·-=· Demysti ing thE Adolescents can be mature one moment and frustratingly immature the next. The nature of brain development helps explain why. Laurence Steinberg n addition to being a transitional time and our understanding of brain maturation in physical, intellectual, emotional, has grown at breathtaking speed. Major and social development, adolescence contributions to our understanding have is a time of important changes in the come from studies using functional magnetic sn-ucture and function of the brain. resonance imaging (fMRI). This teclmique . ScientistsI are beginning to understand how enables researchers to take pictures of indi­ the psychological changes of adolescence are viduals' brains and compare anatomy (brain linked to brain maturation. structure) and activity (brain function). Before the development of brain imaging Some aspects of brain development in technology, scientists could only speculate adolescence are reflected in changes in brain about the workings of the adolescent brain. structure (for instance, certain parts of the Now, however, with the same scanners that brain are relatively smaller in childhood are used to identify tumors and tom liga­ than in adolescence, whereas other parts ments, researchers can see inside the adoles­ are relatively larger). Other aspects of brain cent's brain and watch what happens when development are reflected in changes in brain teenagers think. We now know that, other Junction (for instance, adolescents may use than the first three years of life, no period of different parts of the brain than children do development is characterized by more dra­ when performing the same task). matic brain changes than adolescence. In addition, greater interconnectedness among various regions of the brain allows for What We've Learned from fMRI better communication between parts asso­ It used to be thought that improved intel­ ciated with different functions. For example, lectual functioning in adolescence would be connections between regions of the brain reflected in larger brain size. However, the responsible for logical reasoning become brain has reached its adult size by age 10, better connected with those responsible for making it impossible that changes in thinking experiencing intense emotions; "cross-talk" during adolescence are the result of sheer between these regions enables better impulse increases in the brain's size or volume. control and self-regulation. That's one reason Since 2000, there's been an explosion in that older teenagers are so much better than research on adolescent brain development, younger ones at controlling their emotions. - L .. ' .. ~ ' ' . l 'it cen Bra • '· .... -... '.. ..... ' , .. ....... .:~ ... , '> , )' 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 slows down in early childhood, but the use fMRI to examine patterns of brain rewards of a risky choice than on the development of new synapses continues activity while individuals perfonn a spe- potential costs. throughout life as we learn new skills, build memories, acquire knowledge, and adapt to changing circumstances.1 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 cover that one path is more direct than cific task (for example, recalling a list of A Primer on Brain M aturation others. More people begin using this words, viewing photos of one's friends, Synapse Formation 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 chemicals called neurotransmitters. The elimination of synapses con­ of brain activity differ during different Neurons do not actually touch; there's tinues through adolescence and is tasks (for example, when we actively a miniscule gap between them called normal and necessary to development read as opposed to being read to) and a synapse. When the electrical charge and functioning. j ust 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 somed1ing 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 trans­ In general, the development of syn­ individuals perform tasks either alone or mission is involved. apses is characterized by a period of with their friends watching them, and A key process in early brain devel- growth (when more and more synapses ?'-·- :; 7 --rl .. ~r .ll · ·\o I • _I I t ' i . ' -i! ~ ; ~--- <t. ;. ..- ;;.. .,, . 1 . .,., if.}. ~. - Things that feel good ~ are created) followed by a period of • decline (when more and more syn­ '· feel better during apses are pruned). Although synaptic pruning takes place throughout infancy, adolescence. Scientists ~ childhood, and adolescence, different regions of the brain are pruned at dif­ now understand why. ferent points in development. As a rule, "~ - . .• •...J..;;:.P..·(,.. the brain regions in which pruning is taking place at a particular point in development are the regions associated with the greatest changes in cognitive to regulate our emotions and coordinate you were a teenager? How hard you functioning during that stage. our thoughts and feelings. Maturation laughed with your high school &iends? of the prefrontal cortex is not complete Things that feel good feel better during Myelination until the rnid-20s, a much later point in adolescence. Scientists now understand Initially, neurons are "nude," but in development than scientists had once why. the course of development, white fatty thought. A chemical substance in the brain tissue called myelin encases the projec­ Imaging studies have also shown called dopamine is responsible for the tions of neurons that interconnect them, important changes in the functioning feeling of pleasure. When something a process called myelination . Myelin, of the prefrontal cortex in adolescence. enjoyable happens, we experience what which acts like plastic insulation around Patterns of activation within the pre­ some scientists have called a "dopamine an electrical wire, increases the speed frontal cortex typically become more squirt," which leads to the sensation of of neural impulses and so improves focused. For instance, in experiments pleasure. It makes us want whatever information transmission. Myelination in which participants are presented elicited the squirt because the feeling of occurs in waves, beginning in the with a rapid succession of images and pleasure it produces is so strong. (Some prenatal period and continuing into asked to push a button when a certain stimuli produce so much pleasure that adulthood. As with synaptic pruning, image appears but refrain from pushing we get a dopamine squirt just.antici­ examining where myelination occurs it when a different image appears, ado­ pating the experience.) most dramatically at a particular point lescents are less likely than children to We now know there's a rapid increase in development provides clues about activate prefrontal regions that are not in dopamine activity in early adoles­