, and situated in the center of a large standard
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Surgical and histological procedures
Rats were anesthetized with isoflurane respiratory anesthetic.
Stereotaxic coordinates were based on the Paxinos and Watson(1997) atlas and located in relation to bregma and the horizontal
skull surface. The procedure for making hippocampal lesions was
slightly modified from the technique developed by Jarrard andMeldrum (1993). Using a small dental burr, eight holes were
drilled through the skull directly above the hippocampus in each
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Memory reconsolidation and context specificity

hemisphere. Hippocampal lesions were produced by 10 intracranial microinjections of a solution containing the cellular neurotoxin N-methyl-D-aspartic acid (5 mg/mL PBS per site) into eachhemisphere. The injection coordinates were calculated from a level
head with respect to bregma: 3.1 mm posterior (p), 61 mm lateral(l), and 3.6 mm ventral (v); 3.1 (p), 2 (l), 3.6 (v); 4.1 (p), 2 (l), 4 (v);
4.1 (p), 3.5 (l), 4 (v); 5 (p), 3 (l), 4.1 (v); 5 (p), 5.2 (l), 5 (v); 5 (p), 5.2(l), 7.3 (v); 5.8 (p), 4.4 (l), 4.4 (v); 5.8 (p), 5.1 (l), 6.2 (v); 5.8 (p), 5.1(l), 7.5 (v). The solution was infused at a rate of 0.4 mL/min
through 30-gauge stainless steel needles for 38 sec, using a 10-mL
syringe attached to a motorized infusion pump. The last twoventral hippocampal sites were injected for 2 min each. The
needles were removed 2 min after each injection. In the control
procedure, incisions and burr holes were identical to the lesioned
animals with the exception that there was no penetration of brain
tissue. To facilitate recovery from surgery, all rats were given
intraperitoneal injections of diazepam (10 mg/kg).
At the end of the experiment, rats with hippocampal lesions
were deeply anesthetized with sodium pentobarbital (65 mg/kg)
and perfused intracardially with 0.9% saline followed by 10%
formalin. The fixed brains were removed from the skull and stored
in 10% formalin. Brains were transferred to a 30% buffered sucrose
solution 48 h prior to sectioning. The brains were then frozen andsliced at 40 mm, using a cryostat. Every fifth section was mounted
on a gel-coated slide and stained with neural red.
Lesion size was estimated for each rat by determining thepercent of hippocampal tissue damaged within a cross-section ofbrain slices corresponding to Figures 28, 34, 37, and 40 in Paxinos
and Watson (1997). Sections were examined under a light microscope, and the area of tissue damaged was manually recorded on
templates of the hippocampus for the four figures, correspondingto 3.30, 4.50, 5.20, 5.80 mm from bregma. The volume ofdestroyed hippocampal tissue was determined as a percentage by
dividing the lesioned area by the total area of hippocampal tissue
for the appropriate figure. For each section, the percentage ofhippocampal tissue damaged in the dorsal and ventral regions wascalculated for both the left and right hippocampus. An estimate of
the total hippocampal lesion was also calculated for each brain.

minder and Reminder Conditions. The control rats were tested
24 h after surgery, a delay that allowed sufficient time to recoverfrom the minor surgery and establish the type of memory representation that governed their behavior at that point. The HPC
groups were lesioned at the same times but, as in similar, previous
studies (e.g., Debiec et al. 2002), they were tested seven days lateras that much time is required to recover from the more invasive
surgery. As indicated in the Results section, the behavior of therespective groups was affected by the dominant form of memory
at the time of surgery and not by the amount of time between
surgery and testing.

Acknowledgments
The research was supported by grants from the Natural Sciencesand Engineering Research Council to G.W. and M.M. and from the
Canadian Institutes of Health Research to P.W.F. We thank Dr.
Malcolm Binns for help with data analysis and Chelsea Good, Jacy
Lee, and Lucy McGarry for technical assistance.

