NeuroImage 83 (2013) 593–598

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NeuroImage
journal homepage: www.elsevier.com/locate/ynimg

Hippocampal volumes differ across the mouse estrous cycle, can change
within 24 hours, and associate with cognitive strategies
Lily R. Qiu a,1, Jürgen Germann a,1, Shoshana Spring a, Christina Alm b, Dulcie A. Vousden a,
Mark R. Palmert b,c,d, Jason P. Lerch a,e,⁎aMouse Imaging Centre, Program in Neuroscience and Mental Health, The Hospital for Sick Children, Toronto, Ontario, Canada
Division of Endocrinology, The Hospital for Sick Children, Toronto, Ontario, Canada
Department of Pediatrics, University of Toronto, Toronto, Ontario, CanadadDepartment of Physiology, University of Toronto, Toronto, Ontario, CanadaeDepartment of Medical Biophysics, University of Toronto, Toronto, Ontario, Canadabca r t i c l e

i n f oArticle history:
Accepted 25 June 2013
Available online 3 July 2013a b s t r a c t
Recent human and rodent brain imaging studies have shown that the shape of the brain can be changed byexperience. These mesoscopic alterations in neuroanatomy are hypothesized to be driven by changes atthe level of neuronal processes. To examine whether the shape of the brain changes rapidly, we used MRI
to examine changes in the volume of the hippocampus across the 4–6 day estrous cycle in the female
mouse. It is well known that changing steroid levels across the cycle inﬂuence dendritic spine maturation
and alter synapse density in the hippocampus; our results show that the estrous cycle is associated withapproximately 2–3% changes in hippocampal volume as seen by high-resolution ex-vivo MRI. Changes in hippocampal volume are, moreover, associated with a switch between hippocampal and striatal based navigation strategies in solving the dual choice T-maze in the same mice. A second experiment, using in-vivo
MRI, suggests that these changes in hippocampal volume can occur over a 24 hour period. In summary, weshow that the brain is highly plastic at a mesoscopic level of resolution detectable by MRI, that volumetricincreases and decreases in hippocampal volume follow previously established patterns of changes inneuropil, and that these changes in volume predict changes in cognition.
© 2013 Elsevier Inc. All rights reserved.

Introduction
There are intriguing indications that the volume and shape of thehuman brain can change over short periods of time. Direct evidence
includes increases in local gray matter volume in the parietal andoccipital cortices when learning to juggle (Draganski et al., 2004;
Driemeyer et al., 2008; Scholz et al., 2009) or acquiring similar
dexterity skills (Taubert et al., 2012). There is, however, signiﬁcant
controversy over the reproducibility of plasticity related brainchanges detected by MRI (Thomas and Baker, 2012); and the underlying mechanism is uncertain (Zatorre et al., 2012). Understanding
how the brain changes at a mesoscopic resolution — i.e. at the levelof brain regions and nuclei — is a key challenge for the brain imaging
community, as it will both (i) answer fundamental questions of how
the brain is shaped and how malleable that shape is, and (ii) provide
further understanding of abnormal brain plasticity found in multiple
neurologic and psychiatric diseases.
⁎ Corresponding author at: The Hospital for Sick Children, 25 Orde St., Toronto, ON,
M5T 3H7, Canada.
E-mail address: jason.lerch@sickkids.ca (J.P. Lerch).
1
These authors contributed equally.
1053-8119/$ – see front matter © 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.neuroimage.2013.06.074

