Our ability to see change over time

[originally published March 2, 2005]

Take a look at the following movie (quicktime required). The movie will alternately flash a picture of a desk and a patterned block. Your job is to see if anything about the picture of the desk changes each time it flashes. Don't replay the movie when you get to the end; just stop.

Did you notice any changes? Most people won't spot any changes at all when they watch this movie the first time. But watch the image as you press play again, and you'll see that the desk has changed significantly from the beginning to the end of the movie. I actually rotated it by two degrees at each point along the way. You can drag the handle underneath the movie to fast-forward through the whole thing and convince yourself that the desk was smoothly rotating the entire time.

Andrew Hollingworth of the University of Iowa and John M. Henderson of Michigan State discovered this phenomenon and explored it in "Sustained Change Blindness to Incremental Scene Rotation: A Dissociation Between Explicit Change Detection and Visual Memory" (Perception and Psychophysics, 2004). They found that even though people could easily distinguish between the beginning and end positions of the movie, people can't recognize the change when it is done as a series of small changes (People are able to recognize even small rotations when images are displayed without a "pattern mask" inbetween. By using the pattern mask, we can test people's memory for the position of the objects. Detecting motion is a separate perceptual task from visual memory).

So what is the nature of our visual memory? We clearly remember something about the scene we are shown: we know it's a desk, for example. Perhaps what we're doing is remembering the first position and then comparing it to each subsequent position; the reason that we don't recognize the change each time is because the memory degrades over time. Hollingworth and Henderson devised an experiment to test that question. They showed movies where the desk rotated as in the movie above (but with a nicer picture, and using 1-degree increments). When the desk reached a rotation of 20 degrees from the original position, the pattern mask was flashed one more time, and then the desk was returned to the original position. Participants immediately noticed a change. So clearly, they weren't simply comparing each image to the original picture: from their perspective, what seemed to happen is they were watching the same image over and over again, and then it suddenly shifted 20 degrees counterclockwise.

Somehow, as the image was shifting, so—unconsciously—was the viewers' memory of the image. Why are we unable to detect these shifts in our memory? Imagine a world in which we could. Every time we turned our heads, even slightly, we would seem to see the world changing around us. This, of course, would be a false perception: most of the time, the world is not moving, we are. So we are better off assuming that the world is stable.

There are some interesting connections between this research and the Whitney study I discussed yesterday. Clearly a lot of our visual system is adapted to handle the fact that we are constantly moving through a basically stable world. Both of these studies show that there is a tremendous amount of visual information that our minds simply ignore, in order to build a more coherent picture of the world.


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I noticed very much that it was moving, because one of the things I took note of in the beginning was the fact that the desk was oriented nearly horizontally in relation to the screen. In the original study, did the participants know that they were meant to look for changes in the desk? One pays more attention knowing that, I think.

I definitely noticed it rotating, and I did not take note of the original position. In fact, when I got to the end, I was not 100% sure I was right about it rotating, but I felt pretty sure I had seen it move. So, I could not remember what position it was in originally at all, but as it flashed I saw it rotating to the right :)

Well, I don't know what's wrong with me because I spotted the rotation by the third or fourth frame.

However, one thing I did that may have helped is that I went into something I might call "far focus" instead of spending time focusing on pinpointing any particular part of the desk. Instead, I tried to gestalt it, and was kind of looking beyond it to get a better big picture look.

Doing that somehow made it quite obvious (and yes, this was on my first viewing).

One does pay more attention with knowing but having administered and been administered change detection tasks, I can tell you that change blindness is remarkably resilient. Heck, I'll admit to being completely oblivious to things most humans seem to notice, but it was still impressive when I still couldn't see a change in a natural scene after an experimenter told me that there was a change, told me which quadrant the change was in, and gave 3 minutes in front of an image that switched between two frames (no mask) every 500msecs.

