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acquire about 50 single words, mostly nouns, over the
next eight months. As with birds, there is a period of
development when communication skills are acquired
most efficiently. From 2 to 6 years of age, children learn
about eight words per day. One indication of a sensi-
tive period for language development comes from
studies of humans who learn to produce and under-
stand a second language. When English-language
skills were analyzed in native Korean or Chinese
speakers who arrived in the United States as children
or adults, the youngest subjects performed best
(Johnson and Newport, 1989). A second indication of
a sensitive period comes from studies of deaf individ-
uals who were exposed to sign language from birth to
one year of age. Those individuals who are exposed to
sign language from birth are more skilled than infants
who are exposed even as early as 6 months of age
(Newport, 1990).
In contrast to bird song, human communication is
performed with equal precision in three sensory
modalities. Those born with profound hearing loss
can learn to communicate perfectly with their hands
and visual system using sign language. Those born
without sight can learn to read with their somatosen-
sory system using Braille. Furthermore, the develop-
ment of language seems to be quite natural in any of
these modalities. It has been known for some time that
hearing infants begin to produce speech sounds well
before they can understand words. These vocaliza-
tions, called vocal babbling, are commonly made up of
repeated syllables (e.g., “dadadada”). Whereas deaf
children are unable to produce perfect vocalizations as
adults, similar to deafened songbirds, a remarkable
thing happens: their language ability can be trans-
ferred to another sensory modality.
Early stages of language acquisition were studied in
two infants who were deaf from birth but were con-
tinually exposed to American Sign Language (ASL) by
their deaf parents (Petitto and Marentette, 1991). To
determine whether the infants would “babble” with
their hands, the manual activity of each infant was
codified in some detail, and their production of ASL
hand shapes was analyzed (Figure 10.28). Deaf chil-
dren devote about 50% of their manual activity to ASL
hand shapes, while hearing children only produce
about 10% of this activity, presumably by chance.
Interestingly, the disparity between deaf and hearing
children increases from 10 to 14 months of age, sug-
gesting that deaf children learn language at the same
stage of development as hearing children when given
the opportunity to use their visual system. Finally, 98%
of the manual babbling was performed in front of the
body, presumably within the infant's visual field.
Thus, imitation of a “tutor” and sensory feedback are
A
3 H-MK-801
0.2
0.1
lMAN
0.0
25
50
75
Adult
Age (days)
B
Tutor Song
AP5
lMAN
Learns 50%
of tutor song
AP5
Alternate
tutor
AP5
Learns 20%
of tutor song
Simultaneous
tutor
PRACTICE
32
52
90
Day
FIGURE 10.27 NMDA receptors are implicated in song learn-
ing. A. The number of NMDA receptors was assessed by measuring
the amount of a receptor antagonist ( 3 H-MK-801) that bound to
lMAN during development. NMDA receptors began to decrease
after day 30. B. The influence of NMDA receptors on song learning
was tested by injecting an antagonist (AP5) into lMAN while the
bird was being exposed to a tutor song. In control experiments, AP5
was injected on days when the birds were not exposed to the tutor
song. When AP5 and exposure to tutor song were delivered simul-
taneously, the birds performed very poorly at day 90, only produc-
ing 20% of the tutor song. (Adapted from Aamodt et al., 1995;
Basham et al., 1996)
within area X is not observed until the period of
sensory learning is nearly finished, and may be
responsible for the phase of learning during which
birds practice and adjust the memorized tutor song.
LANGUAGE
Although human communication is far more com-
plicated than bird song, there are some similarities.
Learning is certainly involved at every stage of devel-
opment, from the production and perception of vowels
to the syntax of a sentence. Humans generally speak
their first words between 9 and12 months and slowly
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