Differential Effects of Forward or Simultaneous Conditioned Stimulus–
Unconditioned Stimulus Intervals on the Defensive Behavior System
of the Norway Rat (Rattus Norvegicus)
Francisco J. Esmorı´s-Arranz, Jose´ L. Pardo-Va´zquez, and Gustavo A. Va´zquez-Garcı´a
Universidad de Santiago de Compostela
To test several predictions derived from a behavior-systems approach, the authors assessed Pavlovian
fear conditioning in rats after 30 trials of forward, simultaneous, or unpaired training. Direct evidence of
conditioned fear was collected through observation of flight and freezing reactions during presentations
of the conditioned stimulus (CS) alone. The authors also tested the CS’s potential to reinforce an
instrumental escape response in an escape-from-fear paradigm. On the one hand, rats that received
forward training showed conditioned freezing, but no conditioned flight was observed. On the other hand,
rats that received simultaneous training showed conditioned flight, but no conditioned freezing was
observed. Rats that received either forward or simultaneous pairings showed instrumental learning of the
escape-from-fear response. Implications for several theories of Pavlovian conditioning are discussed.
The main goal of this article is to explore the heuristic benefits
of applying the behavior-systems approach to the study of rats’
Pavlovian fear conditioning (Fanselow, 1994; Masterson & Craw-
ford, 1982). Traditionally, experimental psychologists have fol-
lowed the continuity assumption of evolutionary theory (Darwin,
1859/1968). However, the behavior-systems approach emphasizes
the adaptationist assumption. Rats’ defensive behavior is assumed
to be an adaptation (Williams, 1966), that is, a highly organized,
efficient, economic, and reliable system that has developed
through natural selection to cope with an ecological challenge
(viz., defense from predators). As a consequence of such a natural
selection process, rats have a repertory of defensive behaviors
(species-specific defense reactions [SSDRs]; Bolles, 1971), which
include freezing, fleeing, fighting, and burying. These behaviors
are controlled by spatial (Esmorı´s, Albo, & Me´ndez, 1994) and
temporal cues that may reflect the degree of imminence of a
potential encounter with a predator (Fanselow & Lester, 1988) or
the existence of an escape route (Masterson & Crawford, 1982).
The present article focuses on the implications of the behavior-
systems approach regarding the role of temporal factors on Pav-
lovian conditioning (e.g., Akins, Domjan, & Gutie´rrez, 1994; Silva
& Timberlake, 1997). Since Pavlov’s time (Pavlov, 1927, p. 27),
many researchers (e.g., Heth, 1976; Heth & Rescorla, 1973) have
found conditioning to be weak with a simultaneous conditioned
stimulus–unconditioned stimulus (CS–US) relation in which CS
and US begin at the same time. Researchers have usually found
that the optimal temporal relation occurs in forward conditioning,
where CS onset precedes US onset (Bitterman, 1964; Smith,
Coleman, & Gormezano, 1969). However, it is possible that if
simultaneous procedures do not lead to a good conditioned re-
sponse (CR), this is not due to an acquisition failure, but rather to
a measurement problem (Holland, 1984). Although simultaneous
training does not translate into CS control of the class of behaviors
that researchers usually assess, there are reasons to believe that the
election of other behavioral indexes could reflect good learning
(Holland, 1984; Matzel, Held, & Miller, 1988). Indeed, the main
point of this research was to explore such a possibility on rats’ fear
With traditional indexes of fear, researchers have found good
fear conditioning with forward procedures (e.g., Estes & Skinner,
1941) and poor conditioning with simultaneous ones (e.g., Heth,
1976; Heth & Rescorla, 1973). However, it is possible that using
a different behavioral index (viz., the flight reaction), one should
observe the opposite pattern; that is, poor conditioning after for-
ward training and good conditioning after simultaneous ones. Let
us see the rationale supporting such a prediction.
