It’s said that to trigger the effect, you simply stare at the center of two colored “induction images” for several minutes or more, switching back and forth repeatedly. It works best with green or red lines. Then, when you look at vertical black-and-white lines, you’ll find it appears red, green, or pinkish in places.
Tilting your head 90 degrees may lessen or enhance it. In fact, rotating the induction images and staring at them again may actually reverse the effect. The longer you stare at the original induction images, the longer it’ll last – for hours, days, or even a few months in some cases.
But is that actually true, and what’s causing it if so?
The effect is named after its discoverer, US psychologist Celeste McCollough Howard. She was the first person to ever find a so-called “contingent aftereffect”, which is an illusion that affects your brain for an extended period of time.
