Anyone who's seen a young toddler "at work" can tell that her learning style is a study in chaos. She moves from banging pots and pans to tormenting the cat to demanding food to bursting into tears when she can't open the back door and hurdle off the deck--all in the span of minutes.

But when it comes to, say, the daunting task of mastering language, that same baby is a turbo-charged learning machine. She is building her toolkit of words and phrases by the hour, forming sentences that not only get her needs met but generally comply with rules we would never think to try to teach her.

How does this distractible little creature manage to derive so much information from her surroundings when she seems to be moving too fast to make sense of anything? Why, if we adults are so good at learning and doing things, is she so much better and faster at learning something as complex as, say, a language?

Maybe, suggests an intriguing article published today in the journal Current Directions in Psychological Science, it's because her underdeveloped powers of paying attention, of filtering out distractions and of imposing regular patterns on her experiences. Maybe it's because she lacks the very thing that makes her dad so good at filtering out distractions and getting things done--a fully formed prefrontal cortex.

Babies are born with the foremost part of the brain--the prefrontal cortex--almost completely undeveloped. For children developing normally, it takes about four years for that so-called "seat of higher reasoning" to catch up with the rest of the brain in size and complexity. (For children with attention-deficit hyperactivity disorder, the maturation of the prefrontal cortex takes longer; for those with autism, the prefrontal cortex develops early.) This would explain why toddlers (and some with ADHD) are inattentive, distractible, do not remember what you told them 15 seconds ago, and "live in the moment": when developed, the prefrontal cortex plays a key role in suppressing impulses, focusing on the task at hand and setting priorities among competing demands.

Yet, by the time a typical baby is 4, she will have learned to speak (in fact, she may never be quiet), and will have learned all kinds of complex behaviors--that small creatures with fur are fun to torment, that climbing up a slide's ladder will yield a fun ride down, that stealing a friend's toy will bring momentary triumph but will also cause her friend to cry and a teacher to intervene.

In those crucial four years, a toddler's accumulation of knowledge about her world may be unhampered by the discipline imposed by the prefrontal cortex, suggests a trio of neuroscientists from the University of Pennsylvania and Stanford University. She vacuums up experience raw, the way she'd ingest anything she found on the carpet. Her prefrontal cortex doesn't stand in the way and try to keep her "on task." It won't make her reject the use of a pan as, say, a hat because hats cannot be made of shiny metal.

In language, her underdeveloped powers of attention will keep her from getting bogged down by pesky exceptions to rules of grammar or syntax. So, she'll always apply the most general rules she knows--say, that adding an "s" makes things plural. She'll incorrectly say, "the mouses are running away," because her underdeveloped prefrontal cortex didn't slow her down and take note that the plural of "mouse" is "mice."

The authors call this period of disorderly learning "cognition without control." They note that it happens beyond babyhood as well: deep sleep, during which the prefrontal cortex is generally quiet and the sensory cortex is working overtime, would seem to afford even adults a nightly return to cognition without control--and an opportunity to do what babies seem to do so well: to derive broad inferences from recently learned things without getting too bogged down in petty exceptions and details.

This is a theory, not a finding, note the authors, led by University of Pennsylvania's Sharon L. Thompson-Schill: that evolution may have favored a delay in the maturation of the brain's "braking system" as a means of allowing rough-but-rapid learning of complex matter such as language and social conventions. But it's a theory that might help clinicians and educators begin to identify what are the best windows for teaching very young children and for helping kids with developmental differences to learn as well.

-- Melissa Healy