by creating optical sensations with the repetition of line and shape and the manipulation of high-contrast values and complementary colors, truly is in the eye of the beholder.

I believe the correct answer is movement.

An optical illusion (also known as visual illusion) is an illusion that causes visual percept to appear different from reality. By creating optical sensations with the repetition of line and shape and the manipulation of high-contrast values and complementary colors, the movement is in the eye of the beholder.

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