References
Alberini CM. 2007. Reconsolidation: The samsara of memory
consolidation. Debates Neurosci 1: 17–24.
Anagnostaras SG, Maren S, Fanselow MS. 1999. Temporally gradedretrograde amnesia of contextual fear after hippocampal damage in rats:
Within-subjects examination. J Neurosci 19: 1106–1114.
Biedenkapp JC, Rudy JW. 2004. Context memories and reactivation:
Constraints on the reconsolidation hypothesis. Behav Neurosci 118:
956–964.
Biedenkapp JC, Rudy JW. 2007. Context preexposure prevents forgetting ofa contextual fear memory: Implication for region change in brain
activation patterns associated with recent and remote memory tests.
Learn Mem 14: 200–203.
Bontempi B, Laurent-Demir C, Destrade C, Jaffard R. 1999. Time-dependent
reorganization of brain circuitry underlying long-term memory storage.
Nature 400: 671–675.
Debiec J, LeDoux JE, Nader K. 2002. Cellular and systems reconsolidation in
the hippocampus. Neuron 36: 527–538.
Debiec J, Doyere V, Nader K, Ledoux JE. 2006. Directly reactivated, but notindirectly reactivated, memories undergo reconsolidation in the
amygdala. Proc Natl Acad Sci 103: 3428–3433.
Dudai Y. 2004. The neurobiology of consolidations, or, how stable is theengram? Annu Rev Psychol 55: 51–86.
Dudai Y. 2006. Reconsolidation: The advantage of being refocused. Curr
Opin Neurobiol 16: 174–178.
Eisenberg M, Kobilo T, Berman DE, Dudai Y. 2003. Stability of retrieved
memory: Inverse correlation with trace dominance. Science 301: 1102–
1104.
Frankland PW, Bontempi B. 2005. The organization of recent and remotememories. Nat Rev Neurosci 6: 119–130.
Frankland PW, Cestari V, Filipkowski RK, McDonald RJ, Silva AJ. 1998. The
dorsal hippocampus is essential for context discrimination but not forcontextual conditioning. Behav Neurosci 112: 863–874.
Frankland PW, Bontempi B, Talton LE, Kaczmarek L, Silva AJ. 2004. The
involvement of the anterior cingulate cortex in remote contextual fear
memory. Science 304: 881–883.
Frankland PW, Ding HK, Takahashi E, Suzuki A, Kida S, Silva AJ. 2006.
Stability of recent and remote contextual fear memory. Learn Mem 13:
451–457.
Jarrard LE, Meldrum BS. 1993. Selective excitotoxic pathology in the rat
hippocampus. Neuropathol Appl Neurobiol 19: 381–389.
Kim JJ, Fanselow MS. 1992. Modality-specific retrograde amnesia of fear.
Science 256: 675–677.
Lehmann H, Lacanilao S, Sutherland RJ. 2007. Complete or partial
hippocampal damage produces equivalent retrograde amnesia forremote contextual fear memories. Eur J Neurosci 25: 1278–1286.
Maviel T, Durkin TP, Menzaghi F, Bontempi B. 2004. Sites of neocorticalreorganization critical for remote spatial memory. Science 305: 96–99.
Milekic M, Alberini C. 2002. Temporally graded requirement for protein
synthesis following memory reactivation. Neuron 36: 521–555.
Morris RG, Inglis J, Ainge JA, Olverman HJ, Tulloch J, Dudai Y, Kelly PA.
2006. Memory reconsolidation: Sensitivity of spatial memory to
inhibition of protein synthesis in dorsal hippocampus during encoding
and retrieval. Neuron 50: 479–489.
Moscovitch M, Rosenbaum RS, Gilboa A, Addis DR, Westmacott R, Grady C,
McAndrews MP, Levine B, Black S, Winocur G, et al. 2005. Functional
neuroanatomy of remote episodic, semantic and spatial memory: A
unified account based on multiple trace theory. J Anat 207: 35–66.

No-Reminder Condition
Sixty rats were used in this condition. Of these, 21 rats received
hippocampal lesions and 39 received control lesions. Fear conditioning was conducted in CXT-A as described above. Hippocampal
or control surgery was performed 24 h or 28 d after the fear
conditioning trial. Rats in the control groups were tested the next
day in either CXT-A or CXT-B. Rats in the HPC groups were tested
either in CXT-A or CXT-B 7 d after surgery. A schematic outline ofthe procedure for the No-Reminder Condition and the number ofrats in each group are provided in Figure 1.

Reminder Condition
Eighty rats were used in this condition. Of these, 30 rats received
hippocampal lesions and 50 rats received control surgery. Initially,
all rats received a single fear conditioning session in CXT-A, as
described above. Twenty-eight days later, each rat received a reminder session in which it was transported to the appropriate test
room and placed either in the chamber of CXT-A or in thechamber of CXT-B. Neither the tone nor foot shock was presented.
After the 60-sec reminder session, the rat was then returned to its
home cage.
One day after the reminder session, hippocampal lesion orcontrol surgery was performed. Twenty-four hours later, memory
for the contextual fear response was tested in the control rats
either in the chamber of CXT-A or the chamber of CXT-B,
according to the test procedures described above. Rats with hippocampal lesions were tested seven days later in either CXT-A or
CXT-B.
A schematic outline of the procedure for the Reminder
Condition and the number of rats in each group are provided in
Figure 2.
Given the nature of this study, it was necessary to vary thetime between surgery and postoperative testing in the No-Rewww.learnmem.org

