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the sight of a snake becomes a conditioned stimulus,
and it produces a conditioned response (e.g., freezing).
Although unethical by modern standards, a 9-month-
old baby with no fear of animals was trained to fear
rabbits by pairing the rabbit with a startling noise (a
hammer striking metal just behind the baby's head).
This conditioning eventually caused the baby to cry
every time he saw a rabbit (Watson and Raynor, 1920).
In a typical fear conditioning experiment, an animal
is exposed to a frightening stimulus such as a mild foot
shock, and at the same time a pure tone is presented
from a speaker. How do we know that the electric
shock is frightening? Animals usually stop moving
(i.e., they freeze) and their blood pressure goes up
when they are in frightening situations, and this is pre-
cisely the response to mild foot shock. In contrast, the
pure tone alone does not produce a change in move-
ment or blood pressure. By presenting these stimuli
together several times, the sound alone is gradually
able to elict a fear response. Are developing animals
able to form such associations? Actually, it seems to
depend on the stimulus that the animal is asked to
learn as well as the behavior that it is asked to perform.
For example, rat pups at 15 days or older can learn to
freeze in response to a tone that was paired previously
with mild foot shock (Moye and Rudy, 1987).
For many learning tasks, animals improve with age.
This was explored in rats by first pairing a brief loud
sound that elicited a startle response with a long-
lasting pure tone at moderate intensity (Figure 10.24).
Adult animals learn quickly that the pure tone predicts
the arrival of the loud sound. During test trials, they
produce a much larger startle response when the
pure tone is present, and this is referred to as fear-
Noise-evoked startle response
Training: Pairing tone with loud noise
Tone (1.6 kHz)
startled rat
startle meter
T est : Noise alon e
Tes t : Tone plus noi se
16 day
23 day
75 day
+ noise
+ noise
+ noise
FIGURE 10.24 Emergence of fear-potentiated startle. A. When rats are exposed to a brief, loud noise (red
speaker) they make a sudden movement, called a startle response (left). This can be recorded by a platform
on which the animal stands and displayed on an oscilloscope. When a pure tone (green speaker) precedes
the loud noise, the rats learn that the tone predicts the noise burst (right). In subsequent tests, they give a
larger startle response to the paired tone plus noise, and this is called fear-potentiated startle . B. When trained
in this paradigm, postnatal day 16 rat pups display no potentiation, indicating that they have not learned to
associate the two signals. At 23 days, the animals do display a potentiation due to pairing, although the poten-
tiation displayed by adults is greater still. (Adapted from Hunt et al., 1994)
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