What to notice:
The four center bars are always vertical and straight, and they do not physically change, but the bars appear to wiggle as the surround rotates.
You can slide the lever to change the spacing between light and dark in the background circle. The different-sized bars make the bars wiggle differently. You can also press the button to see what the four bars look like when the background circle is not present.
Brief Comments: As I have said before, many illusions capture our attention because they violate our expectations about how objects behave in the world. In the real world, straight bars don’t typically wiggle when the background changes, but here they do. (Well, I suppose you could argue that a stick appears to bend when you put it into water, but that has an entirely different cause than this illusion).
In many respects, the effect is similar to the Fraser illusions that I wrote about in my last post. The local information (the contrast between the bars and background) indicates that the bars wiggle; the global information (the bars themselves) tells you that the bars are straight. Here, though, the effect seems to be due not only to contrast but also to brightness changes induced into the bars from the surrounding field. (This is a type of grating induction—you can find some excellent research on grating induction at the webpages of Barbara Blakeslee and Mark McCourt at North Dakota State University.)
We seem to expect the global information (i.e., the information about the bars) to be correct and invariant even though the local contrast is equally real. To explore the effect of the contrast, I have included another version of the illusion that allows you to spin the background at your own speed. I find it most compelling to move the background between -45 deg and 45 deg.
I have been a little neglectful of the blog for the past few weeks—sorry. I was out of town, and classes have started.