Unlike the proverbial elephants, babies always forget.

Infants' memories may be wiped clean by the genesis of new brain cells, a study in rodents suggests.



The findings offer an explanation for why people can't recall memories from early childhood, a century-old mystery.

The study's authors "make a very interesting and compelling case," says neuroscientist and psychiatrist Thomas Insel, director of the National Institute for Mental Health in Bethesda, Md.

It's just truly fascinating," he says. "Nobody has actually looked at this very carefully before."

More than 100 years ago, Sigmund Freud speculated that humans' tendency to forget their early years, dubbed infantile amnesia, might have a psychosexual origin. Scientists later thought memories might be rooted in language, because kids typically start making long-term memories around the time they start speaking, says study coauthor Sheena Josselyn of the Hospital for Sick Children in Toronto.

"But the really weird thing is that most animals show infantile amnesia too," she says. "So the development of language can’t be the whole explanation."

Inspired by observations of their own toddler, Josselyn and her husband, study coauthor Paul Frankland, wondered why young children couldn't retain memories of situations or events. These memories — such as what a person ate for dinner — involve the hippocampus, a skinny seahorse-shaped belt of tissue that stretches from ear to ear and houses a cell-making factory about the size of a few blueberries. This little factory is the only part of the brain that normally cranks out new neurons, which scientists believe help make memories.

Josselyn and Frankland knew that such cell production tapers off in childhood. "That's exactly when we start to be able to form long-term memories," Josselyn says. She and colleagues wanted to find out whether youngsters' recollections were somehow tied to brain cell formation. So the team turned to mice, animals that — like humans — harbor blank spots in their early memories. As mice age, the birthrate of neurons slows down. This drop-off matches up with the rodents' ability to remember scary situations, the researchers report in the May 9 Science.

For their tests, the researchers placed adult mice in a chamber noticeably different from their usual homes —stripes on the walls and a vinegary smell — and buzzed the animals with mild foot shocks. The mice learned to fear the room, and even 28 days later would freeze up when put in the chamber.

Infant mice were more forgetful. A day after being shocked, their fear began to fade. The animals' behavior hinted that making new brain cells might be mucking up memory retention.

Next, the researchers boosted neuron production, or neurogenesis, in adult mice.

Josselyn and colleagues shocked adult mice in the striped room and then let them exercise at will on running wheels for days or weeks. Running naturally triggers the growth of new brain cells in the hippocampus, Josselyn says. And just a few weeks of racing on the wheel helped mice forget their fear of the scary room.

Other tricks to turn up the number of new neurons also cleared adult animals’ memories. And the reverse worked too: Dialing down the birth of new neurons in infant mice kept the fear memory alive.

"It was really amazing to us that we could make a memory last much longer in these infant mice just by decreasing neurogenesis," Josselyn says.

The findings give a new twist to the role of neurogenesis in the hippocampus: Instead of merely making memories, as scientists currently believe, spawning brain cells could help animals forget.

The notion is "contradictory to where everyone else in the field has been," Insel says. "That's going to be very provocative."

Josselyn thinks that the new cells could be messing up brain circuits laid down by preexisting neurons. These cells reach out spindly fingers and link up with neighbors. Memories made using older links may be hard to call to mind when new links take over, she suggests.

"Maybe forgetting is not a bad thing," Josselyn says. "Maybe it's good to clear away some memories and forget some things that are not so important."

The hippocampus might be something like a computer cluttered with files, says neuroscientist Richard Morris of the University of Edinburgh. "Every so often we all sit down and do a little tidy-up," he says. "Maybe that's what neurogenesis is all about. It's the hippocampus's very own spring cleaning system."