Part of the answer will come from work in animal models, where
high-ﬁeld MRI can provide the link between the rich literature ofcellular brain plasticity in model systems and the emerging ﬁeld of
human systems-level brain plasticity. We have shown, for example,
that 5 days of training mice on the Morris water maze changes thevolume of the hippocampus and striatum by 2–4% as detected byhigh-ﬁeld MRI and correlates with an immunohistochemical stain
for neuronal process remodelling (Lerch et al., 2011b). Similarly,
maze training in the rat has shown diffusion related brain changes
associated with astrocyte immunoreactivity and synapse staining(Blumenfeld-Katzir et al., 2011; Sagi et al., 2012).
The dominant paradigm in studying mesoscopic brain plasticity has
been to use an external stimulus, such as training on mazes or dexterity
tasks, and use imaging to examine the outcomes of these stimuli on thebrain. Brain plasticity, however, can also occur in response to endogenous signals; to, for example, cycling hormone levels.
Ovarian steroids have been well established to affect the brain(McEwen and Alves, 1999), in particular the hippocampus with its
high density of estrogen receptors (Woolley, 1998). Estradiol induces
maturation in dendritic spines in the rodent hippocampus, and has
been associated with increases in synaptic markers in rodents andprimates (Spencer et al., 2008a; Woolley and McEwen, 1992). Over594

L.R. Qiu et al. / NeuroImage 83 (2013) 593–598

the approximately 4–6 days of the rodent estrous cycle, synapse
markers peak at proestrus when estradiol levels are high, decline rapidly over a 24 hour period as the cycle enters estrus where estradiol
and progesterone levels are low, and then rise gradually over the
next several days in metestrus and diestrus (Spencer et al., 2008a;
Woolley, 1998) (illustrated in Fig. 1a). There is also an intriguing association between the estrous cycle and cognition: during proestrus,
when estradiol is high, rats are much more likely to choose a hippocampal dependent place strategy to solve a maze than during estrus,
when steroid levels are low and a non-hippocampal response
strategy is more common (Korol and Kolo, 2002; Korol et al., 2004).
In this study we thus set out to address the following hypothesis:
the neuronal alterations known to occur with changing steroid levelsa)Materials and methods
We conducted two experiments. In the ﬁrst, mice were assessed
for their estrous cycle stage, trained on a T-maze to determine
which cognitive strategy they used to solve the maze, and thensacriﬁced and their brains imaged with high-resolution ex-vivo MRI.

b)

pTrkB

pAkt

across the 4–6 day estrous cycle will be associated with changes inhippocampal volume as seen by MRI between estrous cycle stages.
Furthermore, we expect that these volume changes will be associatedwith a concomitant alteration in the cognitive strategy used to solve a
maze, with larger hippocampi during proestrus associated with use ofa place strategy.

PSD-95Synaptophysin

EstradiolEx-vivo MRI

Segmentation

In-vivo MRI

Segmentation

Progesterone

proestrus

estrusmetestrus

diestrus

Train

Probe Test

Estrous Cycle Stage

R

P

Goal

c) Experiment 1: behaviour and ex-vivo MRI
Day 0Food deprivation and maze habituation

Start
Day 3

10AM

1PM 1:30PM
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2PM

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d) Experiment 2: in-vivo MRI
Day0

Day 1

3PM
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ec

nj
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10AM

1PM

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n1

ge

n2

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Fig. 1. An outline of hypotheses and experimental design. As has been repeatedly shown, the estrous cycle is associated with an increase in estradiol toward proestrus followed by a
sharp decline in estrus. These changes in steroids affect the morphology of dendritic spines and numbers of synapses as indicated by multiple synaptic markers, as shown in(a). During proestrus, when estradiol peaks, post-synaptic markers (PSD-95) and phosphorylated Akt (pAkt), which leads to new spine formation, are highest, as is the Brain
Derived Neurotrophic Factor (BDNF) receptor TrkB (pTrkB). These values all rapidly decrease in estrus, and begin to rise again through metestrus and diestrus. Conversely,
synpatopysin, a pre-synaptic marker, is relatively low in proestrus and increases toward diestrus. These data are synthesized and adapted from McLean et al. (2012) and Spencer
et al. (2008a). We imaged mice across the estrous cycle and automatically measured volumes of the hippocampus. Sample ex-vivo and in-vivo images and the corresponding atlassegmentations are shown in (b) and the experimental design in (c–d). For the ex-vivo experiment, mice were habituated to the T-maze and food deprived to 85% of their starting
body weight over three days. On the fourth day, mice were trained on the T-maze and their spontaneous navigation strategy assessed on a probe trial. If the mouse made the same
direction turn (i.e. left or right) when starting in the opposite arm for the probe trial it was considered to be using a response strategy; if the mouse used the spatial cues in the roomand made the correct turn for the food reward, it was considered to be using a place strategy. After the probe trial the mice were perfusion ﬁxed for ex-vivo MRI. For the in-vivo
experiment, no behavioural testing was performed. Mice instead were injected with Mn2+ the day before the ﬁrst in-vivo imaging session, and imaged a second time 24 hours later.
This 3 day procedure was performed starting on post-natal day 76 (with imaging on days 77 and 78).

