Behavioural Brain Research 87 (1997) 223 – 232
Assessing the magnitude of the allocentric spatial deficit associated
with complete loss of the anterior thalamic nuclei in rats
E.C. Warburton, A.L. Baird, J.P. Aggleton *
School of Psychology, Uni6ersity of Wales, College of Cardiff, PO Box 901, Cardiff CF1 3YG, UK
Received 1 October 1996; received in revised form 15 January 1997; accepted 15 January 1997
Abstract
The behavioural effects of complete lesions of the anterior thalamic nuclei (ANT), the anterior thalamic nuclei plus the lateral
dorsal nucleus (ANT +LD), and fornix (FX) were compared using a series of tests of spatial memory. All three lesion groups
were found to have an equally severe and long-lasting impairment in the acquisition of a T-maze alternation task when compared
with the control animals (COMB SHAM). In Experiment 2, the control animals were able to perform the alternation task when
the test trial was started from a different location to the sample trial, so demonstrating that they were able to use allocentric cues
in order to differentiate the most recently visited arm. In contrast, all the lesion groups performed close to chance level. In fact,
for this condition the ANT +LD group was significantly worse than the FX group. In contrast, none of the lesion groups was
impaired on an egocentric discrimination and subsequent reversal task (Experiment 3). The control animals came from two
different control procedures, a surgical control sub-group (SHAM) and a group of animals that received injections of
N-methyl-D-aspartic (NMDA) into the fornix (NMDA SHAM). There were no differences in the performance levels of the
NMDA SHAM group compared with the surgical control group in any of the experiments conducted, so showing that the
anterior thalamic lesion effects were not due to non-specific damage to the fornix by NMDA. This series of experiments
demonstrated that complete lesions of the anterior thalamic region impair the ability to process allocentric information, and
provide evidence for a contribution from the lateral dorsal thalamic nucleus. © 1997 Elsevier Science B.V.
Keywords: Spatial memory; Lesion; Rat
1. Introduction
Damage to the anterior thalamic nuclei in rats has
been found to disrupt the acquisition and performance
of a variety of spatial tasks [2 – 4,19]. The same class of
tasks is also severely disrupted following damage to
both the hippocampus and fornix [3,4,19]. In view of
these parallel effects and the dense direct, and indirect,
projections from the hippocampus to the anterior thalamic nuclei via the fornix, it has been supposed that the
hippocampus and the anterior thalamic nuclei form
principal components of a circuit underlying normal
spatial memory [5,18,21]. The magnitude of the anterior
* Corresponding author. Tel.: +44 1222 874563; fax: + 44 1222
874858.
0166-4328/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII S 0 1 6 6 - 4 3 2 8 ( 9 7 ) 0 2 2 8 5 - 7
thalamic lesion effects are, however, typically less severe
than those observed after hippocampectomy or fornix
transection [3,8,9,19]. Explanations of this difference
include the existence of other hippocampal/fornix outputs that can support spatial performance or a failure
to induce sufficiently complete lesions in the anterior
thalamic nuclei. In order to investigate these possibilities, and hence to delineate more fully the hippocampal
system underlying spatial memory, the present study
examined whether damage to the lateral dorsal thalamic nucleus could potentiate the effects of extensive
anterior thalamic lesions.
Interest in the lateral dorsal nucleus principally arises
from the fact that it has many connections that are
similar to those of the anterior thalamic nuclei. Most
significantly, the lateral dorsal nucleus has dense inter-
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connections with the hippocampal formation and the
cingulate cortices [12,23,24]. In fact, the lateral dorsal
nucleus has sometimes been classified as an additional
member of the anterior thalamic group [6]. Furthermore, both the anterior thalamic nuclei and the lateral
dorsal nucleus contain units that are responsive to
particular head directions, independent of the spatial
position of the rat [14,21]. These units appear similar to
those present in retrohippocampal regions [16,21]. For
these reasons, it seems plausible that the lateral dorsal
thalamic nucleus supports spatial processing and that
by sparing this nucleus the disruptive effects of an
anterior thalamic lesion are reduced.
