If you want to test it, look perfectly forward, and raise your right arm to the front, and then start moving it to the right- very slowly, all the time looking forward. You're looking at the hand with your peripheral vision. You will notice as you move the arm, that there is a section in which the hand dissapears, and if you keep going it reappears again- you have found your 'blind spot'.
Since the brain already knows what's to your right (or left, depending on which arm is up) it 'fills in' the gap with whatever's supposed to be there (from visual memory), but this is not a perfect process, and it's fast, as we constantly move our eyes.
But when you're alone, introspective, or meditating, you tend to observe without observing, and this is when you see 'interesting things.'