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Invisibly morphing faces

In the animation below, you can see a ring of four faces gradually morphing between Leighton Meester and Natalie Portman. If you keep your eyes fixated on the green cross in the center, you will notice that the morphing is quite difficult to see—but only when the faces are moving. If you match the movement with your eyes, or if the faces stop moving, you can clearly see the morphing happening.

This illusion was presented twice, in different guises, in the finale of last year's vision sciences illusion contest. Once by Jordan Suchow and George Alvarez, who ended up winning the contest. And once by Rob van Lier and Arnon Koning. So this illusion may not be entirely original, but it is certainly the most aesthetically pleasing rendition out there!

So what's going on here? Previously, I explained this phenomenon in terms of retinal motion. When the faces move, they slide across your retina (the light sensitive part of your eye), at least if you keep your eyes still. So the same face will be 'seen' successively by different parts of your retina. And because the organization of the retina is roughly preserved in visual areas of the brain, the same face will be successively processed by slightly different (sub)areas of your brain. According to the 'retinal motion' explanation, these different brain areas do not communicate effectively enough for changes to be detected. At least not if they are small and gradual.

But, as it turns out, this …

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Boogie-woogie and living diamonds

The following illusion is a variation on the boogie-woogie illusion, described by Patrick Cavanagh and (yes, again!) Stuart Anstis. If you play the video and track the moving dot with your eyes, you will see that the edges of the diamond shape come to life. Specifically, the dots that make up the edges appear to travel along the lines. Kind of like the steps of an escalator.

So what might be going on here? I have to admit that my degree of belief in the explanation that I will outline here is modest. But that being said, here we go: Essentially, this illusion could be an instance of the aperture problem.

Imagine that you are looking through a hole, as in a in the figure below. Through this hole, you can see part of a bar, but not all of it. Now imagine that the bar moves, as in b, c, or d. Can you tell, based on what you can see through the hole, what the exact movement of the bar has been? No! As long as you cannot see the ends of the bar, all three forms of movement look the same.

So what do people perceive when presented with this type of ambiguous motion? Well, they tend to perceive a motion that is orthogonal to the length of the object (d). Perhaps we are biased to perceive orthogonal motion, because that's how objects generally move (do they, though?). Or perhaps it's because orthogonal motion is, in a …

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Stepping feet and tipsy toddlers

I recently came across this awesome optical illusion, first described by Stuart Anstis. Two bars, one red and one blue, move horizontally across the display. If a specific type of background texture is present, the two rectangles appear to move in anti-phase: When the red rectangle moves quickly, the blue one grinds to a halt, and vice versa. This is illusionary, of course, and the effect is gone when the background texture is removed.

The two rectangles resemble a pair of stepping (or shuffling) feet, hence the name: the stepping feet illusion. (The effect is strongest for some people if you don't look directly at the rectangles.)

The explanation for this illusion appears to be fairly straightforward (but see [1]). And, as any good illusion, it provides some insight into how our visual system works.

The crux is that the illusion will not work with just any pair of colours: There must be a luminance difference. Put differently, one stimulus must be bright (the blue rectangle in this case) and the other must be dark (the red one). In addition, there must be a comparable luminance difference in the background, which is achieved here through a pattern of alternating light and dark bands.

Now, let's say that the front and hind edges of the stimuli are on a dark band, as in a) in the figure below. In this situation, there is little contrast between the side edges of the red stimulus and the background (both are dark). Because of …

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The Loch Ness after effect

Last week I was at the annual meeting of the Vision Sciences Society. One of the more exciting events of this conference was the illusion contest. And of the more exciting illusions of this contest was the one that you can see below, created by Mark Wexler. (The winner of the contest, incidentally, was the awesome illusion by Suchow and Alvarez that I described earlier.)

You will see a slowly (or at least not very quickly) clockwise rotating ring. You will also see intermittent counterclockwise rotations, which are brief and much faster.

So what's going on here? Basically, and as you might have guessed, the counterclockwise jumps are illusionary. The only thing that happens is that for a few frames the coloured squares out of which the ring is composed are completely randomized. Obviously, this randomized "noise" is not really rotating in any particular direction. And yet we perceive a fairly clear counterclockwise rotation.

This illusion bears some resemblance to the traditional motion after effect, in which prolonged exposure to a motion in one direction results in a small, but clear after effect of perceived motion in the opposite direction. But usually the illusory motion is much slower than the real motion that induces the after effect. In contrast, in Wexler's illusion the illusory motion is much faster than the motion that induced the effect.

It seems to be that the "noise" amplifies the motion after effect. Even if the line-segments were not randomized, you would still perceive a regular …

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Silence of the changes

Suchow and Alvarez report a very compelling optical illusion in an upcoming edition of Current Biology. The illusion is very simple and I was quite surprised that it actually works. A cloud of colored dots is arranged in a circle around a central fixation point. In one condition, the dots gradually change color, but they don't move. As you would expect, it is very easy to spot the color changes in this condition. However, in another condition, the dots move around as well as change color. Surprisingly, in this condition the color changes are extremely difficult to detect! This is demonstrated quite nicely in the video below (provided by the authors).

How does the illusion work? An important clue is that retinal motion is required. If you match the movement of the dots with your eyes, thus eliminating the retinal motion, it becomes considerably easier to detect the color changes (not as easy as when the dots are static, but this is presumably because it is difficult to match the movement perfectly). Simply put, this suggests that we detect the color changes with neurons that “see” only a small part of our retina. If the dots move around on our retina, they are continuously “seen” by different neurons, and this compromises our ability to detect changes.


Suchow, J. W., & Alvarez, G. A. (2011). Motion silences awareness of visual change. Current Biology, 21, 1–5. [PDF]

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