Hmm. I noticed in lectures, if I stilled my eyes, most of the field of view would grey out, except for areas of motion (eg a lecturer's head or writing arm) which appeared normal. After motion stopped in an area, it would slowly grey out. When a motion started, its area would snap to normal, making it easy to spot onsets of motion. Eventually my eyes would twitch, and the whole field would refresh.
I first read about this back in the 1980s, in an issue of Science Digest. Couldn't find a link or reference on short notice, but here's something from the American Academy of Ophthalmology that explains the phenomenon, with an experiment to see the blood vessels in your eye:
Apparently, the brain tends to ignore visual stimuli that don't change over a short period of time, which allows you see "around" the blood vessels passing through the middle of your eye. By closing your eye, and moving a penlight around against your eyelid, you can make the vessels cast a shifting shadow on your retina that makes them visible.
The reason you usually see everything out in front of you is that various actions cause your eye to shift about just a little, just enough to cause the image on your retina to shift about enough for the brain to notice.
I've done this in the past with bugs in the grass. If I stare at a fixed point, I start seeing each individual bug moving through the grass, whereas normally they would be really hard to spot among all the fine details of the ground and grass blades.
