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The role of vision and proprioception in the aversion of rats tothe open arms of an elevated plus-maze
Juan Carlos Martınez a, Fernando Cardenas b, Marisol Lamprea b,Silvio Morato b,*
a Universidad de la Sabana, Bogota, DC, Colombiab Universidade de Sao Paulo, Ribeirao Preto, SP 14040-901, Brazil
Received 25 September 2001; received in revised form 12 June 2002; accepted 28 June 2002
Abstract
The elevated plus-maze test is usually run with a short edge surrounding the open arms in order to prevent the rats
from falling. The present experiment investigated the role of transparent edges differing in heights: 1 (used as control),
5, 10, 20 and 40 cm, the latter the same height as the closed arm walls. Additionally, this 40-cm high transparent edge
was also studied covered by white translucent or black opaque paper. The data show that the time spent in the open
arms was significantly greater when the edge height was 5, 10 or 40 cm covered by the white or black paper. However,
there were no differences from the 1-cm control edge when the height was 40 cm transparent. A similar effect was
observed when entries in the open arms and total entries were analyzed. The facts that there were no differences when
the open arms were surrounded by 1- or 40-cm transparent edges (which allow thigmotaxis) and that the same 40-cm
edge caused increases in exploratory behavior when covered by papers indicate that vision triggers aversion to open
spaces.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Aversion to open spaces; Elevated plus-maze; Exploratory behavior; Proprioception; Rats; Vision
1. Introduction
The elevated plus-maze, the modification of a
procedure introduced almost five decades ago
(Montgomery, 1955), was first reported by Hand-
ley and Mithani (1984) and has often been used to
investigate anxiolytic and anxiogenic compounds
and to study the involvement of neurotransmitters
in anxiety (e.g. Dorow et al., 1983; Pellow and
File, 1986; Moser, 1989; Cruz et al., 1994; Rodgers
and Cole, 1994; Anseloni and Brandao, 1997;
Anseloni et al., 1995). The method was considered
to be a valid and reliable tool for measuring
anxiety, on the basis of extensive investigation
analyzing several of its behavioral, physiological
and pharmacological aspects (Pellow et al., 1985;
Cruz et al., 1994; Rodgers and Cole, 1994;
Anseloni and Brandao, 1997). The test consists
* Corresponding author. Address: Faculdade de Filosofia,
Av. Bandeirantes, 3900, Ribeirao Preto, SP 14040-901, Brazil.
Tel.: �/55-16-602-3662; fax: �/55-16-633-5668
E-mail address: [email protected] (S. Morato).
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of placing an animal in a plus-shaped maze
elevated above the floor level, with wall-closed
arms and open arms and measuring the frequency
of entries and time spent in each type of arm as
well as other behaviors not related to ambulation,
such as rearing, stretching, head dipping, etc. A rat
explores both the closed and the open arms but
will typically enter more frequently and stay longer
in the closed arms. The percent preference for
open or closed arms, both for entries and duration
(Handley and Mithani, 1984; Pellow and File,
1986) is taken as an index of anxiety: the more
intense the anxiety the lower the percent preference
for the open arms.
In spite of the apparent simplicity of this test
situation, the aversion to the open arms seems to
be influenced by many factors (for a review, see
Hogg, 1996). Some of them are inherent to the
subjects, such as sex (Johnston and File, 1991;
Imhof et al., 1993) and age (Imhof et al., 1993).
Others are linked to experimental procedure, such
as pre-exposure to the maze (File, 1992; Griebel et
al., 1993; Treit et al., 1993) and time of day at
which testing occurs (Gentsch et al., 1982; Griebel
et al., 1993). Still others concern the test situation,
such as the levels of illumination of the test room
(Gentsch et al., 1982; Morato and Castrechini,
1989; Griebel et al., 1993; Cardenas et al., 2001),
and indicate that exploration of the open arms is
increased by low levels of environmental illumina-
tion. Also related to the test situation is the
presence of raised edges surrounding the open
arms, which prevent the rats from falling (Treit et
al., 1993; Fernandez and File, 1996). It is possible
that the presence of edges may also have the effect
of decreasing the aversion to the open arms
because of grasping, since Morato and Castrechini
(1989) have shown that a wire mesh floor (which
allowed grasping) increased the exploration of the
open arms. Still others refer to the manipulation of
experimental subjects, such as the way rats are
transported to the test room (Morato and Bran-
dao, 1996), how they are housed (Maisonnette et
al., 1993) or the place and how long they are kept
there before testing (Morato and Brandao, 1997).
