Cognitive Daily

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


  1. #1 Jared
    November 24, 2008

    I always thought we naturally simulate the rest of the “incomplete” pictures. Our brains tend to extend out unfinished patterns with structures we are familiar with such as the bottle, chair, and glass in the first series and hair, Tower of Pisa, etc. Couldn’t this “false memory” actually just be an extension of our brain’s simulation software extrapolating from the actual visual input?

  2. #2 Brian
    November 25, 2008

    The demonstration video seems more about change blindness than about how quickly boundary extension occurs. If you switched the order so that the narrower picture came second, you also wouldn’t notice a difference. Not because of a rapid anti-boundary extension effect, but because of change blindness.

  3. #3 Paper Hand
    November 25, 2008

    I had to look at those top two pictures several times before I realized that there even *was* a difference between them!

  4. #4 Dunc
    November 25, 2008

    Where exactly is the line between perception and memory? 😉

  5. #5 Dave Munger
    November 25, 2008


    No, you would be more likely to notice a difference if the pictures were switched. Indeed, you’re likely to “notice” a difference if the pictures are the same.

  6. #6 Coki
    November 25, 2008

    Isn´t this related to the Gestalt? We tend to complete the images in our minds and understand them as a whole.

  7. #7 Jen
    November 25, 2008

    I agree with Brian.
    I would like to see the two pictures switched in order in that demo.
    You probably won’t notice a difference as long as there’s masking.

  8. #8 Dave Munger
    November 25, 2008

    Jen and Brian:

    I can put up another demo (which I may very well do) but it’s never going to be as well-controlled as the original study, which clearly finds the effect.

  9. #9 Bowser
    November 25, 2008

    I don’t think it’s appropriate to be calling this a false memory. As others have mentioned here, boundary extension (lots has been done on it by Helene Intraub) is more a perceptual than a memory phenomenon. A false memory is more than a simple mistaken conclusion about the content of a photograph. There are events, history, contexts, etc that are all reconstructed in the process of memory recall. Boundary extension is really more a perceptual effect than a memory effect. It’s just that we employ the same process of reconstruction when describing the picture, and because we do naturally complete objects etc, that tendency comes into play when describing the scene in the extension picture.

    As for “the line between perception and memory”, while not completely distinct, it’s pretty easy to find if you look for it. It’s been inconsistencies in terminology that has lead to confusions over where one starts and the other stops. Going back to Sperling’s whole and partial report studies and perhaps further, the term “iconic memory” has seen widespread use.

    However, this really only refers to the persistence of the stimuli in the receptors before it is overwritten by new stimuli. While the contents can be accessed while present, to call it memory is to misunderstand what it is. We can’t rehearse information in it as with short term/working memory, we can’t encode information into it as in short term and long term memory etc.

    It’s true that our expectations influence our perceptions, but visual inferences or ‘filling-in’ if you like occurs at a lower level in perceptual processing, one below our conscious control.

  10. #10 Erik Giberti
    December 1, 2008

    Found this article linked from Twitter. It seems clear, as others have said, that our minds are simply completing the images we see. In my head I have a clear image of what a coke bottle looks like. The cropped bottle in photo A is simply completed in memory yielding photo B. Amazing stuff though.

  11. #11 annie
    January 30, 2009

    i think its neat you were able to show that. even when i was looking at the pictures side by side i didn`t notice a difference at first. not only did my mind account for the expansion but minds tend to leave out lots of things some times. our minds are conditioned to simplifing information. it will subconciously notice differences and make the changes that simplify something a little more complex. our minds take in so much information at one time they have to dumb down. ex: when you sit in a chair your not thinking about the hard cold seat, or the temperature of the room, or who ever is talking…all at the same time. your mind is conditioned to retain that information, and pretty much for get about it. if its not causing harm or pleasure it is set aside. minds can`t handle but so many things at one time. cute video though i was distracted by the wierd picture between shots…=)

New comments have been disabled.