Study undercuts claims of new neurons in adult brains.

Over the past 20 years, evidence that adult humans can produce hundreds of new neurons per day has fueled hope that ramping up cell birth could be therapeutic. Boosting neurogenesis, researchers speculate, might prevent or treat depression, Alzheimer9s disease, and other brain disorders. But a controversial study in Nature this week threatens to dash such hopes by suggesting that the production of neurons declines sharply after early development and grinds to a halt by adulthood. The unsuccessful search for new neurons in adult human and monkey brains is disappointing to many. But skeptics also say that the study left much room for error, and that the debate is far from over.