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rized. Infant and adult primates were trained on a daily
basis until they could perform the task correctly 90% of
the time. Animals 2 to 3 years of age reached criterion
after eight days of training, but 3-month-old monkeys
required 36 days of training (Figure 10.25). One possi-
ble limitation for younger animals may be the amount
of sensory activity entering the central nervous system
(Bachevalier et al., 1991). In 3-month animals, visually
evoked activity is significantly lower in regions of the
cortex thought to mediate this form of learning, as
measured with the 2-deoxyglucose technique (see Box:
Watching Neurons Think in Chapter 9).
When humans of different ages were challenged
with a similar delayed nonmatch to sample task,
they also displayed a gradual improvement with age
(Overman, 1990). Yet children nearly 3 years of age
take about 10 times longer to learn the task compared
to adults. Furthermore, they forget things more
quickly. The duration of time that children can retain
a simple associative learning task gradually increases
from 2 to 18 months of age (Hartshorn et al., 1998).
Complex learning tasks also emerge at different
periods of development. As discussed above, memory
is commonly divided into recollection of facts versus
performance of skills. One type of factual ability is
the recollection of the spatial environment. Spatial
memory was tested in 2- and 4-year-old children by
asking them to retrieve candy from eight different
locations in an unfamiliar room. It was found that 2-
year-olds revisited locations where they had already
procured the candy more often than did 4-year-olds.
That is, the younger subjects did not remember where
they had been. In a different type of factual learning,
children were asked to recall details of a story that they
had been read, and there was significant improvement
between 5 and 10 years of age. Finally, children were
asked to learn a complex motor task (i.e., a skill), and
their performance was equivalent at 5 and 10 years of
age (Foreman et al., 1984; Hömberg et al., 1993). These
studies point out the diversity of complex associative
learning. Presumably, the improvements that are
observed with age result from the maturation of
sensory function, motor skills, and learning and
memory systems themselves.
of our consciousness. Depending on our position in the
food chain, it fetches us a mate, warns us of danger,
informs of a food source, bonds us in society, and
enriches us with artistry. Perhaps the best studied com-
munication system is that of song birds, where adult
males produce courtship vocalizations to attract con-
specific females. While sex-specific behavior is expla-
ined in terms of genetic and epigenetic factors (above),
the individual songs require learning and practice.
When juvenile birds are reared in isolation such
that they do not hear a normal adult song, they
develop abnormal vocalizations. The vocalizations are
even more degraded by deafening the song bird soon
after hatching (Marler and Sherman, 1983). Yet these
vocalizations still retain a few species-specific charac-
teristics, such as song duration. Even relatively boring
vocalizations, such as those of the crow or rooster,
may be affected by sensory experience. When a
middle ear muscle is detached early in development,
male chickens crow at a higher frequency than control
animals, possibly because low-frequency sounds can
no longer be damped by the middle ear mechanism
(Grassi et al., 1990). These studies illustrate the role of
learning, but suggest that there are intrinsic limita-
tions on the song that any single species of bird is able
to acquire.
Many neuroscientists have settled on the zebra finch
as a model for studies of behavior and nervous system
development. Juvenile birds leave the nest about 20
days after hatching, and they begin to sing a few days
later. As with sparrows, male zebra finches must be
exposed to the species-specific song, and they must be
able to hear themselves sing if they are to produce an
accurate rendition as adults. When males reach about
90 days of age, they produce a stereotyped song that
remains unchanged throughout life, providing they
continue to hear themselves sing. Lesions of the vocal
control nuclei, HVc or RA, have a devastating effect on
song production in adults (Nottebohm et al., 1976).
There is a second pathway from HVc to the anterior
telencephalon, and this projection has been implicated
in song learning. One indication of this special role
in learning comes from the anatomy of the system
(Hermann and Arnold, 1991; Johnson et al., 1995). The
size of one of these telencephalic nuclei, lMAN,
increases when birds first start to practice their tutor's
song, and it eventually decreases in adulthood. The
projection from lMAN to the motor output from the
telencephalon, RA, is also greatest during the early
stages of learning. Since degenerating nerve terminals
can be stained within a few days of lesion, the lMAN
was lesioned at three different posthatch ages, and
the number of degenerating synapses within RA was
assessed. The technique showed that the number of
GETTING INFORMATION FROM
ONE BRAIN TO ANOTHER
The development of animal communication is a fas-
cinating mix of inherited traits and learning. For many
of us animals, communication provides the foundation
of our existence. Some might say that it forms the basis
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