Despite the broad use of this animal model of
anxiety, little is known about the event(s) trigger-
ing the aversion that ultimately will cause rodents
to avoid the open arms of the maze.
It has been demonstrated (Treit and Fundytus,
1989; Treit et al., 1993) that open spaces prevent-
ing thigmotaxis (the trend to stay close to vertical
surfaces), rather than height or novelty, is the
aversive stimulus acting in the elevated plus-maze.
There is also a suggestion that rats do prefer to be
close to vertical surfaces in order to avoid avian
predation (Grossen and Kelley, 1972). In the same
vein, there are reports (Steiner et al., 1986; Milani
et al., 1989) showing that rats with the vibrissae on
one side removed tended to explore an open-field
keeping the vibrissae of the intact side directed to
the walls, which indicate rats might be using the
vibrissae to detect the presence of vertical surfaces.
However, it has recently been reported that this
may not the case in the elevated plus-maze
(Cardenas et al., 2001). There are no further
suggestions as to what could be the sensory
modality used by the rats to detect open spaces,
namely whether they are perceived by one or more
senses, such as vision, touch or hearing.
The present experiment investigated the effects
of different heights of the transparent open arm
edges on rat exploratory behavior in the elevated
plus-maze and the influence of light in the aversion
to the open arms.
2. Materials and methods
2.1. Animals
Male Wistar rats weighing 200�/230 g were
housed 6 to a cage (41�/34�/17 cm3) with food
and water ad libitum on a 12:12 h light/dark
photoperiod (lights on at 7:00 a.m.) for an
habituation period of at least 3 days before being
submitted to the experimental conditions. Room
temperature was maintained at 24�/26 8C in both
the vivarium and the test room, which was
adjacent to it. All tests were performed between
7:30 and 10:30 a.m.
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2.2. Apparatus
Exploratory activity was measured in an ele-
vated plus-maze similar to that previously de-
scribed elsewhere (Pellow et al., 1985; Cardenas
et al., 2001). Briefly, the maze was elevated 50 cm
from the floor and consisted of two closed arms
(surrounded by 40-cm high wooden walls and no
roof) and two open arms arranged in such a waythat like arms were opposite to each other. The
inside of the closed arms was painted with
transparent navy enamel and the floor was made
of dark blue opaque Formica. Each group (N�/
12) was tested with transparent Plexiglas edges of
different heights (1, 5, 10, 20 and 40 cm) surround-
ing the open arms. Two additional groups of
animals were tested with the 40-cm transparentedges covered with white translucent or black
opaque paper (see Table 1 for light intensity
measurements in different parts of the maze, for
the different edge heights). The sessions were
recorded with videocassette equipment and ana-
lyzed later with a software described elsewhere
(Conde et al., 2000).
2.3. Procedure
Each rat was gently placed in the maze facing
one of the closed arms. The number of entries and
time spent in each type of arm and in the central
square was then recorded for 5 min. At the end ofeach session the maze was cleaned with a 10%
alcohol solution and dried with a dry cloth. An
entry was recorded when all four paws were placed
inside one arm. The percentage of entries into each
kind of arm was calculated in relation to total
entries into both arms. The distance run in each
type of arm was assessed dividing the number of
crossed squares by the length of each square (10
cm). In order to do so, the maze was divided in 10-
cm squares on a transparent plastic mask placed
over the TV set screen. A crossing was recorded
every time both hind paws entered a square.
