rewrite this title Why You Can’t Remember Being a Toddler

Think back to the first thing you remember: Candles on your birthday cake, the wiry fur of a childhood dog, the smell of your mother’s perfume. Whatever the memory, chances are you were already at least a couple years old when those events took place. Children younger than three are intensely aware of the world around them—just look at any toddler, delightedly mouthing a toy or screaming at fireworks—and they seem to remember things from the recent past. Until, one day, they don’t.  

Between us and our earliest experiences lies a mysterious barrier. And that’s not just true for  humans. Experiments have shown that mice show a similar pattern of forgetting. Mice that learn to escape a maze when they’re just a couple weeks old forget by the time they are adults. Young mice conditioned to fear a chamber where they’ve received a shock don’t recognize it after they grow up.

In recent years, scientists who study this phenomenon—sometimes called childhood or infantile amnesia—have made some surprising findings that illuminate how this nearly universal form of forgetting works.

Are the memories gone, or just out of reach?

It might seem like the memories of early life simply get erased. But some research suggests that in mice, those memories still exist, and can be brought back. 

At the lab of Paul Frankland, a senior scientist at the Hospital for Sick Children in Toronto, researchers tagged the cells in the brain that were activated as young mice learned to fear a chamber. Three months later, when the full-grown mice had forgotten their fear, the researchers activated those cells again—and suddenly, the mice remembered. 

This suggests that—at least in mice—memories formed in early life are not wiped or destroyed; they’re just out of reach of normal recall. It may be that something about how the brain grows makes those memories inaccessible, locked rooms whose keys have been lost.

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Indeed, animals whose brains tend to add smaller crops of neurons after birth—guinea pigs, for instance—do not show signs of this amnesia, Frankland and colleagues have found. 

The lab of Tomás Ryan, a neuroscientist at Trinity College Dublin, has also revealed that some mice are less prone to forgetting. Male mice whose mothers’ immune systems were activated during pregnancy do not show the same pattern of amnesia as their female siblings or control mice.

What’s more, Ryan’s group has highlighted the role of immune cells in the brain called microglia in infantile amnesia: Knock down microglia activity in a key developmental window, and mice don’t forget the solution to a maze as they mature. This suggests that the immune system may be involved in infantile amnesia.

But what about humans? 

It’s tricky to figure out what is going on inside the brains of babies and very young children. (To name one barrier: Sitting motionless for brain scans is not their strong suit.) However, Nick Turk-Browne at Yale University and his colleagues have managed to scan the brains of a growing number of little kids, and they’ve discovered that kids as young as a year old do appear to be forming memories, in the same way that adults create recollections of past events, called episodic memory. This suggests that humans, too, may be making memories that later just can’t be reached.

Is it worth taking your two-year-old on vacation, if it’s all going to disappear? “I get asked this all the time: ‘What can we do to prevent this from happening?’” says Turk-Browne. “You can talk about it a lot, or show pictures. But the true, pure thing—where he had this memory that he hasn’t thought about in a long time, that you haven’t talked about with him—that will soon be gone, for better or worse.” 

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To get a better sense of precisely when memories are formed and forgotten, Sarah Power at the Max Planck Institute for Human Development and her colleagues built a media room where children have experiences they will never encounter in the outside world. “One of the really important things about the task is that everything only exists inside the lab space. We wanted to make sure it was completely unique in the sense that…the contextual environments don’t exist anywhere outside in the real world, so that we could know that if they did remember these associations, it could only be from the fact that they had been in the lab,” she says. They have so far observed 400 toddlers between the ages of 18 and 24 months, having them form memories of the lab space, and they intend to follow them over time. The project is still in its early stages, but “from the preliminary data, we’ve been very surprised at their ability to encode and retain these episodic-like memories,” she says.

For a smaller experiment Turk-Browne is running, parents filmed footage of events from the children’s perspective. Then, he and his colleagues showed the children their videos and videos from strangers while scanning their brains, at several sessions spread out over two years. The idea is to assess whether videos of a child’s own view of the world elicit remembering that’s detectable on brain scans, and, if so, exactly when that effect disappears. 

Why do we forget?

It’s a mystery why our brains, and those of other mammals, forget our early lives. “I do wonder what it tells us about human memory in general, education, early life, learning…Is this a biological switch, or is this just a product of exuberant learning?” says Ryan. “In other words, is our brain actually intentionally saying we’re going to shut down these memories? Or is it just a byproduct of heavy learning in that period?”