When Casey told her son Will that the Weisses were coming over for dinner that night, his response took her by surprise: "Don't forget the strawberry cupcakes." Three-year-old Will was recalling that the Weiss' son liked the cupcakes Casey had served at a dinner party eight months earlier, when he was only two. As precocious as his memory might seem, though, by the time Will reaches school age, the strawberry cupcakes will be gone, along with his recollections of most, if not all, of his first three years.
Psychologists refer to this as "childhood amnesia." But it only applies to certain kinds of memory — kids retain plenty of information from the early years. If Will's dad teaches him to throw a baseball, he may remember how for the rest of his life. Not only that, but the emotional memories of childhood have a lasting impact. Long after the image of the cupcakes has faded, Will might be left with warm feelings towards sweets and social gatherings. Researchers now know this is because the brain doesn't have just one record button. By studying healthy kids as well as adults who have lost certain kinds of memory completely, psychologists have come a long way in explaining how early childhood can be so formative long after our memories of these years have completely vanished.
The most famous patient in brain research, Henry Molaison, known for decades as H.M., passed away recently in a nursing home in Connecticut. Born in 1926, he suffered from severe epilepsy until, at age twenty-seven, he underwent a radical surgery in which doctors removed two slivers deep in his brain, just at ear level. When H.M. woke up, his seizure disorder was cured, but he had lost the ability to form new memories. He remembered his childhood, but, from the time of the surgery until his death at age eighty-three, new memories stayed in his consciousness for about twenty seconds and then disappeared. A lot of what we know today about how memory develops began with H.M. It turns out that for the first few months of life, babies operate much like the famous amnesiac. Babies then gradually become more skilled at memory as the part of their brain that H.M. lived without begins to grow.
Even though their memories are just beginning to ripen, newborns are surprisingly good at knowing what they've seen or heard before, a skill referred to as recognition. To test the abilities of the tiniest babies, researchers take advantage of the infant sucking reflex. When they are given a pacifier that controls what they see or hear, depending on how fast they suck, babies just days old will adjust their sucking rates to spend more time looking at a picture of mom's face, or listening to the words of a book she read aloud in her pregnancy. The fact that a baby recognizes what is familiar to her (and prefers to see and hear her parents) means that, in some form, she is storing memories of her experiences right from the start.
An experiment designed by Carolyn Rovee-Collier in the 1970s is still used today to test just how long infant memories are stored. Researchers lay a baby on her back so that she's looking up at a mobile. They tie the mobile to the baby's foot with a ribbon so that when she kicks, the mobile comes to life. The baby quickly learns to kick more vigorously to watch the mobile dance. Then, after days have passed without any exposure to the set up, she is put back in her crib with the mobile, but it's no longer attached to her foot. When they see the mobile, most babies start to kick furiously, remembering the fun they had with it days before. Two-month-olds remember if they are tested a day or two later, while six-month-olds remember for about two weeks.
These early abilities do not quite add up to what we think of as conscious memory — the skill that allows us to recite the U.S. capitals or reminisce about summer camp. Performing well on the mobile test does not require conscious memory. In fact, it doesn't require a cerebral cortex. Even though a baby is born with most of the brain cells she will have as an adult, for the first few months of life the cortex (the big grey mass we use to think) is largely offline. Lower parts of the brain and the spinal cord control most of a newborn's behavior, allowing her to form habits and develop motor skills. Our first memories are formed in these primitive parts of the brain.
Lise Elliot, Associate Professor at the Chicago Medical School and author of What's Going On In There? How the Brain and Mind Develop in the First Five Years of Life, explains that these unconscious patterns — like when baby's legs kick at the sight of her favorite toy — are part of what is called implicit memory. "Anything you learn is said to store a memory," explains Eliot. "But we distinguish conscious and unconscious memory. Probably ninety percent of what we do happens at this unconscious level, especially in young children."
This kind of memory takes repetition to develop, but requires little thought once it's in place. Implicit memory is how you tie your sneakers or write an email without thinking about your hands, and without any sense that memory is involved. A little kid's mind is bustling with implicit memories in the early years, but the impact goes further than just motor skills. These lower brain regions cozy up to our feeling centers too, so kids learn emotional patterns way before they become conscious beings.