Frequency and time spent in behaviors other
than locomotion were measured and included
rearing (rising on the hind limbs both touching
and not touching a wall surface), stretching
(elongation of the body), head out (sticking the
head outside the maze border, horizontally or
upwards) and head dipping (sticking the head
outside the maze border and below floor level).
The exact location of the rat when performing
these behaviors was recorded with the mask over
the TV screen, using both hind paws as a reference.
2.4. Statistical analysis
Comparisons between the different edge-size
groups were performed using one-way analysis of
variance followed, whenever appropriate, by the
post hoc Dunnett test for comparing group means
with a control mean, except when stated otherwise.
Significance was set at P B/0.05 for all tests.
Table 1
Amounts of light (Lux) reaching the different parts of the elevated plus-maze (open- and closed-arm extremities, and center) with
different open-arm enclosures
Open arm enclosure Location of the measurement
Open-arm extremities Closed-arm extremities Center
1-cm edge 93 95 47 48 84
5-cm edge 91 100 49 48 81
10-cm edge 99 100 50 48 80
20-cm edge 101 102 50 48 81
40-cm transparent 107 109 50 48 82
40-cm white translucent 95 95 48 48 78
40-cm black opaque 59 66 48 47 67
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3. Results
Fig. 1 shows the percentage of entries into the
open arms, the total number of entries into both
kinds of arms (differentiating between open and
closed arms) and the relevant statistics. It can be
seen that the relative frequency of entries into the
open arms was increased by both the 5-cm edge
and by the 40-cm edge covered with the black
opaque paper. It can also be seen that total entries
were increased by all size edges, an effect that was
caused mainly by the increases in the open arm
entries, since entries into the closed arms were
unaffected by the edges height. Fig. 2 shows the
time spent in the open arms, central square, closed
arms and the corresponding statistical analysis. It
shows that the aversion to the open arms, as
measured by the time spent in them, was decreased
by all edge heights except the 40-cm transparent
one. It is interesting to notice that one difference
between this group and the two others tested with
the 40-cm edge covered either with the white
translucent or black opaque papers is the possibi-
lity of seeing the outside of the maze. There were
no significant changes in the time spent in the
central square, indicating that the increases in the
time spent in the open arms were achieved at the
expense of the time spent in the closed arms, the
profile of which is a mirror image of that of the
open arms. Tables 2 and 3 show that there was an
increase in the mean duration of open arm entries,
that is, each individual entry was longer with the 5-
, 10-cm high edges and with the 40-cm high edge
covered with the black opaque paper. In almost a
mirror image of that, there was a decrease in the
mean duration of closed arm entries (individual
entries were shorter) with the 5-, 10- and 20-cm
high edges and with the 40-cm high edge covered
with both the white translucent and the black
opaque paper. The mean durations in the central
square were not altered by the height of the edges.Fig. 3 and Table 3 exhibit frequency, time spent
and mean duration of stretching, head dipping and
head out. It can be seen that both the frequency
and the time stretching were decreased by all the
edges different from control. Not only did they
decrease but there was a decrease in the mean
duration of stretching, indicating the rats were not
only stretching less frequently but less time as well.
Head dipping and head out were increased by the
two edge heights that allowed these behaviors to
be expressed. It is possible that dipping the head,
Fig. 1. Percentage of entries into the open arms, number of entries into both kinds of arms and the correspondent statistics. F[6,77], F
values for 6 and 77 degrees of freedom; P , probability; C, control group with 1-cm edge surrounding the open arms; 5�/40, groups of
rats with different heights of edges surrounding the open arms; 40W and 40B, 40-cm high Plexiglas edge covered with white translucent
paper and black opaque paper, respectively. (*), Significantly different from the control group (Dunnett, P B/0.05). (8), Significantly
different from all other totals (Dunnett, P B/0.05). Under the last column above the graphs are the groups that are different from
controls (Dunnett, P B/0.05); N.S., non significant (ANOVA, P B/0.05).
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but not sticking the head out, was made more
difficult by the 10-cm high edge.