L.R. Qiu et al. / NeuroImage 83 (2013) 593–598

595This experiment allowed us to relate hippocampal volume to estrous
cycle stage and cognition. In the second experiment, mice were imaged at different stages of the estrous cycle using in-vivo Manganese
Enhanced MRI (MEMRI) to measure hippocampal volume change overa 24 hour period. Methods and experimental design are illustrated inFig. 1. All experiments were approved by the Animal Ethics Committeeof the Toronto Centre for Phenogenomics.

cells; diestrus consisted predominantly of leukocytes (McLean et al.,2012). Staging was identical for the in-vivo study, except that thevaginal secretions were collected at 10 AM, prior to administrationof anaesthesia and imaging in the afternoon.

Mice

After estrous cycle staging, mice were anaesthetized by an intraperitoneal injection of a ketamine (100 mg/kg) and xylazine (20 mg/kg)
solution. Mice were then transcardially perfused through the left ventricle with 30 mL of phosphate-buffered saline (PBS), 1 μL/mL Heparinand 2 mM ProHance at a ﬂow rate of 1.0 mL/minute. This was followed
by infusion of 30 mL 4% paraformaldehyde (PFA) and 2 mM ProHance
at 1.0 mL/minute for ﬁxation. After perfusion, the head along withskin, lower jaw and ears were removed in order to isolate the brain,
still in the skull. Following dissection, skulls were placed in individualvials of 4% PFA and 2 mM ProHance solution overnight at 2 °C. Afterat least 24 hours, skulls were transferred to a solution of 4% PFA,
0.02% sodium azide and 2 mM ProHance.

