Cool Visual Illusions: The Flying Bluebottle Illusion


From Anstis & Casco, 2006, Movie 1, p. 1088

OK, here's a really, really cool illusion published last year, and that I learned about only recently. To see it, go to Stuart Anstis' page here, watch the first movie only, and then come back here.

You should have seen two flies moving in circles with the same radius. The flies' rotations are offset so that one is at 6 o'clock when the other is at 12, but otherwise, the circles they're tracing are identical. Now go back and watch the second movie.

As the caption notes, the two flies are still moving on identical circles, except that they're out of phase as in the first movie. However, moving the background, which is in phase with the left fly (and thus out of phase with the right fly) makes the left flies orbit look much smaller than the one on the right. In the published version of this illusion(1), Anstis and Casco found that people perceived the right fly's orbit to be 2.3 times larger than the left fly's, on average. Again, this is despite the orbits are, in fact, the same size!

Now watch the third video. Again, same to flies with identically-sized, but out of phase orbits, but this time with the background in phase with the "horizontal component" of the left fly's orbit (that is, the background's at 3 and 9 o'clock at the same time as the left fly), and with the "vertical component" of the right fly's orbit (meaning it's at 6 and 12 o'clock at the same time as the right fly). Now the left fly's orbit looks really wide and short, while the right fly's orbit looks really tall and thin. In Anstis and Casco's study, the left fly's orbit looked 2.2 times as wide and 6.3 times shorter than it actually is, while the right fly's orbit was 3.5 times thinner and 2.1 times taller than it actually was.

Anstis conducted a third experiment as well, which unfortunately is not represented on his demos page (at least I can't find it), but if you have access to the Journal of Vision, you can watch it here (it's Movie 4), which basically replicated the second experiment with "interrupted motion."

Anstis and Casco don't offer a detailed explanation for the illusion, but they does provide this suggestive real-world example of an analogous illusion of motion against a moving background:

Johansson (1950) pointed out that when a friend waves to you from a train, his or her hand traces out a horizontally extended sine wave relative to the earth. However, that is not what you see. The visual system decomposes the movement into the linear motion of the train plus an up-and-down movement of the hand. (p. 1087)

In this case, and likely in the bluebottle illusion, your visual system is trying to separate the different sources of motion so that it can represent each with the effects of the other excluded. In the waving-from-a-train example, this makes it look like the person's waving hand is moving just as it would if the person were standing on the train station platform with you. In the bluebottle illusion, then, the visual system tries to subtract the background motion from the flies' motion, but because the background motion is in full or partial phase with the fly's motion, the visual system doesn't subtract the background's from the fly's motion, and you get the illusion.

1Anstis, S., & Casco, C. (2006). Induced movement: The flying bluebottle illusion. Journal of Vision, 6, 1087-1092.

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Is this really an illusion? To me it seems that the brain is working exactly as it's supposed to, observing motion relative to the background. What other frame of reference could the brain possibly use? The interesting thing is that the brain uses the immediately surrounding background, instead of the entire field of vision as a reference.

Just by coincidence, I was recently reading this:

Masland, R.H., The Retina's Fancy Tricks. Nature, 423 ( 22 May 2003) 387-388.

In the light of the work described there, it seems probable that this illusion does not arise in the brain, as one might expect. The segregation of object motion from background motion appears to be "computed" in the retina itself. As other studies (such as much recent work on eye movements*), also show, the traditional/popular view of the eye as acting more or less like a TV camera, sending digitized pictures back to the brain for analysis, is waaay off base.

*E.g.: Rucci, M., Iovin, R., Poletti, M. & Santini, F. (2007). Miniature Eye Movements Enhance Fine Spatial Detail. Nature, 447 (#7146, 14th June 2007), 851-854.

Bompas, A. & O'Regan, J.K. (2006). Evidence for a Role of Action in Colour Perception. Perception, 35, 65-78.

[Incidentally, the Journal of Vision is open access, so everyone should be able to follow your link to the paper there........ But then they would miss the weird cartoon of Rama in the demos on Anstis' web site. What's that all about!]

