Walking on the beach. I'm always amazed at the ability to walk barefoot at a normal pace on a rock covered beach while talking with others, and yet not stepping on any rocks. It seems our visual system has a "low level" background process to watch where we are placing our feet, and to advise us without us noticing consciously that it is doing this. Try walking backwards just a few steps, or forwards with your eyes closed, and you'll quickly see that rock avoidance is not due to rock sparsity, but instead due to some visual processing. Bob points me at this paper. ¶8:24 PM; digg