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Anim Cogn (2006) 9: 62–70
DOI 10.1007/s10071-005-0005-4
ORIGINAL ARTICLE
Sylvain Fiset ·Franc¸ois Y. Dor´
e
Duration of cats’ (
Felis catus
) working memory
for disappearing objects
Received: 25 November 2004 / Revised: 14 June 2005 / Accepted: 28 June 2005 / Published online: 31 August 2005
C
Springer-Verlag 2005
Abstract This study explored the duration of cats’ working
memory for hidden objects. Twenty-four cats were equally
divided into four groups, which differed according to the
type of visual cues displayed on and/or around the hiding
boxes. During eight sessions, the four groups of cats were
trained to locate a desirable object hidden behind one of
the four boxes placed in front of them. Then, the cats were
tested with retention intervals of 0, 10, 30 and 60 s. Results
revealed no significant differences between the groups dur-
ing training or testing. In testing, the cats’ accuracy to locate
the hidden object rapidly declined between 0 and 30 s but
remained higher than chance with delays of up to 60 s. The
analysis of errors also indicated that the cats searched as a
function of the proximity of the target box and were not sub-
jected to intertrial proactive interference. This experiment
reveals that the duration of cats’ working memory for disap-
pearing objects is limited and the visual cues displayed on
and/or around the boxes do not help the cats to memorize a
hiding position. In discussion, we explore why the duration
of cats’ working memory for disappearing objects rapidly
declined and compare these finding with those from domes-
tic dogs. The irrelevance of visual cues displayed on and
around the hiding boxes on cats’ retention capacity is also
discussed.
Keywords Working memory .Retention interval .
Object permanence .Domestic cats .Visual cues
S. Fiset ()
Secteur Sciences Humaines, Universit´
e de Moncton, Campus
d’Edmundston,
Edmundston, New-Brunswick, Canada E3V 2S8
e-mail: sfiset@umce.ca
Tel.: +506-737-5005
Fax: +506-737-5373
F. Y. Dor´
e
´
Ecole de psychologie, Pavillon F´
elix-Antoine-Savard,
Universit´
eLaval,
Qu´
ebec, Qu´
ebec, Canada G1K 7P4
Introduction
The capacity to search for and locate a desirable object that
has visibly moved and disappeared behind an obstacle is of
great adaptive value for several animal species. For exam-
ple, social behaviours, such as feeding, mating or coopera-
tive hunting, require that animals remember the position of
social partners that have moved and disappear. Hunting also
requires from some predatory species (e.g. cats, wolfs) the
ability to pursuit and locate hiding prey. In the last 20 years,
the Piagetian framework of object permanence—the ability
to mentally represent physical and social objects—has pro-
vided an appropriate and valuable support (both theoretical
and methodological) to examine and determine whether a
particular species is spontaneously able to search for and
find a disappearing object (Pepperberg 2002). Object per-
manence gradually develops during ontogeny through the
interaction between the organism and its environment. In
human infants, object permanence progresses through six
stages within the first 2 years of life and its understanding
truly begins at around 12 months of age when the child
reaches Stage 5 (Piaget 1937).
Stage 5 of object permanence is typically assessed with a
visible displacement task [VD task]). In a VD task, the sub-
ject faces a number of identical boxes (normally four) and
sees an attractive object move in front of the boxes and then
disappear behind one of the boxes. Immediately after the
disappearance of the object, the subject is allowed to search
and he is rewarded if he finds the object directly behind the
last box where he saw the object disappear. In a VD task,
the target location changes from trial to trial and further
tests of Stage 5 involve moving the target object behind
a succession of two or more hiding locations within the
same trial. Stage 6 of object permanence is acquired when
the subject is able to solve single invisible displacement
problems in which an object is first moved into a transport
container and then, the transport container is moved inside
one of the boxes in which the object is imperceptibly trans-
ferred from the container to the box. In double invisible
displacement problems, the transport container is moved
63
inside two of the available boxes and the target object can
be transferred to one of two visited boxes.
Animal studies have shown that object permanence
developed at the same rate as the one described in human
infants and reached the upper level (Stage 6) both in
chimpanzees (Mathieu and Bergeron 1981) and in gorillas
(Redshaw 1978). In several mammal species, however,
there are some differences. For example, in cats and dogs,
Stages 4 and 5 are acquired more rapidly than in humans
(Dumas and Dor´
e1989,1991; Gagnon and Dor´
e1994)
and both species are unable to solve invisible displacement
problems of Stage 6 (Collier-Baker et al. 2004;Dor´
e1986).
In fact, very few mammal species are able to cope with in-
visible displacement problems (for a review, see Call 2001).
