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potentiated startle. Thus, if the pure tone enhances the
startle response, then one concludes that the animal
learned about a dangerous situation. When 16-day rat
pups are trained in the same paradigm, they do not
show any sign of learning (Hunt et al., 1994). Their
response to the tone plus noise is nearly identical to
noise alone (Figure 10.24). A similar delay in learning
is demonstrated when a light stimulus is paired with
footshock (Hunt, 1999). For some learning tasks,
neonates perform better than juvenile animals. Rat
pups of 5-10 days can learn to avoid a sugar solution
when it is paired with mild foot shock, yet 15-day pups
fail to learn this task (Hoffman and Spear, 1988). Thus,
learning is not simply poor in young animals and
robust in adults. Rather, it is a complex function of age,
sensory modality, and the motor response that is being
modified by training.
Even extremely simple forms of learning, such as
habituation and sensitization, can emerge at different
times during development. This has been studied at the
level of both behavior and neurophysiology in the sea
slug, Aplysia (Rayport and Camardo, 1984; Rankin and
Carew, 1988; Nolen and Carew, 1988). During habitua-
tion, animals produce a smaller reflexive response
when they are exposed to repeated presentations of an
identical stimulus. This form of learning can be demon-
strated by squirting some seawater on the animal's
siphon while monitoring its contraction. With each
squirt of water the siphon withdrawal decreases, finally
reaching about 30% of its initial amplitude. When stim-
ulation ceases, the response gradually recovers over a
few hours. Habituation can be observed in 5- to 10-day
Aplysia , before most central neurons are born. However,
the stimuli must be delivered with much shorter inter-
vals to produce habituation in young animals. Consis-
tent with these behavioral results, synaptic potentials
that mediate the response decrease in size with
repeated use in neurons from 5- to 10-day animals.
Do other simple forms of learning appear this early?
This question was assessed for sensitization, a form of
nonassociative learning in which an animal produces
a larger reflexive response when it is preceded by a
strong, usually noxious stimulus. For example, when
an electric shock is delivered to the tail, the same squirt
of seawater evokes a much larger siphon withdrawal
response. Sensitization was found to emerge quite late
in development, almost 60 days after the appearance
of habituation. Once again, a neural analog of sensiti-
zation was first observed at roughly the same time as
the behavior. In adult animals, stimuli to the siphon
nerve produce synaptic potentials in a neuron called
R2, and the size of these synaptic potentials can be
increased by delivering stimuli to the nerve emanating
from the tail. However, this synaptic facilitation is
observed only in animals >70 days. Thus, there is some
reason to believe that specific forms of learning emerge
at distinct periods of development owing to the matu-
ration of explicit synaptic mechanisms.
Studies that target simple forms of learning will be
critical for linking behavior with underlying neural
mechanisms. However, it is also interesting to ask how
developing animals learn complex, multistep tasks,
such as how to write a sentence or make a peanut butter
sandwich. Of course, even sophisticated learning tasks
are studied with a formal paradigm. For example, in a
delayed nonmatch to sample task, a primate is first
shown an object that can be moved to reveal a reward,
such as a food pellet (Bachevalier, 1990). After a delay,
the animal is next presented with two objects, one of
which it saw previously. In this case, the animal must
learn to move the new object in order to obtain the
reward (Figure 10.25). The task can be made more com-
plicated by increasing the time between trials or by
increasing the number of objects that must be memo-
Object 1
Object 2
3 years
1 year
6 months
3 months
Time (days)
FIGURE 10.25 Development of memory in primates. A. The
delayed nonmatch to sample task involves remembering the object
presented first and, after a delay, choosing the new object on a test
trial. B. The time to learn this task is shown for primates of differ-
ent ages. At 2-3 years of age, animals learned the task within eight
days of training. However, 3-month-old monkeys did not reach cri-
terion (90% correct responses) until they had received 36 days of
training. (Adapted from Bachevalier, 1990)
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