A type of learning.

For the most part, the brain stops producing new neurons—a process called neurogenesis—soon after birth. In humans, mice, and some other species, however, neurogenesis continues throughout life in a brain region that encodes memories about space and events, called the dentate gyrus of the hippocampus. In adult humans, the dentate gyrus produces roughly 700 new brain cells each day.

Studies in mice have shown that suppressing neurogenesis can impair a type of learning called pattern separation, which allows us to distinguish between two similar but slightly different circumstances. One example is remembering where you parked the car from 1 day to the next, explains René Hen, a neuroscientist at Columbia University who was not involved in the new study.

Although the precise role of neurogenesis in memory is still controversial, more than a decade of research has demonstrated that boosting neurogenesis with exercise and antidepressants such as Prozac can increase rodents’ ability to learn new information about places and events. A few years ago, however, neuroscientist Paul Frankland of the Hospital for Sick Children in Toronto, Canada, noticed that some of the animals in his experiment actually did worse on certain memory tasks when their neuron birth rates had been ramped up. In particular, they performed poorly on tests that required them to retain details about past events.

The result was “way too interesting to ignore,” Frankland says. Because neurogenesis surges in newborn mice and humans and then tapers to a slow trickle by adulthood, Frankland and colleagues wondered if that explosion of new neurons could help explain the widespread phenomenon of infantile amnesia—the inability of adults to remember events that occurred before they were 2 to 4 years old. Some theoretical models suggested that new neurons destabilize memories already stored in the hippocampus by degrading the information there, but the idea had never been explored in live animals.