What this shows is that our visual system, in the process of giving us our impressions of the world, takes in information from all parts of our field of view. Seeing the ceiling and floor helps us to perceive correctly that we are looking at two walls painted the same color but lighted differently, not two paint chips painted different colors. Binocular vision also helps — that’s the process in which the slightly different viewpoints of our two eyes contribute to our perception of depth. Again look into a corner where two walls painted the same color meet. When you close one eye, you eliminate binocular vision; when you block your view of the ceiling and floor, you eliminate information from other parts of your visual field. As a result, you see merely two areas, one lighter than the other.