Photo: pindec (Flickr)
This is a psychology-of-perception demonstration you can experience in a room whose walls are all painted the same color.
Look into a corner of the room, where two walls and the ceiling meet. Almost certainly one wall will be receiving more light than the other from some source, so one wall will look lighter than the other. Yet, somehow, your visual perceptual system correctly tells you that you are seeing two walls, painted the same color, meeting at a corner.
Now close one eye and hold up pieces of cardboard so as to block out the ceiling and the floor, restricting your view to the two walls and the corner where they meet. The scene will then look dramatically different.
You’ll see two flat-looking areas of color meeting at an edge, like two paint sample chips butted against one another. The impression of two walls meeting at a corner will be gone.
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.
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.