For the ex-vivo experiment, 39 female C57BL/6 mice, aged
74–95 days, were used during varying stages of their estrous cycle(proestrus n = 8, estrus n = 9, metestrus n = 13, diestrus n = 9).
For the in-vivo experiment, 20 female C57BL/6 mice were imaged
twice each on post-natal days 77 and 78. On day 77 the estrous cycle
was proestrus n = 5, estrus n = 2, metestrus n = 7, diestrus n = 6.
Mice were obtained through in-house breeding at the Toronto Centre
for Phenogenomics or ordered from Jackson Laboratories.
T-maze habituation
For the ﬁrst ex-vivo experiment mice were food deprived for
3 days prior to and including the day of testing. They were weighedeach day to ensure that no mice fell below 85% of their initial weight.
For each of the 3 days prior to testing, each mouse was given 10 minutes
to openly explore the T-maze with food at the arms to allow habituationof both the maze and food reward. The 20 mice used for the second
in-vivo experiment did not undergo maze habituation, training or
testing.
Maze training
Noldus' Ethovision XT was used to program the protocol for theT-maze. Mice were trained to enter one of the arms to obtain a food
reward until 9/10 consecutive trials were completed successfully.
Immediately after training, a probe trial was administered in which
the start box was rotated 180 degrees from the original location.
Four spatial cues (triangle, circle, etc.) were placed around theT-maze apparatus at ﬁxed locations that did not change from initialtraining to probe, or between trials with different mice. Response
learning was displayed if the mouse incorrectly made the same direction turn (i.e. right or left) during the probe trial as during initialtraining. Place learning was demonstrated if the mouse made thecorrect turn to obtain the food reward, i.e. the opposite turn fromthat during initial training. From this behavior we inferred that themouse used the spatial cues around the T-maze to navigate to thebaited arm. A second observer, blinded to the estrous stage of themice, viewed video recordings of 10 separate probe trials to ensureinter-rater reliability for cognitive strategy assessment.
Estrous cycle stage identiﬁcation
To determine the estrous cycle stage of each mouse vaginal secretions were obtained from each mouse. For the ex-vivo study, mice
were staged on each of the 3 days of habituation to increase theaccuracy of the estrous cycle stage identiﬁcation on the day of training. On the day of T-maze training, mice were staged after the training
was completed (approximately 2 PM in the afternoon). Mice wererestrained by the tail, upside down, to allow placement of a plastic
pipette tip inside the vagina. The pipette was ﬁlled with 30 μL of
saline, and was ﬂushed in and out of the vagina three to ﬁve times.
The ﬁnal ﬂush was placed on a glass slide and viewed under a lightmicroscope under 10 × magniﬁcation. Each stage was characterized
by the dominance of a cell type: proestrus was predominantly nucleated epithelial cells; estrus contained anucleated corniﬁed cells;
metestrus contained leukocytes, corniﬁed and nucleated epithelial

Preparation for ex-vivo brain imaging

Ex-vivo MR imaging
A multi-channel 7.0 Tesla, 40 cm diameter bore magnet MRI scanner(Varian Inc. Palo Alto, CA) was used to acquire images of mouse brains.
Brains were intact in their skulls and placed in Fluorinert, and 16
samples were scanned at one time in a 16-coil solenoid array. Parameters used were: a T2-weighted 3D fast spin-echo sequence, with TR =2000 ms, echo train length = 6, TEeff = 42 ms, ﬁeld-of-view = 25 ×
28 × 14 mm and matrix size = 450 × 504 × 250 (Cahill et al., 2012;
Lerch et al., 2011a).

In-vivo MR imaging
We used manganese enhanced MRI (MEMRI) to measure neuroanatomical changes within mice occurring across 2 consecutive
days. Mice were injected intraperitoneally with 30 mM manganesechloride (MnCl2) solution in fractionated doses (0.4 mmol/kg): one
half of the total dose at 18 hours and the second half at 17 hours before the ﬁrst scan. Mice were scanned 7 at a time in separate transmit/
receive coils two times, 24 hours apart. Prior to each scan, mice wereanaesthetized with 4% isoﬂurane and placed within the magnet bore
for imaging. Mice were maintained at a body temperature of 35 °C on
1% isoﬂurane. Mn-enhanced images were acquired using a spoiled gradient echo sequence with TR = 0.1 s, TE = 3.7 ms, ﬂip angle = 55°,
ﬁeld-of-view = 35 × 21 × 21 mm, matrix size = 280 × 168 × 168,and two averages for a total of 1 hour and 34 minutes scan time.
Post-scanning, mice were transferred to a heated cage to allow forrecovery from the anaesthetic.

Image registration and measures of hippocampal volume
In-vivo and ex-vivo images were analyzed separately. In each casea fully automated image registration based pipeline was used to alignall scans into spatial correspondence (Lerch et al., 2011a). A digitalatlas was then used to compute volumes for 62 separate structuresin the brain (Dorr et al., 2008). The volume of the hippocampus,
deﬁned to include CA1, CA2, and CA3, was used as the main outcome
measure. For the ex-vivo data, hippocampal volume was normalized
and measured as a percentage of whole brain volume; for thein-vivo data, the difference in volume (as a percentage of brain
volume) between adjacent scan days (i.e. day 78 minus day 77) was
computed.

