How babies build a picture of the world

Here's a picture of our daughter Nora at about 3 months of age. She looks like she's fairly aware of the events going on around her (arguably more aware than she sometimes appears now, at age 12). However, as our knowledge of how infants begin to perceive the world around them has increased, we've learned that the world of a three-month-old literally looks different to them than the world we perceive as adults. That's because vision, which seems so obvious and instinctive, is actually an active process. When we perceive the world visually, we're not just passively "seeing" what's there, we're constantly determining where one object ends and the next one begins. We're applying logical rules to help break objects into groups and understand how the two-dimensional image on the inside of our eye corresponds to a three-dimensional physical world.

In the picture of Nora, for example, how do we know that the bonnet isn't part of her body? Because it's a different color, white? But the white buckle is part of the baby carrier. Clearly the set of rules we've learned are not simple. But when do we learn them? And in what order?

Yuyan Luo and Renée Baillargeon developed a set of experiments to test just one aspect of infant perception: how babies recognize when an object moves behind an occluder. They constructed a small "stage set" to test a variety of different types of occlusion. The simplest event involves an object moving behind a screen. Lou and Baillargeon followed up on experiments by Andrea Aguiar and Baillargeon which found that 2.5-month-old babies will not be surprised by an object moving disappearing when it passes behind a small screen and reappearing when it emerges from the other side. However, if you cut a hole in the bottom of the screen and repeat the experiment, 2.5 months old infants are still not surprised when an object disappears behind the screen and reappears on the other side -- "magically" moving past the window without being seen. By the time they are 3 months old, they are surprised when they don't see the object in the window in the middle of the occluder. But three-month-olds aren't surprised in a separate experiment when the middle of the occluding screen is made shorter and a tall object is passed behind the occluder, again magically moving past the window unseen. It's not until 3.5 months old that this trick is surprising.

Aguiar and Baillargeon developed the following flowchart to show how infants perceive objects passing behind occluders:

When infants are 2.5 months old, they have only learned the first step: if the object is behind the occluder, it will be hidden (blue boxes indicate when babies believe objects will be visible). By 3.0 months of age, they have learned another idea: when the lower edge of the occluder is interrupted, they should be able to see an object behind it. It is not until 3.5 months of age when infants expect that they will be able to see tall objects behind a short occluder.

But other researchers suggested that there might be flaws in Aguiar and Baillargeon's system. Perhaps babies don't develop rules, but instead file "videotapes" of events they have seen before, and if they see a new event that appears to match the "tape," they aren't surprised. Younger babies haven't had as many experiences, so they probably just haven't seen the more complicated occluders before. Another explanation suggested that babies were just focusing on different parts of the objects as they passed behind the occluders.

Luo and Baillargeon's new experiments addressed these concerns by presenting the opposite scenario. Instead of testing whether infants were surprised at impossible events, they tested for surprise at possible events. They worked with 3-month-old infants, which Aguiar and Baillargeon had previously found were surprised when an object did not appear in a window at the bottom of an occluder, but weren't surprised when an object did not appear in the window at the top of an occluder. This time they showed two events -- the impossible event from Aguiar and Bailargeon's study, and the possible event, where the object did appear in the window as it passed behind the occluder. Here are the results:

The light blue bars correspond to the original Aguiar and Baillargeon study: as before, 3-month-olds are surprised (measured by looking at the display longer [see this article for an explanation of this method]) when an object does not appear in the window at the bottom, and not surprised when it doesn't appear in the window at the top. The new data tested the reverse: when the object does appear. Now infants were not surprised when the object appeared in the window at the bottom of the occluder, but were surprised when the object appeared in the window at the top of the occluder. In a similar experiment, they found a corresponding effect for 2.5 month-olds.

Luo and Baillargeon argue that these results offer additional support for the model that Aguiar and Baillargeon developed in the original experiments: as infants get older, they learn more and more rules about how objects behave in relation to one another, which allows them to develop a more sophisticated representation of the physical world.

Luo, Y. & Baillargeon, R. (2005). When the ordinary seems unexpected: Evidence for incremental physical knowledge in young infants. Cognition, 95, 293-328.

Aguiar, A. & Baillargeon, R. (2002). Developments in young infants' reasoning about occluded objects. Cognitive Psychology, 39, 116-157.