The main physiological explanation is that we only that there is only a very small area that we can see clearly. Our brain reconstructs that information to give

The main physiological explanation is that we only that there is only a very small area that we can see clearly. Our brain reconstructs that information to give us the illusion that we see a much larger area. We can only do this somewhat reliably because we normally constantly move our eyes. A second cause is that that our eyes do not respond well to a constant stimulus. If you could keep your eye in a constant position after a while you would not see any constant image.