I was always puzzled as to why soldiers standing at attention are trained to lock their gazes straight ahead, especially in the case of the guards outside Buckingham Palace, who are famous for staring straight ahead despite any distraction tourists may offer. It always seemed to me that an alert guard should be scanning the whole field of view constantly, not simply staring at a single spot.
Just recently, I found myself thinking about this while waiting for my wife to complete a transaction at the market, and I decided to try it out. I picked a spot on the wall and stared at that spot, and sure enough, it was hard to stay focussed, especially when someone walked in front of that spot.
But then I tried something else. I tried to pay attention instead to my peripheral vision, all the other things that were going on in my field of view besides the spot right in front of me. Taking in the entire picture, noting the presence and movement of everything, rather than trying to pick out the specific details that we look for when we focus on something.
And suddenly, I began to understand why the guards might be trained to stand at attention that way. Turns out, my field of view spans almost a full 180 degrees! So by attending to the entire image, I can actually monitor more targets than if I were to focus on just the portion on or near the fovea (the part of the retina at the very back which gives the most detail). What's more, while I couldn't make out much detail on anyone in particular, I was surprised to notice just how much I was able to fill in even without that detail. We humans read each other's body language very well, for the most part; I could tell that the tall person in the blue coat to my right was probably looking at his cell phone, without losing sight of my wife in her red coat to my left. My situational awareness felt much keener, studying the entire scene this way, which is precisely what you want in a guard.
Interestingly, I also realized that this would go a long way to explaining why it's so difficult for tourists to distract the guards by flashing their breasts or other such silliness. (Not that anyone flashed their breasts at me at the market. And I think I would have noticed.) I found that since my attention was actually spread more or less evenly across my field of view, things happening right in front of me were easier to dismiss. That is, I didn't exactly ignore it when someone walked in front of me and lingered for a bit, obscuring my view of the letter "E" on the sign I had been gazing towards. I knew they were there, of course, and what they were doing more or less; I just had so many other things going on in my field of view that the person who happened to be front and center didn't monopolize my attention. Also, because I was now tracking my field of view based on the positions of things at the periphery, I didn't need to remain locked on the letter "E" to maintain a steady gaze.
It also occurred to me that there's another reason why this kind of attention is useful for certain kinds of guards. Someone who is intently scanning like a searchlightback and forth may be better able to pick out fine details and identify anomalies, but they have bigger blind spots. Worse, those blind spots are more identifiable, and can be exploited. If you watch a scanning sentry, you may be able to tell when his attention is focused on the far end of his field of view, and use that opportunity to creep forward. A guard who stands, eyes fixed straight ahead, is much harder to read. What's more, the straight-ahead gaze is perhaps better adapted to pick up movement.
I did some modest web searching, to see if there are any documents about why guards stand at attention this way, but haven't found anything to confirm my theory. It makes sense to me, and my informal experiments seem to support it, but really it's just conjecture on my part. As always, discussion is welcome int he comments section.