The Salk researchers do not know why running should have such an enhancing effect on neural development. There are some informed guesses: Running might increase the flow of oxygen and nutrients to brain tissues or release special growth factors that promote new neurons, Gage said. Or it could be that running prompts the nervous system to prepare for an onslaught of new information as an animal navigates unfamiliar terrain in the pursuit of prey or in flight from an enemy.

Meanwhile, at Princeton, neuroscientists Elizabeth Gould and Charles Gross of the university's psychology department brought neurogenesis a step further, finding new neurons not just in the hippocampus of adult rhesus monkeys but also in the more advanced cerebral cortex.

In order to test for the presence of new neurons in the adult brain, Gould and Gross injected the monkeys with BrdU. At different times after the injection, ranging from two hours to seven days, the researchers examined the cerebral cortex and found evidence of BrdU in cells in three different regions, all of which play a role in higher thought.

The researchers were able to detect several different proteins in the cells that are found specifically in neurons. Also, they showed that the cells containing BrdU had the long axon extensions characteristic of neurons.

To get those results, Gould and Gross used a technique called fluorescent retrograde tracing. In this technique a chemical dye is applied to a small region of the brain, and the dye travels from the end of an axon back to the cell body, making the axon visible under a microscope.

And when the monkeys engaged in various stimulating exercises, the number of new cells jumped.

Taken together these recent findings suggest that neurogenesis is found across the range of mammalian species, including human beings.

Gould and Gross reiterate that it's not yet known what purpose the new cells serve in the cortex, but if the newly formed neurons are found to have a functional role, scientists may have to reexamine current theories about how the brain works.

In the not-too-distant future, Gage says, advanced brain imaging techniques will probably show how exercise, both physical and mental, causes new neural development in humans. What we already know, he says, is that the brain is not a one-way street where it simply controls our behavior.