There are, however, other reasons as to why the
effects of anterior thalamic lesions might be less severe
than those seen after fornix transection. These include
the fact that the fornix carries hippocampal afferents
from the medial septum as well as numerous hippocampal efferents. As a consequence, the disruption of these
septal projections following fornix damage could independently add to those deficits due to hippocampal–
thalamic disconnection. A further possible explanation
for the difference between the effects of fornix and
anterior thalamic lesions concerns the problems involved in producing complete anterior thalamic lesions.
This largely arises from the shape and position of the
nuclei, which have meant that lesions often spare the
most lateral and caudal parts of the region [2,3,8]. That
there may be a need to produce near-complete lesions is
supported by a recent study into the effects of lesions
within different regions of the anterior thalamic complex [2]. It was found that small cytotoxic lesions
centred in either the anterior medial or the anterior
ventral/anterior dorsal nuclei were sufficient to produce
only mild impairments on tasks such as T-maze alternation [2]. Combining these lesions produced a significantly greater deficit [2], suggesting that the different
regions within the anterior thalamic nuclei may all
contribute to spatial performance and, hence, that it is
necessary to make a complete lesion in order to produce the full deficit.
In order to test these possibilities, five groups of rats
were tested on a battery of allocentric and egocentric
spatial tasks. Two groups of rats received neurotoxic
thalamic lesions made by the injection of N-methyl-Daspartic acid (NMDA). In one of these groups, the
injections were intended to produce an extensive lesion
of all three anterior thalamic nuclei (ANT). The second
of these groups received the same anterior thalamic
placements but an additional NMDA injection was
placed in the lateral dorsal nucleus (ANT +LD). A
third group of rats received radio-frequency lesions of
the fornix (FX). This group was required in order to
provide a baseline against which to assess the magnitude of any thalamic lesion deficits. The fourth group
of rats also received injections of NMDA, but these
were placed in the fornix rather than the thalamus. This
control group (NMDA SHAM) was included to test
the possibility that any of the observed thalamic deficits
were due to leakage of NMDA into the third ventricle
or due to the disruption of fornical activity following
tract damage and NMDA spread. A fifth group of rats
served as sham surgical controls (SHAM).
The animals were first compared on the acquisition
of a forced T-maze alternation task (Experiment 1).
This task was selected as it is highly sensitive to
hippocampal, fornical, and anterior thalamic damage
[2,3,17]. A modified version of this task was then given
in order to assess the ability of the rats to use allocentric cues (Experiment 2). This modified version, which
was first described by Montgomery [15], was very similar to that used in Experiment 1. Thus, in both conditions, the rat was initially allowed to run into only one
of the arms of the T-maze in the sample stage. On this
modified version, however, half of the choice runs were
then started from a fourth arm that was directly opposite the usual start arm. As a consequence, a strategy
that relied on the alternation of body turns (egocentric)
would produce an incorrect choice, but alternating
away from the sample place (allocentric) would still
lead to selection of the correct arm. The final test,
which also used a four-armed maze, examined the
ability of the rats to use an egocentric strategy (Experiment 3). Each rat was initially rewarded for turning in
a constant direction (right or left) irrespective of starting position. Once a rat had reached a predetermined
criterion, it was reversed, i.e. only rewarded for turning
in the opposite direction. This task shares many features of the allocentric task but it is not affected by
anterior thalamic lesions [2]. As a consequence, it serves
as an appropriate control test for general, non-specific
deficits.
2. Materials and methods
2.1. Subjects
Thirty-three naive, male rats of the pigmented DA
strain (Bantin and Kingman, Hull) were used in this
study. All subjects were housed in pairs under diurnal
conditions (14 h light/10 h dark) and all testing occurred at a regular time during the light period. The
animals were tested for 5 days a week, and were
maintained at 85% of normal body weight by restricting
their daily food intake to approximately 15 g of laboratory diet (Harlan Teklad, Bicester, Oxfordshire). At the
start of testing the animals were aged 4 months and
weighed between 215 and 230 g. All animals had free
access to water.
E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
2.2. Apparatus
2.2.1. T-maze and cross-maze tasks
All testing for the forced-alternation task was carried
out in a modifiable T-maze (Experiment 1). The floors
of the maze were 10 cm wide and made of wood, and
the walls were 17 cm high and made of clear Perspex.
The stem was 70 cm long with a guillotine door located
25 cm from the beginning, so creating a start area. The
cross piece was 140 cm long, and at each end there was
a food well 2 cm in diameter and 0.75 cm deep. The
entire maze was supported by two stands 94 cm high.
