B R I E F C O M M U N I C AT I O N S

The precision of remote context
memories does not require
the hippocampus

this latter view, the expression of the original memory in its
precise form always requires the hippocampus3.
In experimental animals, these memory consolidation processes
have been most successfully modeled using context fear conditioning,in which mice learn an association between a distinctive place (context)
and an aversive event (typically the delivery of a mild foot shock).
When returned to the same context, context memory is inferred froman increase in freezing behavior4. The precision of this context memory
may then be evaluated by comparing freezing levels in the training
context to those in an alternate context5–7 (Supplementary Noteonline). Only hippocampal lesions occurring in the days immediately
after training, but not thereafter, disrupt conditioned freezing in thetrained context4, consistent with the view that context memories may
be supported by extra-hippocampal brain regions at remote time
points. However, levels of generalized freezing in an alternate context
also increase over a similar timeframe, suggesting that the precision ofcontext fear memories declines with time5–7 (Supplementary Note).
Therefore, as memory age and memory precision tend to covary inthese studies, both the reorganization or transformation views canreadily account for spared freezing in the training context after
hippocampal lesions at remote time points. However, these twoviews make distinct predictions about the requirement of the hippocampus in the expression of remote context memories that maintaintheir original precision. The reorganization view predicts that precisememories may eventually become independent of the hippocampus,
whereas the transformation view predicts that the hippocampus isalways necessary for precision.
To produce durable, precise context memories in mice, we used acontext fear discrimination protocol8 (Supplementary Methods andb 80

aFreezing (%)

Figure 1 Delay-dependent decline in the precisionof context memories depends on training.
(a) Context discrimination design. (b,c) Levels ofconditioned freezing (b) were greater in context A(black bars) than in context B (white bars) whentested either 1 or 42 d following training (P o0.001, by paired t test) and levels of discrimination (c) did not differ at these two retention delays
following context discrimination training (P 40.05, by t test). (d) Contextual fear conditioning design. (e) Levels of conditioned freezing weregreater in context A (black bars) than in context
B (white bars) when tested 1 d (P o 0.001, bypaired t test), but not 42 d (P 4 0.05, by pairedt test), following training in the standard version ofcontextual fear conditioning. (f) Accordingly, levelsof discrimination declined in a delay-dependent
manner (P o 0.05, by t test). These andsubsequent graphs show means ± s.e.m.

1dABABABAB

42 dABABAA

B

4020Be 801dAAAB

42 dAc 0.6

B

600d

A

Discrimination index

Although the hippocampus may be important in the expression ofmemories soon after encoding, expression of the same (or at leastequivalent) memory may become independent of the hippocampus at later time points1. One predominant view is that thetransition of the memory from a hippocampus-dependent tohippocampus-independent form reflects a time-dependent processof reorganization that leads to the permanent storage of thememory in the cortex2. An alternative view proposes that thetransition from a hippocampus-dependent to hippocampus-independent form reflects a transformation of the memory from a precise(or detailed and contextually rich) form to a less precise (or genericand context-free) form in extra-hippocampal regions. According toA

B

60
40
20

B0

142
Delay (d)

1f0.4
0.2
0.0
142
Delay (d)42
Delay (d)Discrimination index

Although the clarity of many memories fades with time, somememories may maintain their original precision. Here weused a context discrimination procedure to evaluate whether
the hippocampus is important in maintaining precision as
memories mature. Spared discrimination in hippocampallesioned mice indicated that precise, remote context memories
may be supported by extra-hippocampal brain regions.

Freezing (%)© 2009 Nature America, Inc. All rights reserved.

Szu-Han Wang1–4, Cátia M Teixeira1–3, Anne L Wheeler1,3 &Paul W Frankland1–3

0.6
0.4
0.2
0.0142
Delay (d)1Program in Neurosciences and Mental Health, The Hospital for Sick Children, 555 University Avenue, Toronto, M5G 1X8, Canada. 2Department of Physiology, University ofToronto, 1 King’s College Circle, Toronto, M5S 1A8, Canada. 3Institute of Medical Science, University of Toronto, 1 King’s College Circle, Toronto, M5S 1A8, Canada.
4Present address: Centre for Cognitive and Neural Systems, Edinburgh University, Edinburgh, EH8 9JZ, UK. Correspondence should be addressed to P.W.F.
(paul.frankland@sickkids.ca).

