McCollough

A few minutes spent looking at two coloured patterns can leave your colour vision altered for weeks.²

The effect is well replicated, it attaches itself to one orientation rather than to the whole visual field, and sixty years on there is no agreed account of what the visual system is doing.³

Celeste McCollough reported it in Science in 1965, as colour adaptation of edge-detecting units in the human visual system.¹ It was the first contingent aftereffect described: the tint a viewer then sees on a plain black-and-white grating is not smeared across everything they look at, in the way an ordinary afterimage is, but depends on the orientation of the lines in front of them.³ Horizontal stripes take on one tint and vertical stripes the opposite one, and the pairing follows whichever colours were used to induce it.³

Two properties separate it from a common afterimage. It does not transfer between the eyes: induce it in one eye and the test grating viewed with the other produces nothing, which places the mechanism before the first binocular cells rather than in the eye or in later, two-eyed processing.^{1, 3} And it lasts. McCollough's own report described aftereffects of an hour or more.¹ Jones and Holding gave observers a fifteen-minute induction in 1975 and found that those tested repeatedly over the following days lost it within five, while those left alone and not tested until eighty-five days later still showed it.² Decay tracks testing rather than elapsed time, which is an awkward result for any account resting on simple fatigue.

No mechanism commands agreement.³ The competing explanations run from McCollough's original adaptation account to error-correction and classical conditioning, and the evidence on the site points to early visual cortex without settling on one.^{1, 3}

That combination is worth weighing before going to look for the induction images. A few minutes of adaptation can leave a faint colour on gratings, tiled floors and lined paper for days, and the published evidence is that leaving it alone is what makes it last.²