Lighting was provided by a fluorescent light suspended
164 cm above the apparatus.
For Experiments 2 and 3, an additional arm was
fitted onto the choice point of the T-maze to form a
four-arm (cross-shaped) maze. The dimensions and appearance of the additional arm matched those of the
stem of the T-maze, and this new arm fitted onto the
maze directly opposite to the original stem. A slotted
metal door made it possible to open or close the end of
either of the two stem arms.
2.3. Procedures
2.3.1. Experiment 1: T-maze forced alternation
Testing began at least 2 weeks after surgery. Animals
were given several days pretraining so that they would
run reliably down the stem of the maze to find food
pellets in the food wells in both arms. This was immediately followed by a series of 18 acquisition sessions,
each of six trials.
Each trial was divided into two stages, a ‘sample run’
followed by a ‘choice run’. At the start of each trial,
three food pellets (45 mg Campden Instruments,
Loughborough) were placed in each food well and a
metal barrier was placed at the neck of the T-maze, so
closing one arm. On the sample run, the animal was
placed in the start area and the guillotine door raised.
Because of the metal barrier, the rat could only enter
the one open arm, where it was confined for approximately 10 s while it ate the food. It was then picked up
and confined to the start area for a delay of 10 s while
the barrier at the choice point was removed. The door
to the start area was then raised and the animal allowed
a free choice between the two arms of the T-maze. On
this choice run, the animal was deemed to have selected
an arm when it had placed a hind foot down that arm;
no retracing was allowed. If the rat had alternated, i.e.
had entered the arm not previously visited on the
sample run, it was allowed to eat the food reward and
was then returned to the home cage. If the other arm
was chosen, i.e. the same arm as visited on the sample
run, the animal was confined to that arm for approximately 10 s, and then returned to the home cage.
225
The rats were tested in groups of four with each rat
having one trial in turn, so that the intertrial interval
was approximately 3–4 min. Each animal received six
trials a day, and each day contained a pseudo-random
sequence of correct choices between the two arms.
The subjects were given a total of 18 acquisition
sessions. Following this, the rats were tested on the
forced alternation task for a further 10 sessions in an
identical manner, except that a variable delay period
was introduced between the sample and choice runs.
Three delays (10, 20 or 30 s) were used and these were
presented in a pseudo-random sequence so that each
session contained two trials at each delay.
2.3.2. Experiment 2: allocentric alternation
Testing began 7 days after the completion of Experiment 1. The procedure again consisted of two parts, the
first being identical to that described in Experiment 1,
i.e. on the sample run the animal was forced to run into
one randomly selected cross arm and allowed to eat the
reward. All sample runs began in the same start area as
used in Experiment 1.
The second part (choice run) was also similar to that
used in Experiment 1 in that the animal was rewarded
for entering the arm not previously visited on the
sample run. The critical difference was that now, on
half of the choice trials, the animal was placed in the
start area of the arm opposite to that used on the
choice run. As the location of the correct choice arm
had not altered, an animal that relied on allocentric
cues could still alternate to the correct arm on these
trials. In contrast, an animal relying on egocentric or
motor cues to alternate would continue to turn in the
opposite direction to that used in the sample run, but
this would now bring it back into the arm that had
been entered on the sample run (i.e. an error). A rat
relying on egocentric cues would therefore be expected
to make many more errors on those choice trials that
began from the opposite arm. Although this task was
run in a cross maze, each trial was effectively in a
T-maze as a metal barrier always prevented the animal
from entering the arm directly opposite the start area.
All animals received six trials per day for a total of
15 days. Within each day, three choice runs were
started from the ‘same arm’ as that used in the sample
run, and three choice runs began from the ‘opposite
arm’. Once again there was an intertrial interval of 3–4
min between trials and the interval between the sample
and choice phase was the same as that in the acquisition stage of Experiment 1.
