More optical illusion stuff

This stuff always freaks me out.

Anyone know how this works?

That’s pretty cool! My guess is that it takes advantage of the eye’s ability to adjust for residual images. Essentially, the absence of the pink dot is perceived as green. For example, after looking at the whole ring for a bit, shift your focus to a different part of the page and you should see a residual greenish ring of dots. The part with the disappearing dots is even weirder; your brain is perceiving the pink dots as after-images of the (non-existant) green dot and compensating.

Also, here’s another illusion for you:

  • Alan

That’s cool.

It works because the pink spots are bright enough to leave an imprint on your retina which, as a pink spot momentarily disappears, appears against the neutral grey background as the spot’s negative colour, green. You can get the same effect by looking at bright green grass and then looking at a sheet of paper - you can see a purple image of the grass on the paper.

So, after a few moments of looking at the dots, keeping your eye in the same place, the green imprints on your retina cancel out the purple dots and they seem to fade into the neutral background.

I do. It’s a clever combination of several optical effects. The first is one that most people have encountered: a retinal afterimage. Simple explanation: your rods and cones become fatigued when exposed to moderately intense light for a period of time. Your brain is able to compensate for this (which is one of the reasons why our perception of color is relatively constant), but when you look away (or the stimulus is removed), the photoreceptors that the light was stimulating are no longer doing their thing. So you see an “afterimage” of the stimulus in a color of its spectral compliment (since the only photoreceptors currently functioning in that part of the retina are the ones that weren’t stimulated by the original light source).

So, in the illusion, you stare at a pattern of magenta dots. This saturates and fatigues the cones that are sensitive to red and blue light. Then the illusion removes one of the dots, and because only your green-sensitive cones are unfatigued, you get a green afterimage.

When the illusion does this quickly, in sequence, the green dot afterimage seems to be travelling around the circle. This is also a commonly-known illusion called “persistence of vision.” It’s the principle on which motion pictures and television operate.

Finally, the fading of the magenta dots is a phenomenon called the Troxler Effect. Basically, our brain tends to tune out static stimuli in our peripheral vision (which is why our peripheral vision is so sensitive to movement; it’s probably a trait that evolved because it allows us to better respond to threats). That’s why you have to fixate on the central point in order for it to work. Turning off the magenta dots in sequence enhances the fading effect, because when the magenta dot is turned off you see dark gray in its place (the same color as the background), which also leaves an afterimage (albeit a fainter one)–in light gray–that partly obscures the magenta dot when it appears again.

The fact that you still see the green dot when the magenta dots fade is evidence that the Troxler effect is a product of the brain rather than the eye. There is no actual movement in that image–just the illusion of movement–but your brain continues to perceive the green dots anyway, even when the magenta dots fade.

Edit: that link that Alan posted is also a great demonstration of the Troxler Effect.

Now let’s see this intentionally worked into a shooter.

Holy fucking shit. This is even better than that wicked grey=yellow one from a few weeks back.