New imaging techniques provide extremely detailed pictures of the living brain, revealing how it grows and how its function changes though the teenage years. Before brain imaging was invented, autopsy studies showed that older children had more of a fatty substance, called myelin, on their brain cells. Myelin speeds up the electrical transmission of information between brain cells and is thought to make the brain more efficient as we go through the teen years. Earlier studies also revealed a lively growth of connections in the first two years of life, with a slow elimination of connections thereafter.

Now, imaging technologies let us visualize even more remarkable changes in the brains of children and teens. Magnetic resonance imaging (MRI) studies of human brain maturation during the adolescent years have consistently shown subtle increases in total brain volume along with regionally variable patterns of reductions in gray matter volume and increases in total white matter volume. The spatial and temporal distribution of tissue density changes has also been mapped revealing a pattern of maturational changes that is consistent with what would be expected given findings from postmortem studies of myelination and synaptic

• fMRI
  Functional MRI provides us with information about the location of major brain activity during a behavior. We have come to understand that that the brain is a very dynamic place and continues to be so throughout development and even into adulthood. New synaptic connections continue to form between neurons throughout life.
• MEG and EEG
  The magnetoencephalogram (MEG) and the classical electroencephalogram (EEG) give the best information about brain activity over time as well as about connections between cortical regions. The MEG gives information about brain activity in a way similar to EEG, indicating the activity of neural networks, but it gives more information than the EEG about deeper structures. Combining information from these varied imaging techniques offers a more complete portrait of brain functioning.

• automated linear transformation of theimages into a standard orientation with scaling to remove global differences in head size allowing assessment of local changes in brain size or tissue density
• classification of brain images into gray matter, white matter, and CSF
• removal of nonbrain tissue (i.e., scalp and orbits) and cerebellum from the transformed images
• automated extraction of the cortical surface for each;
• tracing of 23 sulcal and gyral landmarks in each hemisphere on the cortical surface rendering of each individual
• estimating gray matter density or local gray matter proportion over the entire cortical surface of each individual’s brain;
• estimating relative local brain growth measured at each cortical surface point [i.e., the radial expansion or distance from the center (DFC) of the brain near the anterior commissure to each cortical surface point

In this report, for the first time we have mapped the spatial distribution of late brain growth and demonstrated that it does indeed continue in the frontal and posterior temporal lobes during the postadolescent years regardless of whether individual differences in global brain size are controlled. To our surprise, an extraordinary wave of tissue growth spread through the brain, from front to back, between the ages of three and 15. Frontal brain circuits, which control attention, grew fastest from ages three to six. Language systems, which are further back in the brain, underwent a rapid growth spurt around the age of 11 to 15, and then drastically shut off in the early teen years. This language system growth is interesting, as it corresponds to the end of a period when we are thought to be most efficient at learning foreign languages.

Interestingly, the anatomical regions within the frontal lobes where we see the most robust accelerated gray matter density loss are in precisely the same locations where we see the most robust continued postadolescent brain growth. Just before puberty, children lost up to 50 percent of their brain tissue in their deep motor nuclei. These systems control motor skills such as writing and sports. This loss moves like a wildfire into the frontal lobes in late teens. We think it is a sign of rapid remodeling of brain tissue well into the teens and beyond. The strong correspondence in the age effects for gray matter density reduction and increased brain growth in frontal cortex may provide new insight for making inferences about the cellular processes contributing to postadolescent brain maturation. Regressive (i.e., synaptic pruning) and progressive (i.e., myelination) cellular events are known to occur simultaneously in the brain during childhood, adolescence, and young adulthood, both of which could result in the appearance of gray matter density reduction or cortical thinning on MRI.