Freezing is one of the rat’s SSDRs (Bolles, 1971). This reaction
theoretically appears when the rat perceives that a predator is
close, although the imminence of a potential encounter is relatively
low (Fanselow & Lester, 1988; Ratner, 1975). It is tempting to
think that, in forward training, where the CS precedes the onset of
an aversive US, such a CS becomes a danger signal that resembles
those cues that (in a natural situation) indicate to the rat that a
predator is close. Therefore, from such an analogy, it is not
puzzling that rats show freezing to the CS after forward training.
Flight is also one of the rat’s SSDRs. This reaction would
theoretically appear when the rat perceives that the predator is so
Francisco J. Esmorı´s-Arranz, Jose´ L. Pardo-Va´zquez, and Gustavo A.
Va´zquez-Garcı´a, Facultad de Psicologı´a, Universidad de Santiago de Com-
postela, Santiago de Compostela, A Corun˜a, Spain.
Support for this research was provided by intramural funding of Uni-
versidad de Santiago de Compostela to Francisco J. Esmorı´s-Arranz. He
thanks Sonia Cepeda-Fandin˜o for her personal support during this project.
Correspondence concerning this article should be addressed to Francisco
J. Esmorı´s-Arranz, Facultad de Psicologı´a, Universidad de Santiago de
Compostela, Santiago de Compostela, A Corun˜a-15782, Spain. E-mail:
Journal of Experimental Psychology: Copyright 2003 by the American Psychological Association, Inc.
Animal Behavior Processes
2003, Vol. 29, No. 4, 334–340 0097-7403/03/$12.00 DOI: 10.1037/0097-7403.29.4.334
close that the imminence of a potential encounter is relatively high.
Following our analogy, a CS that has been simultaneously paired
with the US could provide a cue that (in a natural situation)
indicates an encounter with the predator is highly “imminent”;itis
in fact ongoing. According to such a view, it is not puzzling that
rats do not show freezing to the CS after simultaneous training.
However, the behavioral outcome that this perspective predicts is
clear; a flight reaction to the CS should be observed. To our
knowledge, this prediction has never been explored in a Pavlovian
fear conditioning paradigm. Thus, the purpose of our experiment
was to test it.
An escape-from-fear (EFF) learning paradigm (e.g., Brown &
Jacobs, 1949; W. R. McAllister & McAllister, 1971, pp. 107–110),
in which termination of the CS is used to reinforce an escape
response, seemed ideal to pursue our goal. However, it is worth
noting that most contemporary Pavlovian fear conditioning re-
search assesses fear by measuring freezing. Usually, this reaction
is recorded automatically and also indirectly. Three common prep-
arations are based on the suppression of (a) appetitively reinforced
operant behavior (Estes & Skinner, 1941), (b) lick behavior (e.g.,
Esmorı´s-Arranz, Miller, & Matute, 1997), or (c) general locomotor
activity (e.g., McKinzie & Spear, 1995). Fear assessment through
such preparations depends, at least partially, on the fact that a fear
emotional reaction elicited by the CS will translate into freezing.
Indeed, freezing will mean suppression of ongoing activity (e.g.,
tube licking or general locomotor activity; Bouton & Bolles,
1980). Those preparations seem adequate to assess the effects of
forward training, as far as freezing is the expected CR after such
treatment. But, the automated paradigms do not seem adequate to
detect flight behavior.
Other fear conditioning experiments measure fear through direct
observation of freezing (e.g., Bolles & Collier, 1976; Phillips &
LeDoux, 1992). That kind of data-collection procedure seems
adequate to detect the presence of other behaviors (e.g., flight).
However, it is important to note that the type of chambers normally
used by researchers could promote the occurrence of freezing over
other SSDRs (Bolles, 1971). Square chambers are common,
and—it has been argued—they are likely to promote freezing
(Bolles & Collier, 1976). Indeed, a flight reaction seems more
likely in a long box (Bolles, 1971; Bolles & Collier, 1976). In
conclusion, contemporary fear conditioning preparations seem to
offer few opportunities to detect an SSDR other than freezing.
