What's happening inside a toddler's brain as she scans a room full of toys or familiar faces? Psychologist John P. Spencer, PhD, at the Delta Center of the University of Iowa, and colleagues have turned to a relatively new — and child-friendly — neuroimaging technique called functional near-infrared spectroscopy (fNIRS) to get a glimpse of visual working memory as it develops (NeuroImage, 2013).

Visual working memory is the core cognitive function that allows us to make short-term mental maps of our surroundings. "It's important for gluing our perception of the world together and is correlated with things like general fluid intelligence," Spencer says.

Spencer and graduate student Aaron T. Buss and colleagues used fNIRS to determine which brain regions jumped into action as 3- and 4-year-olds detected changes in the shapes of objects on a screen. Three-year-olds can keep an average of 1.3 items in their visual working memory at one time, he found, while 4-year-olds can hold onto 1.8. In both age groups, the brain's frontal-parietal network lit up as the kids completed the task.

This same network has been implicated in studies of visual working memory in adults. But kids' brains do things a bit differently, Spencer and his colleagues have found. As adults remember a mounting number of items, activity in this network increases before it levels off at about four objects — the working-memory limit for mature brains. So Spencer was surprised to find that activity in this area continues to increase in children even as they're presented with more objects than they can possibly keep in their visual working memory.

That may be because the system is still so volatile at this stage of development, he says. As new information comes into children's young brains, it may disrupt the data they're already holding in working memory. "There's a lot of chaos in the 3-year-old brain."

Until recently, sorting through that chaos has been technologically challenging because children are notoriously poor at lying still for fMRI studies, which have captured much of what is known about the adult brain. While fNIRS is still fairly new, it's promising for studying basic cognitive processes in the squirmiest subjects. Kids being tested with fNIRS can sit upright and play while wearing a cap that holds sensors against the head. The sensors measure changes in blood volume and oxygenation as light passes through the brain.

Spencer's study is an exciting step, says Lisa Oakes, PhD, a psychologist who studies cognitive development in infants at the University of California, Davis, but was not involved in the research. "This is an incredibly important advance in our understanding of memory development," she says.

The work may also have implications for spotting attention problems in very young children. Spencer's team found certain signatures in the brains of children who held more items in working memory. Such kids, regardless of age, had more activity in the right frontal cortex, for instance. Such differences might serve as early warning signs of problems such as attention-deficit hyperactivity disorder. "Kids who are distractible tend to have lower working-memory scores," he says. "The earlier we intervene, the more likely we are to affect development."

— Kirsten Weir