2.3.3. Experiment 3: egocentric discrimination
Testing began 17 days after the completion of Experiment 2. For this task each animal received 12 trials a
day and, unlike the previous experiments, these were
run consecutively, i.e. one straight after the other. At
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the start of the each trial, the rat was randomly assigned to one of the four arms of the cross-maze and
confined at the end by a metal barrier. The arm opposite the start arm was also blocked, so effectively creating a T-maze. One of the side arms was baited with
three 45-mg pellets so that the animal was rewarded for
selecting the arm in a given direction (always turn right
or always turn left). Following a choice, the rat was
placed at the end of another randomly assigned arm,
confined for approximately 10 s, and the process repeated, i.e. the rat was rewarded for turning in the same
direction irrespective of which of the four arms it had
started from. Care was always taken to appear to bait
both of the possible choice arms, so that the movements of the experimenter would not cue the animal.
The correct body turn (right or left) for the first discrimination was defined by the behaviour of the animal
on the first trial of the very first session. On this trial
only, both arms were baited and the animal was rewarded for entering either arm. On subsequent sessions,
until the reversal, the animal was only rewarded for
turning in the same direction as it had turned on this
very first trial.
Each animal was tested until it reached a criterion
level of 30 or more correct responses over three consecutive sessions (36 trials). Upon reaching this criterion,
the correct body turn was reversed for the next and all
subsequent sessions, e.g. if the animal had been rewarded for always turning left, it would now only be
rewarded for turning right. Each animal again received
12 trials per day, and testing ceased when the animal
reached the 30/36 criterion level.
2.4. Surgical and histological procedures
Each animal was deeply anaesthetised by intraperitoneal injection of pentobarbitone sodium (Sagatal) at
a dose of 60 mg/kg. The animal was then placed in a
stereotaxic headholder (David Kopf Instruments, Tujunga), and the scalp retracted to expose the skull. A
craniotomy was made above the sagittal sinus and the
dura cut to expose the cortex above the target region.
For the anterior thalamic lesions (ANT), injections of
0.2 ml of 0.12 M N-methyl-D-aspartic acid (NMDA)
(Sigma Chemicals, Poole) dissolved in phosphate buffer
(pH 7.2) were made through a 1-ml Hamilton syringe
into two sites in each hemisphere. The stereotaxic coordinates relative to ear-bar 0, with the incisor-bar set
at +5.0 to the horizontal plane, were: AP +5.2, LAT
9 1.0 and AP + 5.2, LAT 1.7. The height at the medial
site was 6.2 mm below the top of the cortex, while that
of the lateral site was 5.6 mm below the top of the
cortex. Each injection was made gradually over a 3-min
period and the needle was left in situ for a further 4 min
before being withdrawn. After completion of the injections the skin was sutured and a topical antibiotic
(Aureomycin) powder applied. Post-operative care also
included systemic analgesia (Temgesic, Reckitt and
Coleman, UK) and fluid replacement (5 ml glucose/saline). The procedures for the anterior thalamic nuclei
plus lateral dorsal nucleus group (ANT +LD) were
exactly the same except that the volume of NMDA
injected into the anterior thalamic nuclei was reduced
to 0.15 ml (medial injection) and 0.12 ml (lateral injection), while an additional NMDA injection (0.15 ml)
was placed in the lateral dorsal nucleus (AP +4.5,
LAT 9 2.3, height − 5.0 relative to the top of the
cortex).
A further group of rats received radio-frequency
lesions of the fornix (FX). The surgeries were identical
to those already described except that a Radionics TCZ
electrode (0.3 mm tip length, 0.25 mm diameter; Radionics, Burlington) was lowered into two sites in the
fornix. The co-ordinates of these placements were AP
+ 5.3, LAT 9 0.7 and AP + 5.3, LAT 9 1.7 The
medial lesion was placed 3.7 mm below the top of the
cortex, while the lateral lesion was 3.8 mm below the
top of the cortex. In each site, the temperature of the
tip was raised to 75°C for 60 s using an RFG4-A
Lesion Maker (Radionics, Burlington).
The surgical control animals (SHAM) received a
craniotomy and the dura was then cut as for the other
surgeries. A Hamilton syringe was lowered to the level
of the fornix using the FX group co-ordinates, but no
injection was made. A final set of animals (NMDA
SHAM) received two injections of NMDA (0.15 ml)
into the fornix in each hemisphere. The co-ordinates
were as follows AP + 5.3, LAT 9 1.0 and 9 1.6;
height − 3.7 relative to the top of the cortex. The 33
rats were randomly assigned to the four groups, resulting in eight SHAM, five NMDA SHAM, seven ANT,
eight ANT+ LD and five FX animals.