In a typical EFF experiment, in contrast, a rat is located in one
of the two compartments of a shuttle box (a long chamber, some-
times divided into two compartments by a wall with an archway in
the lower portion). The archway communicating both compart-
ments is closed during Pavlovian training, which consists of sev-
eral CS–US pairings, with the electric US being delivered through
the grid floor. During the test phase, the archway is open. The
experimenter introduces the rat in the training compartment, and
the CS goes on (no US is delivered during test). The animal may
eventually move to the opposite compartment, and the CS goes off.
The latency to move to the opposite compartment is recorded. It is
assumed that shorter latencies reflect greater amounts of condi-
tioned fear. Presumably, CS onset motivates the rat to move to the
opposite compartment, and CS offset (immediately after such
movement) negatively reinforces such a behavior (D. E. McAllis-
ter & McAllister, 1991). Indeed, the phrase escape from fear refers
to this instrumental escape contingency; that is, moving to the
opposite compartment, the animal escapes from (i.e., interrupts)
the aversive emotional state (fear) induced by the CS and the
contextual cues (presumably, after Pavlovian training, these stim-
uli became secondary negative reinforcers). Among the effects of
instrumental reinforcement on movement to the opposite compart-
ment, one should find a decreased latency (W. R. McAllister &
Beyond measurement of latencies of instrumental escape re-
sponses, the EFF method potentially allows measurement of clas-
sically conditioned reactions (e.g., freezing and flight). Indeed, to
use a long chamber with a “safe”compartment might increase
(with respect to conventional square chambers) the probability to
observe flight reactions (Bolles, 1971). Yet, there should be room
to observe freezing in EFF if a forward CS–US relation is arranged
(Fanselow & Lester, 1988).
The following experiment used an EFF paradigm. The effects of
forward and simultaneous CS–US relations on freezing, flight, and
instrumental escape responses were explored. Our predictions
were as follows. Forward training should lead to conditioned
freezing (but poor flight). Simultaneous training should lead to
conditioned flight (but poor freezing). Both temporal arrangements
should convert the CS into a conditioned negative reinforcer;
therefore, both procedures should lead to EFF learning.
The subjects were 45 adult male and female Sprague-Dawley derived
rats (Rattus norvegicus) brought—from the university animal colony to the
stall of our laboratory—in sets of 3 individuals of the same sex. Food and
water were delivered ad libitum. Temperature was held at 22 ⫾2°C. There
was a 14:10-hr light–dark cycle, with light onset at 0700 hr. Experimental
manipulations were done from 0900–1900 hr (approximately).
The 3 animals were housed in a standard home cage, that is, a polycar-
bonate cubicle filled with wood shavings and covered with a metallic grid.
Each of the animals was transported alone, from the stall to the experi-
mental room, into a holding cage (identical to the home cage but with fresh
wood shavings). Once the experimental manipulations of each session were
over for any animal, the animal was returned to the holding cage and
transported to the stall, where it remained until all rats had completed the
manipulations. Then, the whole set of rats was returned to the home cage.
Training and testing took place in a Letica shuttle box, 52 cm ⫻26
cm ⫻24 cm (length ⫻width ⫻height). The box was divided in two
equal-sized (proximal and distal) compartments. The division was made
with a black Plexiglas (middle) wall that was 26 cm away from the
proximal (or distal) wall. The middle wall had an archway in the lower
portion, 9.5 cm ⫻11 cm (width ⫻height). In the proximal compartment,
one of the side walls was made of clear Plexiglas, allowing us to observe
and videotape the animal’s behavior. The other side wall, the proximal one,
and the ceiling were made of black Plexiglas. The distal compartment
differed from the proximal compartment in at least two ways. First, the
clear Plexiglas side wall was covered with red cellophane (leading to
decreased luminosity inside the compartment but allowing observation of
the animal’s behavior). Second, both black Plexiglas walls were covered
with white plastic. The floor of both compartments was an electrifiable
grid, built with 0.4-cm diameter rods, spaced 1 cm apart (center to center).