Fig. 4 and Tables 2 and 3 show frequency and
time spent in rearing plus the respective statistical
analyses. As a general effect, it can be seen that the
higher edges increased both rearing frequency and
time in both the open arms and the central square.
As control rats did not emit this behavior at all, we
performed an additional statistical analysis only
with the groups in which the averages were
different from zero: groups tested with 20- and
40-cm edges (transparent or covered with white or
black paper). It revealed that the rats tested with
the 20-cm edge or the 40-cm edge covered with
black paper reared more often and for a longer
time when compared to the groups tested with 40-
cm edge transparent or covered with the white
paper (F[3,47]�/9.58, P B/0.001, followed by the
Bonferroni t -test for comparisons between the
means). It is interesting to notice that rearing
was increased by the 40-cm high transparent edge,
which did not happen to the other behaviors,
mainly the entries. There were no significant
effects in the closed arms.
Fig. 5 depicts the distance run in the arms of the
maze. It can be seen that with the exception of the
transparent 40-cm high edge group, increasing the
height of the edge made the rats run longer
distances in the open arms without significantly
altering the distances run in the closed arms.Finally, Tables 2 and 3 show that grooming had
its frequency and time in the open arms increased
by the 40-cm high edge when covered with the
black paper. There was also an increase in time,
but not frequency, in the central square with the
10-cm high edge. There were decreases in time, but
not frequency, spent grooming in the closed arms
caused by the 5-, 10-cm high edges and by the 40-cm high edge covered with both the white translu-
cent and the black opaque paper. At last, total
time grooming was decreased by the 5-cm high
edge and by the 40-cm high edge covered with
both the white translucent and the black opaque
paper, apparently because of decreases in the
duration of individual grooming episodes.
4. Discussion
When we started the experiment, we expected tofind a relation between open arm edge height and
exploratory behavior: the higher the edge the more
exploratory behavior. Recording and analyzing
the behavior of the very first rats we got an inverse
relation, with more intense exploratory behavior
obtained with the lower edges (5 and 10 cm) and
Fig. 2. Time spent in the open arms, the closed arms, the central square and the correspondent statistics. F[6,77], F values for 6 and 77
degrees of freedom; P , probability; C, control group with 1-cm edge surrounding the open arms; 5�/40, groups of rats with different
heights of edges surrounding the open arms; 40W and 40B, 40-cm high Plexiglas edge covered with white translucent paper and black
opaque paper, respectively. (*), Significantly different from the control group (Dunnett, P B/0.05).
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less exploratory behavior with the higher edges (20
and 40 cm transparent). The enhanced exploration
by the rats of groups 5 and 10 and the absence of
enhanced exploratory behavior by the rats of
group 40 was totally unexpected because the first
were supposed to explore in a similar way as the
controls and the latter were supposed to explore
the ‘open’ (surrounded by transparent Plexiglas)
arms in a similar manner as that of the closed
arms. That (a) led us to the idea that light could
be a determining factor, (b) prompted us to
include groups 40W and 40B to control for the
effects of diffuse horizontal light (40W) and no
horizontal light at all (40B), like a conventional
closed arm (made of wood), and (c) to consider
an aversion-inhibiting effect of grasping (proprio-
ception).
The results of the present paper seem to support
the notion that rats have a preference for staying
close to vertical surfaces, the absence of which
correlates with open spaces and could provoke the
aversion that will prevent them from exploring the
open arms of an elevated plus-maze (Treit and
Fundytus, 1989; Treit et al., 1993). In our experi-
ment, in general, with the exception of the 40-cm
high edge, the presence of edges in the open arms
facilitated exploratory behavior in them. This,
however, does not indicate what is the role of
vertical surfaces. In the next paragraphs we argue
in favor of three points. (a) That the main effect of
vertical surfaces (i.e. the walls enclosing the closed
arms) in a conventional elevated plus-maze is to
prevent the rat from perceiving the light coming
horizontally from the surrounding environment.
