Keeping track of multiple objects: How and when is it learned?

How many moving objects can you keep track of at once? Clicking on the image below will take you to Lana M. Trick's web site, where she has a nifty demo of a multi-object tracking task. You're asked to keep track one to four of the smiley-faces as they move randomly around the screen. Then when the faces stop moving, you click on the ones you were supposed to follow. Go ahead, give it a try!

You'll notice there are four levels of difficulty. Most adults can, with a little practice, track four out of ten randomly moving objects for ten seconds -- they fall apart when there are more than four objects to track or more than ten total objects (the "most difficult" trial features four objects to track and twelve total). But when do kids develop the ability to track multiple objects? Very young infants can track a single object moving by itself quite easily, but what about several objects moving among others?

Trick's research team developed a task that could be followed by kids as young as five. Previous studies of multiple-object tracking used colored shapes, which were uninteresting to young kids, who became distracted during the task. Trick's team told kids they'd be looking for sinister "spies" among normal, happy people (just like the demo you just tried). They found that five-year-olds understood the task, and reliably completed it when the faces weren't moving. Then they performed the same test on kids ranging in age from 5 to 19. The results are below.

As expected, 19-year-olds were accurate at tracking "spies," even when there were four of them. But younger kids' abilities tapered off at different levels. Six-year-olds, for example, were just 55 percent accurate when asked to track three items.

This information alone doesn't tell us how many objects they successfully tracked, because they might have guessed on some of the objects. That's what the gray lines on the graph are for. The light gray line indicates the expected result if the viewer guessed on one object. For example, if a child was supposed to follow two objects and successfully followed one of them, she'd be guaranteed to be 50 percent correct, but she'd have a 1 in 9 chance of guessing the second item as well, so her expected average score in this case would be about 56 percent. The dark gray line shows the expected scores if participants guessed on two of the objects.

Six-year-olds, by this logic, can successfully track two objects, because their score is above the chance result when tracking two objects, but below it when tracking three. Eight-year-olds, however, can track three objects. Only 19-year-olds could track four objects successfully.

So how do kids learn to track more objects? They might learn based on their environment. What if a child plays lots of action-oriented video games? Trick's team actually asked parents to indicate the types of video games their kids played, and those who played lots of intense action games (as opposed to things like racing games where keeping track of several objects wasn't as important) did perform significantly better on the task. Kids who played action sports like soccer and basketball (as opposed to swimming or golf) also performed better -- though not as well as video gamers. Clearly this is a skill that can be learned.

But perhaps it's simply a matter of visual working memory. Maybe the number of moving objects you can track is simply related to the number of static objects you can remember. Trick et al. argue that this isn't likely, since even five-year-olds can easily remember the locations of as many as ten static objects. Another possibility is that older kids are better at ignoring distractors. In a separate study Trick's team found that 19-year-olds couldn't track four objects when there were 14 objects on the screen (compared to the 10 in this study), so it's likely that younger kids would also have trouble with more distractors. Yet this experiment doesn't tell us whether the number of distractors is the key facto. The authors recommend that future studies vary both the number of items tracked and the number of distractors.

We've reported on additional work on multiple object tracking here.

Trick, L.M., Jaspers-Fayer, F., & Sethi, N. (2005). Multiple-object tracking in children: The "catch the spies" task. Cognitive Development, 20, 373-387.