In the distal compartment, the grid floor was covered with a white plastic
tray (identical to the plastic covering the walls), filled with wood shavings
(resembling the safe home cage). Under the grid floor of each compartment
(proximal and distal), there was a tray with wood shavings. Grid floor
cleaning was done with a piece of water-wet cotton after any animal was
removed from the chamber. Wood shavings were always renewed before a
rat was introduced into the shuttle box.
Inside the proximal compartment, and during the training sessions, each
rat received a 15-s light-plus-tone stimulus compound CS. A 15-s, 0.5-mA
constant current scrambled footshock (delivered through the grid floor)
served as the US. It should be noted that, to explore simultaneous condi-
tioning, it was necessary for us to use a shock that far exceeded the duration
of that typically used in fear conditioning. The intensity of the CS light
was 56.4 lx, when measured in the middle of the grid floor of the proximal
compartment (because of photometer characteristics, it was only possible
to measure with the ceiling of the shuttle box opened). The intensity
was 49.2 lx when measured in the distal compartment. CS light was
provided by a 1.5–2.0-W lamp, located on a clear Plexiglas disk, 4 cm in
diameter, and located in the upper portion of the proximal wall (16.5 cm
above the grid floor). Background illumination was provided by the lamps
located at the ceiling of the room, and its intensity was 52.8 lx when
measured in the proximal compartment and 46 lx when measured in the
distal compartment. The auditory component of the CS was a complex tone
with three simultaneous sinusoidal components at 1000, 2000, and 3000
Hz. The intensity of the CS tone was 88.5 dB (A-scale), as measured at grid
floor level. It was delivered through a hidden speaker located in the black
Plexiglas side walls, in their intersection with the middle wall. The back-
ground noise was 63.5 dB.
Programming equipment was located in a room adjacent to the experi-
mental room. A video camera, located in the experimental room, allowed
us to follow the course of the experiment through a TV monitor located in
the management room and to videotape the animal’s behavior. The video
camera was 45 cm away and was oriented toward the clear Plexiglas walls
of the shuttle box.
Animals were randomly assigned to one of the following groups: for-
ward, simultaneous, and unpaired. Replications with 1 subject per group
were run until 15 subjects had been tested. The order of groups into the
sessions was counterbalanced for each replication.
Day 1: Handling and exploration. Each rat was handled and allowed
to explore the shuttle box, according to the following schedule: handling (5
min), exploration (5 min), and handling (5 min). Handling consisted of
taking the animal from the holding cage and moving it into the shuttle box,
where it could move freely. A while later (about 5–10 s), the animal was
taken from the shuttle box and returned to the holding cage, where it
remained for another 5–10 s. Thereafter, the whole handling protocol
started again, until the 5-min handling period was over. Exploration
consisted of allowing the rat to move freely through the shuttle box, with
no experimenter’s interruptions. Once the 5-min exploration period was
over, the last handling period started (5 min).
Days 2, 3, and 4: Pavlovian fear conditioning training. First, each rat
was allowed to explore the shuttle box for 5 min. Once the former period
was over, the animal was located into the proximal compartment, and the
archway in the middle wall was covered with an aluminum piece, so it was
impossible to move between compartments. Thereafter, the rat received 10
tone-plus-light (CS) and 10 footshock (US) presentations (i.e., 10 trials per
day, during 3 days of training, led to a total number of 30 trials). Forward
rats received the CS followed by the US. Simultaneous rats received the CS
and the US at the same time. Both forward and simultaneous rats received
trials spaced by a fixed 300-s intertrial-interval (ITI; time from offset of
last event to onset of the first event of the next trial). Unpaired rats received
CS and US according to an explicitly unpaired schedule. US presentations
were scheduled with a fixed 300-s ITI (onset of last US to onset of next
US), with first US presentation 180 s into the session. CS presentations
were scheduled with a fixed ITI of 315 s, with the first CS presentation
300 s into the session. The shortest US–CS interval (offset to onset) was
105 s. The shortest CS–US interval (offset to onset) was 45 s.
