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What: This illusion serves as an instance of the reverse phi phenomenon. It deceives your brain into perceiving motion when there is hardly any, meaning the colors are changing while the main object remains still. This example I've created is inspired by the Twitter user, @Jagarikin, who has shared multiple videos of similar effects involving spirals and other shapes, which I also aim to recreate. However, I desired an interactive version.

How To: As you move your mouse (or tap) around the central circle, you should have the sensation that the circles on the page are changing direction, and in a way, that they're moving in that particular direction. There are also several interesting things you can do with the settings, such as adjusting the speed of the color change, or the size of the circles, and changing the background, which will greatly modify the effect. For me, the different speeds combined with the interaction produce a great effect. It's also fascinating to observe that the effect endures and sometimes feels more intense when in grayscale. When in grayscale, you'll also be able to discern the "trick" of the effect a bit more easily, as the colors are part of the distraction.

Explain it: First, I'll provide a link to a great paper about this effect and a short quote, and then I'll do my best to explain how it works based on how I constructed this (again, based on Jagarikin's video). Reversed apparent motion (or reversed phi) can be witnessed during a continuous transition 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 placed one above the other. In this case, we have a collection of RGB gradients that alternate between light and dark at the 25%, 50%, and 75% points of the total gradient (100%)... we then apply this gradient to the circles with a small 1 - pixel inset at the 25% and 75% points, which are rotated in the chosen direction. This implies that at any given moment, the circle will alternate between light and dark on its directional edges, which makes the mind (when the right background is set!) create an illusion of movement. You'll also notice that the sense of movement seems even stronger in the center and when changing direction, perhaps because of the alternating patterns of the arrows and the circles.