Casual Fridays: Troxler's not for everyone

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)

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You know, I'd never seen the inside of the ring change color with this illusion before this version. I guess I can't go back and find out which version did it for me, but do you have stats for which ring version gave the most people that effect?

Interesting. The interindividual variability differs markedly between illusions and is often poorly understood, which is of course related to that we don't really understand most illusions themselves. Here, however, I suggest that the interindividual variability can be traced to eye movements. People are very different in their fixation behaviour, and very steady fixation is required for a Troxler effect, _especially_ if the contours are sharp rather than gradual in their luminance profile.

Perhaps some for whom the ring did not disappear might experience it when they understand about the role of eye movement here. Also, I find that the instruction to "stare intently" may lead or more fixation tremor than the idea to "let you eyes hang" (in the correct gaze direction, of course ;-)). This removes the stress with the potential reduction in eye tremor.

Firstly, I think the phenomenon is also related to your observation distance to the picture. For me, the phenomenon is more fast to appear when my eye is not so near nor not so far from the picture. It seems too close distance or too far distance could weak the appearance of phenomenon. It might be caused by our individual visual system limitation. Furthermore, this phenomenon might be more significant for the people owning a more focal attention. On the other hand, people whose attention is more global should be difficult to detect or slowly to detect this effect.

Could we get the similar result when we use one eye for each comparing with using two eyes? :)

I only tried this illusion just now, and on my extremely high-resolution screen, where the image appears to be very small, the ring doesn't disappear at all. I had a hunch that the apparent closeness of the ring and the dot were making a difference, since I've successfully experienced a similar illusion in a book of similar stuff, so I enlarged the image to the size that it probably appears on the average viewer's monitor or LCD. And then it mostly/partially disappeared. Whee!

I call shenanigans on the majority of the "Yes, a little bit." responses; I suspect they are lazy with focusing on the center dot/letting saccades get out of control. I'll come out of the anonymity closet and say it fully disappeared for me. The basis for my accusation is the fact that my own attention-starved eyes kept hopping around, and my answer was going to be/could easily have been "a little"... until I really metafocused (can coin that? focused on focusing?), at which point it fully disappeared.

This explains why I no longer see ads on the internet :O)

By Berry Zito (not verified) on 05 Oct 2009 #permalink

This actually amazed me and I know different people see different things but I felt I saw something unusual. The ring was vibrating for a start and I saw phases of both partical and full disappearance. Then there was the appearance of a 'light shadow' of a ring on top and blinking away it turned pinky red. Also, after seeing 71 (which is neither 74 nor 21) I am wondering if I landed on what of the UFO some of you spotted!

I tried to post a comment to this thread several days ago now (Saturday or Sunday I think), and got a message back that it had been held for moderation. Presumably that was because I did include a (single) link in the body of the comment. However, it was in no way spam. The link was to another illusion, on a non-commercial site, that is presumably related to the Troxler illusion, but also different in some interesting ways that I wanted to comment on and ask about.

I expected the comment to appear the next day, or maybe the day after, but it has still not appeared. Should we now expect that all comments containing links will automatically be trashed? If so, I think there ought to be some warning to that effect that might stop people wasting their time.

Bryan, I think you are correct. I was unable to force my eyes to hold still for 15 seconds--every time I started to see a 'little bit' of fading my eyes would shift slightly and the ring would snap back into view.

By Dave Empey (not verified) on 10 Oct 2009 #permalink

Lori, the "light shadow" you describe is negative palinopsia. I think you'd find that if you stared at the green ring (not the red dot) for 30 seconds or so, and then transferred your gaze to a blank white space, the pinky-red ring would appear. Positive palinopsia is a lot less common and is when the ghosted image is the same colour as the original.

Like Bryan, my eyes moved too much to effectively see the fade for long. I also tried focusing on focusing, but could not even maintain that for particularly long. (I may possibly have undiagnosed Attention Deficit Disorder, and the inability to stay focused on something when it holds no interest to me is an old familiar foe.) My attention and therefore my eyes kept darting to the ring itself, and as my focus slipped before I dragged it back on task, the familiar negative palinopsia appeared in lurid pink at the margins of the ring, jittering and pulsing and seeming to rotate around the ring as I struggled to pull my wayward eyes back to the damned red dot. Even at the stillest I could get my eyes, my focus inevitably drifted down, brought to my attention by the herald of that pink ring.

Even maintaining a loose focus, after reading the comments, did not help the ring disappear completely, although it did fade considerably. I noticed the white areas of the screen appearing greenish, rather than their wonted white, and the red dot appeared to bleed its own color into the area surrounding it.