Day 5: Test. The whole session was videotaped for latter analysis of
flight, freezing, and EFF behaviors. For the entire session, the aluminum
piece (covering the archway in the middle wall) was removed, and the rats
were allowed to move through the shuttle box, from the proximal to the
distal compartments. Test protocol was as follows. First, each rat was
allowed 5 min of exploration inside the shuttle box. Second, the animal was
taken to the holding cage, where it remained for 30 s. Third, the rat was
moved, one more time, to the proximal compartment of the shuttle box,
where it received the first of 25 CS presentations (with no US). CS went
on when the forelegs of the rat touched the grid floor. Each CS presentation
had a scheduled duration of 1 min. However, if the rat moved (all of its
body but the tail) to the distal compartment during CS presentation, the
stimulus went off (i.e., without completion of the 1-min programmed
duration). Once the rat moved to the distal compartment in less than 1 min,
it was allowed to stay there for 30 s. If the rat did not move to the distal
compartment during the 1-min CS presentation, it was taken from the
shuttle box to the holding cage, where it remained for 30 s. Once this “out
of the proximal compartment”period was over (either in the distal com-
partment or in the holding cage), the animal was taken again to the
proximal compartment, and the CS went on, repeating the former protocol
until 25 CS presentations were completed.
Preanalysis Treatment of Data
Data of 1 forward, 1 simultaneous, and 1 unpaired animal were lost
because of technical failures. Also, 1 simultaneous animal became too
aggressive to be handled (during the first four CS presentations of the test
phase) and was removed from the experiment.
Flight and EFF data. Each animal’s videotaped behavior was ob-
served during the 25 CS presentations. Latency to move from proximal to
distal compartments was measured. Timing started once the CS went on
and stopped once the animal arrived at the distal compartment and CS went
off. Latencies were averaged in 5-trial blocks. Raw mean latency of the
first 5-trial block went through transformation (log
; in order to promote
normal distribution and justify parametric tests), and it was used as an
index of flight behavior. We built EFF learning curves for each experi-
mental condition with log mean latencies of the five 5-trial blocks. An
outlier criterion was used to remove 1 simultaneous and 1 unpaired animal
that showed, during the first 5-trial block, latencies close to the 1-min
ceiling time. (These animals were removed from both the EFF and freezing
analyses.) Considering log mean latencies of the first 5-trial block, we
defined outliers as those animals whose values were 7 standard errors
above or below the group mean. (The simultaneous and unpaired rats
were 1.93 and 1.83 standard deviations above their respective means.)
Freezing data. One experimenter (Jose´L. Pardo-Va´zquez) observed
each rat’s videotaped behavior during CS presentations. To score freezing
behavior, we used a time-sample procedure. During each CS presentation,
the experimenter had to judge—at 5-s intervals—the presence of freezing
(yes–no choice). Freezing was defined as complete absence of movement,
but respiration. Each yes was recorded as one freezing score. A second
experimenter (Gustavo A. Va´zquez-Garcı´a), blind to experimental condi-
tions, scored the freezing behavior of a sample of rats. The correlation
between the scores of Jose´L. Pardo-Va´zquez and the blind observer’s ones
was .97. Flight, EFF, and freezing data were analyzed through an analysis
of variance (ANOVA). Planned comparisons were made when adequate.
The criterion for statistical significance was an alpha level of .05.
