New brain cells may erase childhood memories
What is your earliest memory? It is highly likely that you do not remember your first birthday or anything before your third birthday, and you probably have only a few memories from between the ages of 3 and 7 years old. An adult’s inability to remember early life events, including his or her birth, is called childhood amnesia. The term was initially coined “infantile amnesia” by psychologist Sigmund Freud in 1899 and now researchers have found what could be causing it: the generation of new neurons.
The study published in the journal Science on May 8th, 2014 suggests that neurogenesis, or the generation of new neurons, could play a significant role in this infantile amnesia, which occurs across a wide range of species, including humans and rodents.
“Previously, people would argue that neurons only help make new memories,” said neurobiologist and study author Paul Frankland of the Hospital for Sick Children in Toronto. “But as you’re adding neurons, you’re also wiping away older memories.”
Before, scientists thought memories might be rooted in language, because kids typically start making long-term memories around the time they start speaking, said neuroscientist and study co-author Sheena Josselyn. “But the really weird thing is that most animals show infantile amnesia too,” she said. “So the development of language can’t be the whole explanation.”
Inspired by observations of their own kids, researchers wondered why young children could not 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 houses a cell-making factory about the size of a few blueberries. This little factory is one of the only parts of the brain that normally crank out new neurons, which scientists believe help make memories.
The scientific team knew that such cell production tapers off in childhood. Fresh neurons form rapidly in the brain after birth and into young childhood, but the process slows to a crawl once we reach adulthood. They 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. 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. They 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 said. 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 said.
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.
Frankland and his wife Josselyn have observed the fragile, fleeting memory of children in their own 5-year-old daughter. When she was 2 or 3, they would quiz her about, say, past trips to the zoo or to her grandmother’s house. If they asked within a day or two, she was very much able to recall the experiences.
“It’s clear she can make these memories and tell us details about the trips,” Frankland said. “But within a couple of months, if we ask about the zoo, it’s, ‘We didn’t go to the zoo. I don’t remember that.’ ”
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,” she said. “Maybe it’s good to clear away some memories and forget some things that are not so important.”
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