Arthur Toga has a rare perspective on his 20-year-old daughter, Elizabeth. Since she was 6 years old, Dr. Toga has been mapping her brain every year or so in a medical magnetic resonance imaging scanner at UCLA. There was nothing wrong with her. Dr. Toga, the founder of the university's Laboratory of Neuro Imaging, just liked to watch her brain growing up.

Most parents can display photographs of progeny in stages from diapers to braces to graduation cap and gown. The ghostly gray images on Dr. Toga's computer hard drive, though, are unique family portraits of a neural work in progress — 100 billion brain cells arranging themselves into the patterns of a healthy mind. While keeping his daughter's identity secret in his published work, Dr. Toga's scans represent one of the longest sequences ever attempted in the growing field of research on the neurobiology of youth. They form part of a composite portrait of brain development prepared by several research teams in recent years.

The ability to document the brain as it matures, made possible by harmless, noninvasive imaging techniques, is transforming our understanding of what it means to come of age.

Not so long ago, scientists were convinced that critical periods of brain development occurred only during the first few years of childhood. Long-term imaging surveys, however, reveal that adolescence also is a crucial time in the life of the brain.

By most measures, the teenage years are the healthiest and most resilient time of life; yet they are also among the most volatile and vulnerable. The mental and emotional turbulence of adolescence may reflect dynamic waves of change in parts of the brain associated with impulse control, judgment, attention and anxiety.

Prompted by puberty, impressionable teenage brain cells radically rewire themselves, researchers have learned. At a neural level, consequently, adolescents often process information differently from either children or adults because the anatomy of reason and decision is in such flux. Unused neural circuits are discarded during normal growth, and young adults end up with less of the gray matter of neurons than a newborn, even though their brains may become three or four times as big.

All the while, adolescent neural circuits are becoming more efficient, as nerve fibers become better insulated — a process called myelination, which typically continues into our 20s. The speed at which the fibers conduct the electrical impulses of thought between brain regions increases 100-fold, especially in an area associated with abstract reasoning and decisionmaking called the prefrontal cortex.

Even subtle variations in the pace of brain development can affect behavior.

The impulsivity and poor judgment of clinical attention-deficit disorders in many cases may be caused by a momentary lag in the timing of cortical growth, researchers at National Institute of Mental Health reported last week in the Proceedings of the National Academy of Sciences. Within a few years, they determined, a normal brain will correct itself.

The researchers could detect this anatomical delay only by monitoring hundreds of youngsters for years, to measure how maturing brain tissues thickened and thinned at 40,000 points across each cortex.

The pace of neural development may also matter. Young people with superior intelligence, as measured by IQ tests, stand out in neural scans by how early in life sections of brain regions associated with cognition begin to thicken and thin, researchers at McGill University reported last year in Nature.

Even the normal pace of development can exasperate parents and teachers. The scatter-brain qualities of the normal teenager arise, in part, because neural circuits that control our ability to focus mentally on more than one thing at a time don't finish developing until late adolescence, researchers at the University of Minnesota reported recently in the journal Child Development.

"Different regions develop earlier and later. They grow; they shrink," Dr. Toga said. "It is a very dynamic process. Different kids follow different trajectories of brain development."

Dr. Toga has three children. All of them have taken regular turns in the brain scanner over the years, not only to further his research but also to help them better understand his work. Elizabeth's first MRI brain scan began as an elementary school "share day" project. "You should bring a picture of your brain," he recalled telling her at the dinner table. "Honestly, that's what started it."

So much technical information about her brain development no doubt advanced his understanding as a neuroscientist. But did it make him a better father? Did such intimate knowledge of her changing cortex help him raise a high-spirited daughter? Did he have more parental patience and understanding during adolescent rebellions?

Toga was sheepish. "It is sad but true: It didn't help me at all," he said. "You'd think it would make me more tolerant. I should have been, because I knew what was going on as a matter of neural development."