Why we can't remember things before age 3-4

Credit: Pixabay.

Ever tried to pinpoint your very first memory? If you have, then you know how difficult this is. There’s a whole slew of events from your early childhood that seem to stream through your head, but which one was first? If you’re lucky, maybe you can recollect events from as early as age three, but before that, it’s rather well accepted that you can’t. Some people claim they have memories from an even earlier age, but this can’t be proven. With this in mind, one might ask: why can’t I remember anything before age 3-4?

The place where our early childhood disappears

Canadian researchers tackled this very important question in neuroscience and found that what’s commonly referred to as “child amnesia” is due to an overload of the hippocampus — the area of the brain responsible for filing memories from the short-term to the long-term storage.

“The hippocampus matures slowly and probably doesn’t reach any reasonable maturity until we’re 3 or 4,” says Dr. Eric Kandel, Kavli professor and director of the Kavli Institute for Brain Science at Columbia University and senior investigator at the Howard Hughes Medical Institute. “While 2- and 3-year-olds can remember things for a short time, the hippocampus is required for long-term storage of those memories.”

If you’ve ever raised kids, then you might have noticed that they form memories just fine even as toddlers. They’ll remember that trip they had by the seaside a few weeks back, but surprise, surprise they’ll wipe it right off in a mere few months’ time. This can be frustrating for some parents, but the truth of the matter is this is not only natural, but necessary since their brains, and most importantly their hippocampus, is still developing.

To explain why this happens, though, the researchers led by Paul Frankland, a senior scientist at the Hospital for Sick Children in Toronto, hypothesized that the memories themselves actually ended up in the long-term “memory storage room” — it’s just that the hippocampus lost track of where these were put. Imagine a tiny room that rapidly grows into a whole warehouse, but you only have a limited amount of resources to keep track of and successfully store and tag goods for later retrieval — you’ll get a lot of boxes tucked away with no idea what’s inside them, and most likely you’ll leave them like that.

This is what happens to the hippocampus as it matures and huge numbers of new neurons come online and need to be hooked into existing circuits. As growth slows down, the brain is better at keeping pace and can do a better job of tracking where memories get stored, which is why long-term memories become better as youngsters get older.

“They can’t form stable memories of what happens in the first few years,” Frankland says. “I have a daughter who is 4 years old and because we were working on this study, I would always ask her questions about her memories of places we visited 2, 3 months ago. It’s clear that she can form memories with quite some detail. But four years from now she won’t remember anything.”

Think of memory as like orzo, Patricia Bauer, a professor of psychology at Emory who studies early memory.

“It’s not like one big piece of lasagna noodle. Memories are made up of these little tiny bits of information that are coming in literally across the entire cortex. Parts of the brain are taking those little bits of information and knitting them together into something that’s going to endure and be a memory,” she said.

Scientists liken the childhood hippocampus to a rough-meshed colander that lets orzo creep through. Credit: IndustryEats.

Adults have a fine-mesh net to catch the orzo. Babies have a big-holed colander: the orzo slips through.

“What’s happening with the baby is that a lot of the information is escaping even as the baby is trying to get it organized and stabilized.” In early infancy, a lot of experiences never become memories—they slip away before they can be preserved.

To test this theory, the researchers worked with baby mice, which have the same problem as baby humans. Teach a baby mouse to solve a maze and he’ll forget his way around it within a few days.

A few infant mice were genetically engineered to have a slower neuron build-up in their hippocampus. It was found then that these baby mice were more successful at retrieving long-term memories and could solve the maze after a much longer period of time since they first learned to navigate it.

The findings are pretty clear: toddlers face a hippocampus overload issue. Interestingly enough, the researchers might even have a chance at testing their theory out on humans directly. There are many children suffering from brain cancer who are prescribed medication, which, as side effects, slow down the generation of new neurons.

“We can check to see if the treatment preserves memories of things that happened just before the chemotherapy, just as it did in the mice,” Frankland says.