Ever heard of the McCollough effect? It’s this weird trick of the mind where, after staring at a colored grating (alternating lines), your brain starts to see a pinkish tinge or other colors when looking at black-and-white lines.
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.
Over the years, there have been a number of studies done on the effect. Back in 1975, two researchers tested five groups of 16 people and, amazingly, one of the groups showed no lessening of the effect after five days. In fact, the effect remained better than half strength for four groups up to 2,040 hours later – or almost three months.
You can test the effect for yourself, with the images on the next page. Note, there is a chance it can affect your vision for a while – although it only really gets triggered when you see vertical or horizontal lines afterwards. For the most part, it appears to be harmless. Up to you.