Even as science inches toward a fuller understanding of how memories are made, the details of how the brain associates memories temporally—that is, according to the timing by which they happened—has remained a mystery. New neurons that are born into the memory-encoding circuit of the hippocampus may provide the explanation.

That is the theory posited by Fred Gage, a Salk Institute neurobiologist and one of the world’s experts on neurogenesis, the process by which new nerve cells are generated, survive, and integrate into the surrounding neural network.

Gage’s 1998 Nature Medicine article reporting ongoing neuron birth in the adult human hippocampus is widely regarded as the evidence that tipped the scales in favor of neurogenesis, upending what had been a central tenet of neuroscience: that unlike cells in the rest of the body, the specialized cells of the central nervous system do not regenerate. The nerve cells you get at birth (and shortly thereafter), went the thinking, are the same ones you will have when you die.

That thinking has been tossed out as reports from multiple research groups worldwide have flowed in, revealing the lifelong genesis of neurons in specific brain areas of birds, rats, mice, and nonhuman primates, in addition to the landmark human report by Gage and his colleague Peter Eriksson.

As a result, a nascent area of neuroscience has exploded. A torrent of recent research has illuminated not only how these new neurons develop but also what influences their genesis, survival, and integration into the brain.

Gage recalls presenting data on neurogenesis 10 years ago at the Society for Neuroscience meeting, at a time when fewer than a dozen researchers presented posters devoted to the subject and skepticism and debate about the idea of newly generated cells in the brain were widespread, he said. In contrast, the 2006 meeting had aisle after aisle of posters on adult neurogenesis, in addition to slide sessions and a mini-symposium devoted to it.

A Different Question: Why?

Yet even as scientists have taken the study of how neurogenesis occurs to bold new levels, the pesky question of why it occurs has stubbornly remained unanswered. What is the functional significance of these new cells in normal brain behavior? Simply put, what are they doing?