Magic Tic-Tac-Toe

Our visual system can interpret colors and shades in surprising ways. With this 3×3 Tic-Tac-Toe grid I would like to show that our sensibility to color brightness can be easily fooled. Well, do you notice something particular in the grid below?

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My optical illusion “Magic Tic-Tac-Toe” has been chosen to be among the top 10 finalists for the “2019 Best Illusion of the Year Contest