Visual stimuli in the edge of my field of view really distracts me - if I'm looking at a pair of computer monitors I want any movement or blinking that I'm not expecting to be in the center of my field of view so I can quickly dismiss it. If it's at the edge of my field of view it (i.e. at the top or bottom of my screen) when I focus on it I'll be looking further away from my active work and it is more likely to drop me out of focus.