# Casual Fridays: Why most people didn't see this illusion

Just about two weeks ago, I posted this visual illusion (if you haven't seen it yet, make sure to watch it with the sound turned ON):

How many flashes do you see?

In fact the dot only flashes once, but according to the study I report on in the post, the two beeps are supposed to throw you off. If you are like Shams et al's participants, you would most likely see two flashes, not one. In fact, just 23 percent of CogDaily readers reported seeing two flashes.

Much speculation ensued in the comments, but I had a couple ideas of my own, so last Friday I developed a quick study to test two possible explanations. The first explanation is that there is a practice effect: the more you see the illusion, the more likely you are to experience it. The second explanation is that clicking "play" distracts you from the display, and you aren't focusing enough to experience the illusion. My study tested both explanations.

Participants were divided into two groups; each group would see the same 10 displays, but one group would have to press play each time, while the other group would have the displays play automatically. The actual illusion was only shown three times; the others were distractors where the dot flashed twice, or there was one or no beeps instead of two. So, was there a practice effect? Take a look at this chart of the results:

Doesn't look like a practice effect. The number of flashes seen remained quite steady whether the illusion was seen at the beginning, middle, or end of the experiment. So practicing with the illusion doesn't appear to affect whether it's seen: either you see it or you don't.

But what about pressing "play"? If you have to play the movie yourself, are you less likely to see the illusion?

Apparently you are! An average of 1.37 flashes were seen when the movie was self-played, compared to 1.55 when it played automatically. Interestingly, when we initially ran the poll on the illusion, 1.31 flashes were seen, so our slightly more formal study matches the initial poll quite well.

So why don't viewers see an average of 2 flashes, as in Shams et al found in their original study? I have a couple more thoughts. First, in the initial study, viewers were told to focus on a point some distance away from the site of the dot flashing; this is because the illusion is stronger when viewed with peripheral vision. Second, there may be some hardware / software difficulties. In the lab, Shams' team could ensure that everyone saw the same illusion, but online, it's much more difficult to control for everything.

One commenter thought that perhaps my instructions about having the sound turned up weren't clear enough, so halfway through the experiment I added more explicit instructions. Was there a difference in the results after I improved the instructions? No.

Another commenter suggested that the refresh rate of users monitors may differ. Indeed, Shams' web site offers several different movies to account for this possibility. But there could be other problems as well: distracted users, software incompatibility, or even taking the test in a noisy room could affect the results.

So what's the ideal movie for viewing the illusion? Barring hardware and software problems, it should force viewers to focus away from the center of the visual field, and it should play the movie automatically. The link below should take you to such a movie. Just make sure you focus on the cross when the movie starts to play:

And, of course, don't forget to come back next week for another Casual Fridays study.

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this is cool!!!
my (fake) theory is that you blink when you here the click. i don't really know the real reason is because i didn't read the whole article. nevertheless it's still cool.
~nora

I totally saw two that time.

knowing about the illusion probably causes you to be more immune...and we certainly knew a great deal baout it even by the time of the first test.

By Katherine (not verified) on 03 Nov 2006 #permalink

The illusion _sort of_ worked for me. Without reading the rest of the article or knowing the nature of the illusion, I caught myself wondering if I'd seen it once or twice, and played it again to recheck.

But then again. I've been supervising/directing animation for the past year, so I'm getting very sensitive to visual artifacts happening for 1/24th of a second. ;)

Katherine,

We actually asked participants whether they had seen the illusion before, and there was no significant difference between those who had seen it (average perceived 1.44 flashes) and those who hadn't (average perceived 1.47 flashes).

Though, oddly, in the autoplay condition, there was a difference for the third viewing of the illusion, where those who hadn't seen it saw an average of 1.60 flashes, while those who had seen it saw an average of 1.37 flashes. I can't for the life of me figure out why seeing it a week before would affect the third viewing of the illusion but not the other two, and why only in the autoplay condition.

Nice stuff. I did 'see it twice'. Though the experience is a bit odd, because it as Left Wing Fox said, 'it appears as if you see it twice, but you're not really sure'.

Is this some kind of modality interference? Information on one modality (audio) interferes with encoding of information on another modality (vision)?

Pretty cool :)

By Fuzzy Logic (not verified) on 05 Nov 2006 #permalink