In each square millimeter of tissue, Dr. Shankle said, the number of neurons rises by a third from birth to 3 months as new cells are added. Then the number plummets back to birth level between 3 and 15 months. After this point, the number increases rapidly, doubling by the age of 6 years. It probably continues increasing, although at a slower rate, up to age 18 or 21, Dr. Shankle said. The brain enlarges by making new columns, not by making existing ones larger, Dr. Shankle said.
Children progress through the same stage of development at same rates independent of their culture.
This rapid growth and construction of brain tissue may help explain why children whose left or right brain hemispheres are removed entirely seem to develop more or less normally, Dr. Shankle said. The rate of growth, or plasticity, is so large early on, they can learn to do most things with their remaining brain tissue. But Dr. William T. Greenough, a University of Illinois neuroscientist, said he was not yet convinced that Dr. Shankle had proved that the growth was from new nerve cells and not from supporting cells called glia. Meanwhile, Dr. Steven A. Goldman of Cornell Medical Center in New York City is studying human brain tissue removed from epilepsy patients and has found progenitor cells in the ventricles. About 10 percent of cells in this zone are progenitors that give rise to other cell types, he said.