Received 13 November 2008; accepted 18 December 2008; published online 1 February 2009; doi:10.1038/nn.2263

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1B R I E F C O M M U N I C AT I O N S

40LesionAB

60
40202000142
Delay (d)

c 1.0

Sham

Sham
80

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Lesion
80

60

60

60

60

4040

4040

20202020

e 1.5

Sham

Lesion0.50.0

–0.5
1d 80

042
Delay (d)142
Delay (d)0A C0A AB1

Discrimination index

60b 80Freezing (%)

AB

Discrimination index

Sham

Freezing (%)

Freezing (%)

a 80

C0.50.0–0.50A

Lesion1.0A/CA AB1f 80

A/AB1AAB1

NeuN-positive area

Time spent (%)

Supplementary Fig. 1 online). Mice were trained for three consecutive hippocampal dependence of precise context fear memories at differentdays in context A (always paired with shock) and context B (never times following training.
Mice received complete NMDA lesions of the hippocampus either 1
paired with shock), according to protocols approved by the Animal
Care Committee at The Hospital for Sick Children. Conditioned or 42 d following training in the context discrimination procedure (forfreezing, defined as the absence of all movement except for breathing, training data, see Supplementary Results and Supplementary Fig. 3
was then examined in both contexts either 1or 42 d later (Fig. 1a). When trained in thisSham
LesionLesiona 60bmanner, mice froze more in context A (the(day 5)(day 5)(day 9)40404050shock context) than in context B (the noshock context) at both delays (Fig. 1b). Nota40
303030bly, the degree of discrimination did not differ
30at these two time points (Fig. 1c), suggesting20202020
Shamthat the mice maintained precise representaLesion
10101010
tions of the two contexts over this period.
0The persistent discrimination contrasted
000012345678with a second group of mice that wereTraining day
Target Other
Target Other
Target Other
trained using a more standard context fear–Zone
Zone
Zone
6
conditioning protocol . During training,cSham
LesionShamLesiond 45,000 Shamthese mice received three shocks in context
A and were then tested in both context A andLesion
35,000context B either 1 or 42 d later (Fig. 1d).
1.5 mm
2.5 mmConditioned freezing levels were greater in
25,000context A than in context B when tested 1 dafter training. At the longer delay, however,
15,000conditioned freezing levels were similar in
1.7 mm
3.0 mmboth contexts (Fig. 1e). The delay-dependent
5,000
decline in discrimination (Fig. 1f) suggests
Dorsal
DorsalVentralVentralthat the precision of context memories
2.0 mm
3.5 mmdentate gyrus CA dentate gyrus CA
declined over time following training in thisArea
more standard context fear–conditioning
protocol5–7 (significant training protocol  Figure 3 Behavioral and histological quantification of hippocampal lesions. (a) In the water maze, thedelay interaction, F1,88 ¼ 5.68, P o 0.05). time required to reach the escape platform declined in sham-operated mice (closed circles), but not in
Because the discriminative (rather than non- hippocampal-lesioned mice (open circles), across days (P o 0.001, by ANOVA, significant day  surgery
discriminative5,9; Supplementary Results and interaction). Probe tests (arrows) were conducted on days 5 and 9. (b) Only sham-operated mice spent
more time searching the target zone (black) compared with other (white) zones in the probe test on day 5
Supplementary Fig. 2 online) fear condition- (P o 0.001, by paired t test). (c) Representative images from sham- and lesioned-mice at different
ing protocol prevented the time-dependent levels of the hippocampus stained for the neuronal specific marker NeuN (mm posterior to bregma).
decline in the precision of context memories, (d) NMDA lesions led to almost complete neuronal depletion in the dorsal and ventral CA fields andwe used these procedures to evaluate the dentate gyrus.
Latency (s)© 2009 Nature America, Inc. All rights reserved.

Freezing (%)

Figure 2 Hippocampal lesions disrupt recent, but not remote, context discrimination. (a) In sham-operated controls, the levels of60
conditioned freezing were greater in context A (black bars) than in context B (white bars) when tested either 1 or 42 d following
training (P o 0.005, by paired t test). (b) Levels of conditioned freezing were similar in context A (black bars) and context B
40(white bars) in the group of mice that received hippocampal lesions 1 d after training (P 4 0.05, by paired t test). In contrast,
the levels of conditioned freezing were greater in context A (black bars) than in context B (white bars) in the group of mice that20
received hippocampal lesions 42 d after training (P o 0.005, by paired t test). (c) Hippocampal lesions preferentially disrupted
context discrimination when conducted 1 d, but not 42 d, after training (significant surgery  delay interaction, P o 0.05).
0(d) Discrimination of context A (black bars) versus novel context C (white bars) or context A (black bars) versus hybrid context
ShamLesion
AB1 (white bars) was spared following hippocampal lesions 42 d after context discrimination training (P o 0.005, by pairedt test). (e) Levels of discrimination were similar in sham and hippocampal-lesioned mice, but differed between experiments (P o 0.001, by ANOVA).
(f) Discrimination of contexts A and AB1 was abolished in hippocampal-lesioned, but not sham-operated, mice following re-testing (P o 0.001, by t test).

