When you look out the window and then look away, how do you remember what you saw? Do you hold a picture of the window in your head, frame and all? What about a photo? Do you remember the physical photo, or do you imagine the real scene it represents? If you remember the scene, and not the photo, then how do you form the boundaries of the scene? Does your memory end precisely where the photo does?
Here’s a little test to see how accurate your short-term memory of a photo is. When you play the movie below, you’ll have a second to get ready, then a photo will flash for just a half-second. It will be replaced by a random pattern for two seconds, and then the photo will reappear. It might be the same as the original, or it might have been cropped or enlarged slightly. Your job is to say if the area depicted in the second photo is the same, larger, or smaller than the original.
Click to play movie (QuickTime Required)
What do you think? Let’s collect the answers in a poll (don’t repeat the movie until you’ve responded):
Even playing the movie repeatedly it might be difficult to tell, so I’ll display both photos side-by-side at the end of the post. What we’re exploring here is a phenomenon that’s been investigated for several years by Helene Intraub and her colleagues:
It’s called boundary extension, and it has been robustly found in a variety of circumstances — even in blind and deaf people. Intraub believes the phenomenon is related to the way we construct memories of scenes. Rather than remembering scenes on a pixel-by-pixel basis, we remember just enough information to reconstruct that scene later. Since items in a picture may extend beyond its border, our memory, too, usually extends beyond the boundaries of a picture. If we see the same picture later on, we usually believe it has been cropped. Did I crop the picture in the movie I showed you? I’ll let you know at the end of the post.
Even when we see a picture for a very short period of time, and even when the picture is removed from vision for just two seconds, boundary extension is still observed. In a new experiment, Intraub’s team asked viewers to focus just on the center of the photo (and tracked their eye movements to make sure they did). Even in this case, significant boundary extension occurred — viewers believed the picture they saw was 50 percent bigger in area than it actually was. When allowed to move the borders of an image to reconstruct the view they thought they had seen, each edge was extended by around 20 percent.
But perhaps the boundary was only extended because viewers weren’t allowed to look at it. To explore this possibility, Intraub’s team designed a second experiment. 250 milliseconds after the photo was displayed, an arrow appeared, directing viewers towards an object on the left or right side of the picture. Before viewers could move their eyes all the way to the object, the photo disappeared. Again, they were allowed to reconstruct the boundaries of the picture. Here are the results:
There was significant boundary extension on three of the four borders of the picture: the top and bottom, but also the side viewers were cued to look at. The only side where no extension occurred was the side where they didn’t look!
Boundary extension occurred precisely where viewers were looking. Thus, the researchers argue, it’s not due to inadequate information about boundaries, but an active process whereby our memory actively extends beyond the boundaries of a scene. In a third experiment, viewers were sometimes cued to look one direction or the other, and sometimes cued to remain focused on the center of the photo. When focus remained on the center, there was no significant boundary extension to either side of the picture, but the top and bottom boundaries were still extended.
So boundary extension does not occur when we’re actively “not looking” in a particular direction, but it does occur when we’re looking in a particular direction. This again supports the notion that boundary extension is an active process of the mind, and that our memories are actively constructed, rather than mere passive reflections of reality. In other words, you make your own memories; they aren’t made for you.
So what about the photos I showed you in the movie above? The second photo was zoomed out 5 percent wider on each side, for a total of 21 percent greater area depicted than the first photo:
So even if you thought that the two photos were the same, you were still showing boundary extension (and, of course, if you said the second photo was cropped, you were clearly extending the boundary).
Intraub, H., Hoffman, J.E., Wetherhold, J., & Stoehs, S. (2006). More than meets the eye: The effect of planned fixations on scene representation Perception & Psychophysics, 68 (5), 759-769