Something is amiss, are they moving or not? How can this be.

What: This illusion is an example of the reverse phi phenomonen, fooling your brain into seeing motion when there is very little, that is, colors are changing but the primary object is not at all.

This example I’ve built is inspired by the twitter user, @Jagarikin [1] who has posted multiple videos of similar effects including spirals and other shapes, that I also hope to re-create. Although I wanted an interactive version.

How To: As you move your mouse (or tap) around the central circle you shoudl experience what will feel like the circles on the page changing direction, and in some ways the feeling that they’re moving in said direction. There’s a few fun things you can do with the settings too, like changing the speed of the color change, or the size of the circles, as well as changing the background which will significantly alter the effect.

For me the different speeds along with interaction give me a great effect, but its also cool to see that the effect persists and sometimes feels stronger when in grayscale. When in Grayscale you’ll also be able to see the “trick” of the effect a little easier, as the colors really are part of the distraction.

Explain it: Firstly I’ll link to a great paper about this effect [3] and a quick quote, and then do my best explaination of how it works based on how I built this (again, based on Jagarikin’s [2] video).

Reversed apparent motion (or reversed phi) can be seen during a continuous dissolve between a positive and a spatially shifted negative version of the same image. Similar reversed effects can be seen in stereo when positive and spatially shifted negative images are presented separately to the two eyes or in a Vernier alignment task when the two images are juxtaposed one above the other.

In this case, we have a RGB gradient collection that alternates between light and dark at the 25%, 50% and 75% marks of the total gradient (100%) … we then apply this gradient to the circles with a small inset of 1 pixel to the 25% and the 75% which are rotated to the direction chosen. This means that at any given moment the circle will alternate on it’s directional edges light and dark, which causes the mind (when set to the right background!) to create a false movement.

You’ll also notice that the movement feels even stronger in the center, and when changing direction, perhaps due to the alternating patterns of the arrows and the circles.

Cites and Extras:

I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via, be kind!