Shadow Boxing

The Massachusetts Institute of Technology (MIT) is a leading institution in the study of all things science, technology, engineering, and math — which includes, apparently, vision science.  After all, the picture above — which you may already be familiar with — comes from Edward H. Adelson, a professor of vision science at MIT.  (See a larger version here.)  Called “checkershadow,” the picture contains 25 squares, some obscured by a green cylinder in the upper right quandrant.  Two of the squares are labeled, one “A” and another “B.” The cylinder casts a shadow toward the bottom left of the field, engulfing the lighter gray square, B, while the darker gray square, A, remains untouched.

Kind of.  “A” is not really the darker square.  In fact, “A” and “B” are the same color.  Our eyes — or perhaps more correctly, our brains — are the victim of the MIT professor’s incredible trick.

If you don’t take his word for it, Adelson offers proof — two gray bands which demonstrate that “A” and “B” are identical, as seen here.  Not enough? Adelson suggests printing and cutting out the image and comparing the squares by hand, or, if you are skeptical of your printer (or simply looking for an easier proof), use software such as Photoshop or this Google Chrome extension to automatically grab the color information and confirm his assertion.

Why do our brains malfunction here? Adelson cites a few factors.

First — and this is probably not surprising — we “see” colors, in part, based on how they contrast with their surroundings.  Square “A” is bordered by squares lighter than it, so our brains register it as being “dark.” On the flip side, square “B” is bordered by squared darker than it, so in our mind’s eye, it’s the light square.

The more incredible part comes from the soft edges of the shadow. Our brains have developed in such a way that we tend to ignore soft, gradual changes in color. We do this because those types of changes (as is true here) are caused by shadows, and shadows typically (but not in this case) would mislead us into thinking that a color is darker than it is, typically. The sharp edges of the squares, on the contrary, are treated as true indicators of a change in color.

But while these signals work beautifully in the real world, they fail us here. The two boxes in question are identical in hue, even if our brains — manipulated by vision scientists at MIT — tell us otherwise.

Bonus fact: Our eyes (in concert with our brains) can also take control of our hearing. Let’s say that someone is making the sound “bah” repetitively, but for some reason, their mouths move in a way which would typically be done to make the sound “fah.”  Your brain will think that the person is saying “fah” — that is, what your eyes see trumps what your ears hear, due to something called the “McGurk Effect.” Need proof?  Check out this incredible video — and for further evidence, listen to the part from (roughly) 0:50 to 1:00 without watching the images.

From the Archives: Hidden Messages: The above is about seeing what isn’t there.  This is about not seeing what is.

Related: “Sleights of Mind: What the Neuroscience of Magic Reveals about Our Everyday Deceptions” by Sandra Blakeslee. 19 reviews averaging 4.5 stars. Available on Kindle.