For example, squirrel monkeys are unable to solve invisible
displacement problems (De Blois et al. 1998), whereas rhe-
sus macaque (De Blois and Novak 1994) can solve single
invisible displacement problems but failed to solve double
invisible displacements. Nevertheless, several mammal
species have the mental capabilities to represent an object
and its hiding location because they can fully solve the VD
tasks, that is, they can pursuit and retrieve objects that move
and disappear behind one or successive spatial locations in
the environment [cats (Dor´
e1986; Dumas and Dor´
e1989,
1991; Gruber et al. 1971;Thinus-Blanc et al. 1982; Triana
and Pasnak 1981); dogs (Gagnon and Dor´
e1992,1993;
Triana and Pasnak 1981); and nonhuman primates (Call
2001; De Blois and Novak 1994; De Blois et al. 1998;
Mathieu et al. 1976; Natale et al. 1986; Neiworth et al.
2003;Parker1977; Redshaw 1978; Wood et al. 1980)].
Recently, Fiset et al. (2003) have investigated the du-
ration of the memory mechanisms underlying search be-
haviour for disappearing objects. More specifically, they
determined the retention interval that domestic dogs can
tolerate after the object’s disappearance with a VD-like ob-
ject permanence task: a delay was introduced between the
disappearance of the object behind one of the four boxes
and the beginning of the search by the animal. To succeed
in this task, the animal must encode and maintain an active
mental representation of the hiding location for a limited
period of time in working memory, and later recall this spa-
tial information to locate and find the object. In addition, the
encoded information must be reset from working memory
after each trial because the hiding location changes from
trial to trial (Goulet et al. 1994). Results revealed that the
performance of the dogs was very high (between 80 and
90% of mean percentage of success) in the shorter delays
(0 and 10 s) but it gradually declined between 10 and 60 s
to reach a mean percentage of success around 60%. Then,
it remained stable but well over chance level for delays of
up to 240 s.
In the present article, we investigated the duration of
working memory for disappearing object in another mam-
mal species that demonstrates the ability to spontaneously
locate and find hidden objects in a standard VD task: the
domestic cat. From a comparative perspective with the do-
mestic dog, the study of the duration of domestic cats’
working memory for disappearing objects is of interest.
Indeed, although cats and dogs are closely related species,
there are evidence that cats’ and dogs’ spatial encoding pro-
cesses for hidden objects differ. For example, in a standard
VD task, Fiset and Dor´
e(1996) demonstrated that domestic
cats solely rely on one source of spatial information, that
is, linear egocentric information (directional information
derived from the animal’s position) to find a disappear-
ing object. By contrast, Fiset et al. (2000) revealed that
domestic dogs are more flexible than cats. Although they
primarily use egocentric spatial information, dogs also en-
code and use allocentric spatial information (relationships
between the surrounding objects and the target location)
when egocentric information is not available. Therefore,
the present experiment represents a further step to investi-
gate the distinctive cognitive capability of cats for search
of disappearing objects.
The delayed-response (DR) task has been used at the
beginning of the 20th century to investigate the duration of
cats’ memory. The DR task is relatively analogous to the
VD task used by Fiset et al. (2003) because the animal sees
a food reward hidden in one of two locations (occasionally
three) and then after some delays is allowed to search for it.
Yarbrough (1917) found that cats can maintain in memory
a spatial location for a duration of 4 s in a three-choice
DR task and up to 18 s in a two-choice DR task. Cowan
(1923) showed that cats remember the target location for
a duration of 20–30 s in a two-choice DR task. Finally,
Meyers et al. (1962) tested cats’ memory with intervals
of 1, 2, 4 and 16 s in a two-choice DR task. Their results
revealed that cats’ performance rapidly dropped between
1 and 16 s of retention interval and although no statistic
analyses were provided, it seems that cats’ performance
with retention interval of 16 s was at random. However, the
DR used in these later studies differs to some extents from
the VD task used by Fiset et al. (2003): (1) the number of
potential hiding locations was smaller, (2) the animal did
not visually track the movement of the target object in front
of the other hiding locations, and (3) the target location was
visible during the entire delay because no opaque screen
was introduced between the animal and the target location.
By consequence, it is hazardous to extrapolate these
previous results using the DR task in order to estimate the
duration of cats’ working memory for disappearing objects.
The first purpose of this study was to investigate the cats’
retention capacity to memorise the location of a hidden
object by using the VD-like object permanence task used
by Fiset et al. (2003). Specifically, the disappearance of the
object behind one of the four boxes and the onset of cats’
searching was separated by one of four retention intervals
(0, 10, 30 or 60 s). It was expected that cats’ performance
would decline gradually from 0 to 60 s.
