There’s an illusion popular on youtube.com right now here. Have a look – it’s a motion after-effect illusion. These are discussed in the book (Hack #25). The basic story is the same for all after-effects – continuous exposure to something causes a shift in sensitivity. For continuous motion this means that the visual system shifts its baseline so that, subsequently, stillness looks like movement in the opposite direction to the adapted-to direction. The nice thing about this demo is that is shows that you can have separate motion after-effects in different parts of your visual field. My top tip is to look at your hand at the end of the video for an extra-weirdness effect.
Also today someone asked me how the moving green dot illusion works. Answer: again, i think, it is an after-effect. The purple dots create a colour after-effect, a green dot. All the separate after-effects are joined together by the phi-phenomenon (Hack #27) to give an illusion of one single, moving, green dot.
To understand why we get after-effects, check out Hack #26 (‘Get Adjusted’). Which makes this post the biggest plug for the book I’ve done in a long while!