Is it moving?
When you look at a moving object, signals go to a special part of your brain. Damage to this area can stop you seeing movement, even though your sight is otherwise normal. One woman, who suffered such damage through a stroke, described what it was like. If she poured out a cup of tea, it appeared frozen in mid-air, like ice. When walking down the street, she saw cars and trams change position, but not actually move.