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Kids start remembering by second year, study finds

By Marsha Walton

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(CNN) -- Harvard researchers have found that by age 2 children have developed memories strong enough to repeat tasks shown to them months earlier.

The study, published Wednesday in the British journal Nature, gets closer to unraveling how memory develops by establishing that 2-year-olds have recall, something that their 1-year-old counterparts do not. But it doesn't explain why most adults can remember only as far back as age 3, 4 or 5.

"We want to know what's happening between about the middle of the second year of a child's life, and age 4 or 5, when permanent autobiographical images pop up," said researcher Conor Liston, who studied a dozen babies in three age groups: nine, 17 and 24 months old.

It's a kind of Twilight Zone that scientists call "infantile amnesia," that period of two years or more, between ages 2 and 5, when most humans have little or no recollection of specific events.

Scientists believe many factors aid the development of long-term memory. Many of those factors go beyond the physical structures of brain development. Language skills, socialization, environment and self awareness could play a part.

Many answers are difficult to come by because memories, in the young and old, are often tied to stories that have been told and embellished for years. For example, does someone really remember riding a pony at his grandma's house when he was 3? Or has he just heard the story and seen the picture so many times that he thinks he remembers?

Because many questions remain, neuroscientists advise parents to provide their children ample stimulation. Talking, reading and playing all might help an infant create and keep good memories as early as possible.

How the study worked

Liston, who worked on the study as a Harvard undergraduate, said studying child memory is complicated by the inability of kids to put their thoughts in words.

The study analyzed infants while they sat at home on their mothers' laps and watched Liston complete simple, three-step tasks.

They included: putting a rock and a driver in a toy dump truck and making it move; putting a ball and a lid on a tube and making it rattle; and wiping a spill with a paper towel and tossing it in a basket. Liston repeated the task six times for the nine-month-olds, and four times for the 17- and 24-month-olds. During the demonstrations, he also repeated a simple phrase, such as "make a rattle" or "clean up time." Then the child would try to complete the same task. The tests each lasted about 45 minutes, until the child lost interest in the process.

Four months later, Liston returned to each home and spread out the objects the youngsters had worked with before. He'd give the same verbal instructions, such as "make a rattle," but he wouldn't do another demonstration.

The older babies, who had reached 21 and 28 months, were like old pros with their "assignments," he said. More than 90 percent of the kids in those age groups completed at least one of the tasks they had been shown months before.

But infants in the youngest group, who were 13 months old by Liston's second visit, performed no differently from babies of the same age who had never seen the objects. Their ability to recall and repeat tasks hadn't kicked in yet. Sometime between nine and 17 months, neurons in the pre-frontal cortex of the brain connect to many other parts of the brain, and another area of the brain, the hippocampus, is becoming fully functional, the study found.

The research indicates "that there is a neurobiological component to memory enhancement across the second year, contrary to early assumptions that this is entirely attributable to experience," the researchers wrote.

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