Figure 1 represents flight (top panel) and EFF (bottom panel)
data, as revealed through the log mean latencies of each 5-trial
336 BRIEF COMMUNICATION
block to move (once the CS went on) from proximal to distal
compartments. The top panel suggests that simultaneous rats
showed conditioned flight, as revealed through shorter latencies
than the unpaired rats. Also, it suggests that forward rats showed
retarded flight, as revealed through longer latencies than the un-
paired rats. The bottom panel suggests that both forward and
simultaneous (but not unpaired) animals showed EFF learning, as
revealed through decreased latencies through the 5-trial blocks.
A 3 (group) ⫻5 (block) ANOVA was run on the log mean
latencies. The group factor was significant, F(2, 36) ⫽3.65. A
Fisher least significant difference (LSD) test run on the first 5-trial
block showed that simultaneous latencies were significantly
shorter than unpaired ones. Therefore, the data suggest evidence of
conditioned flight for simultaneous animals. Regarding condition-
ing in the forward group, although visual inspection of the top
panel of Figure 1 suggested retarded flight in these animals (they
showed longer latencies than the unpaired rats), the statistical
significance was only borderline ( p⫽.052). The block effect, F(4,
144) ⫽12.26, from the ANOVA, as well as the Group ⫻Block
interaction, F(8, 144) ⫽2.17, were also significant, providing
statistical support to the suggestion of the bottom panel of Fig-
ure 1. That is, both forward and simultaneous (but not unpaired)
rats showed decrements in their escape latencies through the 5-trial
blocks. LSD tests confirmed the former claim. Namely, in forward
group, the latency from the first 5-trial block was significantly
longer than the latency from any other block. Blocks 2–5 did only
differ from the first block. Regarding simultaneous animals, the
latency from the first block was, once again, longer than the
latency from any other block. The latency of the second block was
different from the latencies of the first and last blocks. Blocks 3
and 4 only differed from the first block. Finally, in the unpaired
condition, no block showed a latency significantly different from
the latency of any other block.
From an operational perspective, the only pure measure of the
Pavlovian flight reaction would come from the first trial. After
that, any measure could be affected by the instrumental operation
(viz., the negative reinforcement of the flight reaction provided by
the CS and contextual cues offset after fleeing to the opposite and
safe compartment). Therefore, we ran a one-way ANOVA on the
first trial log latencies (for which the means were 0.72, 0.37,
and 0.31 for the forward, simultaneous, and unpaired groups,
respectively). The group effect fell short of significance, F(2,
36) ⫽2.851, p⫽.07. LSD tests showed that forward animals had
significantly longer latencies than unpaired ones. However, no
significant difference was found between simultaneous and un-
Figure 2 represents freezing data, as revealed by the percentage
Figure 2. Means (columns) and standard errors (brackets) of the percent-
ages of freezing scores during the first 5-trial block. It is assumed that the
greater the percentage, the stronger the fear. Simultan ⫽simultaneous.
Figure 1. Top panel: Means (columns) and standard errors (brackets) of
the latencies (log s) to move from the proximal to the distal compartments
of the shuttle box during the first 5-trial block. It is assumed that the shorter
the latency, the stronger the fear. Bottom panel: Means (square dots) and
standard errors (brackets) of the latencies (log s) to move from the prox-
imal to the distal compartments of the shuttle box during the complete
series of five 5-trial blocks. It is assumed that the shorter the latency, the
stronger the instrumental escape-from-fear learning. Simultan ⫽simulta-
neous; b ⫽block.
of scores of this behavioral category registered through the time-
sample procedure during the first 5-trial block. Visual inspection
clearly suggests conditioned freezing after forward (but not simul-
taneous) training. Because some of the rats performed flight re-
sponses before freezing could be measured, the figure reflects data
from 13, 5, and 9 rats in the forward, simultaneous, and unpaired
groups, respectively. This makes the test of the hypothesis that
simultaneous rats would freeze less somewhat conservative, be-
cause the measure excluded 12 rats that fled rather than froze.
