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: smorato@ffclrp.usp.br (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

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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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