Posted on 05/09/2014 4:11:12 PM PDT by LibWhacker
New neurons may explain why adults can’t remember being infants
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.”
I suppose that those of us who can didn't get the new brain cells.
How far back can most people go?
I remember things from age 4 very clearly....but with the exception of one small memory, its really a fog before that.
I can recall being pushed in my stroller through a gravel parking lot at a restaurant and a conversation between my Mother and a neighbor lady with an infant also in a stroller. When I mentioned it years later, my Mom told me that I was around one year old at the time.
Kind of ironic, devising a Room 101 for rodents...
I can remember being a baby and being in a baby swing. I fell out on my head.
I remember isolated images from before about the age of 4. I remember events from about age 4 on.
This does make sense, although whether it is borne out in further research remains to be seen. New neurons in the brain would have to plug themselves in to the neurons already in place; that disruption of neural connections could very well disrupt memories.
I can remember being confined in my crib and my mom wouldn’t let me out. I was positively enraged. I was 19.
LOL, no, I was about 2 years old, I suppose. It was after the war and we were living in a tenement building in NYC on 24th St. My crib was pushed up against a window overlooking an alley. My dad said I got so mad about being confined that one day I threw some of my mom’s perfume out the window. He said she retaliated by throwing my teddy bear out. Don’t remember that. But I do remember the crib and the alley. There were alley cats down below. Lots of other memories start emerging after that.
Haha, good point.
Interesting: “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.”
I have a couple of memories from before age 3. One is of the underside of my grandparents’ dining room table as seen from below. The earliest, strongest memory is of my father returning from WWII when I was just over 3.
I can remember things like that too. Maybe we didn’t run enough.
If we could, we ran. I was a tree climber, too. I remember climbing the black walnut tree outside the yard. I never fell from it even though the wind was blowing the top back and forth. My dad worked on cars and had a thingy shaped like a child’s swing set. He used it to pull the motor out. I was walking on the top of it, fell off and knocked the wind out of me. My mom, her sister and others there thought it had killed me. It did not stop me though. That same aunt and her family lived in a house which had a corral. I would walk on the top rail. It is a miracle I am still here. : ). Great memories.
And I can remember being less than 1 year old. I used to get really angry when people wouldn't believe me, but I was able to describe too many details, and they all came around. For example: there were black and white tiles on the floor, and a few were loose; it must have been summer because it was hot in my crib; it must have been winter because it was cold and ice was on the ground.
My earliest memory is of workers putting the roof on the house that I grew up in. The roof was about half finished. I must have been under two years old.
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