To the neurotypical among us, this is indeed an illusion. To us it looks like an object with a dark grey top and a white bottom, and our brains insist on seeing the bottom as lighter than the top. When we place our fingers over the seam, breaking up the appearance of a 3D object, we are surprised to note that the absolute grey value of the top and bottom are the same.
As a graphics geek, I know all about the illusions exploited by illustrators, but I still feel the twang in my brain when I cover the seam and uncover it, as my brain interprets the image either way. I don’t know how I’d cope if my head-meat didn’t try to trick me like that. To see things as they actually are isn’t always a blessing.