Table 2
Frequency and duration of several behaviors shown by rats exploring an elevated plus-maze with edges of different heights and
transparencies surrounding the open arms
Behaviors Edge height
C 5 10 20 40 40W 40B
Entry duration (s)
Open arms 9.19/0.6 12.59/0.9* 13.39/1.3* 11.29/1.6 9.59/0.7 129/1.2 139/0.9*
Closed arms 299/3.4 169/2.2* 189/2.2* 189/1.9* 239/3.9 14.09/0.7* 149/1.0*
Central square 2.49/0.3 1.99/0.4 2.29/0.4 1.69/0.3 1.99/0.3 2.49/0.4 2.49/0.4
Rearing duration (s)
Open arms 0.09/0.0 0.09/0.0 0.09/0.0 1.89/0.1* 1.19/0.2* 1.29/0.2* 1.89/0.1*
Closed arms 1.39/0.1 1.19/0.1 1.39/0.2 1.49/0.1 1.69/0.2 1.49/0.1 1.59/0.2
Central square 0.69/0.2 1.29/0.3 1.29/0.1 1.19/0.1 1.39/0.1 1.89/0.3* 2.19/0.3*
Total 1.39/1 1.29/0.2 1.29/0.1 1.59/0.1 1.59/0.1 1.59/0.1 1.89/0.1*
Grooming
Frequency-open arms 0.09/0.0 0.89/0.3 0.89/0.3 1.89/0.9 0.99/0.3 1.89/0.7 3.09/0.8*
Frequency-closed arms 7.49/0.7 4.79/1.1 4.69/1.1 5.19/1.1 6.79/1.3 3.79/0.8 4.19/0.5
Frequency-central square 0.99/0.2 1.59/0.4 3.49/0.8 2.89/0.7 1.99/0.6 1.89/0.6 2.39/0.5
Total frequency 8.39/0.7 6.99/1.2 8.89/1.1 9.69/1.3 9.59/1.4 7.29/1.0 9.49/1.0
Time-open arms 0.09/0.0 0.59/0.2 2.29/1.2 1.99/1.0 2.19/1.5 3.69/1.9 5.79/1.2*
Time-closed arms 42.79/9.5 16.09/5.7* 19.19/5.4* 19.79/5.4 37.39/9.5 12.09/3.7* 10.19/2.3*
Time-central square 3.39/1.5 3.89/2.4 19.19/5.7* 7.79/3.2 3.29/1.1 7.09/4.0 8.29/2.2
Total time 46.19/8.8 20.49/5.5* 40.49/6.4 29.49/4.4 42.69/8.8 22.59/3.9* 24.09/2.3*
Grooming duration (s)
Open arms 0.09/0.0 0.39/0.1 1.79/1.0 0.59/0.3 0.89/0.5 1.19/0.5 2.09/0.5
Closed arms 5.69/1.5 3.19/1.0 3.99/0.7 3.09/0.7 4.59/0.9 2.89/0.7 2.49/0.4
Central square 3.09/1.5 1.69/0.8 4.29/0.8 1.89/0.7 1.19/0.3 2.49/0.7 2.69/0.7
Total 5.69/1.0 3.19/0.7* 4.69/0.5 3.59/0.6 4.59/0.6 3.39/0.5* 2.79/0.2*
Values are means9/S.E.M. C, 1-cm control edge; 5�/40, height (cm) of transparent Plexiglas edges; 40W, 40B, 40-cm high Plexiglas
edge covered with white translucent paper and black opaque paper, respectively.
* Different from control (Dunnett, P B/0.05).
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(b) That such light perception is the event trigger-
ing aversion to the open arms. And we also (c)
raise the hypothesis that having where to grasp, or
more likely, the proprioception arising from grasp-
ing inhibits the triggering of aversion by the
perception of light.
