Symptoms first. I stood up from my office chair and stared up at a large wall map above my head. Since I sat down a portion of my vision is somewhat blurred (can't see exactly what i'm looking at, just next to it). When i close my eyes i see a patern where the 'hole' is in my vision when i open my eyes. I see the same pattern in both eyes, though not in exactly the same quadrant. It dissipates with time, but only after approxiamtely twenty minutes or so. Question: What is it, why did it happen and how can i prevent it? thanks!