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made to formant frequencies in order to produce vari-
ants (Figure 10.29B). Do infants treat the variants as a
member of the group, as adults do? Infants were first
exposed to the prototype vowel and then presented
with a variant. If she perceived the variant to be dif-
ferent from the prototype, the infant was trained to
turn her head. The data show that American infants
are more likely to treat variants of /i/ as a member of
that group, but are less likely to treat variants of /y/
as members of that group. The opposite result is found
for Swedish infants. These results suggest that experi-
ence with one's own language in the first 6 months of
life allow for improved perception of unique speech
sounds.
Is it possible that language-specific activity is
present in the brain as the infant is becoming sensitive
to the unique attributes of human speech? A functional
magnetic resonance imaging study in 3-month infants
suggests that it is (Dehaene-Lambertz et al., 2002).
Human adults exhibit the greatest activation along the
left superior temporal sulcus when exposed to their
native language, but the response is much smaller
when the speech is played in reverse (i.e, “my dog has
fleas” versus “saelf sah god ym”). When 3-month-old
infants were tested with their native language
(French), they also displayed greater activation of the
left superior temporal gyrus (Figure 10.30). Forward
speech elicited greater activation for one brain region
on the left side, the angular gyrus, as compared
to reversed speech. However, forward and reverse
speech were equally effective at activating the tempo-
ral lobe, which is quite different than adults. Thus, left-
hemisphere dominance for speech processing appears
to be already present by 3 months of age, yet the sen-
sitivity to phonological cues (e.g., forward vs. reverse
speech) are immature.
A
B
Left vs. Right
Forward vs. Reverse
L
R
L
R
FIGURE 10.30 Speech activation of the human infant brain.
fMRI images were obtained from 2- to 3-month-old infants during
presentation of native speech. A. A transparent brain view (top) and
an axial section (bottom) map the relative sound-evoked activity for
left versus right temporal cortex. The activation was significantly
greater on the left side. B. An activation map showing the relative
sound-evoked activity in response to forward speech versus reverse
speech. While there was no difference in the temporal cortex, there
was an asymmetry within the angular gyrus. (From Dehaene-
Lambertz et al., 2002).
ecules, or systems of nerve cells, take on new proper-
ties that were not expressed by the single molecule or
nerve cell. While the genetic dissection of behavior is
an important strategy, it should also be recognized
that multiple gene products inevitably contribute to
each phenotype, including behavior. Furthermore, the
expression of many genes is influenced by the envi-
ronment (i.e., neuronal activity). Therefore, a rich
understanding of the relationship between brain and
behavior is fundamental to interpreting all results.
Studying animal behavior is one of the best ways to
measure the properties of a system of nerve cells. It
provides the most sensitive and universal indicator of
a successfully assembled nervous system. All types
of developmental errors (i.e., inappropriate fate, ion
channel mutations, pathfinding errors, weak synapses)
will affect the computational abilities of individual
neurons. This is precisely why behavioral measures
have long been used to tell clinicians when the nervous
system is broken (e.g., schizophrenia, sleep apnea,
delayed learning). It will, therefore, not be too great of
a surprise when behavioral analyses reemerge as one
of the most powerful tools available to developmental
neuroscientists.
SUMMARY
The great strides we have made in molecular and
cellular neurobiology have underscored the impor-
tance of revisiting the behavior of animals, particularly
during development. The maturation of neural pro-
cessing, or the ability of a neuron to respond accurately
to its synaptic inputs, depends on all the building
blocks being in place. What can be gained from study-
ing the system as a whole? If we learn the alphabet, are
we not able to understand sentences? Of course, the
blemish in this logic is simple to grasp: systems of mol-
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