The second purpose was to determine whether the
presence of visual cues displayed on the hiding location
or on the area surrounding it would help cats to maintain
the hiding location in working memory when a delay was
introduced between the onset of the trial and the animal’s
search behaviour. Several earlier studies using the DR
task suggest that the addition of visual cues (e.g. colour,
form, size, brightness) to the target location improved the
retention in memory of chimpanzees (Nissen and Harrison
64
1941), raccoons (Michels and Brown 1959), rats (Bliss
1960; McCord 1939) and rhesus monkeys (Meyer and
Harlow 1952). In the present study, two types of visual cues
were investigated: (1) intrinsic features of the boxes behind
which the object could be hidden and (2) local cues, this is,
features of the immediate area surrounding the boxes. The
intrinsic features were geometric symbols (e.g. X), which
were painted on the boxes and the local cues were geomet-
ric patterns (e.g. series of black dots), which were painted
on the floor surrounding the boxes. Cats were divided into
four groups. In Group I, the boxes could be discriminated
on the basis of their intrinsic features. In Group L, the
boxes could be discriminated by the local cues surrounding
them. Group I–L was exposed to intrinsic features and
local cues, whereas in Group C, no cues were available.
It was expected that the encoding of one or both types of
cues would enhance cats’ retention of the object location.
Method
Subjects
Subjects were 24 domestic cats (Felis catus), 11 females
and 13 males, which had been purchased from a labora-
tory supplier. Their exact age was impossible to determine
but it was estimated to be near or above 1 year. Before
their arrival, they stayed 21 days in the central animal care
facility of the university, where they received a health ex-
amination, were treated for endo- and ectoparasites, and
were vaccinated. Cats were housed individually on a 16:8-h
light—dark cycle with lights on at 6:00 a.m. They were
taken out of their cages daily for 30–60 min during which
they interacted with toys, one experimenter, and other cats.
They received about a quarter of their food ration during
each experimental session and the rest of it immediately
after a session.
Twelve of the cats used in this experiment were naive,
whereas the other 12 cats had participated, 2 months be-
fore, in an unrelated experiment using boxes that could
be discriminated solely on the basis of their spatial
position.
Apparatus
The bare experimental room (158 cm wide ×390 cm deep)
was painted white. Part of the floor length (Fig. 1) was cov-
ered with a grey plywood stand (36 cm high ×158 cm
wide ×120 cm deep), thus forming two empty spaces
(158 cm wide ×87.5 cm deep; 158 cm wide ×182.5 cm
deep) at each end of the room. The experimenter (E1) who
performed the manipulations sat in the pit, just behind the
stand. A vertical grey panel (158 wide ×40.5 cm high)
concealed his arms and hands; the other experimenter (E2),
who restrained the cat by its shoulders during the manipu-
lations, sat on a bench in the other empty space.
A wall (158 cm wide ×90 cm high) separated the stand
from the subject’s starting position and from the pit where
E1
CAT
E2
DOOR
HOLDING
BOX
BENCH
OBSERVATION/DOOR
VERTICAL PANEL
WALL WINDOW
158 cm
87.5 cm
120 cm
182.5 cm
Fig. 1 Schematic illustration of the experimental room and appa-
ratus used. E1: experimenter who performed the manipulations; E2:
experimenter who held the cat; open boxes: potential hiding boxes
E2 was. A holding box (33.5 cm wide ×33 cm high ×
45 cm deep) was fixed on a bench (33.5 cm wide ×36 cm
high ×90 cm deep) perpendicular to this wall. One opening
of the box placed at the centre of the wall was connected
to the stand by a door (20 cm wide ×40 cm high). A
transparent Plexiglas door, used to confine the cat into the
holding box during the manipulations, could be slid into
this opening from right, whereas a similar opaque white
Plexiglas door, used to block the cat’s view of the apparatus
during intertrial and retention intervals, could be slid into
the opening from the left. E2 sat on the bench behind the
holding box, held the cat by passing her arms through the
other opening of the box and watched the cat through the
transparent top of this box. Above this box, a window made
of transparent Plexiglas (35 cm wide ×57 cm high) was
embedded in the wall, so E2 could see the whole apparatus.
Both experimenters had access to the stand through a small-
concealed door on the right of the wall.
The target object was a red wooden cube (3.7 cm) with
a metal ring on its top and a piece of cloth on its bottom.
65
A bent stick, terminated at one end by a hook that could
be inserted into the metal ring of the cube, was used to lift
and move the cube silently. The boxes that served to hide
the object were four rectangular opaque boxes (25.5 cm
high ×20.5 cm wide ×7.5 cm deep) without back panel.
They were fixed on the grey stand at a distance of 20 cm
from each other and they were arrayed in a semicircle so
that all were equidistant (100 cm) from the cat’s starting
position. One set of four identical white boxes was used
during shaping and one set of different boxes was used
during training and testing.
Procedure
The experiment was divided into four successive steps:
Familiarisation, Shaping, Training and Testing.
