groups of receptor cells in the eyes play an important
role in helping to structure some parts of the brain in-
volved in vision. This activity cannot be a response to
visual input, since the eyes are closed at this age. In-
stead, it appears that one part of the nervous system
can create a kind of ‘‘virtual environment’’ specifically
to aid the formation of other, later developing, parts.
The Effect of Experience on Brain
Development
Once a baby is born, the external world can begin
to influence the activity of neurons and thereby the
pattern of brain development. According to Mark
Johnson and his colleagues, for example, newborns
less than one hour old tend to orient their heads and
eyes to look at faces more often than many other com-
plex patterns. This reaction is like a reflex and may
well be controlled not by the cortex but by evolutio-
narily older, subcortical parts of the brain. All of this
staring at faces serves a critical purpose in providing
the necessary input for training some of the slower-
developing ‘‘higher’’ brain areas within the cerebral
cortex. Thus, infants themselves play an important
and active role in determining the subsequent organi-
zation of the cerebral cortex.
One way that experience affects brain develop-
ment is by determining which synapses are retained
during the process of synapse elimination. Useful syn-
apses are kept, while surplus ones are lost. This type
of learning through selective synapse elimination is
thought to happen only at certain points in develop-
ment. This means that there are some types of learn-
ing that may only occur during certain points in
development, sometimes called sensitive or critical
periods. If certain synaptic connections are not laid
down early in life, they are less likely to become estab-
lished later in life. For example, some children are
born with cataracts (a clouding of the lens that pre-
vents patterned light from reaching the eye’s receptor
cells) and experience visual deprivation during the
first months of life until the cataracts are treated.
These children, even when tested years after vision
has been restored, show some difficulties in face rec-
ognition, according to a study by Daphne Maurer and
her colleagues. Thus, visual experience in the first
months of life appears critical for the ability to recog-
nize faces and cannot be replaced even by years of
later experience.
The sensitivity of the young brain to the inputs it
receives means that different patterns of brain orga-
nization can occur in infants with different types of
experience. One example is individuals who are deaf
from birth and thus do not receive typical auditory in-
puts. While some aspects of their visual processing re-
Spinal cord neurons. Neurons must travel from the spot where they
are born to arrive at the particular region where they will be used
in the mature brain. (Frank Lane Picture Agency/Corbis)main unchanged, their processing of visual motion
and information in the visual periphery are enhanced
and reorganized. One interpretation is that there are
surplus visual connections that are normally eliminat-
ed during development but that, in the absence of au-
ditory input, remain and take over what would
normally be auditory cortex.
Of all the cortical areas, the frontal areas appear
to develop the slowest, as many functions attributed
to the frontal lobe, such as planning for the future, do
not mature until adolescence. This does not mean,
however, that the frontal lobes are not working early
in life and suddenly are ‘‘switched on’’ in adolescence.
For example, if a seven-month-old baby watches an
object being hidden in one of two locations she can
remember a few seconds later where it is hidden. In
contrast, a monkey with an injury to the frontal lobe
has difficulty with this task. That human infants can
perform a task that monkeys with damage to the fron-
tal lobes cannot suggests that the frontal lobe is begin-
ning to work already in young infants. TheBRAIN DEVELOPMENT 67