Because I'm too lazy to find an actual citation at the moment, I found a recreation of Rensink's change blindness paradigm online instead: http://www.usd.edu/psyc301/Rensink.htm. You can change the speeds of the frames, as well as the scene, by right clicking on the applet. I grant that this is change blindness in objects in natural scenes (which are far more demanding on attention), rather than in orientation, so perhaps... I'm just really, REALLY oblivious. :)

It's possible that changes in orientation would be more immediately noticeable if it were a natural scene. A chair that started flipping around in, say, a desk scene seems like it'd be much more obvious... but then, you'd think that about entire people and palm trees, too.

Wow! Great post. Can someone make an argument about what kind of "memory" is tested by this task, and whether you would expect to find individual differences in "jnd" (just noticeable difference) of rotation?

If you think there would be correlations, what would it correlate with?

This experiment seems only to be demonstrating that people tend not to notice small changes, unless, of course, they are looking for them. Expectations are clearly important, as is made clear by the commenter above who said they noticed the rotating. If you expect to see rotation then it's much easier to notice. Personally from the introductory paragraph I had assumed that the desk would change its shape or configuration in some way and so I saw nothing.

The authors use this evidence to support the idea that visual memory is somehow separate from whatever mechanism is involved in change detection. An obvious alternative hypothesis is that we don't notice the change at all during the incremental stage - even at an 'unconscious' level. But, when we see the big counter-clockwise movement, we realise its been moving all along and our mind fabricates a plausible explanation.

In other words: how do we know visual memory is really being 'implicity updated' as the authors claim?

This makes sense to me. I am not a scientist, but several years ago worked with a couple or neurobiologists at Berkeley in an attempt to commercialize an improved codec for processing images. These guys were working on chemical reactions within the retina, which led them to try to isolate the signal sent from the retina through to the brain (this is where I learned the retina is simply an extension of the brain). They were successful (in case you were wondering they used frogs) and isolated a very simple signal that could incorporate extremely simple and very complex images. The signal helped us identify the "most important" visual information that, through evolution, was naturally optimized. It was extremely cool and interesting.

The implication after looking at this stuff for awhile, and hence back to your observation, was how much of what we "see" is actually visual memory within the brain and how much of the brain is apparently devoted to this -- perfectly consistent with observers not noticing slight shifts in objects.

The masking prevents continuous tracking of movement, so no rotation could be tracked. Instead it simply relies on memory, if you had taken notice of the original position of the desk or not. Since I was expecting the colored details to change, I watched these and had no awareness of the orientation of the desk at any point. Someone observing the gestalt of the whole image, (i.e. the "forest" rather than the "trees") would notice.

Have you read James Gibson's Ecological works on the philosophy of vision?

It's underrated, fabulous and explains this phenomenon as well as many others. Based in evolution, it assumes a complicated environment and that seeing is to inform action. Only once symbols, pictures and language enter the picture does Gibson think we manage to get quite so confused about how vision happens, and his solution for that is also quite simple and flawless.

Really fascinating. I knew that it had moved and yet I wondered if it had not.

Part of the answer is inattention blindness (remember the basketball players and the woman in the gorilla suit that walks in front of them while you're counting passes). I surmise that when we are not told where to focus ie. what is going to change, we instead look for changes in what we think is most likely to change. In my case I didn't notice the rotating at first, but once I saw a couple of flashes and noticed that the desk wasn't changing shape, colour, attributes, etc. I looked for something else and only then did I start noticing that it was rotating.

I used to have treatment goals that were much more idealistic, but, over time, as a psychologist, I've learned that change, in human beings, is slow at best, interventions aimed at initiating change must be introduced incrementally. Patterns of abusive behavior and self-destructive behavior are especially difficult to extirpate, as I've indicated in the following song about a victim of domestic violence:

Mrs. Jones, Mrs. Jones, Mrs. Jones
words and music by Dr. Bruce L. Thiessen, aka Dr. BLT

aka Dr. BLT
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