Have you ever been told about an incident that happened when you were little that you cannot recall? Perhaps the time you had an unstoppable crying fit at the mall, ate your first piece of pizza or hit your head on the coffee table and had to get stitches.
This inability to remember specific events from the earliest years of our lives, called “infantile amnesia” by Sigmund Freud over a century ago, happens to us all. Now researchers have found what could be causing it: the birth of additional neurons — nerve cells — in the brain.
“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.”
Fresh neurons form rapidly in the brain after birth and into young childhood, but the process slows to a crawl once we reach adulthood. Frankland and his colleagues discovered in experiments with mice that by accelerating the production of new cells in the hippocampus — an area of the brain crucial for memory formation — they induced higher levels of forgetting.
They speculate from their findings that the abundant birth of neurons during our early years could explain the mysterious amnesia we experience in relation to childhood memories. The study was published online May 8 in the journal Science.
But psychologist Justin Rhodes of the University of Illinois at Urbana-Champaign is hesitant to believe that the link between new neurons and lost memories is that simple. “I’m still a little skeptical,” said Rhodes, who was not involved in the study. “Neurogenesis is a field where there’s a lot of controversy, and we don’t know exactly what these new neurons are doing.”
Both adult mice and pups — equivalent to infants in human years — initially learned to fear a certain room by associating it with mild foot shocks. After the animals had spent time back in their cages, Frankland and his colleagues returned them to the room; if they remembered it as being a bad place, the rodents froze. While the adults recognized the scary room up to four weeks later, the majority of pups froze during testing only within a day of the bad experience. After two weeks, almost no pups could remember the room as a bad place.
In the next experiment, two groups of adult mice were taught to fear the same room. Then the researchers induced the birth of new neurons, called neurogenesis, by giving one cohort access to a running wheel.
Exercise in rodents and humans has been shown in previous studies to promote neurogenesis. When compared with sedentary mice without wheel access, the runners more easily forgot their fears. The incorporation of new cells seemed to alter the connections between existing neurons.
The reverse held true when suppressing neuron growth in pups. When their neurogenesis was reduced by the use of a common chemotherapy drug, the young mice were better able to recall their fears.
Last, the team tested two other types of rodents — guinea pigs and degus. They differ from mice in that much of their neurogenesis happens while still in the womb, so they do not experience infantile amnesia. But after neurogenesis was induced in the pups of both species, they could no longer remember as well.
Adults generally fail to recall anything that occurred before age 3, and any memories from earlier than age 7 tend to be fuzzy — although children are clearly capable of making memories. But somehow, these experiences are wiped away quickly.
Frankland and his wife, neuroscientist and study author Sheena 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,” he 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.’ ”
A study of 140 children revealed a similar phenomenon. The subjects were asked to describe their three earliest memories, with their parents confirming the details. When interviewed again after two years, the children who were initially between 4 and 7 years old spoke about completely different events during the follow-up, even after being given hints.
Meanwhile, older children (10-13 years old) were more likely to recall their stories.
Many studies have found that reducing neurogenesis in mice will impair their ability to learn. Frankland and his colleagues flipped this idea in their experiment, wanting to see how changing levels of neurogenesis affected a memory already in storage.
The idea of neurogenesis dislodging old memories could explain forgetting in adulthood as well, since neural stem cells in the hippocampus remain active throughout life. However, the rate of growth slows considerably, so other mechanisms are likely to be at work.
Memory comes in many forms — automatic procedural memories, such as riding a bike, or the short-term storage of working memory — and forgetting could be the same.
One theory in psychology, called interference, states that memories too similar in nature compete and create distortions in recollection. For instance, confusing new and old telephone numbers, or having a fuzzier memory of “Oceans Eleven” as a result of watching the two sequels.
Although inconvenient at times, forgetting in general does seem to be beneficial. Frankland believes the memory-storage capacity of the hippocampus can reach a saturation point — and then it’s time to clean house.
“We know much of what we experience is wiped away,” he said. “But the formation of new memories is facilitated by clearing away the clutter, if you like.”
However, Rhodes argues that the hippocampus is typically known as a structure of temporary storage that integrates incoming information for tucking away into long-term storage but does not act as the data bank itself.
“It helps process and form new memories,” he said. “But those memories are solidified in separate parts of the brain.”
Thus, he believes new neurons in the hippocampus are unlikely to disrupt old memory circuits.
Kim is a freelance science journalist based in Philadelphia.