When you pour yourself some coffee, how do you determine which object is sufficient for containing the liquid? When the coffee is poured into an object, such as a mug, what tells you that the liquid has been contained within the mug? The coffee has, after all, disappeared from sight. So what makes you believe that it is still present?
These are the kinds of questions about objects that we don’t consciously think about. We know that coffee stays in the mug because mugs are nonporous containers. However, when we pour ourselves some coffee, we don’t think about such explanations. We act automatically, allowing the innate knowledge of the brain to work as our guide.
When organisms, including humans, encounter statistical regularities, natural selection builds such information into their brains, making it an integral part of the survival system. What is innate in this situation is the mechanism for learning about a specific domain of knowledge, not the knowledge itself. Thus, the learning mechanism filters the experiences, guiding the organism to attend to some events in the environment, but not others. Returning to our earlier example, we are born with a learning mechanism that allows us to recognize objects such as coffee mugs and make predictions about their behavior, whether they will move or stay in one place, fall apart or stay together, and so forth.
When a human infant sees a new toy, she reaches for it. Novelty is interesting and draws the infant’s attention. Now play a little trick. As the infant reaches toward the toy, place a screen in her path. Not only will she drop her arms, but she will stop searching for the lost object. This finding, discovered by the famous psychologist Jean Piaget, was used to argue that infants under the age of nine months lack object permanence-the capacity to represent an object in mind when it is no longer in view. Using the reaching task and several other experimental manipulations, Piaget found that infants’ knowledge of objects is impoverished before they reach their first birthday. As children grow older, they build on the experiences of seeing, touching, and moving to construct the domain of object knowledge.
Piaget’s results on object permanence, and object knowledge more generally, have been challenged on theoretical and methodological grounds. If object knowledge is constructed in each child as a result of personal experiences, then each child should reach an understanding of objects at different developmental periods. Children growing up in cultures with frequent opportunities to interact with objects should acquire an understanding of objects at an earlier age than children given fewer opportunities. In contrast, if an innate mechanism is at work, then such varied experiences will have little effect on the principles of object knowledge.
Cross-cultural analyses of child development reveal universality in the timing of object knowledge. For example, most normal children begin reaching for hidden objects at around nine months. Furthermore, infants’ knowledge of objects is far more sophisticated than what would be expected from the experiences they have encountered. This suggests that for human infants, the acquisition of object knowledge is facilitated by a set of principles, innate mechanisms that guide the learning experience.
Piaget designed his experiments to elicit explicit responses such as reaching. But if the child is immature, reaching may not be a good measure of what she knows. To find out what human adults know, we typically ask questions. Although our linguistic responses may not always provide accurate representations of our thoughts, we generally assume a high degree of correspondence between what we know and what we say about our knowledge. Unfortunately, we cannot use language to pose similar questions to infants. Researchers have met this methodological challenge by drawing on a technique that does not require reaching or language. Rather, it draws on the logic of magic and the simple fact that we look longer at events that intrigue us.
When you watch a magician saw a body in half or cause a ten-ton box to levitate*, you are drawn in, determined to figure out how the trick is pulled off. You know it is a trick, not because someone told you so, but because you have an understanding of objects. When you see a solid object levitate, your expectations have been violated. Solid objects can move in space only if they are launched, and they can remain stationary in the air only if they have an internal mechanism that allows them to do so. When we watch a hummingbird do the equivalent of Michael Jordan’s pre-slam dunk space walk, we are not surprised because we know that this ability emerges from the speed with which the wings have been designed to move. In contrast, when we see a magician move his arm and cause a chair, watermelon, or human to hang in space with no strings attached–we stare. More generally, whenever we witness events that violate our expectations, we focus our attention and do so because we are surprised. Events that are consistent with our expectations, however, enlist much less attention. This is the crux of the research method used with infants, what those in the psychology trade call the ‘expectancy violation procedure.’
In 1985 the psychologist Renee Baillargeon designed an object permanence experiment for three to four-month-old infants using the expectancy-violation procedure. Infants watched a solid screen rotate through an arc of 180 degrees until they were bored and looked away. Baillargeon then introduced a wooden block in the path of the rotating screen, and set up a curtain to hide both the block and the lower part of the screen. If infants are able to maintain a representation of the block in their mind, then when the screen rotates toward the block, they should expect it to stop at some point. In other words, although they can’t see the block, they should expect the screen to make a rotation of less than 180 degrees; the screen should stop at the point of contact with the block. This pattern is expected on the basis of a core principle of the object domain: one solid object cannot occupy the same space as another at the same time. In one condition, the screen stopped short of the full 180-degree rotation, whereas in a second condition the magic trick the screen rotated straight through the 180-degree arc. The magic trick represents a physical violation because the screen cannot move through the block.
If three-to four-month-old infants exhibit an understanding of object permanence, as Piaget claimed, then the magic trick isn’t magic at all: once the curtain is in place, the block is no longer present in the infant’s mind, and thus the screen is free to rotate through. In Baillargeon’s experiment, therefore, a Piagetian would expect infants to look longer when the screen rotates less than 180 degrees because this represents a novel rotation. If, however, the infant has some understanding of object permanence, then the 180-degree rotation is surprising because it violates a physical principle; infants are expected to look longer at violations than at events that are consistent with the physical world.
Infants look longer at the magic trick. Their eyes are telling us that they have detected a physical violation. More specifically, three-to four-month-old infants appear to understand that an object that is out of sight is still in the mind. Scientists have yet to determine whether this kind of knowledge about object permanence is different from what infants know by the age of nine months, when they begin reaching for hidden objects.
*nonporous: not allowing liquid or air to pass through.
*levitate: to rise into the air and float, in apparent defiance of gravity.
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