We called them fairy rocks. They were just colorful specks of gravel—the kind you might buy for a fish tank—mixed into my preschool’s playground sand pit. But my classmates and I endowed them with magical properties, hunted them like treasure, and carefully sorted them into piles of sapphire, emerald, and ruby. Sifting the sand for those mystical gems is one of my earliest memories. I was no older than 3 at the time. My memory of kindergarten has likewise been reduced to isolated moments: tracing letters on tan paper with pink dashed lines; watching a movie about ocean creatures; my teacher slicing up a giant roll of parchment so we could all finger-paint self-portraits.

When I try to recall my life before my fifth birthday, I can summon only these glimmers—these match strikes in the dark. Yet I know I must have thought and felt and learned so much. Where did all those years go?

Psychologists have named this dramatic forgetting “childhood amnesia.” On average, people’s memories stretch no farther than age three and a half. Everything before then is a dark abyss. “This is a phenomenon of longstanding focus,” says Patricia Bauer of Emory University, a leading expert on memory development. “It demands our attention because it’s a paradox: Very young children show evidence of memory for events in their lives, yet as adults we have relatively few of these memories.”

In the last few years, scientists have finally started to unravel precisely what is happening in the brain around the time that we forsake recollection of our earliest years. “What we are adding to the story now is the biological basis,” says Paul Frankland, a neuroscientist at the Hospital for Sick Children in Toronto. This new science suggests that as a necessary part of the passage into adulthood, the brain must let go of much of our childhood.

Sigmund Freud gave childhood amnesia its name in the early 1900s. He argued that adults forgot their earliest years of life in the process of repressing disturbing memories of sexual awakening. While a handful of psychologists saw merit in this claim, the most commonly accepted explanation for childhood amnesia was that children simply couldn’t form stable memories until age 7—even though there was little evidence to support this idea. For nearly 100 years, psychologists assumed that memories of infancy did not endure because they were never durable in the first place.

The late 1980s marked the beginning of a reformation in child psychology. Bauer and other psychologists began to test infant memory by performing a series of actions—such as building a simple toy gong and striking it—and then waiting to see if a child could imitate the actions in the right order, after a delay ranging from minutes to months.

“Mice performed worse on certain kinds of memory tests after living in a cage with a running wheel.

This work laid bare the contradiction at the heart of childhood amnesia: Infants can create and access memories in their first few years of life, yet most of these memories eventually vanish at a rate far beyond the typical forgetting of the past we experience as adults.

Maybe, some researchers thought, enduring memories require language or a sense of self, both of which we lack as infants. But although verbal communication and self-awareness undoubtedly strengthen human memories, their absence could not be the whole explanation for childhood amnesia. After all, certain animals that have large and complex brains relative to their body size—such as mice and rats—but do not have language or, presumably, our level of self-awareness, also lose the memories they make in infancy.

Perhaps, then, researchers reasoned, the paradox had a more fundamental physical basis that was common to people and other big-brained mammals. The question was, what?

Between birth and our early teens, the brain is still laying down some of its fundamental circuitry and thickening its electrical pathways with fatty tissue to make them more conductive. In a massive surge of growth, the brain sprouts innumerable new bridges between neurons. In fact, we have far more links between brain cells in our earliest years than we end up with in adulthood; most are pruned away. All that excess brain mass is the wet clay from which our genes and experiences sculpt a brain to suit its particular environment. Without such limber brains, young children would never be able to learn as much as quickly as they do.

As Bauer and others discovered, this adaptability comes with a price. While the brain undergoes this prolonged development outside the womb, the large and complex network of disparate brain regions that collectively create and maintain our memories is still under construction, Bauer explains, and not as capable of forming memories as it will be in adulthood. As a consequence, the long-term memories formed in our first three years of life are the least stable memories we ever make and highly prone to disintegrating as we age.

In early 2018, Frankland and his colleagues published a study indicating another way the brain relinquishes our childhood memories: not only do they degrade, but they also become concealed. A few years back, Frankland and his wife Sheena Josselyn—also a neuroscientist at the Hospital for Sick Children—started to notice that the mice they studied performed worse on certain kinds of memory tests after living in a cage with a running wheel.

“Our earliest memories are often insoluble blends of genuine recollections, narratives we sponged up from others, and imaginary scenes dreamt up by the subconscious.

