Boundary extension is a phenomenon we’ve discussed a lot on Cognitive Daily. It’s typically described as a memory error: We remember scenes as having bigger boundaries than what we originally saw. Take a look at these two pictures of Jim:
If you only saw picture A by itself, then later you’d remember seeing a picture that looks more like picture B. If you look at them side-by-side, it’s easy to see that picture A is cropped closer than picture B, but if you see the pictures separately, then it’s likely you’ll misremember the first picture has having broader boundaries than it really has. That’s boundary extension.
But how quickly does boundary extension occur? Very quickly, as this movie demonstrates:
Here I’ve separated the two shots of Nora by only 1/2 second. To my eye, they don’t look any different — it’s as if I saw the same picture twice. But the second viewing (with no filler image inbetween) shows that I actually extended the boundaries. If this is repeatable, can we honestly now say that boundary extension is a “memory error”? How quickly does boundary extension actually occur?
Helene Intraub and Christopher Dickinson showed volunteers similar displays to my example but took a more systematic approach to see if boundary extension really can take place in such a short period of time. Some of the time, the same picture was repeated, with either a 42-millisecond or 250-millisecond interval between pictures where a distracting pattern was shown. Sometimes, the boundary was extended (a close-up was followed by a wider-angle shot, as in my example). Sometimes, it was contracted, with the wider-angle shot preceding the close-up. Viewers were asked to rate the second picture in each sequence compared to the first picture. The scale ranged from -2 (much closer up) to +2 (much farther away). Here are the results:
It might take a bit of explanation to convince you, but this graph clearly shows that boundary extension occurred in both conditions. When viewers saw the same picture repeated, they were significantly more likely to say that the second picture was “closer” than the first one. This could only happen if they were expecting a wider image — that is, if they had extended the boundaries of the image. When the second picture was actually wider than the first, viewers rated it as less wide than an equivalently close-up second picture was rated close. Again, boundary extension occurred.
The 42-millisecond interval between the pictures was chosen deliberately: it’s the amount of time your eye takes to saccade (move) from one view to another. Your eye can only accurately focus on a very small area at one time — about the size of your thumbnail at arm’s length. To build an accurate representation of what you see, you need to reconstruct the results of many saccades. So this experiment shows that boundary extension can occur over the duration of a single saccade.
In a second experiment, Intraub and Dickinson showed a new set of volunteers the same images while their eye movements were tracked. The computer flashed the first picture, then flashed a pattern where the second picture would appear, on the other side of the computer screen. Viewers were told to look to that spot as quickly as possible. The computer detected the eye motion and displayed the second image before the saccade was completed. By the time viewers focused on the new spot on the screen, the new image had already appeared. Again, they were asked to rate whether this second picture was wider or closer than the first. The results were nearly identical to the first experiment. Boundary extension was observed occurring as quickly as viewers could move their eyes to focus on a new location.
Intraub and Dickinson say this suggests that boundary extension is a fundamental part of the process of visual perception. While it’s related to memory in the sense that memory is required to build a complete visual representation of a scene, it’s occurring literally as fast as we perceive the scene.
But they also say that this isn’t necessarily a problem. The “edges” of a photo or a scene are human constructions. The world doesn’t literally end where a photograph does, or at the edge of the window we’re looking through to perceive it. We function better, not worse, when we can readily imagine what lies beyond those boundaries. And we can, in just a fraction of a second.
Helene Intraub, Christopher A. Dickinson (2008). False Memory 1/20th of a Second Later: What the Early Onset of Boundary Extension Reveals About Perception Psychological Science, 19 (10), 1007-1014 DOI: 10.1111/j.1467-9280.2008.02192.x