This optical illusion exploits a flaw in your eyes to fool you

‘I believe only what I see’, said the philosopher St Thomas Aquinas. No, we did not want to open this article with a quote to sound pompous, but, on the contrary, to show you that even the greatest intellectuals can be wrong.

Indeed, an optical illusion will make you doubt what you see; and perhaps St Thomas Aquinas would have turned his tongue seven times in his mouth if he had had access to Twitter in the 13th century.

Does this square remain the same colour throughout?

This optical illusion is by Akiyoshi Kitaoka. Akiyoshi Kitaoka is a professor of psychology at Ritsumeikan University in Kyoto, Japan. His Twitter account, followed by tens of thousands of followers, is full of optical illusions and visual discoveries.

One of these illusions shows a square moving sideways on a coloured background, going from an orange to a blue hue. So far, nothing unusual except that, as the professor points out:

[The] moving square seems to change colour, although the colour is constant.

Chromatic induction

You can take a screenshot and use image editing software to make sure, but the colour of the square remains the same from start to finish. So why does our brain assure us otherwise? The answer is two words: chromatic induction.

In an article, the French Institute of Education gives the following scientific explanation:

We can consider that the shift of the perceived colour towards green, when the initial colour is placed in a red environment, comes from a desensitisation of the L receptors caused by exposure to a colour of long wavelengths,

The L cones are then more saturated and more desensitised than the M ones and the perceived colour results from a stronger activation of the M ones, it thus appears ‘greener’.

If you want to know more about chromatic induction, we invite you to consult this 2016 study which explains the phenomenon in more detail.

This article was translated from Gentside FR.

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