It's hard to know for sure, but some argue that a baby's conscious, or explicit, memory starts to develop at about eight months. Older infants not only recognize something when it's in front of them; they can actually conjure up mental images of the people, places, or objects in their lives. If you show a nine-month-old baby how to use a certain toy (but don't let her touch it) and then give the toy to her twenty-four hours later, she will know what to do with it, presumably because she has the mental picture of you playing with it. Amazingly, at fifteen months, a baby can actually remember the toy tricks after a four-month delay.
These leaps are possible because more signals are starting to reach the cortex and important memory regions are coming online. One of these regions is the hippocampus, an area of the brain that doctors removed to cure H.M.'s epilepsy. "Everything has to go through there to get shuttled into permanent storage," says Eliot. At the time, scientists didn't know what this region did, but they would soon find out that it is necessary for conscious recall — it allows you to remember a friend's birthday or what you had for breakfast.
Scientists were surprised, though, when they realized that H.M. was still capable of forming certain kinds of memory. He could learn new skills and get better at certain tasks. As part of a research project, for example, he got very good at drawing a figure while looking at its reflection in the mirror. Every time he did this it was as if for the first time, and it surprised him that he could do it so easily. His implicit memory was intact, so he could learn and improve, but without a hippocampus he couldn't recall ever having done the drawing exercise before.
A baby's hippocampus does work to some degree — allowing her to recognize faces, for example (which H.M. was unable to do), but this brain region is immature, and it gradually becomes faster and more connected during the first few years of life. As it does, conscious memories are able to hang around for longer and longer periods, giving toddlers impressive recall for single events that happened months before. But, as most of us know, the earliest permanent memories we have are of age three or four.
Freud, who coined the term "infantile amnesia," explained this by saying that we repress these early memories into our unconscious, where they continue to lurk even through adulthood. But researchers now know that memories never get stored in the first place until regions like the hippocampus develop. "The recording apparatus just isn't up to speed to store things in a permanent way," says Eliot. Even though the amount of time a child can remember things continues to increase as the hippocampus matures (allowing Will to make his dessert recommendation after eight months), it isn't until preschool and beyond that memories have a chance of becoming permanent. Even then, memories aren't likely to stick unless they are repeated or have a big emotional impact.
The fact that we don't remember our first three years, Eliot explains, has to do with more than just immature brains. A lot of memories depend on cues from our environment. We see, hear, or smell certain things and a memory is triggered. Some psychologists believe that this is part of the reason for childhood amnesia — our environment changes so much that we don't get the same cues that we did when we were little. "You're never that small again," Eliot says. "You never have that perspective on the room and other people, so there is no stimulus to evoke recall. There are no crib bars to be looking out."
Once a child starts to talk, in the middle of the second year, long-term memory gets a big boost. Language comes around the time kids develop self-awareness and start to understand how events fit together. "When you're little, you don't have that cognitive framework," says Eliot. "Experiences happen, but there's nowhere to slot them, so they're less likely to be rehearsed." In the preschool years, children start to be able to put together narratives — thinking about the world in terms of who, what, when, where, and how all these things fit together, which bodes well for memory's staying power.
Researchers have found, in fact, that the way parents reminisce affects their kid's memory. A child's memory will be stronger if mom helps her put together a story and make connections between things (for example, "Remember what we saw at the park last week? Yes, a dog! And what was the dog doing? He barked loud and you started to laugh!"). As opposed to asking repetitive questions that don't involve much detail ("What did we see? Yes, a dog! And what else did we see?"). When parents flesh out a story, rather than just go after simple facts, memories can find their home in a child's mind.
H.M. never recognized the researchers who studied him for over five decades, but he did learn new skills and associations. We now know that this is because memory doesn't have just one address in the brain. The way we remember how to ride a bike is separate from the way we remember the day our parents took off the training wheels. Without a mature hippocampus, babies and toddlers are mainly creatures of short-term memory. But the unconscious memories that they form right from the start may be the most important ones. These are the emotional patterns that we learn — that we are safe, that when mom picks us up we feel happy, or that when we knock over a tower of blocks and turn to look at dad, he will be smiling back at us. This is why many people say that the first few years of life are the most important — because way in the back of our brains is where we learn (unconsciously) that the world is a good place.