Familiarization
Individually, each cat freely explored the experimental
room and the retention box during 15 min. Both experi-
menters were present, but the boxes, the visual cues, and
the target object were absent.
Shaping
Each daily session was divided into 30 trials in which the
cats were trained to touch the target object with their paw
in front of the four identical white boxes. In the first step
of shaping, E1 simply moved the target object in front
of the cat, while E2 held the cat by its front shoulders
in the holding box as it watched this movement through
the transparent sliding door. As soon as E1 put down the
object (anywhere on the carpet but not close to or behind
the boxes), E2 opened the transparent sliding door and
released the cat. Each time the cat left the box and touched
the object with its paw, it was reinforced with a piece of
commercial dry food (Science Diet, Hill’s Pet Nutrition,
Topeka, KS) by E2. When the cat had touched the object
in five successive trials, the target object was gradually
positioned closer to the four identical opaque boxes as the
cat succeeded. Finally, the object was moved between two
of the four boxes but never behind them and the cat had
to touch it in 9 trials out of 10. The 12 naive cats needed
a mean number of 65.92 trials (SD =23.13) to complete
the shaping phase and the 12 cats from the previous study
needed a mean number of 18.42 trials (SD =31.99).
Training
Before training, the cats were randomly divided into four
equal groups (n=6) that were equivalent in terms of the
mean number of trials required to reach the shaping crite-
rion, F(3,20) =0.38, P=0.771, and each group included
three naive cats and three cats that had already participated
in a prior study. In Group C (Control), the apparatus was
the same as in Shaping: Four identical rectangular white
boxes were used. In the other three groups, visual cues
were displayed on the white boxes and/or in the grey area
surrounding them. In Group I (Intrinsic features), a differ-
ent geometric symbol (a circle, an S, an X, or a diamond
crossed by a vertical bar) was painted black on the front of
each box. In Group L (local cues), a different set of geomet-
ric patterns (semicircles, horizontal bars, vertical bars, or
full circles) was painted black on the stand, in front of and
behind each box, as well as on the surface of the vertical
panel concealing E1’s arms and hands. In Group I–L (in-
trinsic features and local cues), each box was identified by
a geometric symbol as in Group I and by a set of geometric
patterns as in Group L.
Training began the day following the end of shaping.
The cats were submitted to a training phase where they
learned to find the target object in each of the four posi-
tions on the stand. Through this procedure, the cats learned
that each position on the stand and each box had an equal
probability of being a hiding location. Each training ses-
sion was preceded by three warm-up trials during which the
object was placed between two boxes as in the last step of
shaping.
At the beginning of a training trial, E1 placed the object
on the stand in front of the transparent door of the hold-
ing box, while E2 restrained the cat by grasping its front
shoulders. With the help of the bent stick fixed at the top of
the cube, E1 lifted the object, captured the cat’s attention,
moved the object visibly in front of each of the four boxes,
and finally hid the cube behind the target box. When the tar-
get box was located to the cat’s right, E1 moved the object
from left to right, and when the target box was located to
its left, E1 moved the object from right to left. If the cat did
not watch the movement throughout the entire sequence,
the trial was interrupted and repeated. Once the object had
disappeared, E1, to prevent cuing, looked at E2 who was
sliding the opaque door in front of the transparent door and
remained immobile. Then, E2 immediately removed her
hands from the retention box, opened both doors simulta-
neously and rapidly, and released the cat. The purpose of
this manipulation was to habituate the cats to the manip-
ulation of the opaque sliding door that was used later in
testing. If the cat made no search attempt during the minute
that followed its release (no choice), the cat was called back
in the holding box for the beginning of the next trial. If the
animal found the object behind the target box and touched
it (success), it was called back by E2 and was reinforced.
However, if the cat chose a non-target box (error), the trial
was immediately interrupted, without a chance to search
for the object behind a second box.
Training included eight sessions of 25 trials and within a
session, the object was randomly hidden at least six times
behind each of the four potential hiding locations. Overall,
the cats had to find the object 50 times behind each box.
To avoid a bias of reinforcement toward a specific box, we
repeated all failed trials (no-choices and errors) at the end
of the session until the cat was successful. Each trial was
separated by a short intertrial interval of 30 s.
66
Testing
Testing began the day following the end of training. Each
testing session began with three warm-up trials, which were
identical to those of the last step of shaping. On each testing
trial, E1 hid the target object behind one of the four boxes,
as was done during training. Then, the opaque door was
slid in front of the transparent door for 0, 10, 30 or 60 s.
During the retention interval, E2 removed her hands from
the box and the cat was free to move inside the retention
box. At the end of the retention interval, E2 opened both
doors and released the cat. To prevent cueing, E1 looked at
E2 during the cat’s searching behaviour.
