Have you ever heard of the McCall Effect? This is a strange feature of our brain - looking at alternating lines of different colors, we begin to see some shade of a completely different color.
To trigger the effect, you need to look at the center of two colored "exciting images" for several minutes, constantly moving your gaze between them. This works best with red and green lines. Then, when looking at vertical black and white lines, you can find that they have acquired a red, green, or pinkish tint.
Tilting the head 90 degrees can increase or decrease this effect. In fact, if you rotate the images and look at them again, you will notice that the hue has changed. And the longer you look at the original images, the longer this effect will last - hours, days, or even months in other cases.
But is it really so, and why is it happening at all?
The effect is named after its discoverer, Celeste McCall Howard. She was the first person to discover the so-called "extra aftereffect", which is an illusion that affects our brains over a long period of time.
Over the years, a number of studies of this effect have been carried out. In 1975, two researchers tested five groups of sixteen people each, and, surprisingly, one group did not show any effect after five days of study. In the other four groups, it manifested itself half-heartedly until the record of 2040 hours - or almost three months.
You can check the McCall effect for yourself, below are the necessary images. Note that there is a chance that this will affect your vision for a while - although for the most part it only works when you are looking at vertical or horizontal lines. You decide.
So what's causing it? There is no definite answer. There are three main hypotheses, the first is that something is happening to the neurons in the visual cortex. The other is that the brain is trying to color the world around correctly and comes to a dead end, and the third hypothesis is that this is like a cancellation effect, like a hangover from an overabundance of color.
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There was one particularly interesting study from 1995. In it, the researchers examined a patient with significant brain damage. According to scientists: "he could hardly distinguish colors."
After being shown the black and red gratings, although he could hardly distinguish them, the patient reported that the effect worked when looking at the black and white grating. The researchers concluded that this effect appears outside the visual cortex, somewhere in the path from the eye to the brain.
This is also confirmed by the thesis presented by Julien Cyroux at the University of Edinburgh. He wrote that "the processing mechanisms associated with EM (McCall effect) are mainly located in the primary visual cortex, even if excitations in this area of the visual system lead to a subsequent change in the activity of the higher regions of the cerebral cortex."
Several studies have tried to find out how common this effect is. In one experiment in 1969, it was found that any streaks painted in red and green had an effect. I also found that if you were looking at the green grid, you would see a red tint on the vertical white lines, and green on the horizontal ones. If you were looking at the red grid, then vice versa.
Oddly, it’s the rich red and green that work best. "Images with bright blue and bright yellow stripes, or pale red and pale green, had little impact," the study noted. As far as one can tell, there is no clear understanding of why exactly red and green are so good at creating the McCall effect.
Why lattices? This may be due to the fact that “neurons in the visual cortex respond best to a particular orientation and spatial frequency,” as one study puts it. It also offers an interesting theory based on the fact that the JPEG format uses a "checkered" structure, which is two grids superimposed on each other.
“Perhaps the effectiveness of this kind of visualization means that something similar is being used in our brains?” The researchers write.
And this seems to be true. Something is happening in the visual cortex and the brain is somehow deceived. The exact mechanics of what is happening are unknown, but it seems that this is a trick of the brain, not the eye.
Ilya Kislov