University of Wisconsin neurobiologist Ronald Kalil was among those who pursued the question. His studies in young cats showed that entire networks of brain cells could be rerouted around damaged areas. Young animals whose primary vision centers were destroyed could still learn to see normally, he found, because cells in another part of their brains took up the job of processing vision.

Neurotrophic factors are like super-nannies to brain cells. They are there when the cells are first born, making sure they are nourished, grow and make the right connections. They are there throughout the life of the cells, guarding their health and repairing damage. And they are there to ensure that the brain cells do the jobs they were created for, to learn and remember.

When neurotrophic factors decline or disappear, brain cells quickly fall down on the job, shrink and eventually die. This happens because the factors are no longer there to protect cells from being chopped up by free radicals, molecular piranhas created by normal body chemistry. Nor are the factors there to turn genes on and off to maintain the cells' communications lines or make sure the cells are sending out their "I'm OK, you're OK" messages to other cells.