Fear Motivation 47rodent species, and it is characterized by the absence of all
movement except for breathing. In the wild, rodents often
freeze when they encounter a predator. This behavior is an ef-
fective defensive strategy because many predators have diffi-
culty detecting an immobile target, and movement can act as
a releasing stimulus for predatory attack (Fanselow & Lester,
1988). In the laboratory this behavior is prevalent when ro-
dents are presented with a CS that has been paired with foot
shock (e.g., Fanselow, 1980). Rats usually freeze next to an
object (thigmotaxis) such as a wall or corner. This behavior
occurs even when the fear stimulus is present and the rat is
not next to the object. Thus, part of the freezing response may
be withdrawal to a rapidly and easily accessible location to
freeze (Sigmundi, 1997). Thus, the freezing sequence con-
tains a component of flight.
Conditional Analgesia. Rodents become analgesic
when they encounter learned fear stimuli. Although triggered
by fear stimuli, this analgesia becomes useful if the animal
suffers injury from a predatory attack. Reduced pain sensitiv-
ity permits the animal to express defensive behaviors and
forego recuperative behaviors when predatory imminence is
high (Bolles & Fanselow, 1980).
Circa-Strike Defensive Behaviors
Rodents engage in circa-strike defensive behaviors when all
other defensive strategies have failed. Thus, these behaviors
are prominent when predatory imminence is relatively high.
Flight. Another defensive behavior that is common to
rodents and many species is flight. In circa strike, flight con-
sists of a rapid burst of activity away from the predator. If
cornered, a rat will vocalize, bare its teeth, or jump beyond or
at the predator (Blanchard & Blanchard, 1989). The activity
burst to electric shock and the potentiated startle response of
an already frightened rat to a loud noise are other examples of
this behavior.
Fighting. When other defensive behaviors have failed,
rodents often resort to defensive fighting when the predator
attacks. In the laboratory this behavior emerges when two co-
horts receive a series of inescapable foot shocks (Fanselow &
Sigmundi, 1982). Fighting emerges only after many presen-
tations of foot shock. Presumably, the attacks are an attempt
to halt shock delivery, and rats attribute the delivery of shock
to their cohort.
In the analysis of defense it may be important to distin-
guish between immediate and subsequent behaviors. Let us
consider a hypothetical situation that involves a rat encoun-
tering a threat. When a rat receives a shock via a shock prod,
the animal’s initial response is to retreat from the shock
source and then exhibit freezing behavior. Later the animal
may return to the shock source’s vicinity, and then it may ex-
hibit freezing, stretch-attend, and defensive burying behav-
iors. The animal may also move away from the shock prod in
a manner that resembles retreat to a burrow.
In the previous section we described the functional behav-
ior systems view of defensive behavior. This view suggests
that defensive behavior is organized by a continuum of per-
ceived danger: When the threat is perceived, rats express
specific sets of defensive behaviors that are qualitatively dif-
ferent from those expressed when the threat has not been
detected. This discrimination may also vary with time if ani-
mals continually update their concept of perceived danger.
This updating process may then contribute to the selection of
defensive behaviors in the shock prod scenario: Initially, rats
move away from the shock source and freeze, and later on
they freeze, bury, and stretch-attend. Notice that the move-
ment away from the shock prod expressed immediately dif-
fers from the flight expressed later. Thus, the immediate
response to shock delivery may differ qualitatively from sub-
sequent responses to the environment because the animal has
updated its concept of perceived danger. Such updating likely
depends on the basic principles of extinction, or possibly the
reconsolidation phenomenon that has recently received atten-
tion (Nader, Schafe, & LeDoux, 2000).Neural Substrates of Learned Defensive BehaviorMammalian species share fundamentally similar brain cir-
cuits that underlie fear behavior. Indeed, in humans, rats,
mice, rabbits, and monkeys the amygdala is a prominent
component of the fear circuit. To date, more is known about
the brain circuits that support learned fear owing to the pop-
ularity of Pavlovian fear conditioning as a model for experi-
mental analysis. Less is known about innate fear circuitry,
although evidence seems to suggest that these circuits over-
lap (e.g., Walker & Davis, 1997). Fendt and Fanselow (1999)
have provided a comprehensive review of the neural struc-
tures of defensive behavior. Numerous brain structures me-
diate the acquisition and expression of Pavlovian learned
fear.The AmygdalaThe amygdala consists of a cluster of interconnected nuclei
that reside in the medial temporal lobe. Brown and Schaffer
(1886) provided the first evidence that implicated the amyg-
dala in emotional processing. They demonstrated that large