When Do Babies Develop Memories?
Oct. 30, 2002 -- What's your earliest memory?
Chances are if you think your earliest memory dates from your first year or even early in your second year, it's not real — or at least not one you formed from the actual experience.
Researchers have learned that the area of the brain thought to play a key role in encoding long-term memory matures in spurts. And a study published this week in the journal Nature demonstrates that a major spurt happens after a person's first year and then takes a second year to fully mature.
"Components of early memories may be accurate," says Conor Liston, a graduate student who conducted the Nature study while at Harvard University. "But memories recalled from the first or second year of life are probably not that reliable."
Cleaning Up and Making a Rattle
To test young children's ability to remember, Liston taught three groups of children sequences that were prompted by specific toys and sounds. A call for "Clean Up Time," for example, was followed by wiping a table with a paper towel and then throwing the towel into a basket. "Make a Rattle" was followed by the motion of inserting a ring into a slot in a bottle and then shaking the bottle.
Liston taught 9-, 17- and 24-month-old babies three to five different sequences so that each child could do the actions after prompting. He then waited four months and tested each child's ability to re-enact each sequence following the same prompts.
The differences between the youngest group and the two older ones were striking. Both groups of older children were quickly able to repeat the sequences while the youngest group had a near-zero score.
"We know that neurons are beginning to grow at the frontal lobe around 8, 9 months," says Jerome Kagan, a Harvard University professor of psychology, Liston's adviser and co-author of the study. "This bolsters the work of others that has shown most memories from at least the first nine months become lost."
Kagan explains that one hint that a child is starting to develop memory begins at the age of 9 months when children become less willing to leave their parent. Missing one's mother, he says, is a sign that the child has a clear memory of his or her mother just being there and so the child notices when she leaves.
"If you're 5 months old, it's out of sight, out of mind. You're less likely to cry because you just forgot that your mother was ever there, so it's not as frightening," he says.
Tests of older children reveal they can form memories, but later they don't always realize they have them.
Sweaty Recognition
Nora Newcombe, a psychologist at Temple University in Philadelphia, recently tested the ability of 11-year-olds to recognize pictures of former classmates from their preschool years. She showed them a series of pictures of 3- and 4-year-old children, including some images of children they knew seven years earlier.
Most 11-year-olds claimed not to recognize any of their former classmates. But when Newcombe wired up their hands to measure sweating — also called a galvanic skin response — the children showed biological signs of remembering the faces of those with whom they'd attended preschool.
As they looked at pictures of children they had never known, the instrument measured no sweating responses.
"It was like an unconscious emotional memory existed even when there is no conscious memory," says Newcombe.
Newcombe and her colleagues are now working with 3- and 4-year-old children and testing their ability to remember scenes. She's finding that most children at this age are good at remembering central figures in picture scenes, like an elephant in a jungle, but they're not adept at remembering secondary details, such as the green jungle around the elephant.
"I think what happens after nine months is a growth in the ability to form explicit, conscious memories," she says. "It's clear that this is a step by step process that takes years to develop."
Early Trauma Erased?
Endel Tulving, a psychologist at the University of Toronto, believes that children develop different forms of memory at different phases. First, he says, they encode primitive memories, such as sights and sounds. Then comes semantic memory, the accumulation of general knowledge, such as concepts and language. The final kinds of absorbed memories are episodic, or autobiographical memories, which are recollections of personal experiences.
Understanding when and how memories form has implications beyond neuroscience. Kagan points out that knowing when children start to retain long-term memories could be useful in courtroom cases where a child's memories are used as evidence. Also, knowing that children younger than 9 months are poor at retaining memories could be a comfort to some adopting parents who might worry about early traumatic experiences in their adopted children's lives.
"Some people have argued that a child's first six to seven months can have a profound influence," he says. "But if experience recorded before the frontal lobe matures can't even be retrieved later, this is unlikely."
The recent studies fill in a long-standing gap in understanding of children's brain development since until recent years, most work had focused on adult brains and memory. Liston says after finishing these studies, he started to understand why.
"Babies' schedules aren't as reliable — it's not like working with adults," he says. "So I couldn't count on them always showing up at the lab on time. They get sick sometimes and then there's always nap time."