In general, our results show that high edges tend
to increase the exploration of the open arms, as
compared to controls. This was more evident in
the time spent in the open arms. But when the
open arms were surrounded by the 40-cm high
transparent walls the rats behaved like control rats
Table 3
Statistical results of one-way ANOVAs applied to several behavior measures to test for significance of differences between the various
edge heights
Behaviors Statistics Different from control (Dunnett, P B/0.05)
F D.F. P
Entry duration
Open arms 2.596 6, 77 0.024 5, 10, 40B
Closed arms 4.095 6, 77 0.001 5, 10, 20, 40W, 40B
Central square 1.367 6, 77 0.239 Non significant
Rearing duration
Open arms 39.192 6, 77 B/0.001 20, 40, 40W, 40B
Closed arms 1.442 6, 77 0.210 Non significant
Central square 4.923 6, 77 B/0.001 40W, 40B
Total 3.159 6, 77 0.008 40B
Stretching-central square
Frequency 8.579 6, 77 B/0.001 5, 10, 20, 40, 40W, 40B
Time 10.007 6, 77 B/0.001 5, 10, 20, 40, 40W, 40B
Mean entry duration 4.498 6, 77 B/0.001 5, 10, 20, 40, 40W, 40B
Head dipping-open arms
Frequency 5.079 2, 33 0.012 5
Time 4.39 2, 33 0.020 5
Mean duration 5.045 2, 33 0.012 5
Head out-open arms
Frequency 13.4 2, 33 B/0.001 5, 10
Time 8.232 2, 33 0.001 5, 10
Mean duration 7.487 2, 33 0.002 5, 10
Grooming
Frequency-open arms 3.066 6, 77 0.010 40B
Frequency-closed arms 2.084 6, 77 0.065 Non significant
Frequency-central square 1.992 6, 77 0.077 Non significant
Total frequency 0.990 6, 77 0.438 Non significant
Time-open arms 2.657 6, 77 0.021 40B
Time-closed arms 3.829 6, 77 0.002 5, 10, 40W, 40B
Time-central square 2.989 6, 77 0.011 10
Total time 2.967 6, 77 0.012 5, 40W, 40B
Grooming duration
Open arms 1.892 6, 77 0.093 Non significant
Closed arms 1.829 6, 77 0.104 Non significant
Central square 1.400 6, 77 0.225 Non significant
Total 2.702 6, 77 0.020 5, 40W, 40B
F, F values; D.F., degrees of freedom; P , probability. 5-40, groups of rats with different heights of edges surrounding the open arms;
40W and 40B, 40-cm high Plexiglas edge covered with white translucent paper and black opaque paper, respectively (ANOVA, P B/
0.05).
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(with 1-cm high edges surrounding the open arms)
and avoided the ‘open’ arms, in spite of being able
to stay close to the transparent walls and touch
them as well. This is an unexpected and important
finding. It is unexpected because, since the ‘open’
arms are now surrounded by 40-cm high trans-
parent Plexiglas walls, the rats should behave like
when inside arms surrounded by wooden walls.
Instead, they behaved like when in the open arms
of a conventional plus-maze. It becomes clear that
the sheer presence of vertical surfaces is not
enough to prevent aversion, as suggested elsewhere
(Treit and Fundytus, 1989). One element the open
arms surrounded by either 1- or 40-cm high edges
have in common is the capability of seeing outside,
suggesting that the visual input is more important
for triggering aversion than just being away from
vertical surfaces. Thus, our rats avoided the open
arms when they could see outside (group 40), not
so much when the light coming in was diffuse and
hindered images to be formed (40W), but did not
avoid the open arms when no light entered
horizontally (group 40B). In fact, group 40B rats
depicted a number of entries and time spent in the
Plexiglas arms analogous to those they exhibited in
the wooden (closed) arms. Accordingly, when rats
are tested in the dark, when visual input is scarce
or not possible at all (Morato and Castrechini,
1989; Griebel et al., 1993; Cardenas et al., 2001),
there are large increases in exploratory activity in
the open arms, as measured both by the frequency
of entries and the time spent there.