The object was hidden five times behind each box for
each of the four retention intervals, for a total of 80 tri-
als that were distributed over four sessions. The sessions
were counterbalanced among cats and were administered
on four consecutive days. To avoid the negative effect that
a succession of long retention intervals might have on the
performance in trials with short retention intervals (Fletcher
1965), trials were assigned semi-randomly to the four ses-
sions: no trial with a given interval was followed by a trial
with the same interval. As in training, each trial was sepa-
rated by a short intertrial interval of 30 s.
Results
For all statistical analyses, a criterion of α<0.05 was used
for rejection of the null hypothesis. In training and testing,
the percentage of success expected by chance was 25% be-
cause if cats searched randomly, they should have searched
equally behind each of the four boxes. Preliminary factorial
ANOVAs not presented here revealed that the performance
of naive cats and that of cats that had participated in a pre-
vious study did not differ among groups in training and
testing trials. Consequently, data from the naive and expe-
rienced cats were pooled within their respective group in
the following analyses.
Analysis of success
Training
Data from training were expressed as a percentage of the
number of successful trials in the 25 first trials of each of
the eight daily sessions. The trials repeated at the end of
each session, therefore, were not included in the data. As
one can see in Fig. 2, the mean percentage of successful
trials for each group was similar but it increased as a func-
tion of the training sessions. Our data, however, violated
the assumption of sphericity according to the Mauchly’s
test, W=011, P<0.001. Consequently, the degrees of free-
dom for the repeated factors in the ANOVA were adjusted
according to the procedure of Huynh–Feldt. A factorial
ANOVA (Group ×Session) with repeated measures on
the last factor revealed a significant effect of the Session,
F(4.72,94.47)=31.49, P<0.001, but no significant effect
0
10
20
30
40
50
60
70
80
90
100
123 456 78
Session
Mean percentage of successful
trials
C
I
L
I-L
chance level
Fig. 2 Mean percentage of successful trials of each group as a
function of the training sessions. Error bars: standard deviation
of Group, F(3,20) =0.64, P=0.599, or of the interaction,
F(14.17,94.47)=0.935, P=0.526. A posteriori Newman–
Keuls test (P<0.05) showed that the mean performance of
cats gradually improved from 62.3% (SD =18.4) in the
first session to 88.6% (SD =9.1) in the fourth session;
then, it remained stable during the following four sessions
and success reached a high rate of 94.3% (SD =5.2) in the
last session. Finally, a series of one-sample t-tests showed
that the cats’ mean percentage of success (because their
percentage of success did not differ in training, the four
groups were pooled) was greater than what would be ex-
pected by chance (25%) for each training session (sessions
1–8), ts(23) =9.95, 15.90, 21.72, 34.18, 42.49, 64.23, 67.86
and 64.89 (all Ps<0.001), respectively.
These analyses show that the cats rapidly learned to suc-
cessfully locate the hidden object behind each of the four
potential hiding locations on the stand. Interestingly, the
visual cues displayed on the boxes themselves (Groups I
and I–L) and/or on the area surrounding them (Group L and
I–L) did not facilitate the cats’ search behaviour because all
groups learned at the same rate. One reason for this could
be a ceiling effect. Maybe the training task was too easy for
the adult cats in order to be sensitive to the perceptual cues.
Nevertheless, it is still possible that the boxes’ intrinsic
features and/or the local cues provided perceptual support
to search behaviour when the objects’ disappearance and
the onset of searching were separated, as in testing, by a
retention interval.
Testing
Figure 3illustrates the mean percentage of successful trials
for each group as a function of the length of the retention
interval in testing. Firstly, it seems that cats’ performance
in testing trials with a0sretention interval was influenced
by the presence of trials with longer retention intervals: It
was significantly lower than in the last session of training,
F(1,5)=17.72, P=0.008. Secondly, groups did not differ.
A factorial ANOVA (Group ×Interval) with repeated mea-
sures on the last factor revealed that in testing, there was
a significant effect of Interval, F(3,60) =71.58, P<0.001,
67
0
10
20
30
40
50
60
70
80
90
100
010 30
60
Interval (sec.)
Mean percentage of successful
trials
C
I
L
I-L
chance level
Fig. 3 Mean percentage of successful trials of each group as a
function of the retention intervals. Error bars: standard deviation
but no effect of Group, F(3,20)=0.71, P=0.559 or of the in-
teraction, F(9,60)=0.67, P=0.734. A posteriori Newman–
Keuls test (P<0.05) revealed that the percentage of success
was higher witha0sretention interval than with any of
the other three retention intervals and that it was higher
with a 10 s retention interval than with a 30 or a 60 s re-
tention interval, which did not differ. In addition, a series
of one-sample t-tests was computed to estimate whether
the cats’ mean percentage of success for each interval was
significantly higher than chance (because their percentage
of success did not differ, data from the four groups were
pooled). As a group, cats performed significantly higher
than chance (25%) for all retention intervals (0, 10, 30,
60 s), ts(5)=17.70 (P<0.001) [M=72.92, SD =13.26],
6.43 (P<0.001) [M=44.38, SD =14.77], 3.48 (P<0.002)
[M=35.42, SD =14.66] and 3.61 (P<0.001) [M=33.96,
SD =12.16], respectively.
