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Rotating colour pie

In the image below (which I have adapted from an illusion by Kitoaka) there are three greenish “spokes” rotating counterclockwise. (You can download a script to generate this type of images here.)

Or are there? Actually, the greenish and blueish areas are the same “aqua” colour. The only difference is that in the greenish areas, aqua is interspersed with orange, whereas in the blueish areas, aqua is interspersed with pink. This becomes clear if you look at the image below, in which all pink has been replaced with orange. By the way, the illusion works equally well in a static image. I just added the rotation to make it look extra cool.

Initially I thought the illusion had to do with colour opponency. There are visual neurons which respond to yellow and are inhibited by blue, and vice versa. Because pink is essentially orange with some added blue, you might think that if aqua is interspersed with pink, it becomes more “yellowish” and, conversely, if aqua is interspersed with orange it becomes more “blueish.” However, on closer inspection the illusion works the other way around: Aqua actually looks more blueish next to pink, than next to orange!

Monnier and Shevell describe an illusion which is similar to the one presented here and offer a tentative explanation. Essentially they say that the colours appear to “spill over.” That is, if aqua is next to pink it simply “borrows” some of the blue that is present in the pink. However, this …

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Bulging checkerboard

One of the great creators of optical illusions is Akiyoshi Kitaoka and the following cool illusion is by his hand (he is also the creator of the awesome “rotating snakes” illusion). As you can see in the image below, the checkerboard gives a strong sensation of “bulging out the screen”. Of course, it's just a regular checkerboard. I made the picture fuzzy, so that (to me) it appears like a mysterious sphere hovering in a dark space, but that's really just being fancy. All that is needed for the illusion to work are the little dots (a script to generate this type of images can be found here).

How does this illusion work? According Michael Bach (who's website, incidentally, contains an impressive collection of optical illusions) this is unclear. Kitoaka has a book on optical illusions, which might contain the secret, but I don't have this book in my possession and, at any rate, it's Japanese. So I will venture a guess. I think it has to do with the fact that we tend to see lines in everything. If we consider, for example, the squares in the top-right part of the checkerboard, the pattern looks like (a) in the figure below. Because we tend to interpret the margin around the small white spots as lines (b) and because these perceived lines are not perfectly aligned, we perceive a slight tilt (c). By cleverly positioning the small dots differently in different quadrants of the checkerboard, the whole checkerboard appears to …

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Reverse spokes

Another cool illusion is the “reverse spokes” illusion, which has been studied by Anstis and Rogers (and probably discovered as well. Anstis describes the illusion very modestly as “new, or newish”.). As you can see in the image below, the spokes (the grey lines separating the “pie-slices”) appear to be rotating clockwise. On closer inspection you can clearly see that this is not the case. In fact, what is happening is that a “colour wave” traverses the wheel in a counterclockwise direction. That's weird. The actual movement is in the opposite direction of the movement that we perceive! (you can download a script here to generate these types of animations)

Unlike the illusions that I described in previous posts, the reverse spokes-illusion can actually be explained fairly easily. The spokes are occasionally “gobbled up” by the pie-slices, because two of the pie-slices are of the same colour as the spokes. In the figure below you can see a close-up of two pie-slices and a spoke. Initially, the spoke is clearly separate from the pie-slices (step 1). As the pie-slices become brighter, the spoke becomes the same colour as the pie-slice on the right. Essentially this means that the spoke becomes part of the right pie-slice. As a result, the boundary between the two pie-slices is placed a bit to the left (step 2). In the next time-frame, the spoke is the same colour as the pie-slice on the left and, as a result, the boundary between the pie-slices appears to …

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Café Wall

Another very striking illusion is the famous Café Wall illusion, first described by Gregory and Heard in 1979. The story goes that a colleague of Gregory and Heard felt that the black and white tiling on the wall of a local bar (they worked in Bristol) conveyed a wedge-like impression. They brought this initial observation into the laboratory, refined it and came up with something like the image below. As you will no doubt agree, the illusion that the horizontal lines (which, of course, are perfectly parallel) form a wedge-like pattern is very strong (you can download a script to generate these type of images here).

To the best of my knowledge, this illusion has never been fully explained. There are, however, some clues. One important factor appears to be that the grey line (all tiles are separated by grey “mortar”) is interpreted differently depending on whether it separates two tiles of the same colour or two tiles of a different colour. If the grey line separates two tiles of the same colour it is interpreted for what it is: A grey line. However, if the grey line separates a black and a white tile it is interpreted as a slight “blurring” of both tiles and not as an actual line (this also explains why the illusion only works if the luminance of the “mortar” is intermediate between the luminance of the tiles).

However, the question remains why defective “border-locking” (as Gregory and Heard call it) results in the particular …

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Pinna - Brelstaff

One optical illusion that I find particularly compelling is the relatively new (newly discovered that is, the illusion was there all along of course) illusion of relative motion, first described by Pinna and Brelstaff. I'm sure you've already figured out how the illusion works, but here it goes: Fixate on the central dot in the image below and move your head slowly towards and away from the image. The circles will appear to rotate in opposite directions. (The example below is actually a variation by Gurnsey and colleagues. The original illusion uses geometrical shapes rather than Gabor patches, but the principle is the same. You can download a script to generate these type of images here, it's fun to play around with.)

How does this illusion work? To the best of my knowledge this is not entirely clear, although Pinna and Brelstaff as well Gurnsey and colleagues provide a discussion which goes some way towards an explanation.

One way to think about how this illusion might work is to consider what happens if the circles really move as they appear to do in the illusion. Let's consider the patch at the bottom of the inner circle (see (a) in the figure below, for clarity the patch is larger that the patches used in the illusion). If the circle rotates counter clockwise (as it appears to do if you move your head towards the image) the patch will move to the right (b).

Now let's consider what happens if you move …

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