Whenever I create a demo for this site, there’s always a balance: Do I make the demo dramatic, so it’s most surprising when it works? Or do I make it less dramatic, so it works for more people? (There are other things to balance as well, such as my time and technical proficiency)
The Troxler effect is a particularly dramatic and simple demonstration of how much our visual system adapts to the environment. Just stare at the center dot in this figure for about 15 seconds, and the outer ring should fade completely from view!
But the illusion, like nearly all illusions, doesn’t work for everyone. Some people only see a partial fading. For some people, the center region turns green, but they still see the outer ring. Others see a variety of effects, from double-vision to a pulsing, color-shifting mess.
Last week for our Casual Fridays study we presented a variety of different versions of the illusion, along with another illusion, a “color-blindness” test, and a few semi-random questions about habits and tendencies. We thought we might find a pattern in the responses. What we certainly did find is a lot of variation in responses. This graph shows the response for the image I showed you above:
Fewer than half of the respondents experienced the illusion completely, but almost 80 percent had some experience of the effect.
But we also showed subtler versions of the figure. Here are three more versions (I reduced them to fit on this page so you probably won’t be able to try the illusion).
I converted the viewer response to each figure into a score (3 = ring completely faded, 0 = no fading at all). Here are the results for all four versions of the figure:
The results were significantly different for each version of the figure. The light, blurry ring gave the best results, with over 70 percent of respondents seeing the ring completely disappear, and over 90 percent experiencing at least a partial illusion. But of course it’s not as dramatic to see a rather faded ring disappear compared to the original, dark green ring.
We also showed some viewers a different-colored ring: blue, instead of green. There was no difference in the responses, except for one interesting trend. We gave respondents a quick “color-blindness” test:
If you see the number 74 in this figure, then you probably have normal trichromatic vision. However, if you see the number 21, then your vision is likely dichromatic and you may not be able to distinguish between green and red. Dichromats saw the illusion more often when it was presented in green than when it was blue, but this difference didn’t rise to the level of significance.
We asked a few other random questions, but the results didn’t affect whether they saw the illusion. Results are presented here for your amusement only:
(Just a reminder: All Casual Fridays studies are non-scientific. This doesn’t mean we can’t use scientific principles to assess what’s going on, but we can’t make general claims based on the results)