You may have had an MRI exam to
whether the ways in which the presence opment is the development of connec-
diagnose the underlying cause of some
of friends changes brain activity differs
tions—synapses—between neurons. By
sort of pain. Although the technology
between teenagers and adults. We’ve
age 2, a single neuron may have 10,000
used in this sort of imaging is the
found that the mere presence of peers
connections to other neurons. The for-
same as that used by neuroscientists
activates adolescents’ reward centers—
mation of some synapses is genetically
who study brain development, the “f”
but not those of adults. This may make programmed, but others are formed
in fMRI refers to the use of the test
teenagers more inclined to take risks
through experience. The rate of synapse
to examine how the brain functions,
when they’re with their friends because formation peaks at about age 1 and
and not just its anatomy. Researchers
they’re more likely to focus on the
use fMRI to examine patterns of brain
rewards of a risky choice than on the
activity while individuals perform a spe- potential costs.
slows down in early childhood, but the
development of new synapses continues
throughout life as we learn new skills,
build memories, acquire knowledge,
and adapt to changing circumstances. 1
Researchers can
now see inside the
adolescent’s brain and
watch what happens
when teenagers think.
Synaptic Pruning
Initially, the brain produces many more
connections among cells than it will use.
The number of synapses in the brain of
a 1-year-old is about twice the number
in the adult brain. However, soon after
birth, unused and unnecessary synapses
start to be eliminated, a process called
synaptic pruning. As a general rule, we
tend to assume that “more is better,”
but that’s not the case here. Imagine
a meadow between two patches of
forest. Hundreds of lightly trodden
paths connect one side to the other (the
unpruned brain). Over time, people dis-
cific task (for example, recalling a list of A Primer on Brain Maturation
words, viewing photos of one’s friends, Synapse Formation
cover that one path is more direct than
others. More people begin using this
path more often, so it becomes wider
or listening to music). Participants in an The human brain contains approxi-
and deeper. Because the other paths are
fMRI study are asked to perform tasks
mately 100 billion neurons, cells that
not used anymore, the grass grows back
on a computer while they lie inside
carry information by transmitting elec-
and those paths disappear. That’s what
a brain scanner. With this setup, it’s
trical charges within the brain by means synaptic pruning is like.
possible to study both how patterns
of brain activity differ during different
tasks (for example, when we actively
of chemicals called neurotransmitters.
Neurons do not actually touch; there’s
a miniscule gap between them called
The elimination of synapses con-
tinues through adolescence and is
normal and necessary to development
read as opposed to being read to) and
a synapse. When the electrical charge
and functioning. Just as pruning a rose
whether people of different ages show
travels through a neuron, it stimu-
bush—cutting off weak and misshapen
different patterns of brain activity while lates the release of neurotransmitters,
branches—produces a healthier plant
performing the same task. Many of the
chemicals that carry the signal across
with larger flowers, so synaptic pruning
most important brain changes that take the synapse from one neuron to the
enhances the brain’s functioning. It
place during adolescence are not in the next. Anytime we perceive something
makes the brain more efficient by trans-
brain’s structure, but in how the brain
(for example, feel an itch); move some-
forming an unwieldy network of small
works.
thing (scratch the itch); or process infor- pathways into a better organized system
At Temple University, we’re studying mation (wonder where the itch came
of superhighways.
how patterns of brain activity vary when from), this process of electrical transindividuals perform tasks either alone or mission is involved.
with their friends watching them, and
A key process in early brain devel-
In general, the development of syn-
apses is characterized by a period of
growth (when more and more synapses