Following the completion of the experiment, subjects
were anaesthetised with Euthatal and transcardially
perfused with saline followed by 10% formol–saline.
The brain was removed and post-fixed in formol–saline
for a minimum of 2 h, before being transferred into
20% sucrose in 0.2 M phosphate buffer and left
overnight. Coronal sections were cut at 60 mm on a
freezing microtome and stained with cresyl violet, a
Nissl stain.
3. Results
3.1. Histological analysis
Three of the eight ANT+ LD showed some bilateral
damage to the dentate gyrus region of the hippocampus. In addition, two of the ANT animals had lesions
that were substantially smaller than the remainder of
the group. These animals were excluded from further
analysis.
E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
227
Fig. 1. Diagrammatic reconstructions showing the cases with the largest (grey) and smallest (solid black) lesions in ANT + LD group (left
column). For comparison purposes, the right column contains normal coronal sections. The numbers refer to the corresponding sections in the
atlas of Swanson [20].
In all remaining five ANT+ LD animals, a complete
bilateral lesion of all three anterior thalamic nuclei
(anteromedial, anteroventral and anterodorsal) was
achieved (Fig. 1). In the same five cases, there was
extensive bilateral damage to the lateral dorsal nucleus
(Fig. 1). As a consequence of the extensive lesions in
the anterior thalamic nuclei and lateral dorsal nuclei,
there was encroachment into adjacent thalamic regions.
Thus, bilateral damage to the rostral portion of the
medial dorsal nuclei was found in two animals and
unilateral damage to the medial dorsal nuclei was
found in two other ANT +LD cases. All animals
showed bilateral damage to the rostral midline thalamic
nuclei including nucleus reuniens. The most rostral
portions of the reticular nucleus were also damaged.
Some unilateral cell loss was observed in the granule
cells of the dentate gyrus in three cases (see Fig. 1). This
unilateral damage involved the ventral half of the dentate gyrus and extended caudally to the level of the
posterior thalamus.
The seven remaining animals in the ANT group
showed extensive bilateral neuronal loss to the ante-
rior thalamic nuclei (anteromedial, anteroventral and
anterodorsal), the only exception being one case in
which there was some sparing of the anterodorsal nucleus (Fig. 2). One subject had some additional, minor unilateral damage to the rostral portion of the
lateral dorsal nucleus. Three of the ANT cases
showed bilateral damage to the rostral region of the
medial dorsal nucleus and a further case showed evidence of unilateral medial dorsal damage. In all cases,
there was damage to rostral midline nuclei, including
nucleus reuniens. There was no evidence of cell loss
in the hippocampus in any of the animals in this
group.
All five cases in the FX group revealed a complete
bilateral transection of the fornix. In three animals,
there was additional damage to the dorsal margin of
the anterodorsal and anteroventral thalamic nuclei,
which in one case extended caudally to include the
lateral dorsal nucleus unilaterally (Fig. 2). Finally the
infusions of NMDA into the fornix in the five
NMDA SHAM cases produced no apparent damage
to either the fornix or surrounding tissue.
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E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
Fig. 2. Diagrammatic reconstructions showing the cases with the largest (grey) and smallest (solid black) lesions in ANT group (left column) and
fornix group (right column). The numbers refer to the corresponding sections from the atlas of Swanson [20].
3.2. Groups for beha6ioural analysis
Following histological analysis, the study consisted
of eight SHAM, five NMDA SHAM, seven ANT, five
ANT+ LD and five FX animals.
3.3. Comparison of SHAM groups
The performances of the SHAM control group and
NMDA SHAM group were compared for each of the
behavioural tasks conducted in this study (T-maze acquisition, T-maze delay performance, allocentric alternation, egocentric discrimination and reversal). A series
of analyses of variance (ANOVA), using the same set of
comparisons as were subsequently used for the lesion
groups, found no evidence of a difference between the
two control groups (all comparisons F B 1.0). As a
consequence, the two control groups were combined
into one sham group (COMB SHAM; n = 13) for the
subsequent analyses.
