I made this post in another thread, but thought it was worth having a thread of it's own so it doesn't get lost. I think it's a great way to see just how important head posistion is to picking up the two visuals. It also demonstrates how, despite what a few others have said, moving your head in any direction can instantly change the visuals.
Draw a large circle on a blank sheet of paper, and tape the paper on a wall roughly at eye level.
Now stand about 6' or so away from the paper, and hold your left arm straight in front of you at eye level. You may have to close your non-dominant eye (if you have one) to do this correctly.
Point only your left index and middle finger at the circle. Your middle finger should point to the left edge of the circle. This is the center-to-edge line. Your index finger should be pointing at the right quarter of the circle. This represents the aiming line, which in this case, is pointed at C.
Now simply move your head to the left or right. You'll notice that even the slightest movement will change the perception of where your fingers are pointing, despite not moving your arm.