For most of this century, scientists have believed that the human brain grew until a certain age, then stopped - making learning more difficult in later years.
Now, researchers have the clearest evidence yet that the part of the brain most closely associated with complex learning and memory continually regenerates itself.
On one level, the findings reported in today's issue of the journal Science may help researchers working on treatments of brain damage or brain diseases, as well as ways to stimulate learning.
But on a more basic level, they suggest that the mental failings once assumed to be an unavoidable pitfall of aging are little more than stereotypes and have no basis in biological fact.
"There is a new sense of possibilities and hope," says Paul Grobstein, a neurobiologist at Bryn Mawr College in Pennsylvania. "The biology validates that."
Scientists have been building toward this conclusion for several decades. The model of the static brain began to crumble as early as the mid-1960s and early '70s, when experiments showed that the nervous systems of slugs and other animals demonstrated the ability to change their structure over time by altering their network of brain cells.
Subsequent experiments turned up evidence of the generation of new brain cells - a process called neurogenesis - in adult rats, mice, dogs, and other lower-order mammals. In this decade, scientists have also found neurogenesis in several types of adult monkeys and even in adult humans. But these findings were confined to less evolved, older portions of the brain that are believed to play a less-crucial role in the highest brain functions.
Despite this evidence, many neurologists continued to believe that neurogenesis did not exist in the cerebral cortex of higher-order primates and humans. Their feeling was that the stability of the cerebral cortex was crucial to memory and identity, both of which require a strong degree of constancy.
"People thought: If the cerebral cortex is important in memory, how could it change?" says Charles Gross, a professor at Princeton University in New Jersey and one of the study's authors. "In fact the opposite view is at least as plausible: If memories are formed from experiences, these experiences must produce changes in the brain."
In the study announced today, Professor Gross and his colleagues at Princeton successfully tracked neurogenesis in the cerebral cortex of a higher order primate, the adult macaques monkey. People and monkeys have nearly identical brain structures, so it is considered likely that the same process may be at work in the human brain, too.
From an evolutionary standpoint, this superior ability to translate experiences into structural flexibility may be one of the critical differences between humans and many other animals, say researchers.
"The cerebral cortex is the last area of the brain to develop. It seems to have retained much of its ability to adapt to the environment," says Marian Diamond, a biologist at the University of California at Berkeley. "Therefore it maintains its plasticity. We call it soft-wiring instead of hard-wiring."
In this case, society may be far ahead of science in its assessment of the brain. The concept of life-long learning is well-entrenched in America. More and more companies offer to underwrite university coursework for their employees in the belief that they make for smarter workers. Seniors are storming the ivory tower in search of intellectual stimulation.
No biologists are ready to claim that eight-year-olds and 65-year-olds stand on equal footing in Japanese class.
"It's clear that in the first 10 years of human life it's much easier to do everything concerned with mammalian brain function because the cerebral cortex is growing so quickly during that time," explains Ms. Diamond.
But according to Diamond and others, the most current research holds that aging is not a hindrance to continued education. "Your memory function stays very intact throughout your life span," says Kathryn Braun, a gerontologist at the University of Hawaii's Center on Aging. "It might take you longer to retrieve something from memory, but your ability to learn is not impaired."
(c) Copyright 1999. The Christian Science Publishing Society