3.3.1. Experiment 1: spatial forced alternation
3.3.1.1. Acquisition. Each animal performed a total of
108 trials over 18 acquisition sessions. The scores were
grouped into six blocks, each of three sessions, and the
mean percent correct scores for each of the four groups
are shown in Fig. 3 (left). Analysis of these blocked
scores using an ANOVA revealed a highly significant
group effect (F3,26= 91.72, PB 0.001). Subsequent
Newman–Keuls tests showed that both thalamic
groups (ANT, ANT+LD) and the FX group all had
significantly lower scores than the COMB SHAM control group (all comparisons PB 0.01), but the three
lesion groups did not differ significantly from one another. The analysis also revealed a significant effect of
session (F5,130= 2.87, P B 0.05), but no group by session interaction (FB 1.0).
3.3.1.2. Delays. Following the last acquisition session,
the performance of the subjects was further challenged
by the imposition of a variable delay between the
sample and test phases. Fig. 3 (right) shows the effect of
increasing delay on performance accuracy. The performance of the COMB SHAM group was characterised
by a decline in mean percentage accuracy from 93% at
0 delay to 86.5% at the 30-s delay. An ANOVA conducted on all data revealed a significant main effect of
delay (F3,26=5.18, PB0.01) and a significant main
effect of lesion group (F3,26= 15.18, PB 0.01) but no
lesion by delay interaction (F6,52=1.26). Post hoc
analysis with Newman–Keuls tests showed that the
performance of all three lesion groups (ANT, ANT+
LD, FX) was significantly lower than that of the
E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
229
Fig. 3. Left: Experiment 1. T-maze alternation: the graph shows the mean percent correct scores of the COMB SHAM, ANT, ANT +LD and
fornix groups over the initial 18 acquisition sessions, grouped in blocks of three sessions. Right: T-maze alternation with retention delays of 10,
20 and 30 s. The data are collapsed across 10 sessions.
COMB SHAM control group at all delay periods, but
these groups did not differ significantly from each
other. Inspection of the scores (Fig. 3, right) shows that
all three lesion groups were in fact performing at, or
close to, chance level irrespective of delay.
3.3.2. Experiment 2: allocentric alternation
Each group performed 90 trials, 45 when the start
arm was the ‘same’ as that used in the sample run, and
45 when the start arm was ‘opposite’ to that used in the
sample run. An ANOVA using the total scores for each
group in both types of condition revealed a significant
group difference (F3,26 = 27.91, P B0.01). Subsequent
Newman–Keuls tests revealed that all lesion groups
performed significantly worse than the COMB SHAM
group (P B 0.01), as shown in Fig. 4. In addition, the
ANT+ LD groups performed significantly worse than
the FX group (PB0.05). There were no other group
differences. There was a significant decrease in performance of the task when the test began from the start
arm ‘opposite’ to the sample run (F1,26 = 27.96, PB
0.001), but there was no interaction between lesion
group and type of trial (same vs. opposite) (F B 1.0).
The performance of the COMB SHAM group was
above chance in the opposite condition from the first
session (mean percent correct 77%), demonstrating that
they were able to use the allocentric cues immediately.
3.3.3. Experiment 3: egocentric discrimination
The total number of errors made by the COMB
SHAM group and the three lesion groups to reach the
acquisition criterion for the initial discrimination and
its subsequent reversal were examined. An inspection of
the scores clearly indicates that none of the lesion
groups were impaired (Fig. 5) and a subsequent
ANOVA using these scores confirmed the lack of any
group difference (FB 1.0). Although the reversal led to
the expected increase in errors (F1,26= 40.7, PB0.001)
there was no evidence of a group by reversal interaction
(FB 1.0). The same pattern of results was found when
sessions, rather than errors, to criterion were compared.
4. Discussion
The present study had several aims. The first was to
assess the severity of the spatial memory deficit associated with complete lesions of the anterior thalamic
nuclei, and to compare it with the consequences of
Fig. 4. Experiment 2: allocentric alternation. The group scores
(COMB SHAM ANT, ANT+LD and FORNIX) are presented as
the total number of correct choices for those trials in which the same
start arm was used for the sample run and choice runs (SAME) and
those in which the arm opposite was used for the choice run (OPPOSITE). The dotted line marks chance performance level.
