Here is an interesting eye test/experiment that stresses the importance of scanning and not staring/fixating on a specific point.
http://www.msf-usa.org/motion.html (http://www.msf-usa.org/motion.html)
Thats was quite instructive. I've always known that (scanner training in CAP, crewchief training in the Army), but it helps to actually see it.
I spend quite a bit of time in the back of a Twin Otter with the door open. I am consistently amazed at the number of my fellow enthusiasts who seem to think that "airspace clearance" means "Look straight down and make sure no clouds obscure the DZ".
I am, unfortunately, less frequently at the door when the red light goes. When I am, however, the door goes open, and my head goes out of the plane. Straight down, ID the DZ. Lean out and look under the plane for traffic off the right side. Scan the entire left-side from straight down vertical to about 45 degrees out, as far forward and aft as possible.
More than once I've spotted a spam can down low (3500 ft or so) headed toward the local controlled airpatch 6 miles away in a position where if he makes a turn of some sort, he'll be over our airspace pretty quickly. I call "Traffic" and people are scrambling to look and find it. And they can't. The funny thing is, they act like I'm seeing things. The pilot thought I was trying to pull a fast one on him once, I said "No, there's traffic." He said "I don't see it.." "Its a Baron ... " That reminded him that I know what I'm talking about and that I actually _did_ see something.
But I had a guy ask me one time why I "scanned' (and I make a very obvious "left-right/right-left" scan using my head when I'm looking) like I did when I'm in the door. I explained off-center vision to him. Now I'll point him to this web page.