For most people, this illusion shows an ever-growing void and invokes a feeling that you may be about to enter a tunnel.
But the image isn’t actually moving at all.
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Overall, the participant’s pupils consistently dilated when they saw the black hole against any lighter-colored background.
For an inverted version with holes of lighter colors, the opposite is true: The pupils shrank to take in less light.
It might be due to an evolutionary instinct that prevents our eyes from becoming overwhelmed by a sudden influx of light or darkness, the researchers write.
Though the holes are not actually getting larger, the brain thinks they are and tries to prepare itself to not get caught off guard by what could be a sudden change in light.
Study co-author Bruno Laeng told The New York Times that this could be due to the fact that some perceive the illusion as two dimensional, based on their past experiences.