I wonder same as Flaky, if this should be named "illusion".

If we don't stop somewhere, we would need to call every similar case where a thing moves vs. a background in movies illusion, because we see those things as moving vs. that background, and not relative to the TV screen.


Place a ruler (or string etc) horizontal and edgewise midway down across the last 2 images.

Are the perceived difference in radii really an illusion?

Or is the "experimenter" simply moving the original focal point in image 1 without enlarging the "window" in images 2 & 3 thus and predictably restricting the observer's visual range?

Our eyes shift several times a second, and each time send a different image to the retina. But the brain (or is it the retina) assumes that the world is not jerking around and so makes it look like the background is static, while what is in front of it is moving, a reasonable assumption as that is almost always the case.

And example of this is when you stand say ten feet from the side of a train, so it fills your whole vision, and the train quietly starts moving. It is very easy to become disoriented and think that the train is static and your own body is falling over.

By bobo the chimp (not verified) on 04 Nov 2007 #permalink

I had pretty much the same illusion Bobo is describing earlier today. I was about to back out of my space in a parking lot. I'd shifted into reverse, but still had my foot on the brake. The car to my right (unexpectedly) started to back out, and the impression that I was moving forward was so strong that I slammed on the brake to "stop" myself.

I also had some problem accepting this as an "illusion", but it's an interesting effect nonetheless. As to the background used in the movies though.. is that V.S. Ramachandran, and what's he doing in the picture?!

Over a period of five years I have been in love with X, but during this time, both of us have been involved with other partners. This has caused problems. I have now separated from my girlfriend.
X has been married for a number of years but plans to separate after her daughters A levels in the summer.
During this time, there have been questions of love and trust. X is a wonderful woman and loves dancing but has a propensity to drink a little at belly dancing nights. This has now extended to Latin dancing classes.
As an experienced black male musician, I am well aware of the pitfalls of a western woman in their mature years entering the arena of salsa clubs- a sexual- social ritualised world dominated by black men. Often, I am afraid to say, western ladies fall into a minefield of sexual exploitation, whether they are there with genuine reasons to learn the dances or not. Western women are always viewed as easy sexual and economic prey in these situations. With over twenty years experience as a black musician, I am ashamed to say this is the case throughout the world. Cuba, New York, London-at every level-dance classes, clubs and bars etc.

I have tried not to let this affect my relationship with X, but it has. X assured me that she will not end up another 'victim' of this world, but after an arguement, ended up with a man at a social event and had unprotected sex.

After health checks at the hospital proved negative, I was angry, but realised that my initial rejection of X caused her to feel unwanted. I felt responsible for her drifting away to this world of latin music that I initially painting as a world of musical riches(which it is)

After long denial, X confessed she was 'chasing shadows' and this is why she would go dancing by herself in clubs and drinking. X is a beautiful heart and soul, but extremly fragile. We love each other, and so we went abroad for a break to consolidate our feelings.

After a great night out where we had a meal and discussion. On the way out of the resturant, X approached a stranger-a young boy of twenty at a bar and kissed him-She said she did not know why-she said she was merely happy in my company and wanted to share that before offering her chair to this stranger.

These sort of inexplicable events have coloured the course of our relationship. From what X says to me, she is repeating the same behaviour as her mother, who had a history of getting of with other men when married.

X lost her father to cancer when she was young and married a much older man who became impotent-but in every other way gave the best of his love to X and their children.

I feel that I have become the sexual cliche, in so far as I am younger than X and fulfilled part of the sexual needs of X, but cannot really trust her.

X wants me to be there for her when she finally separates later in the year but the damage caused to all(partners, family, ourselves) may be too much to rebuild.

I have asked X to consider counselling. X says she will once she is divorced.

In short, all of this has made me unsure.

Do u think X has a sort of psychological/validation/needs condition that can be worked on with professional help?

I hope I have not made X sound a freak or anything-she is the love of my life.