Fear Motivation 45to perform these responses via reinforcement. Bolles in-
cluded a response selection rule in the original formulation of
SSDR theory. He suggested that SSDRs were organized in a
hierarchy but that the hierarchy could be rearranged by expe-
rience. If fleeing is ineffective in avoiding shock, that SSDR
will be suppressed by punishment, and as a result the animal
will switch to the next SSDR in the hierarchy. Upon further
examination of this idea, however, Bolles and Riley (1973)
concluded that freezing could not be punished by shock, and
as a result the punishment rule could not explain how an ani-
mal switched between different SSDRs when threatened.
The Organization of Defensive Behavior:
Predatory Imminence Theory
As an alternative to Bolles’ explanation of defensive behav-
ior, Fanselow (1989) developed the theory of the predatory
imminence continuum.In this theory, Fanselow retains the
basic tenets of the SSDR theory: Animals use innate SSDRs
in defensive situations. However, Fanselow proposed a dif-
ferent response selection rule that determines which SSDR
an animal will perform at any given moment. This rule sug-
gests that the selection of specific defensive responses is
related to a continuum of the physical and psychological
distances between the predator and prey. Thus, given that
danger signals elicit fear, response selection is mediated by
fear directly. Specifically, high levels of imminence vigor-
ously activate the fear motivational system, whereas low lev-
els of imminence activate the fear system weakly. The relative
activation of the fear motivational system thereby determines
the selection of defensive behaviors.
Just as there are responses that are particular to each stage
of predatory imminence, there are sets of stimuli that tend to
be correlated with each stage. These relationships can be il-
lustrated by considering four situations from the rat’s natural
environment that differ in predatory imminence.
1.A safe burrow.When a rat rests in a safe environment
such as a burrow, predatory imminence is relatively low.
In this environment the animal may not exhibit any sort of
defensive behaviors because none are needed. Alterna-
tively, the act of remaining in the burrow could itself be
classified as a defensive behavior because it significantly
reduces the threat of predation.
2.A preencounter environment.As a rat leaves its burrow to
forage for food, predatory imminence increases because
the probability of encountering a predator increases. Rats
engage in preencounter defensive behaviors when their
circumstances might lead to an encounter with a predator,
but the predator has not yet been detected. These behaviors
include changes in meal pattern foraging, thigmotaxis,
dark preference, defensive burying, retreating to a burrow,
and leaving the burrow via investigative, stretch-approach
behavior.
3.A postencounter environment.Predatory imminence in-
creases further when a rat encounters a threat, and it will
engage in postencounter defensive behaviors. The rat’s
prominent postencounter defensive behavior is freezing.
Rats freeze when they encounter predators, and also when
they encounter aversive stimuli. Other postencounter de-
fensive behaviors include conditional analgesia.
4.A circa-strike situation.When the rat’s postencounter de-
fensive behaviors have failed, a predator will typically at-
tack. As the predator makes contact with the prey, the rat
switches to circa-strike defensive behaviors. These behav-
iors seek to reduce predatory imminence by either escap-
ing the attack or fending off the predator. When attacked,
the rat engages in a rapid bout of flight called the activity
burst, and it may also engage in defensive fighting.Notice that two factors change across the predatory im-
minence continuum. First, the physical distance between
predator and prey typically decreases as predatory imminence
increases. Second, the psychological distance decreases as
the perceived danger of the threat increases. This feature ac-
counts for situations where the prey may fail to detect the
threat, although the absolute physical distance between them
is small. Thus, if a rat does not notice a cat, it may not freeze
or flee despite the close proximity of the predator.
The utility of predatory imminence theory lies in its ability
to predict the form of defensive behavior based on these two
selection principles. One challenge of the theory lies in dis-
covering the specific defensive behaviors for each species. It is
entirely possible that similar species use different SSDRs and
that these SSDRs may be organized along the predatory immi-
nence continuum is different ways. For example, although the
dominant postencounter defensive behavior for a rat is freez-
ing, hamsters may exhibit flight when threatened (Potegal,
Huhman, Moore, & Meyerhoff, 1993).Defensive Behaviors on the Predatory
Imminence ContinuumIn the last section we explained the predatory imminence con-
tinuum, the basis of a functional behavior systems approach
to defense. This continuum is divided into three functional
classes of defensive behavior: preencounter, postencounter,
and circa-strike defensive behaviors. In this section we de-
scribe and organize these behaviors according to the preda-
tory imminence continuum. In many cases, a particular