As the couple knew, exercise on a running wheel promotes neurogenesis—the growth of whole new neurons—in the seahorse-shaped hippocampus, a brain region that is essential for memory. But while neurogenesis in the adult hippocampus likely contributes to the ability to learn and remember, Karl Deisseroth of Stanford University and others had suggested that it might also necessitate a certain amount of forgetting. Just as a forest has room for only so many trees, the hippocampus can hold only so many neurons. New brain cells might crowd the territory of other neurons or even replace them altogether, which could in turn break or reconfigure the small circuits that likely store individual memories. Perhaps, then, the especially high rate of neurogenesis in infancy was partly responsible for childhood amnesia.

To put this notion to the test, Frankland and Josselyn transferred infant and adult mice from the plastic shoebox-sized cages they had known all their lives to larger metal cages they had never seen before. In these new containers, they zapped the rodents’ feet with mild electric shocks. The mice quickly learned to associate the metal cages with the shocks, stiffening with fear whenever they were returned to those enclosures.

While baby mice began to forget about this connection after a single day—relaxing when they found themselves in the shock cages—adult mice never forgot about the danger. But when adults ran on a hamster wheel after the shocks—thereby stimulating neurogenesis—they started to mirror infants in their forgetfulness. Prozac, which also encourages neural growth, had the same effect. Conversely, when the researchers hindered neurogenesis in infant mice with drugs or genetic engineering, the young animals formed much more stable memories.

To get a really close look at how neurogenesis might change memory, Frankland and Josselyn used a virus to insert a gene encoding a green fluorescent protein into the DNA of the mice’s newly sprouted brain cells. The glowing dye revealed that the new cells were not replacing old ones; rather, they were joining existing circuitry. That suggests that, technically, the many little circuits of neurons that store our earliest memories are not wiped out by neurogenesis. Instead, they are thoroughly restructured, which probably explains why the original memories become so difficult to recall. “We think it’s an accessibility issue,” Frankland says, “but it’s sort of a semantic issue too. If a memory becomes impossible to access, then it is effectively erased.”

This restructuring of memory circuits means that, while some of our childhood memories are truly gone, others persist in a scrambled, refracted way. Studies have shown that people can retrieve at least some childhood memories by responding to specific prompts—dredging up the earliest recollection associated with the word “milk,” for example—or by imagining a house, school, or specific location tied to a certain age and allowing the relevant memories to bubble up on their own.

But even if we manage to untangle a few distinct memories that survive the tumultuous cycles of growth and decay in the infant brain, we can never fully trust them; some of them might be partly or entirely fabricated. Through her pioneering research, Elizabeth Loftus of the University of California, Irvine has demonstrated that our earliest memories in particular are often insoluble blends of genuine recollections, narratives we sponged up from others, and imaginary scenes dreamt up by the subconscious.

In one set of groundbreaking experiments conducted in 1995, Loftus and her colleagues presented volunteers with short stories about their childhood provided by relatives. Unbeknownst to the study participants, one of these stories—about being lost in a mall at age 5—was mostly fiction. Yet a quarter of the volunteers said they had a memory of the experience. And even when they were told that one of the stories they had read was invented, some participants failed to realize it was the lost-in-a-mall story.

When I was a toddler, I got lost in Disneyland. Here is what I recall: It was December and I was watching a toy train loop through a Christmas village. When I turned around, my parents had disappeared. Dread dripped down my body like cold molasses. I began blubbering and wandering the park, searching for them. A stranger approached me and took me to a giant building filled with TV screens playing feeds from security cameras all over the park. Did I see my parents on any of the screens? I did not. We went back to the train where we found my parents. I ran into their arms, overcome with joy and relief.

Recently, for the first time in quite a while, I asked my mom exactly what she remembers about that day in Disneyland. She says it was spring or summer and that she and my family last saw me beside the remote control Jungle Cruise boats, not the railroad near the entrance of the park. As soon as they realized I was missing, they went straight to the Lost and Found Center. A park official had indeed discovered me and brought me to the center, where I had been placated with ice cream.

It was unsettling to have what I thought was a pretty accurate memory so thoroughly contradicted, so I asked my mom to search our family photo albums for some hard evidence. All she could find were pictures from an earlier trip. We will probably never have any tangible proof of what happened. We are left with something far more elusive: those tiny embers of the past, embedded in our mind, twinkling like fool’s gold.

Original article here