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E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
Fig. 5. Experiment 3: egocentric discrimination. The group scores
(COMB SHAM ANT, ANT+LD and FORNIX) are presented as
mean errors required to reach the acquisition criterion for the body
turn discrimination and the subsequent reversal.
cutting the fornix. This comparison is of interest as the
fornix provides the route for many hippocampal connections, including the substantial projections to the
anterior thalamic nuclei. The second aim was to examine the possible contribution of the lateral dorsal thalamic nuclei to the processing of spatial information. This
was achieved by combining lesions of the lateral dorsal
nucleus with lesions of the anterior thalamic nuclei. A
third aim was to verify whether the spatial memory
deficits associated with neurotoxin injections into the
anterior thalamic nuclei were affected by diffusion or
leakage of NMDA into the fornix or third ventricle.
This concern arises because the injection tracts to the
anterior thalamus pass through these regions.
Both phases of the spatial alternation task (initial
acquisition and subsequent performance with increased
retention delays) revealed severe, and apparently permanent, deficits in the anterior thalamic lesion group
(ANT), the anterior plus lateral dorsal thalamic lesion
group (ANT + LD) and the fornix lesion group (FX).
In contrast to the control group, whose performance
level was around 90% throughout, all three experimental groups performed at or close to chance. The second
experiment strongly indicated that the control rats were
using allocentric cues to perform the alternation task,
as they were able to select the correct arm even when
the test trial was started from a different location, from
the very first test session. An intriguing aspect of this
allocentric procedure was the finding that the scores of
the ANT+ LD group were significantly worse than
those of the FX group. In Experiment 3, none of the
lesion groups differed from the control group in their
ability to acquire an egocentric rule (always turn to the
right or left) and then to reverse the reinforced turn.
This result serves to highlight the selectivity of the
allocentric spatial deficit found on the T-maze tasks.
Finally, in none of the three spatial experiments was
there any evidence of a difference in the performance
levels of the control subjects that had received NMDA
infusions into the fornix and those that had received the
standard sham surgical procedure. This finding helps to
confirm the importance of anterior thalamic damage
for inducing impairments in spatial information processing.
The very severe impairments in the animals with
anterior thalamic lesions contrasts with a number of
earlier studies that had failed to demonstrate spatial
alternation deficits following lesions of the anterior
thalamic nuclei when short delays between sample and
choice runs were used [8,10]. Two factors may contribute to this apparent inconsistency. The first concerns the extent to which the animals may learn to use
egocentric cues to solve the task and the second concerns the extent of anterior thalamic damage. Inspection of these previous studies suggests that the failure to
find a clear deficit is associated with surgeries that only
produced partial anterior thalamic damage. For example, Beracochea and Jaffard [8] only found a spatial
alternation deficit in mice with anterior thalamic lesions
when an extended retention delay was placed before the
choice trial, but in their surgeries the anterior ventral
nuclei were largely spared. More direct support comes
from the few studies that have deliberately attempted to
place lesions in specific nuclei within the anterior thalamic complex. Thus, it was found that neither anterior
ventral nor anterior medial electrolytic lesions affected
continuous alternation performance by rats [10], while
neurotoxic lesions placed in either the anterior medial
or the anterior ventral/anterior dorsal nuclei produced
only mild deficits on T-maze alternation [2]. In contrast,
a combination of the two lesions (anterior medial and
anterior ventral/dorsal) resulted in a marked alternation impairment [2]. Likewise, although Beracochea et
al. [9] found that lesions of the anterior ventral and
anterior medial nucleus had no effect on the performance of a radial arm maze task, it is evident from the
reconstructions that much of the anterior and posterior
portions of the nuclei were spared. It was also the case
that the surgeries were carried out in two stages, with a
1-week interval. This may have ameliorated any lesion
effect. Finally, Beracochea and Jaffard [7] reported that
ibotenic acid lesions, involving about 70% of the anterior medial and anterior ventral nuclei in mice failed to
affect spontaneous alternation.
