Memories for places and things are first formed in an area of the brain called the hippocampus. In 1965, scientists discovered that new cells were being generated in the hippocampuses of adult rats, but this was not believed possible for humans. Last year, new cell growth was discovered in five human brains, which was a huge find. Scientists are discovering new evidence that the number of neurons in the brain, thought to be fixed, changes throughout life. The number of neurons rises for the first three months of life, then falls as connections are pruned, then rises sharply until about age 6.
Some evidence suggests new neurons are formed well into adulthood.
The traditional view of human brain development is based on experiments done in the mid-1960's on macaque monkeys. At the time, based on available techniques for detecting dividing cells in brain tissue, there was no evidence that new cells were being born in the monkey brain. Scientists concluded this must be true of all primates, including humans. According to the theory then developed, brains develop by pruning and sculpturing, not by building networks with billions of new cells. The fact that many people do not recover the ability to speak or walk after having strokes or other traumatic brain injury cemented the view that adult brains did not add new cells.