Cognitive Daily

Casual Fridays: Why most people don’t see this illusion

Posted by Dave Munger on October 27, 2006
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On Monday, I posted a recently-discovered visual illusion with a quick poll to see how many of our readers could spot the illusion. As it turned out, not very many of them did. This was surprising to me, because the team that discovered the illusion, led by Ladan Shams, found that the illusion was very robust, experienced nearly all the time by most viewers. So why didn’t our readers see it?

I’ve designed a quick study to test a couple of hypotheses. Normally we restrict Casual Fridays to just 5 questions, but this time I had to stretch the rules just a bit — you’ll have 11 quick questions. It still should take no more than a minute of your time.

Click here to participate (make sure your SOUND is turned on)!

If you haven’t seen the illusion yet, please participate in the study first (if you have seen it, that’s no problem either; you’re welcome to participate). Then, if you’re interested in Shams et al’s research, you can check out our report on the illusion, and my follow-up post.

As usual, you will have until 11:59 p.m. Eastern time on Wednesday, November 1 to participate — or until we have 500 responses, whichever comes first. Then don’t forget to come back next Friday for our analysis of the results!

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Comments

  1. #1 Deacon
    October 27, 2006

    I just realized that one of the main stumbling blocks to psychological research is that the subjects “expect some sort of trick,” even though they might not be able to figure out what it is. It’s similar to those tests where one choice is “obviously” correct but the actual correct answer turns out to be the other option. After enough exposure to those situations, the subjects “expect a trick” and compensate for it when they answer, and that skews the test results.

  2. #2 peggy
    October 27, 2006

    You might want to tell people to make sure they have the sound turned on before they start. I usually surf with the sound off, and it was just chance that the sound was on when I took the poll.

  3. #3 Dave Munger
    October 27, 2006

    The instructions *do* say to have the sound turned on. I’ll add it to the main post, too.

  4. #4 Zachary Weiss
    October 27, 2006

    Very interesting illusion. It worked for me the first time around (I thought to myself when I watched the first video “that definitely flashed twice”) but as deacon said, I expected it to be more complicated than that. So I watched it again, and lo and behold, the illusion did not work for me anymore. I wonder how long I would have to abstain (perhaps ‘abstain’ isn’t the right word, let’s say ‘forget about’ instead) from looking at this illusion in order for it to work on me again…

  5. #5 Eric Irvine
    October 27, 2006

    I will also repeat that the first time I saw the illusion it worked, but upon viewing it again it didn’t.

    Maybe it has something to do with quick and novel information, or being surprised. Possibly the tectopulvinar (more primitive) vision system incorporates the sound into the interpretation more strongly than when the vision system is primed by knowing what the stimulus will look like?

    When things happen unexpectantly the brain has less time to incorporate information and react; since the tectopulvinar and auditory systems both converge in the tectum perhaps they incorporate reflexive interpretations of novel stimuli?

    … just hypothesizing.

  6. #6 Harlan
    October 28, 2006

    I suspect the type of monitor you use has a huge effect, as does the use of Quicktime to display the illusion. The software that cognitive psychologists use to do this sort of experiment synchronizes changes in the display with screen refreshes. I noticed that the Quicktime video does not, so (for example) the top half of the circle was visible for one more screen refresh than the bottom of the circle, making it appear more solid and putting a line in the middle of the circle. That’s a no-no in rapid visual displays. I use E-Prime for my experiments, and it specifically synchronizes the screen with changes in the display.

    Also, the latency and persistence of images on a CRT screen (like the one I’m using here) and LCD screens are very very different. LCD screens respond a little more slowly, and images fade much more slowly. People who do experiments requiring very precise or rapid visual displays tend to get CRT (not LCD!) monitors and drive them at 100 Hz refresh speed (instead of the usual 70 Hz or so).

    Either or both of these issues could make this Quicktime demo not work at all.