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L.R. Qiu et al. / NeuroImage 83 (2013) 593–598

Statistical analysis

Results

For the ex-vivo data, estrous stage was treated as an orderedfactor based on reported estradiol and progesterone levels (estrus b
metestrus b diestrus b proestrus) (McLean et al., 2012; Spencer etal., 2008a,b), and a linear model with F(3, 35) degrees of freedom
used to assess the relation between hippocampal volume and cycle
stage. For the in-vivo data, the transition between stages (i.e. metestrus to diestrus) for each mouse on the 2 scan days was determined.
Mice grouped into four possible categories: (1) starting in an hypothesized high hippocampal volume stage of the estrous cycle (proestrus
and diestrus) and remaining in a presumed high hippocampal volume
stage, (2) starting in a high hippocampal volume stage and going to
low hippocampal volume stage (estrus and metestrus), (3) startingin a low volume stage and remaining in a low volume stage, and(4) starting in a presumed low volume stage and transitioning to ahigh volume stage. A Welch two sample t-test with 10.2 degrees of
freedom assessed whether volume changed when going from ahypothesized high to low volume stage versus moving from an
hypothesized low to high volume stage. For the behavioral data,
differences in proportions of mice using a place or response strategy
in each stage of the estrous cycle was assessed using a χ2 test with
3 degrees of freedom.

Behavior
There was no difference by estrous stage in the number of trials
needed to reach criteria (p = 0.78, Fig. 2a). The probe trial revealed
a signiﬁcant difference in the proportion of mice using a place strategy(p = 0.03, Fig. 2b). There was perfect consistency between raters on
assessing navigation strategy on the probe trial (10/10).
Hippocampal volume
Hippocampal volume, as assessed by high-resolution ex-vivo MRI,
was signiﬁcantly related to estrous cycle stage (p = 0.004, Fig. 2c),with the maximum difference between adjacent stages reaching
2.8%. Hippocampal volume furthermore predicted navigation strategy
on the T-maze (p = 0.03), with larger volume predicting the use of aplace strategy and smaller volume the use of a response strategy.
Twenty different female mice were then imaged twice eachin-vivo and differences in hippocampal volume across the 24 hour
time period between adjacent scans computed. The difference inhippocampal volume across 24 hours increased when going from astage with low baseline hippocampal volume (estrus or metestrus)a) Trials to Criteriab) Navigation Strategy
Strategy

ANOVA, p=0.78

Response

1.0
Chi−squared, p=0.03

0.8

Proportion

Trials to Criteria

30

Place

20

0.6
0.4
0.2
0.0

10
proestrus

estrusmetestrus diestrus

proestrus

Estrous Cycle Stage

3

ANOVA, p=0.004

4.4

metestrus diestrusd) Hippocampal Volume Changet−test, p=0.01

Hippocampal Volume(% change over 24 hours)

Hippocampal Volume (% Brain Volume)c) Hippocampal Volume
4.6

4.5

estrus

Estrous Cycle Stage

2
1
0
−1
−2
−3

4.3
proestrus

estrusmetestrus diestrus

Estrous Cycle Stage

D/P−to−E/M E/M−to−E/M E/M−to−D/P D/P−to−D/P

Estrous Cycle Transition

Fig. 2. Results. (a) There was no difference across estrous cycles stages in the mice's ability to solve the T-maze (p = 0.78). Nevertheless, the strategy used to solve the maze
differed across the cycle (p = 0.03), with mice in estrus least likely to use a place strategy (b). The estrous cycle stage each mouse was in further related directly to ex-vivo measured hippocampal volume (p = 0.004), with volume being lowest in estrus and increasing until proestrus (c). A separate cohort of mice was then imaged in-vivo and differencesin hippocampal volume determined across a 24 hour time period. As seen in (d) volume increased when mice were in estrus/metestrus transitioning to diestrus/proestrus, anddecreased if transitioning from diestrus/proestrus to estrus/metestrus (p = 0.01). All error bars are SEM.

