Mind, Vision, and a Challenge

Dr. Dale Purves delivered an excellent lecture today in MCB80 as a guest lecturer. The past few lectures have focused on vision and the biology behind it. Dr. Purves presented the class with a unique perspective. The above photo (from Dr. Purve's website), was probably one of the most intriguing shown. Believe it or not the gray at the top and the bottom are the same gray. Don't believe me? Cover up the space where the top and bottom meet and you'll see. Your finger or a pen will do just fine. See it yet? Copy the image, open it in Photoshop and use the color picker to see for yourself. The top and bottom grays are the same.

Looking at the image as is, it turns out your mind is telling you a lie! Why you may ask? Well its all about experience. Not just yours, but your father's, father's, father's experience as well. Rooted in your brain is the experience that when light shines on a surface, such as the top above, the color should have more luminance. A surface in the shadow, such as the bottom above, should appear darker than normal. What you normally see is dark gray on top and white on bottom. The odds of seeing equal grays on both top and bottom in the real world is probably quite low. Experience has taught us this. Take a picture of a dual colored cube with light shining above. Good luck getting the lighting just right such that the luminance on the top and bottom are the same. So there's my challenge. Reproduce this phenomenon in the real world - no Photoshopping. I can do a bit of that myself. I actually used Gimp, but who's asking.