In summary, these analyses indicate that cats’ accuracy
to locate a disappearing object rapidly declines between 0
and 30 s. In addition, cats’ capacity to memorize a hiding
location is not enhanced by the presence of visual cues
and/or local cues displayed on the boxes or on the area
surrounding it.
Analysis of errors
To determine why cats’ duration of working memory for
disappearing object declined so rapidly, we examined the
hypothesis that the cats may have been subject to intertrial
proactive interference. Intertrial proactive interference is
inferred when the information used by an animal in the
trial prior to the current trial interfered with its capacity
to maintain in memory the target location (Hampton et al.
1998). Intertrial proactive interference is most likely to oc-
cur when the different retention intervals are all distributed
within the same testing session and the intertrial interval
is short (Edhouse and White 1988; Grant 1981; Roberts
1980). The present study met these two conditions because
the four retention intervals were mixed within each single
session and the intertrial interval between each successive
trial was of 30 s. In the present experiment, intertrial proac-
tive interference was inferred whenever cats returned to the
spatial position rewarded prior to the current trial.
In the statistical analyses, the number of search attempts
made at the location the cats were rewarded prior to the cur-
rent trial was expressed as a percentage of the total num-
ber of errors. A non-significant within-subject ANOVA,
F(3,20)=0.304, P=0.822, revealed that the percentages of
search attempts made at the location rewarded prior to the
current trial were stable for each interval (note that data
from one cat were eliminated from this analysis because it
did not make any errors during test trials of 0 s). Because the
number of search attempts made at the location rewarded
prior to the target trial did not differ from one interval to
another, they were pooled. In addition, the errors made in
the first trial of each of the four testing sessions were taken
away from the total of errors because intertrial proactive
interference was not possible in these trials. We used a bi-
nomial test (P=0.25) to determine whether the number of
search attempts made at the spatial location rewarded prior
to the current trial, expressed as the total number of errors
committed by each cat, differed significantly from the num-
ber of search attempts expected by chance. Data revealed
that only one out of 24 cats searched above-chance level
at the location they were rewarded prior to the current trial
(this cat made 11 search attempts out of 27 errors at the
location where it was rewarded the trial prior the current
trial). Consequently, it appears that the cats did not return to
the location where they were rewarded during the prior trial
and were not subject to intertrial proactive interference.
Because the cats’ decline of performance was not
the effect of intertrial proactive interference, we examined
the hypothesis that the cats’ errors in the present VD task
were due to difficulties to remember the exact location
of the hiding location. According to this hypothesis (Bjork
and Cummings 1984; Call 2001), the distribution of the per-
centages of search attempts to the non-target boxes should
be a function of the proximity to the target box. Proximity
was analysed according to two spatial configurations of the
boxes. Firstly, the target box was either on the far right or on
the far left of the row of boxes (10 trials for each interval).
With this spatial configuration, there was a first (1st), a sec-
ond (2nd), and a third (3rd) adjacent non-target box relative
to the target box. Secondly, the target box was either the sec-
ond box from the far left or the far right of the row of boxes
(10 trials for each interval). With this spatial configuration,
there was one non-target box (1st-1) on one side of the tar-
get box and two nonadjacent boxes (1st-2 and 2nd-2) on the
other side. Given the small number of trials (n=10) for each
interval in both types of spatial configuration of the boxes,
it was statistically risky to evaluate the distribution of the
percentages of search attempts made behind the three non-
target boxes as a function of the retention intervals. Con-
sequently, the percentages of search attempts made behind
each non-target box were pooled for each interval. In addi-
tion, because the distribution of the percentages of search
attempts made behind each of the three non-target boxes
was not independent, a non-parametric Friedman two-way
ANOVA by ranks was used. For each cat, the percentages
of search attempts made behind each of the three non-target
68
boxes were transformed by ranks from 1 to 3. Rank1 was
given to the lowest percentage and rank 3 was given to the
highest percentage. Significant Fr values were followed by
a series of multiple comparisons performed on the differ-
ence between the rank sums (Siegel and Castellan 1988).
