Just what do infants and toddlers glean from the Baby Einstein disks and Blues Clues episodes that they ingest so rapturously?

Not much, it would seem. Very young children have what researchers have termed a video deficit: they’re unmistakably bad at learning about a 3-D world via a 2-D medium. For instance, a 2010 study led by University of Virginia psychologist Judy DeLoache found no evidence that, after viewing a popular word-learning video about 20 times, 15-month-olds could point to a real-world object upon hearing a word they’d been exposed to any better than could children who hadn’t watched the video at all. (Fifteen-month-olds who’d been taught the same set of words by their parents did, however, perform better than the control group.)

And yet all is not lost for educational programming. Studies show that slightly older children, including preschoolers, can benefit from judicious portions of educational media. In other words, children appear to “grow out” of their video deficit after their second birthday.

A daunting amount of relevant research exists on this topic—after all, everyone from television executives and marketers to pediatricians and psychologists has a stake in the answer to this question. Thankfully, a few years ago Vanderbilt psychologist Georgene Troseth compiled and summarized much of this literature.

Troseth reports that visual perception is probably not responsible for the deficit. Quite young infants appear to have little trouble detecting the differences and similarities between actual objects and their two-dimensional depictions. Even newborns can distinguish between 3-D and 2-D events. And, by just five months old, infants who have been exposed to a doll will later “recognize” a photograph of that doll, demonstrating less interest in it than in a photo of a novel doll. (Jaded babies.)

The problem must be one of interpretation, in that children must internalize the fact that a videotaped event is both a 2-D object in and of itself and a representation of a 3-D object, which, to complicate matters further, has some but not all of the properties of said 3-D object. One missing element is responsiveness: actors and avatars can’t smile directly at infants, or meet their gaze, or react to loud noises in the infant’s environment. This may actually teach infants to temporarily mistrust video, or at least regard the information they learn from it as (often quite rightly) not applicable to them.

After acquiring some combination of cognitive skills and video exposure, children do begin to learn the relationship between object and image. According to work by Troseth and her colleague Sophia Pierroutsakos, nine-month-olds often attempt to manipulate objects on a screen, rubbing or grabbing at them. But by 15 months—and certainly by 19—attempts at manipulation have been replaced by pointing. (Though, as with so many developmental achievements, there are adorable lapses: Stanford emeritus professor John Flavell has reported that many three-year-olds will claim that, say, a balloon or a bowl of popcorn on a television screen could be touched if the top of the television were removed, and could “come out” if the set were shaken.)

Still, most infants and toddlers can learn some things from video. Indeed, many psychologists who study cognitive development rely on videos—which can be easily manipulated and carefully controlled—to test pet theories about how infants learn. The real problem with educational programming, at least for children under the age of two, has to do with the richer learning experiences youngsters are not having—with 3-D people, 3-D toys, and 3-D books—while plopped in front of the television.