The truth, with shades of gray

If you’ve ever claimed to know the facts because you’ve “seen it with your own eyes,” you probably haven’t come to grips with the ugly truth: Our vision lies to us.

Professor Edward H. Adelson of MIT offers a case in point. The image shown in Adelson’s Checker-Shadow Illusion, to your eye and mine, doesn’t seem to present anything terribly illusory–a green cylindrical object casting a shadow across a gray and white checkerboard. Done.

Or it would seem…

When the brain interprets the visual information in this setup–specifically, the color information–it does so while considering context. We know the square marked A is definitely gray, and the square marked B is clearly white. And we know this because A and B are defined by their surrounding local colors–white and gray, respectively. But you’ve already been made the fool, yet again, by your visual system! Take a pair of scissors to the checkerboard, cut out B, place it on A.  They’re the same.

Really, they are.  See for yourself. Don’t trust what you see? Here’s the proof.

But before you get too paranoid that your brain and eyes are in cahoots against you, we leave you with this bit of reassurance, courtesy of Dr. Adelson:

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

Stay tuned for Glimpse‘s Visions issue, where we’ll take you beyond tricks of the eye into the realm of the visually inexplicable…


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