It could be argued that the rats were attracted to
the less bright areas of the maze and that could
explain why they preferred the arms closed with
wood to the ones closed with the transparent
Plexiglas. However, light incidence at the extremi-
ties of the transparent-walled arms was similar
Fig. 3. Frequency, total and mean duration of stretching, head-dipping and head-out behaviors. C, control group with 1-cm edge
surrounding the open arms; 5 and 10, groups of rats with different heights of edges surrounding the open arms. (*), Significantly
different from the respective control group (Dunnett, P B/0.05).
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when they were surrounded by edges either 1-cm,
40-cm transparent and 40-cm transparent edges
covered with white translucent paper. In fact, light
incidence did not correlate with any behavior
studied. In spite of this, exploratory behavior in
these arms was different. When the animals could
not form images because of the white paper, they
explored more; when they could form visual
images (transparent edges or no edges) exploration
of the open arms was similar and less than in the
Fig. 4. Number of rearings and time spent rearing in the open arms, the closed arms, the central square and the correspondent
statistics. F[6,77], F values for 6 and 77 degrees of freedom; P , probability; C, control group with 1-cm edge surrounding the open arms;
5�/40, groups of rats with different heights of edges surrounding the open arms; 40W and 40B, 40-cm high Plexiglas edge covered with
white translucent paper and black opaque paper, respectively; (*), significantly different from the control group (Dunnett, P B/0.05).
Under the last column above the graphs are the groups that are different from controls (Dunnett, P B/0.05); N.S., non significant
(ANOVA, P B/0.05).
Fig. 5. Distance run in the open arms, the closed arms and the correspondent statistics. F[6,77], F values for 6 and 77 degrees of
freedom; P , probability; C, control group with 1-cm edge surrounding the open arms; 5�/40, groups of rats with different heights of
edges surrounding the open arms; 40W and 40B, 40-cm high Plexiglas edge covered with white translucent paper and black opaque
paper, respectively; (*), significantly different from the control group (Dunnett, P B/0.05).
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first condition. The fact that similar exploration
was observed both with the arms surrounded by
Plexiglas walls covered with black paper and by
wooden walls also suggests that the presence of
light in the environment is important. An interest-
ing possibility, thus, is that two processes are
operating to produce aversion to open spaces. One
triggered by increasing the light levels reaching the
retina horizontally, as indicated by more intense
exploration when the transparent edges were
covered with black opaque paper than when they
were covered with the white translucent paper
(allowing light to pass). And the other, triggered
by image formation in the retina, as indicated by
the same level of avoidance of the arms observed
both with the 40-cm transparent Plexiglas edges
and 1-cm high edges. Alternatively, they may be
two different levels of one and the same processes.
In agreement with this proposition, other
authors (Anseloni et al., 1995) have compared
mazes with wooden closed arms to mazes with
transparent Plexiglas closed arms and concluded
that the transparent walls rendered the maze more
aversive than the conventional maze. Although
their comparison was not as direct as ours, in the
same maze, they seem to support the notion that
the perception of light triggers aversion.
Another result that was unexpected to us was
the large facilitatory effect exhibited by the 5-cm
edges and, to a lesser extent by the 10- and 20-cm
edges, in most measures in the open arms. We first
expected a decrease in aversion proportional to the
border height, i.e. the higher the border the more
exploratory behavior in the open arms, because we
thought the height of the vertical surface was the
determining factor. It turned out to be the
opposite: the order of aversiveness for the edges
was 20 cm�/10 cm�/5 cm. A possible but counter-
intuitive explanation is that proprioception can
inhibit the triggering of aversion by the perception
of light, a possibility already suggested not so
clearly elsewhere (Morato and Castrechini, 1989).
Such an inhibition of the triggering of aversion
would come from grabbing or grasping, as dis-
cussed elsewhere (Cardenas et al., 2001), rather
than from the tactile sense from the vibrissae.
However, in spite of this being an exciting
possibility, more specific experiments are neces-
sary to establish that this is the case and, if so, how
does light perception and proprioception relate to
each other.
Other behaviors correlated with anxiety in
factorial analyses studies (File, 1992; Cruz et al.,
1994; Rodgers and Cole, 1994; Anseloni and
Brandao, 1997) also point in this direction.
