How the Brain Deletes Old Memories

Do you remember your first birthday? How about what you ate for breakfast weeks ago? For most people, such events slip through the sieve of memory, never to be retrieved. Now, the first study of its kind in mice suggests that the brain may clear away that old information in the process of forming new memories.

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.

Psychologists have long considered the process of forgetting as key to a healthy mind, yet neuroscientists haven’t paid much attention to it in the past, Frankland says. “If you embrace the idea that forgetting is healthy,” then it makes sense that neurogenesis may contribute to the clearing out of old memories, he says. Although it’s pure speculation at this point, he says, it’s possible that one way that antidepressants help people with depression, a condition linked to reduced neurogenesis, “is to promote some sort of clearing or forgetting,” he says.

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