A first Friedman two-way ANOVA by ranks revealed a
significant effect of the proximity to the target box when
there was a 1st, a 2nd, and a 3rd adjacent non-target box,
Fr(2)=12.45, P=0.002. Multiple comparisons tests showed
that the cats searched significantly more frequently at the
1st non-target box (mean rank=2.48) than at the 3rd adja-
cent box (mean rank =1.48). Search attempts made at the
2nd non-target box (mean rank =2.04) did not differ from
the other two non-target boxes. A second within-subjects
ANOVA indicated a significant effect of the proximity to
the target box when there was one non-target box (1st-1) on
one side of the target box and two nonadjacent boxes (1st-2
and 2nd-2) on the other side, Fr(2)=23.89, P=0.0001. Mul-
tiple comparisons tests revealed that the search attempts
made at the 1st-1 non-target box (mean rank =2.21) and at
the 1st-2 non-target box (mean rank =2.56) did not differ
but were more frequent than the search attempts made at
the 2nd-2 non-target box relative to the target box (mean
rank =1.23). Put together, these results reveal that when
the cats committed an error, they searched as a function of
the proximity to the target box. These last analyses strongly
suggest that in the present memory task, cats’ errors were
due to difficulties in remembering the exact spatial position
of the hiding location.
Discussion
This experiment investigated the limits of cats’ working
memory in search for disappearing objects when the hiding
locations could be discriminated on the basis of visual in-
formation. Overall, the cats’ performance rapidly declined
between 0 and 30 s but still remained slightly above chance
at 60 s. The analysis of errors also indicated that whenever
the cats committed errors, they searched as a function of
the proximity to the hiding location and were not subject
to intertrial proactive interference. Finally, this experiment
revealed that the visual cues displayed on the hiding loca-
tions or on the surface surrounding them do not facilitate
the memorization of the hiding position in cats.
First, the decrease of cats’ performance as a function of
the length of the retention interval is in accordance with pre-
vious studies that used the DR task to investigate the limits
of the working memory in cats (Cowan 1923; Meyers et al.
1962; Yarbrough 1917). This supports the hypothesis that
cats encode and maintain a mental representation of the
hiding location in working memory during the retention
interval and that the memory trace of the hiding location
declines during the 30 s following the disappearance of
the object behind a target box. Nevertheless, because the
cats were free to move inside the retention box during the
entire interval, one could argue that the cats relied on a non-
mnemonic strategy (head and/or body orientation) to locate
the hiding position. Although we were aware of this possi-
bility when we designed this experiment, we did not register
the body or head orientation of cats during the retention in-
tervals. Two reasons motivated our decision. Firstly, even
if Yarbrough (1917) had found some evidences of a body
orientation strategy in cats in a DR task, his observations
had not been supported by Cowan’s (1923) and Meyers
et al. (1962) studies. Secondly, prior works in our labora-
tory (Goulet et al. 1989; Fiset and Dor´
e1996) revealed that
when they were confined inside an opaque retention box,
the cats displayed different behaviours suggesting that they
did not use physical orientation to locate the hiding loca-
tion: they fixated the corner of the box by which the opaque
door was closed, they attempted to open the opaque door
with their paws, they moved and turned on themselves, they
lied down in different directions or they washed. Actually,
we did observe the same behaviours in the present experi-
ment for all retention intervals. This strongly suggests that
the cats did not rely on a non-mnemonic strategy to remem-
ber the hiding location. Similarly, Vallortigara et al. (1998)
observed that chicks kept moving in diverse directions dur-
ing confinement inside an opaque box after they saw an
imprinting object disappear behind an opaque screen. In
the present experiment, the analysis of errors also showed
that the cats searched as a function of the proximity to the
hiding location. This suggests that when they committed
an error, the cats did remember an approximation of the
spatial position of the target location but forgot the exact
position. In addition, if the cats relied on a non mnemonic
strategy, they should have succeeded the 0 s intervals with
the same level of success as in training. However, our data
showed that the performance of the cats was lower in the
tests, revealing that the memory trace of the hiding location
at 0 s was significantly influenced by the presence of longer
retention intervals within the same testing session. We do
not see how a non-mnemonic strategy, such as body orien-
tation, could had been influenced by the longer intervals.
The most likely explanation, therefore, is that the cats relied
on a memory trace of the hiding location rather than on a
non-mnemonic strategy for finding a disappearing object.
In the present study, our four groups of cats did not
differ either in Training or in Testing, It appears that the
discriminative visual cues displayed on the boxes did not
help in finding and remembering the location of the disap-
pearing object. Therefore, our hypothesis predicting that
the box’s intrinsic features or local cues would be encoded
and would enhance retention of the object’s location was
not confirmed. Our results are similar to those observed by
Vallortigara et al. (1998) who used a DR task to estimate the
duration of chicks’ memory for imprinting objects. These
authors did not find any increase of performance in chicks
when the imprinting object was hidden behind screens that
could be discriminated on the basis of colour and pattern.
