Thursday, September 4, 2008

Four Bars: wiggle wiggle

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.


Will said...

No need to apologize. I'm glad you're posting again! This was interesting and informative, as usual.

Anonymous said...

I notice that the spinning bars in the background change size--they are widest when they are horizontal. Is this necessary for the illusion to work?

opiated said...

Kip, I dont think the bars need to change size as they spin. This should be evident by looking at the illussion where the background is stationary.

Anonymous said...

great post. It is an interesting illusion.

Barbara said...

This raised several questions for me and I would really like to know what you think:

I didn't see the bars wiggling until I read the blurb below telling me about it, but then it was really hard to dispel the illusion.

(And once I perceived the bars wiggling, I was taken in by the other illusions, don't know if this would've happened without the first illusion taking hold).

So I'd love to know, have you experienced other people not 'seeing' the illusion? If so, I wonder if those people have any other traits in common?

I'm particularly interested because my take on it is, as a trained visual artist, I'm in the business of seeing, and to draw illusionistically (or create an illusion by other means), I have to see without illusion. I wonder if all people with a strong drawing training would have a similar experience.

I noticed at art college that although I was with extremely bright people, most of them were dyslexic to some degree. And since my training I respond to written language differently too.

I'd really love to know if there's any been work done on the effects of training on (especially visual) perception. Maybe you would kindly post an answer to the board, or some links relevant to these questions?

And thanks for this fascinating site!

Anonymous said...

Hello there , my name is Ryan Seah. I 'm from Malaysia and i am doing a research about optical illusions on my assignment in college and could really use your help. Can I have you e-mail so we can communicate a little smoother ?? Please ?

Unknown said...

I was drawn to this page by receiving a traffic ticket this morning. I have lived in the same town (Laguna Beach, CA), eaten at the same breakfast place and made the same legal u-turn to come home for over 10 years.

Two months ago three signs were prominently posted that would now prevent a left or u-turn. One was on the right side of the road, one attached to the light standard above and one on the left. There were very few cars on the road and the officer said that the sign was there to aid in traffic control in the summer. I never saw the signs.

When pulled over, I had no idea why. When I went back to look, the signs are indeed there. For two months they gave only warnings, but now we are in month #3.

My question is whether there is any study that I could present to the judge that would add credence to the claim that I reasonably couldn't have seen this new addition to the scene after 20 years of driving the road and over 10 years of frequenting the same breakfast establishment.

Any help would be appreciated. The mere fact that the motorcycle policeman is hiding in the bushes of the street where they recently added the signs seems to indicate that they are less concerned with public safety than revenue generation.

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