Adapted from B. Laeng and T. Endestad, PNAS Early Edition (2012)

ScienceShot: Brightness Is in the Eye of the Beholder

Stare into a camera's flash or walk into the sunlight after watching a movie, and your pupils will contract. But will the same thing happen if you just look at an image of a bright object? Yes, according to a new study, published online today in the Proceedings of the National Academy of Sciences. Researchers asked volunteers to stare at a series of visual illusions of brightness—including teardrop shapes arranged in a circle so that the center looked brighter than the outside (left, above. Image on right is the control) and then measured what happened to the subjects' pupils. The team found that the brighter an object appeared to be, the more the pupils contracted. After the initial contraction, the pupils gradually dilated to reflect the actual amount of light reaching the eye. The results add to previous research that has shown that our pupils dilate when we look at something that fascinates or intrigues us, suggesting that the so-called pupillary light reflex isn't merely an automatic response (like jerking your hand back from a hot stove). Instead, inputs from higher brain functions, like those responsible for interpreting what we see, also play a role.

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