While researchers agree that the birth of new neurons plays an important role in the adult brain, they have long debated to which aspects of learning, memory and behavior the process contributes. A new study published today (January 30) in Nature has used a gene knockout approach to link adult neurogenesis to spatial learning.

The paper showed that adult mice that were deficient for a neurogenesis-linked gene , Tlx, had diminished learning and memory capabilities compared to their wild-type counterparts. The researchers, led by Ronald Evans at the Salk Intsitute for Biological Studies, demonstrated that knock-out mice took, in some cases, 10% longer than the controls to navigate a water maze.

In the past, researchers have irradiated the hippocampus or used drugs that blocked neurogenesis to observe the outcome on behavior in animal models. But using a genetic approach adds a lot of power to the study, Elizabeth Gould of Princeton University, who was not involved in the study, told The Scientist; those other methods may have in fact prompted neurogenesis to compensate for the impairment, Gould added.

But Martin Wojtowicz's group at the University of Toronto has shown conflicting results . His studies suggest that neurogenesis does not play a role in spatial learning, but instead strongly affected contextual fear conditioning (a learned fear response to a place). "It's puzzling why they got the opposite results," Wojtowicz told The Scientist, but added that differences in methodology -- Wojtowicz used irradiation, not genetics -- and the choice of which gene to knock out could all yield varying results. While the current paper used mice, the Wojtowicz group's work used rats, which may account for differences in the water maze test, but not the fear conditioning test; rats are better swimmers than mice, but both rats and mice easily learn to be afraid of a specific place.

"The problem is different methods are being used, many different time points are being examined, different learning paradigms used, and the results are not consistent across studies," said Gould. There is no debate that neurogenesis does play some role in the adult brain, she said, but exactly what that role is remains to be seen.