Staring at two squares of opposing horizontal and vertical black, red and green lines for a few minutes, can permanently rewire an individual’s brain.
Take a look at this image first.
Now stare at this set of images for a couple of minutes.
Now go back to looking at this image again.
What you'll see is a green haze around the horizontal lines, and a magenta haze around the vertical lines. The intensity of this effect varies between individuals. If you don't see this, go gaze at the colored grids for a while longer.
While it might seem like a regular case of after-imagery, the catch is, that even after going back to a black and white striped image, the green-magenta effect refuses to vanish.
Staring at the set of coloured images for more than a few minutes can lead to afterimages that strangely last for days. Ten minutes of looking at it can affect your vision for 24 hours.
The effect of the coloured bars is such that, when the brain is shown lines that are really black and white, it will still subtract the colours, creating an illusion where the coloured lines are the opposite way around.
The McCollough effect is not your regular optical illusion. It doesn’t work on a retinal level but rather a cognitive one, and can leave an individual seeing white gratings as coloured for up to three months after staring at the image for anything between 15 seconds and 15 minutes.