(WHTM) — There’s nothing like a bewildering optical illusion to get the weekend off to a good start. There are all kinds to choose from: the image that looks like one thing but also looks like another, lines that look parallel when they’re not (or not parallel when they are), still images that look like they’re moving, pictures that trick your eye into seeing colors that aren’t there, the list goes on and on.

But how do these illusions work? Scientists have been trying to figure that out for years. Recently, three psychologists, Bruno Laeng and Shoaib Nabil of the University of Oslo, and Akiyoshi Kitaoka of the Ritsumeikan University in Osaka, ran an experiment to try to figure out why one particular optical illusion does what it does. Their results were published in the online journal Frontiers in Human Neuroscience.

Without further ado, here’s the illusion:

Example of illusorily expanding central region or “hole.” (Nota Bene: viewing the image full screen is best for eliciting the effect). (From the original article.)

This is an example of a dynamic optical illusion, where an image gives the impression of movement when there is none. In this case, it gives the impression the black hole is slowly expanding.

The researchers’ hypothesis was that the illusion works by fooling the brain into thinking it was about to move from a bright area into a dark one, like going down a tunnel, and that the brain was preparing for it by dilating the pupils of the eyes.

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To test this, they showed versions of the images to 50 volunteer test subjects. The images had the black spot in front of blue, cyan, green, magenta, red, white, and yellow backgrounds. They also showed the subjects images with white spots, as well as colored spots-twenty-six patterns in all. An infrared camera recorded the expansion (or lack thereof) of the pupils.

The researchers report in the article that the results of the experiments are “consistent with our hypotheses that, when the central region is black, the pupils adjust proportionally to the motion illusion to prepare for a change in luminance.” There was little if any change in the size of the pupil with white or colored spots.

They also found something else. Of their 50 test subjects, seven saw little or no expansion of the central hole. “It remains unclear what could be the reasons behind these individual differences.” the researchers say.

To read the original article, click here.

Want more optical illusions? click here.

For a list of types of optical illusions, click here.

For more about optical illusions in general, click here.