Until the development of sophisticated imaging techniques, the human brain was a black box. Overt behavior could be observed and even manipulated by clever psychologic experimenters, individuals could provide subjective descriptions of their mental states, psychoanalysts could attribute pathology to unobserveable, subconscious events. But no one could look inside the brain and say: This is what's happening when we are frightened, or angry, or learning something new; This is how unusually clever children or children with ADHD differ from others; This mental attribute is shaped primarily by the environment (nurture) and that by inheritance (nature).
In the last 20 years, however, various techniques have allowed us to image the live, working brain in dynamic, real time. For pediatricians, the most exciting work of this kind has looked into the process of development, of how infants grow and change through childhood and adolescence. Sixteen years ago, the Child Psychiatry Branch of the National Institute of Mental Health began a longitudinal study of young children: serial, prospective, anatomic brain magnetic resonance imaging (MRIs), taken at 2-year intervals. Subjects included 300 to 400 normal healthy children, sets of identical and fraternal twins, 300 hyperactive children, and others with schizophrenia, obsessive compulsive disorder, or sex chromosome abnormalities. Genetic analyses are done on every subject, and the children are followed clinically as well.
Researchers learned that in normally developing children, the cerebral cortex gets thicker (that is, acquires more neuronal connections) in childhood, then prunes down (like a well-tended apple tree), to discard un-needed connections as the brain matures. The process looks different in ADHD children than in those without ADHD, although it happens in both; those ADHD children who do well as they mature hang onto their thick, unpruned cortex longer than ADHD children who continue to have trouble. Brains of schizophrenic children and their normal siblings both look abnormal in very early childhood, but the brains of the healthy siblings normalize over time, while those of their less fortunate siblings do not.
According to Judith Rapoport, MD, head of the Child Psychiatry Branch at the NIMH and originator of the imaging project, parents should understand that these findings are statistical measures of groups of children only. "There is nothing to suggest it would be worthwhile to pay for an MRI on a healthy child; no individual MRI or series of MRIs would be useful in any way." But to pediatricians interested in development, the work is fascinating, and there's much more still to come. You can see a graphic showing the developmental trajectory of the waxing and waning of cortical thickness at http://www.nih.gov/news/pr/mar2006/nimh-29.htm.