728

Learning & MemoryDownloaded from learnmem.cshlp.org on October 30, 2009 - Published by Cold Spring Harbor Laboratory Press

Memory reconsolidation and context specificity

Sara SJ. 2007. Retrieval and reconsolidation: Toward a neurobiology ofremembering. Learn Mem 7: 73–84.
Squire LR, Alvarez P. 1995. Retrograde amnesia and memory consolidation:
A neurobiological perspective. Curr Opin Neurobiol 5: 169–177.
Sutherland R, Lehmann H, Spanswick S, Sparks F, Melvin N. 2006. Growth
points in research on memory and hippocampus. Cdn J Exp Psy 60: 166–
174.
Sutherland RJ, O’Brien J, Lehmann H. 2008. Absence of systems
consolidation of fear memories after dorsal, ventral, or complete
hippocampal damage. Hippocampus 18: 710–718.
Suzuki A, Josselyn SA, Frankland PW, Masushige S, Silva AJ, Kida S. 2004.
Memory reconsolidation and extinction have distinct temporal andbiochemical signatures. J Neurosci 24: 4787–4795.
Wang SH, Teixeira CM, Wheeler AL, Frankland PW. 2009. The precision of
remote context memories does not require the hippocampus. Nat
Neurosci 12: 253–255.
Wiltgen BJ, Silva AJ. 2007a. Memory for context becomes less specific withtime. Learn Mem 14: 313–317.
Wiltgen BJ, Silva AJ. 2007b. Neural circuits mediating the retrieval of
specific and generalized context fear. Poster presented at the Annual
Meeting of the Society for Neuroscience, San Diego, CA.
Winocur G, Moscovitch M, Fogel S, Rosenbaum RS, Sekeres M. 2005.
Preserved spatial memory after hippocampal lesions: Effects of extensive
experience in a complex environment. Nat Neurosci 8: 273–275.
Winocur G, Wojtowicz JM, Sekeres M, Snyder JS, Wang S. 2006. Inhibitionof neurogenesis interferes with hippocampus-dependent memory
function. Hippocampus 16: 296–304.
Winocur G, Moscovitch M, Sekeres M. 2007. Memory consolidation ortransformation: context manipulation and hippocampal
representations of memory. Nat Neurosci 10: 555–557.

Moscovitch M, Nadel L, Winocur G, Gilboa A, Rosenbaum SR. 2006. The
cognitive neuroscience of remote episodic, semantic, and spatial
memory. Curr Opin Neurobiol 16: 179–190.
Moses SN, Winocur G, Ryan JD, Moscovitch M. 2007. Environmental
complexity affects contextual fear conditioning following hippocampal
lesions in rats. Hippocampus 17: 333–337.
Nader K. 2003a. Memory traces unbound. Trends Neurosci 26: 65–72.
Nader K. 2003b. Neuroscience: Re-recording human memories. Nature 425:
571–572.
Nader K, Hardt O. 2009. A single standard for memory: The case forreconsolidation. Nat Rev Neurosci 10: 224–234.
Nader K, Schafe GE, Le Doux JE. 2000. Fear memories require protein
synthesis in the amygdala for reconsolidation after retrieval. Nature
406: 722–726.
Packard M, McGaugh JL. 1992. Double dissociation of fornix and caudate
nucleus lesions on acquisition of two water maze tests: Further evidence
for multiple memory systems. Behav Neurosci 106: 439–446.
Paxinos G, Watson C. 1997. The rat brain in stereotaxic coordinates. Elsevier
Academic Press, Amsterdam, The Netherlands.
Phillips RG, Ledoux JE. 1994. Lesions of the dorsal hippocampal formationinterfere with background but not foreground contextual fear
conditioning. Learn Mem 1: 34–44.
Remondes M, Schuman EM. 2004. Role for a cortical input to hippocampal
area CA1 in the consolidation of a long-term memory. Nature 431: 699–
703.
Rosenbaum SR, Winocur G, Moscovitch M. 2001. New views on oldmemories: Re-evaluating the role of the hippocampal complex. Behav
Brain Res 127: 183–197.
Ross RS, Eichenbaum H. 2006. Dynamics of hippocampal and corticalactivation during consolidation of a nonspatial memory. J Neurosci 26:
4852–4859.
Sara SJ. 1973. Recovery from hypoxia and ECS-induced amnesia after
a single exposure to training environment. Physiol Behav 10: 85–89.

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Received April 6, 2009; accepted in revised form August 25, 2009.

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