Over at Motley News yesterday, Michelle posted a great bunch of optical illusions. (The second one is my favorite.) Many such collections tend to feature the same old illusions that we’ve all seen before, but she managed to find new ones. New to me, anyway. I’d only seen one of them before.

Still thinking about optical illusions this morning, I suddenly remembered a certain type of picture that was a big fad at my office for a while. I’m guessing it was maybe in the late ’80s. The pictures looked like meaningless, repetitive patterns until you changed your focus to either a few inches in front of the picture or a few inches behind. Then a previously hidden 3-D image appeared. After at least a half hour of searching with various parameters, I finally found what they were called, along with some examples. Autostereograms. Or just stereograms. (Or a brand name, Magic Eye.) I remembered their hidden images as being much more complex than the examples I was able to find. Disappointing, I’ll admit.

This autostereogram was in Wikipedia, and I grabbed it because it comes with a map of the hidden image. To me the 3-D image here appears to be concave instead of convex, but I may be focusing the wrong way. Click for a larger image.

Rather than spoil your attempts to see the 3-D image for yourself, I’ll just let you click this link if you give up and want a hint.

Anyone else remember these? I thought they’d come and gone in that one period, but now that I know what they are called, I see they are all over the Internet. Isn’t everything?

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I can’t resist posting this optical illusion I came across while looking for the autostereograms. I’d not seen this before. It’s called Lilac Chaser. Yes, it’s a .gif and the gap is moving around the circle, but that’s not the point. Stare at the black cross in the middle. In the moving gap, a green dot should appear. Keep staring, and all the lilac dots should disappear in sequence.

Wikipedia has a great list of optical illusions, with the name of each effect and details about how and why it works — if you want to burden the fun with scientific explanations. For example, this is the explanation of the lilac chaser illusion:

The lilac chaser illusion combines three simple, well-known effects:

1. When a visual event occurs briefly at one place in the visual field, and then a similar event occurs at an adjacent place in the same visual field, we perceive movement from the first place to the second. This is called apparent movement or beta movement, because no actual movement has occurred. Beta movement is the basis of moving neon signs, film, and video. We see movement because such displays stimulate receptors (called Reichardt detectors) in our brains that encode movement. The visual events in lilac chaser initially are the disappearances of the lilac discs. The visual events then become the appearances of green afterimages (see next).
2. When a lilac stimulus that is presented to a particular region of the visual field for a long time (say 10 seconds or so) disappears, a green afterimage will appear. The afterimage lasts only a short time, and in this case is effaced by the reappearance of the lilac stimulus. The afterimage is a simple consequence of adaptation of the rods and cones of the retina. Colour and brightness are encoded by the ratios of activities in three types of cones (and also the rods under mesopic conditions). The cones stimulated by lilac get “tired”. When the stimulus disappears, the tiredness of some of the cones means that the ratios evoked by the grey background are the same as if a green stimulus had been presented to these cones when they are fresh. Adaptation of rods and cones begins immediately when they are stimulated, so afterimages also start to grow. We normally do not notice them because we move our eyes about three times a second, so the image of a stimulus constantly falls on new, fresh, unadapted rods and cones. In lilac chaser, we keep our eyes still, so the afterimages grow and are revealed when the stimulus disappears.
3. When a blurry stimulus is presented to a region of the visual field away from where we are fixating, and we keep our eyes still, that stimulus will disappear even though it is still physically presented. This is called Troxler’s fading. It occurs because although our eyes move a little when we are fixating a point, away from that point (in peripheral vision) the movements are not large enough to shift the lilac discs to new neurons of the visual system. Their afterimages essentially cancel the original images, so that all one sees of the lilac discs is grey, except for the gap where the green afterimage appears.

These effects combine to yield the remarkable sight of a green spot running around in a circle on a grey background when only stationary, flashing lilac spots have been presented. Occasionally it seems as though the green afterimage has eaten up the lilac discs, this resemblance’s to Pac-Man accounting for the illusion’s alternative name.

Heh, bet half of you skimmed over that part …