There was a particularly interesting study done on it back in 1995. Researchers then examined a single patient, one who had experienced significant brain damage. According to the researchers, he was “able to see color but little else”.
After showing him red-and-green gratings (although he was not really able to notice the gratings), the patient still reported that the effect worked when using a black-and-white grating after. The researchers concluded that the effect thus was probably occurring within the visual cortex, or somewhere between the eye and the brain.
A thesis submitted by Julien Ciroux to Edinburgh University agreed with this. He wrote that “the processing mechanisms involved in the ME [McCollough effect] are mostly located in the primary visual cortex, even if the change in this early area of the visual system drives subsequent modification in the activity pattern of higher cortical areas.”
Some studies have looked into just how prevalent the effect is. One way back in 1969 found that various stripes colored red and green produced the after-effects. Weirdly, it found that if your induction image was green, then you’d see red colors on vertical grates and green colors on horizontal grates. If a red induction image was used, then those colors reversed.
These are the two induction images that can trigger the effect, if you alternate between staring at their centers for a few minutes. Fredifortakeoff/Wikimedia