Nonetheless, a one-way ANOVA on the freezing data revealed a
significant effect, F(2, 24) ⫽7.46. LSD tests showed that forward
values were significantly greater than the unpaired and simulta-
neous ones. Therefore, conditioned freezing was demonstrated for
forward animals. The same test did not show significant differ-
ences between simultaneous and unpaired values. Clearly, no
conditioned freezing was found for simultaneous animals.
The three predictions were confirmed. First, forward CS–US
training led to conditioned freezing but little conditioned flight.
Second, simultaneous CS–US training led to conditioned flight but
little conditioned freezing. Third, both preparations led to the
acquisition of negative reinforcer properties by the CS, as far as
EFF learning was observed.
Our finding of forward pairings between a CS and an aversive
US leading to conditioned freezing is congruent with a large body
of research. Conditioned suppression (presumably based on con-
ditioned freezing) after forward training has been documented
from early (e.g., Estes & Skinner, 1941) to contemporary studies
on Pavlovian fear conditioning (e.g., Esmorı´s-Arranz et al., 1997).
One may also find direct evidence of conditioned freezing after
forward training (e.g., Phillips & LeDoux, 1992). Therefore, the
sensitivity of our paradigm to replicate such a well-documented
fear conditioning phenomenon has been demonstrated.
However, the main contribution of our study is to provide direct
and indirect evidence of first-order, excitatory, Pavlovian fear
conditioning with a simultaneous CS–US preparation. As far as the
direct index of conditioned fear was conditioned flight (instead of
freezing), simultaneous fear conditioning was observed. Accord-
ing to the indirect index of conditioned fear (i.e., EFF learning),
both training procedures (forward and simultaneous) led to Pav-
Prior to this article, evidence of simultaneous fear conditioning
was poor. Some of the evidence was based only on indirect
measures, using sensory preconditioning (Matzel et al., 1988) or
second-order conditioning (Barnet, Arnold, & Miller, 1991) prep-
arations. Heth (1976) provided evidence of simultaneous fear
conditioning, also using an indirect measure of conditioned fear.
He assessed simultaneous fear conditioning after different amounts
of pairing trials. Conditioned fear was assessed through CS capac-
ity to punish instrumental behavior. A greater number of simulta-
neous pairings led to smaller CS punishing properties. That is,
presumably, simultaneous fear conditioning decreased as far as the
number of training trials increased. There is also some direct
evidence—revealed through freezing—of simultaneous condition-
ing, based on one-trial first-order simultaneous fear conditioning
(Albert & Ayres, 1997). It must be emphasized that, when the
behavioral index was the flight reaction, our data showed simul-
taneous—but not forward—conditioning. This observation is at
clear variance with the extended view of forward CS–US intervals
as being optimal to observe conditioning.
Unfortunately, the three experiments reported by Fanselow and
Lester (1988, pp. 194–201) to address the existence of conditioned
flight did not assess flight behavior directly. Instead, flight was
assessed indirectly through indexes like time in contact with a
barricade (either steel plate or rods) that divided the chamber in
two compartments, head poking, escape attempts directed at a
hole, and locomotion. Neither escape attempts nor locomotion (not
to say time in contact with the barricade, or head poking) were
functional flight responses, that is, the rats could not leave the
place through these behaviors. Therefore, a very important differ-
ence between Fanselow and Lester’s work and our study is that we
measured functional (i.e., effective) flight behavior in a direct
We took several steps to increase the probability of observing
conditioned flight (some, but not all, were also used by Fanselow
& Lester, 1988, pp. 194–201). First, we (but not Fanselow &
Lester) used a simultaneous CS–US interval. As explained above,
according to the predatory imminence theory, a simultaneous
interval might increase the likelihood of observing a conditioned
flight reaction. Second, we used a long box (Fanselow & Lester
also did so, using a two-compartments chamber), which seems to
promote flight reactions over other SSDR (Bolles, 1971; Bolles &
Collier, 1976). Third, we (but not Fansleow & Lester) used an
archway door instead of a barrier (as is commonly used in the
shuttle box) to separate both compartments, therefore, providing
“an easily . . . available... escape route”(Fanselow & Lester,
1988, p. 192). Our preliminary research with the barrier suggested
that, in its presence, rats were more likely to show freezing instead
of fleeing. Fourth, using red cellophane paper in the distal com-
partment, we designed the safe compartment as a relatively dark
place. Rats have a preference for darkness when they perceive
danger (Allison, Larson, & Jensen, 1967). Therefore, by providing
a dark place, we were trying to facilitate the rats to flee from the
danger and bright compartment to the safe and dark one. Fanselow
and Lester also worked with a dark (and safe) compartment. Their
animals were allowed to explore both (danger and safe) compart-
ments of the conditioning chamber before Pavlovian training.