In the present study, the T-maze alternation deficit in
the anterior thalamic group (ANT) was as severe and
apparently as long lasting as that found after fornix
transection. Previous studies of large neurotoxic anterior thalamic lesions have typically found that the
anterior thalamic alternation deficit is slightly less
severe than that found after fornix transection [3,4],
suggesting that there might be other fornical outputs
E.C. Warburton et al. / Beha6ioural Brain Research 87 (1997) 223–232
that are capable of supporting task performance. In
both of these previous studies, there was, however,
sparing of the most lateral parts of the anterior ventral
nucleus and parts of the anterior dorsal nucleus [3,4]. In
view of the present results, it would now appear that
this previous difference between fornix and anterior
thalamic lesions reflected the sparing of these small
regions of anterior thalamic tissue. It should, however,
be added that in all of these cases, the anterior thalamic
lesions involved rostral midline thalamic nuclei which
are also interconnected with the hippocampus. Although damage to these midline nuclei does not appear
sufficient to induce a clear deficit on these spatial
working memory tasks [3], it is quite possible that their
dysfunction contributed to the severity of the deficit.
Some of the animals in the present study displayed
unilateral and bilateral damage in the rostral portion of
the medial dorsal thalamic nucleus. It is, however,
unlikely that damage to this nucleus contributed to the
present deficits. This is because neurotoxic lesions of
medial dorsal nucleus that spare the anterior thalamic
nuclei do not disrupt standard T-maze alternation acquisition and performance [10,11,22]. Furthermore, the
scores of ANT and ANT+LD subjects with and without medial dorsal damage were compared, and on none
of the tasks did these two sub-groups differ significantly. Similarly, some subjects had unilateral damage
to a restricted portion of the dentate gyrus which might
have had an effect on performance. Further statistical
analyses revealed, however, that these subjects were not
significantly different from those animals without dentate gyrus damage. A final comparison was made between those animals with both unilateral hippocampal
and medial dorsal thalamic damage, and those without.
Once again, there were no statistically significant differences in performance. Hence, it appears that the large
behavioural deficits in the ANT group were principally
due to the completeness of the anterior thalamic lesions.
It had been predicted that the addition of a lesion to
the lateral dorsal nucleus might increase the magnitude
of performance deficit in the spatial alternation task for
two related reasons. First, this nucleus has direct interconnections with the hippocampus [12,23] and second,
it contains cells that appear to code head directional
information [13]. Furthermore, the activity of
hippocampal head direction cells appears to depend on
the integrity of the lateral dorsal thalamic nucleus [13].
It should be added that head direction cells are also
found in the anterior thalamic nuclei [21]. Although no
differences were observed in the present study between
the ANT and ANT+LD lesion groups, there is good
reason to believe that these comparisons were affected
by floor effects. Similarly there were no differences
between the T-maze scores of the anterior thalamic
lesion groups and the FX group. The ANT+ LD group
231
was, however, significantly worse than the FX group in
the cross-maze task which was used specifically to test
the animals’ ability to process allocentric information.
This result is of especial interest as it shows that the
lack of difference between the ANT and FX groups
cannot all be explained by floor effects, while the
greater impairment in the ANT+ LD group accords
with the fact that the lateral dorsal nucleus, unlike the
anterior thalamic nuclei, receives a nonfornical input as
well as a fornical input from the hippocampus [1,12].
None of the groups was impaired on the egocentric
task, showing that all three experimental groups were
sensitive to the reinforcements and were capable of
learning tasks in other spatial domains. It is, however,
very likely that initial acquisition of the egocentric task
was affected by negative transfer effects following the
allocentric spatial tests in the T-maze. For this reason,
it was important to include a reversal condition, but
again, all three experimental groups were able to reverse the rewarded body turn within normal limits.
The results from the present experiment underline the
importance of the anterior thalamic region in the processing of spatial information. This study has demonstrated that lesions including all of the anteroventral,
anterodorsal and anteromedial nuclei produce severe
and long-lasting impairments in the T-maze spatial
alternation task and that the deficit is the result of an
inability to use allocentric cues. The importance of the
lateral dorsal nucleus has yet to be fully resolved as the
present study was partly compromised by the presence
of floor effects, and thus future similar experiments
should include spatial memory tasks less vulnerable to
floor effects, such as the Morris watermaze. Finally, the
evidence that an extensive anterior thalamic lesion including the lateral dorsal nucleus can result in significantly poorer spatial memory performance than that
found after fornix transection highlights the need to
explore the behavioural effects of selective lesions
confined to just the lateral dorsal nucleus [13].
Acknowledgements
This research was supported by the Wellcome Trust.
The authors wish to thank Mike Davies for his assistance with the figures.
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