Our results in cats are surprising because a study by
Dodwell, Wilkinson, and Gr¨
unau (1983) showed that cats
learned to discriminate visual patterns that were similar
to the geometric figures used in this experiment. It seems
that, although cats are able to discriminate such cues, they
do not encode them in working memory for memorizing
the location of a hidden object. One might argue that cats’
69
performance could have been higher if we had manipulated
the shape, the colour and/or the height of the hiding loca-
tions. However, a series of experiments (Fiset unpublished
data) revealed that intrinsic features of the hiding location,
such as shape, height and width, are irrelevant to the cats
for locating a hidden object in a standard VD problem.
Our results are also in accordance with a study by Dor´
e
et al. (1996) showing that the presence of hiding boxes
that could be discriminated on the basis of their dimension
and the visual cues displayed on their front panel did
not increase cats’ success in invisible displacement
problems. Our data are also compatible with several
studies showing that humans as well as animals favour the
spatial information provided by the environment at large
rather than the visual properties of the spatial location or
the local cues (Cheng 1986; Cheng and Gallistel 1984;
Hermer and Spelke 1996; Brodbeck 1994; Gouteux et
al. 1999; Sovrano et al. 2002, 2003; Vallortigara et al.
1990). Because cats live in a plentiful visual world where
each spatial position is characterised by various types of
visual features, it may be useful for the cats to merely
process and record the spatial information of the hiding
location rather than encode and use the visual properties
of the target position. Nevertheless, as shown by Dodwell,
Wilkinson and Gr¨
unau’s (1983) study, cats may encode
discriminative visual information in circumstances where
the use of spatial information is impossible.
Finally, even though we did not plan to explore inter-
species differences of memory for disappearing objects
within the same experimental paradigm, when we designed
the present study, we carefully replicated the number of
retention intervals (and their length) used by Fiset et al.
(2003) in dogs and we proportionally adapted the distance
between the boxes as well as the size of the boxes to the
cats. This provides us the opportunity to compare cats’
and dogs’ duration of working memory for hidden objects.
Fletcher (1965) rightly argued that interspecies compari-
son of memory can only be made if the species are tested
under the same conditions. Otherwise, memory differences
observed between different species can be the result of dif-
ferent experimental conditions rather than the effect of real
interspecies differences. Interestingly, our data suggest that
the processes of forgetting for disappearing objects in cats
are similar to those of dogs because the cats were not sub-
ject to intertrial proactive interference and they searched
as a function of the proximity of the hiding location when
they committed an error. The retention capability of cats
for disappearing objects, however, seems to be lower than
the one observed in dogs. In cats, the decrease of perfor-
mance rapidly occurred between 0 and 10 s, whereas in
dogs, it gradually occurred between 0 and 30 s. In addi-
tion, the cats’ mean percentage of success remained just
above-chance level for delays of 30 and 60 s, whereas it
was well above chance for the same delays in dogs. Never-
theless, although the data from these two studies strongly
suggest quantitative differences between the limits of cats’
and dogs’ working memory for disappearing objects, these
findings might be confounded by age (Milgram et al. 1994)
or breeds differences in dogs (Head et al. 1995). Conse-
quently, future comparative studies are called for investi-
gating the potential impact of these confounding variables
on the duration of cats’ and dogs’ working memory for
disappearing objects.
In summary, our study supports the VD task, coupled
with the introduction of a retention interval between the
disappearance of an object and the onset of animal’s search
behaviour, as a valuable procedure to investigate the length
of time that animals can retain the spatial position of
a disappearing object in working memory. Future inter-
species comparisons using the present memory task, how-
ever, should be made with caution due to the possible im-
pact of methodological differences on animals’ memory,
such as the number of boxes and the number of retention
intervals, as shown by a large body of evidence in ear-
lier studies using the DR task (for an extensive review,
see Fletcher 1965). The present study also reveals that the
visual properties of the hiding location are irrelevant in-
formation for the retention of a hidden object in cats and
comparative studies are needed for determining whether
this last conclusion can be generalized to other species that
have already demonstrated the cognitive ability to locate a
disappearing object. Finally, the present study supports the
distinctive cognitive capacities of cats and dogs for hidden
objects, which have already been demonstrated by different
studies on the understanding of object permanence (Dor´
e
et al. 1996) and the encoding of spatial information (Fiset
and Dor´
e1996; Fiset et al. 2000).
Acknowledgements This research was supported by an operat-
ing research grant from the Natural Sciences and Engineering Re-
search Council of Canada (NSERC). The experiments received ap-
proval from the Comit´
e de protection des animaux de laboratoire
de l’Universit´
e Laval, which is responsible for the application and
enforcement of rules of the Canadian Council on Animal Care.
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