However, rats were not allowed to move into the dark chamber
during the test phase. The archway was closed with a barricade,
and time in contact with this barricade was taken as the index of
flight. It is important to note that, in Fanselow and Lester’s first
experiment, backward animals showed relatively high times of
contact with the rods of the barricade. This observation suggests
that those animals might have crossed to the dark compartment
(i.e., flight) if they were allowed to do so. Fifth, we put wood
shavings (similar to those ones from the home cage) in the safe
(but not in the danger) compartment (Fanselow & Lester, 1988,
also did so). It was assumed that, when danger was perceived, rats
would prefer to flee toward a place with familiar properties (like
wood shavings). Sixth, we (and Fanselow & Lester) provided the
rat many opportunities to explore the whole shuttle box (see our
Procedure section), therefore, providing “[a] highly practiced es-
cape route”(Fanselow & Lester, 1988, p. 192). It seems that the
existence of a well-known route of escape will promote the prob-
ability of fleeing (Fanselow & Lester, 1988; Masterson & Craw-
338 BRIEF COMMUNICATION
The present findings are inconsistent with several theoretical
positions. First, they challenge the distraction hypothesis formu-
lated by Rescorla (1980) to explain the poor performance observed
after simultaneous training. According to such an interpretation,
simultaneous CS and US presentations should lead to small pro-
cessing of the CS, due to the high saliency of the US. Therefore,
poor CS acquisition of behavioral control should be expected. The
prediction is contrary to our observation of conditioned flight after
simultaneous (but not forward) training. It is also contrary to the
CS acquisition of negative reinforcer properties after simultaneous
training. Second, observation of simultaneous conditioning chal-
lenges the informational approach to understand rats’Pavlovian
fear conditioning (e.g., Kamin, 1969). According to this view, the
CS must have some informative value in order to acquire behav-
ioral control. Forward pairings endorse the CS with such an
informational property, allowing the rat to anticipate US occur-
rence. However, after simultaneous pairings, CS onset does not
inform the rat of the future occurrence of the US. Therefore (and
contrary to our observations), no CS acquisition of behavioral
control should be observed after simultaneous training.
Our observations are congruent with research supporting the
idea that some Pavlovian fear conditioning phenomena reflect
differential performance processes instead of differential acquisi-
tion processes (e.g., Akins et al., 1994; Barnet et al., 1991; Den-
niston, Savastano, & Miller, 2001; Matzel et al., 1988). Prior
reports have shown that associations are acquired after simulta-
neous training, although some special treatments are needed to
translate such internal representations into behavioral outcomes.
As mentioned above, Matzel et al. (1988), with a sensory precon-
ditioning procedure, and Barnet et al. (1991), with a second-order
paradigm, provided indirect evidence of the acquisition of CS–US
associations after simultaneous training. Our observations of good
simultaneous conditioning, especially as revealed through condi-
tioned flight behavior, suggest that weak conditioning commonly
attributed to simultaneous training is in part due to a performance
(but not an acquisition) process. In conclusion, this research sug-
gests that a behavior-systems approach, with its emphasis on the
adaptive organization of behavior, may have an important heuristic
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Received December 18, 2001
Revision received May 1, 2003
Accepted May 14, 2003 䡲
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