or his dissertation Pelphrey used both the table-object test as well as the computer-visor test with children aged 6, 8, 10, and 12 months. The study (conducted in Reznick's lab at the Frank Porter Graham Child Development Center) showed that at 6 months of age, babies locate the hidden object more than 50 percent of the time—a rate slightly higher than chance. The percentage increased with the age of the baby tested.

"You see a gradual, linear increase in their ability to perform the task, so that by twelve months, the babies are good at both versions of the test," Pelphrey says. He, Reznick, and their colleagues believe that working memory develops all the way up to puberty. Other researchers have shown that the ability begins to deteriorate with age or with diseases such as dementia.

Reznick hopes these findings will lead to longitudinal studies that look at how and when working memory initially develops—starting even younger than age six months using just the visual procedure. "The only way for a baby to complete the manual task is for him or her to reach—an act that involves telling the hand, ‘Okay, I need you to get out there and grab that cloth,'" Reznick explains. A baby doesn't become capable of doing that until about age six months.

Reznick is also studying whether individual differences in working memory might be related to developmental disabilities. He's collaborating with researchers at the Frank Porter Graham Child Development Center to assess working memory in infants who have a type of mental retardation called Fragile X syndrome. And graduate student Andrea LeGore is conducting a study in Reznick's lab of working-memory development in preterm infants.

hat is a baby's existence like before it develops working memory? "Working memory is such a pervasive part of our mental life that it's hard to imagine being without it," Reznick says. But he suggests that it might be like living always in the present, without the ability to think about what will happen next. When the working memory does become available, a fundamental shift in the infant's mental capabilities likely takes place. "It must be a profound change," Reznick says.

Understanding the underlying physiology of working memory might provide better insight into how it develops. A piece of the puzzle, definitely, but not the be-all and end-all of the story, Reznick says. "Perhaps more important than how it's happening is how it affects the baby's life." Chances are working memory is vitally important for the skills that come later such as learning and understanding language. That, of course, makes the ability to measure working memory and an understanding of its development all the more important.

"We've opened the door into what seems to be a very interesting house," Reznick says. "We're shining a flashlight into this first room, looking for a light switch."

Kevin Pelphrey is now a postdoctoral fellow at the N.C. Neurodevelopmental Disabilities Research Center.