From Science:
Give a mouse more room and a few toys, and good things happen. New neurons sprout in the hippocampus, while spatial memory improves and anxiety eases. As tempting as it might be to tie the new neurons to the behavioral changes, a new study finds no link between them. The results contradict a popular assumption among scientists that new neurons in the hippocampus contribute to the cognitive boost that comes with a more stimulating environment.
The notion that two parts of the brain–the hippocampus and olfactory bulb–continue to produce new neurons into adulthood has been widely accepted since the late 1990s. But just what role those new cells play in cognitive function remains a mystery. Recent studies have found that animals housed in larger cages with opportunities for exercise and social interaction generate more new neurons in the hippocampus than do animals in more cramped quarters with no playmates. Scientists in the lab of Columbia University neurobiologist René Hen hoped to find the link between hippocampal neurogenesis and certain behaviors such as learning and memory that involve the hippocampus.
More here.