Stretching a category similar to but simpler than
the ones described elsewhere as ‘risk assessment’
(Cruz et al., 1994) or ‘stretched-attend posture’
(Anseloni and Brandao, 1997) decreased in fre-
quency and duration in all conditions investigated.
Increases in this measure have been reported
(Anseloni and Brandao, 1997) after injections of
anxiogenic drugs, like pentylenetetrazol, while
decreases have been reported after injections of
anxiolytic drugs (Cole and Rodgers, 1993, 1994;
Cruz et al., 1994; Anseloni and Brandao, 1997;
Rodgers et al., 1997). This indicates that, in the
present data, increasing the height of the edges
decreases the aversion to the open arms and
inhibits stretching.
The literature shows that head dipping corre-
lates with an anxiety-related factor and increases
after anxiolytic treatment and decreases after
anxiogenic treatment (Cole and Rodgers, 1993,
1994; Cruz et al., 1994; Anseloni and Brandao,
1997; Rodgers et al., 1997). In our experiment,
both head dipping and head out increased in
frequency and duration, indicating that the open
arms with higher edges were less aversive to the
animals.
Rearing, which loaded on a factor that has to do
with general activity or locomotion (Cruz et al.,
1994; Rodgers and Cole, 1994; Fernandez and
File, 1996.), was increased by the height of the
edges. It is interesting to notice, once more, that
the smaller increases occurred with the 40-cm
transparent walls. In fact the 20-cm walls and 40-
cm walls covered with black paper produced more
intense exploratory rearing than the 40-cm walls
either transparent or covered with white paper. In
the remaining conditions, even being able to rear,
the rats preferred not to rear at all. Rearing was
J.C. Martınez et al. / Behavioural Processes 60 (2002) 15�/2624
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reported to be decreased by anxiogenic drugs
(Cruz et al., 1994) but not altered by benzodiaze-
pinic anxiolytic drugs (Cole and Rodgers, 1993;
Cruz et al., 1994). It is possible that rearing, as all
exploratory behaviors, may be decreased or even
suppressed by intense aversion, as suggested pre-
viously (Morato and Brandao, 1996, 1997). The
present results suggest that rearing can also be
sensitive to decreases in aversion. A similar result
was obtained with the distance run in the maze,
which increased only in the open arms but, again,
with the exception of the 40-cm high transparent
edge; no changes were observed on the closed
arms.
The mechanisms proposed to trigger aversion
could be important for rats. On the one hand,
avoidance of high levels of light would ensue that
the rats do not leave the burrow during daylight.
On the other hand, when besides being in bright
environments, the animal can also see (a condition
detected most likely by image formation in the
retina), aversion is more intense. There could be a
survival value to this characteristic, namely, when
a nocturnal prey animal can see (i.e. form images),
predators can also see it. The use of the sense of
vision was long ago hypothesized as being im-
portant in the rats’ negotiations with the environ-
ment. It has been reported (Commins, 1932) that
the greater ease with which rats master elevated
mazes, as compared with floor mazes, results from
the greater use they make of vision and visual cues.
Also, another author (Miles, 1930), after control-
ling for olfactory cues, postulated that rats trained
upon high relief mazes used visual cues in finding
their way about. Also pointing in this direction,
there is a recent report (Doron and LeDoux, 1999)
demonstrating the existence of thalamic nuclei
receiving visual input from the superior colliculi,
connected to the amygdala and providing it with
rapid visual information, allowing for fast beha-
vioral responses to threatening visual stimuli. This
pathway is probably involved in the processing of
visual perception and, as a consequence, in the
modulation of exploratory behavior. However, the
role of this pathway in rat exploratory behavior in
the plus-maze has not been assessed yet. Likewise,
more studies are needed in order to clarify the role
of proprioception in the inhibition or reduction inaversion observed in our conditions.
Acknowledgements
This research was supported by grants from
FAPESP (#98/11187-2) and CNPq (#523094/95-7)
to SM. FC and MR are the recipients of fellow-
ships from FAPESP.
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