Activity Description
A movie plays showing a blue stick moving horizontally, back and forth, behind a purple block. The top and bottom of the stick are visible but the middle of the stick is covered up by the block. The user is asked to look at the moving stick display and then click to continue.
The next screen shows the same display of the blue stick moving back and forth behind the purple block. The user again clicks to continue.
The next screen shows the same display, again, for the third time. The text reassures the user that this is not a bug on the website. The user again clicks to continue.
The next screen shows the blue stick moving back and forth, but without the purple block covering it. With the block removed, it is now clear that the stick is actually broken into two perfectly aligned pieces with a gap between them. The purple block was covering up the gap between the two pieces of the stick, making it look like a single stick when it was, in fact, two sticks.
What’s Going On?
You just participated in an adult version of an experiment usually done with human infants as participants. The experimental procedure is called habituation, and it relies on the universal human instinct to become bored with anything you encounter over and over again. You probably interpreted the first movie as a single blue stick moving back and forth behind a purple block. The first time they are asked to examine the movie, most people look at it for a decent amount of time. As the experiment progresses, however, their looking times tend to go down.
After seeing the same movie three times, you saw a different movie, which you probably interpreted as two separate but perfectly aligned sticks moving back and forth (with no occluding block). Most people look longer at this movie than they did at the previous one, because they find it interesting that something has changed. An increase in looking time is a strong indication that observers can see the difference between the two movies.
Of course, it would have been much more efficient to simply ask you whether or not you saw the difference between the two movies. But what if we wanted to test a 4-month-old infant? Obviously, we cannot question such a participant directly. To get around infants’ lack of verbal abilities, researchers have used the procedure you just experienced.
Habituation Procedure
In the experiment demonstrated here, the infant participant was seated in his or her mother’s lap, facing something like a puppet stage. Once the infant was in a calm state, a blind was opened to reveal a stick moving behind an occluding block (like the movie at top left, although in the actual experiment the infant saw real 3-D objects). An experimenter sat behind the stage, looking through a peephole. When the experimenter saw the infant first look at the stick/block, he or she started a stopwatch, stopping the watch when the infant looked away from the action. At that point, the blind was closed and the infant was given a short rest. Next, the blind was opened again and the experimenter recorded how long the infant looked at the display during the second exposure.
This sequence of events was repeated until the infant was so bored that he or she barely glanced at the display. Then, the blind opened to reveal one of the two displays showing either a stick with a gap in the middle or a whole, unbroken stick. The experimental question was which of these two displays the infant would find more interesting. If the infant, like adults, saw the initial movie as a single stick moving behind the occluder, than he or she should have found the display with the broken stick more interesting, because the single stick had been transformed into two separate sticks. In fact, this is just what the researchers found: infants had a strong tendency to dishabituate (look longer) at the broken stick display, whereas when they showed infants the whole stick display, they looked about as long as they had to the initial display with the occluding block.
This result is interesting because the visual input in the first display contains no explicit signal that the top and bottom halves of the stick are actually connected. To perceive them as connected, the 4-month-olds must have already been using sophisticated strategies (such as the good continuation and common fate grouping principles) to organize the visual input into objects.