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B R I E F C O M M U N I C AT I O N Sonline). We assessed conditioned freezing 7 d after surgery in bothcontext A and context B. In control mice, conditioned freezing levels
were greater in context A versus context B at both delays (Fig. 2a),confirming that the precision of context memories can be maintained
for several weeks following this type of training. In contrast, hippocampal lesions that occurred 1 d after training reduced freezingand abolished discrimination, with mice showing similar levels offreezing in context A and context B. However, discrimination waspreserved when these lesions were performed 42 d after training, withhippocampal-lesioned mice freezing significantly more in context A
than in context B (P o 0.005; Fig. 2b). The spared discrimination inthese lesioned mice suggests that the expression of precise context
memories does not require the hippocampus at remote time points(significant surgery  delay interaction, F1,30 ¼ 7.31, P o 0.05; Fig. 2c).
This temporally graded pattern of retrograde amnesia resembles thatobserved in memory-impaired patients with hippocampal damage,where recently acquired memories are typically more affected thanremotely acquired memories. However, one critical distinction between
the reorganization and transformation views of memory consolidation
is whether spared remote memories are equivalently detailed inmemory-impaired patients and control subjects, and it is possiblethat more thorough interrogation might reveal differences10,11. Accordingly, we trained additional groups of mice in the context discrimination procedure (for training data, see Supplementary Results andSupplementary Fig. 4 online), lesioned the hippocampus 42 d laterand examined conditioned freezing in either context A versus a novelcontext (C) or in context A versus a hybrid context (AB1) that
contained components of both context A and context B (Supplementary Fig. 1). Both sham-operated and hippocampal-lesioned micefroze significantly more in context A than in the novel or hybridcontexts (P o 0.005; Fig. 2d). Notably, the degree of discrimination did
not differ between sham-operated and hippocampal-lesioned mice(Fig. 2e), indicating that discrimination was preserved in hippocampal
mice even under these more rigorous testing conditions. However,additional testing led to pronounced reductions in conditioned
freezing levels and discrimination in the hippocampal-lesioned
mice (Fig. 2f; Supplementary Fig. 3). Therefore, although precise
context memories may be supported by extra-hippocampal regions atremote time points, these additional analyses revealed that thesememories are nonetheless not identical to those in control mice;
without the hippocampus, the memory is more fragile, suggestingthat the integrity of the hippocampus is important for robust
memory expression12.
To verify the effectiveness of the lesion, we next trained mice in thewater maze. As expected, hippocampal-lesioned mice were severely
impaired relative to controls, taking longer to locate the platform (days1–4; Fig. 3a). In a probe test on day 5, control mice searched selectivelyin the region of the pool that formerly contained the platform, whereas
hippocampal-lesioned mice searched nonselectively (Fig. 3b). Even
with an additional 4 d of training, there was no further improvement inescape latency (Fig. 3a) or in probe test performance in hippocampallesioned mice (Fig. 3b). Subsequent histological analyses revealed that

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lesions resulted in more than 95% depletion of neurons in the dorsal
and ventral CA fields and dentate gyrus, with negligible extrahippocampal damage (Supplementary Methods and Fig. 3c,d). The
completeness of these lesions therefore excludes the possibility
that spared discrimination might be supported by residual hippocampal tissue3.
In conclusion, spared discrimination following hippocampal lesions
is inconsistent with the view that the hippocampus is always required
for the expression of precise (contextually rich) memories3. However,
the observed fragility of this remote context memory is also counter tothe view that such a memory can be supported independently by extrahippocampal regions2. Indeed, our data point to a prolonged requirement for the hippocampus in the expression of these types ofmemory12, with the hippocampus perhaps being involved in theintegration of new information into existing extra-hippocampal networks13,14. The fragility of the spared memory in our study contrasts
with two recent reports that involved much more extensive preoperative experience in the training environment14,15. This suggeststhat the amount of pre-operative experience determines whether
memories become partially or completely independent of the hippocampus at remote time points.
Note: Supplementary information is available on the Nature Neuroscience website.

ACKNOWLEDGMENTS
We thank S. Josselyn, B. Wiltgen and A. Corvelo for comments on thismanuscript. This work was supported by grants from the Canadian Institutes ofHealth Research (MOP-77561) and the EJLB Foundation (P.W.F.). C.M.T. andA.L.W. received support from the Ontario Ministry of Research and Innovationand the Ontario Mental Health Foundation, respectively.
AUTHOR CONTRIBUTIONS
S.-H.W. and P.W.F. conceived and designed the experiments. S.-H.W., C.M.T. andA.L.W. performed the behavioral studies and carried out the statistical analyses.
S.-H.W. conducted the lesions and histological analyses. P.W.F. supervised theproject and wrote the paper with S.-H.W.
Published online at http://www.nature.com/natureneuroscience/
Reprints and permissions information is available online at http://www.nature.com/reprintsandpermissions/

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