L.R. Qiu et al. / NeuroImage 83 (2013) 593–598

to high baseline hippocampal volume (proestrus or diestrus) anddecreased when transitioning from a baseline of high hippocampal
volume to low hippocampal volume. Total hippocampal volume
change in transitioning between stages, as measured by in-vivo
MRI, was 2.2% and was signiﬁcant (p = 0.01, Fig. 2d).
Discussion
Here we show that hippocampal volume in female mice changes
during the estrous cycle, mirroring known alterations in steroids
across the cycle and relating directly to behavior. This constitutes
direct evidence that the mesoscopic anatomy of the brain is capableof changing in ways detectable by MRI over short periods of time.
We used two separate experiments to relate hippocampal volume tothe estrous cycle. In the ﬁrst experiment, a cohort of 39 young adultfemale mice were tested on a T-maze to assess their propensity to use
a hippocampus-dependent place strategy to solve the maze. Aftermaze training, which takes approximately 30–45 minutes per mouse,
the stage of the estrous cycle was assessed, following which the animal
was sacriﬁced and the brains prepared for high resolution ex-vivo MRI.
This experiment allowed us to determine that, given a population ofyoung female mice, hippocampal volume differs dependent on the
estrous cycle. These differences follow known steroid levels, withhippocampal volume lowest in the estrus phase, and as estradiol andprogesterone levels increase toward their peak in proestrus anddiestrus respectively so does hippocampal volume. These alterations
in hippocampal volume tie into behavior; we showed that mice in
estrus are most likely to use a response strategy to solve the T-maze(as previously reported in rats (Korol and Kolo, 2002; Korol et al.,2004)), with likelihood for using a place strategy increasing with hippocampal volume and known presence of ovarian steroids during proestrus and diestrus.
The ﬁrst experiment suggested that hippocampal volume changes
between the four stages of the estrous cycle. In the second experiment we determined that changes in hippocampal volume acrossthe estrous cycle can be detected in-vivo and occur within a
24 hour period. We divided our mice into four groups based on
expected hippocampal volume on day 1 and day 2. We found thathippocampal volume did indeed decrease between the two scans if
the mouse started in a stage with presumed high volume (proestrus
and diestrus) and transitioned to a stage with presumed low volume(estrus and metestrus) whereas volume increased if going frompresumed low to high volume stages.
The effects of estrogen and to a lesser extent progesterone on the
hippocampus have been well studied at a cellular level, giving us important clues about the origin of the volume changes we have shown
here. The dominant, often reported morphological change is an approximately 30% increase in dendritic spines in rats in late proestrus(when estradiol is high) compared to late estrus (when estradiol islow) (Woolley, 1998; Woolley and McEwen, 1992, 1994). Alterationsin spine numbers in mice across the estrous cycle are not known,
though multiple synaptic markers (PSD-95, pAkt, Synpatophysin)
alter during the cycle (Spencer et al., 2008a) and an increase in estradiol results in greater numbers of multi-synaptic boutons and inlarger “mushroom” shaped spines (Li et al., 2004; Spencer et al.,2008b). The alterations in synaptic markers are concentrated in area
CA1 of the hippocampus in rats, but appear to affect the entire hippocampus more evenly in mice (Spencer et al., 2008b). There is also
some suggestion that estradiol increases astrocyte volume (Spencer
et al., 2008b), though this is not as well replicated (Woolley, 1998).
In summary, the large alterations in spines and synapses that havebeen well replicated in the rodent are highly likely to underlie thevolumetric changes we report using MRI.
The purported link between spines/synapses and mesoscopic
volume is well in line with the two previous studies that have linked
rodent MRI brain plasticity with cellular markers (Blumenfeld-Katzir

