That's what I've been paying attention to. When I orient quickly (either startle reflex or just fast orientation) on something behind me, both head and eyes move. I wasn't able to find a study that discussed this when I looked a few days ago - ran into too many talking about the visual blanking during saccades. I may try again with a search in a journal engine, to see if keywords are more useful there.First picked it up in combat shooting, in re-targeting. I think it has to do with sacades of the human body. (you might want to start there)
I don't remember what studies I've read, but they're out there, and there were a lot of them if I remember correctly. What I do remember is testing it out. The eyes move pretty quickly from right to left and vice versa in a short field in front of you. Like if you look to the wall in the room you're sitting in right now and go left to right. But further from that front spectrum, like to your far right or left, the head snaps quicker than the eyes do, seems more naturaly as well. If you get startled from the side, what moves - your eyes, or does your head snap that way?