Move your mouse over the image above and the "missing slice" appears to be revealed. The two images are identical. When you move your mouse over the image it is simply rotated by 180º. It is only one's experience and familiarity with the representations that can cause one's brain to interpret the images differently. A slice of Brie, or cake is more commonly seen lying flat on a plate and viewed from above. Typically one's brian does not see a slice standing up on its end, but that would be an equally valid interpretation of the first image.