After six weeks, the wheels were removed and all of the animals became sedentary as they eased into adulthood.

When they were about 7 months old, which is middle-aged for rodents, the researchers injected the animals with a chemical that binds to newborn neurons in the brain and marks them. They then placed the rats in a specialized cage and lightly shocked them several times. This process, known as fear conditioning, creates strong memories in the hippocampus. When animals are reintroduced to that cage, they will typically freeze repeatedly as they remember their earlier experience.

The scientists waited two weeks before setting a number of the animals back into that cage. Some had been runners while young; others had not.

Another group of the rats, runners and not, were placed into a cage that was similar to but not precisely the same as the scary cage, while a final group was settled into a cage that was nothing like that earlier one.

The scientists noted how often each rat froze. Then they microscopically examined brain tissue, counting the total number of newborn neurons in each animal’s hippocampus.

The scientists also determined whether any of these new neurons had fired during the fear reconditioning, based on certain gene markers. The researchers wanted to see to what extent the new cells had helped the animals to identify and respond to their environment. In effect, had these new neurons aided the animals’ thinking?

And they had, although the results were not precisely what the researchers had expected. They had thought that the runners’ brains might contain more newborn neurons over all, meaning that their youthful workouts had amplified later neurogenesis. Instead, the runners’ brains contained the same number of new neurons as those of the sedentary animals.