597

et al., 2011; Lerch et al., 2011a,b; Sagi et al., 2012). When we trained
mice on a water maze we were able to detect hippocampal volume
increases of ~ 3–4% which correlated with GAP-43, a marker ofneuronal process remodelling (Lerch et al., 2011b). Similarly, when
Blumenfeld-Katzir and colleagues trained rats on a water maze, alterations in water diffusion properties were linked with changes inastrocyte as well as synapse staining intensity in the hippocampal formation (Blumenfeld-Katzir et al., 2011; Sagi et al., 2012). The 2–3%
increase in hippocampal volume reported in the current study,
where endogenous estrous cycling with its well established effecton synapses likely drove the volumetric changes, further cements
the link between alterations at the level of neuronal and glial processes
and MR detectable volume.
There are a few caveats to our study in so far as our experimental
setup could subtly alter natural cycling and/or steroid levels in mice.
The food deprivation required for the behavioural testing might be disruptive as could, more severely, the anaesthesia involved in the in-vivo
MRI protocol. Indeed, in the in-vivo experiment, not all mice transitioned
to the next stage in the cycle, though this could also be due to each cycle
stage not necessarily being 24 hours long. We were therefore underpowered to look at all possible stage transitions and thus grouped our
mice into two cycle stages (proestrus/diestrus with presumed highsteroid levels and hippocampal volume, and estrus/metestrus with
presumed low steroid levels and hippocampal volume) and transitions
between them. The experimental protocols, however, require food deprivation and anaesthesia and thus will unavoidably be disruptive to someextent. A further caveat is that we did not measure steroid levels in individual animals. The relation between cycle stage and steroid levels has
been well established, yet future work could break down variation within stages by correlating brain morphometry against each mouse’s estrogen and progesterone levels.
In summary, the experiment described above adds to our understanding of mesoscopic brain plasticity as follows:• Mesoscopic brain plasticity can be rapid – we found the hippocampus
changes in volume within 24 hours.
• The fact that mesoscopic brain plasticity is rapid suggests that it may
be driven by membrane-initiated signalling, since new gene products
from nucleus-initiated signalling take over 24 hours (Spencer-Segal
et al., 2012), and is related to dendritic spine maturation and synapse
numbers (Li et al., 2004; Spencer et al., 2008a).
• Brain shape is rapidly modiﬁed by endogenous neuromodulators(i.e. ovarian steroids), as suggested by human imaging studies(Protopopescu et al., 2008), as well as by the previously establishedlearning/training paradigms (Blumenfeld-Katzir et al., 2011; Lerchet al., 2011b).
• The changes in hippocampal volume tightly relate to changes incognition. Previous data from our group showed that directed
spatial learning in a water maze grows the hippocampus if themice are forced to rely on distal spatial cues to ﬁnd the escapeplatform (Lerch et al., 2011b); in the current study, on the other
hand, endogenous signalling changes the volume of the hippocampus which in turn biases learning strategies when both aplace (hippocampal) or response (striatal) strategy are available.
Our work has important implications for neurocognitive investigations using structural MRI to measure changes in the brain. Most
importantly, the fact that the brain can change so rapidly suggeststhat examining changes in the brain within hours or days, as opposedto the usual time period of weeks or months, could be promising. For
human imaging studies, where conventional longitudinal paradigms
designed to detect changes in the brain image every few weeks
or months, designing experiments examining short term changes
in the brain could be signiﬁcantly more powerful, since isolatingchanges to just the experimental manipulations as opposed to all
the other activities of daily living the subjects carry out will be598

L.R. Qiu et al. / NeuroImage 83 (2013) 593–598

signiﬁcantly easier. As the methods used by the brain imaging community become more sensitive, whether due to improved acquisition
methods at higher ﬁelds or better data analysis algorithms, we believethat the brain will be found to be constantly but subtly changing in response to endogenous (i.e. steroids) as well as external factors.
Conﬂicts of interest
There are no conﬂicts of interest to declare.
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