Your eyes make up colors just for you

The McCollough effect is a strange visual illusion that can quietly rewrite your color vision for hours, days, and — in some documented cases — months. Discovered by American psychologist Celeste McCollough in 1965, it is one of the longest-lasting perceptual aftereffects science has ever measured. You stare at two colored line patterns for a few minutes, look away, and suddenly plain black-and-white lines are tinted with ghostly colors that no one else can see. It feels harmless. It is not entirely.

Here is what makes the McCollough illusion different from every other optical trick you have ever seen. A normal afterimage — the green blob you see after staring at a red dot — fades in seconds. The McCollough effect is measured in hours. In lab studies, it has persisted for more than three months in participants who were re-exposed periodically, making it the closest thing neuroscience has to a permanent, self-inflicted software update on the human visual cortex.

So what is the McCollough effect, in plain terms? It is a contingent aftereffect: a color illusion that only appears when paired with a specific orientation. You train your brain by alternately staring at a grid of horizontal green lines and vertical red lines (or magenta and green — the classic pairing) for about five minutes total, glancing between them every few seconds. Afterward, when you look at a plain black-and-white test image with horizontal and vertical lines, the horizontal lines look faintly pink and the vertical lines look faintly greenish. The colors are not there. Your visual cortex is inventing them.