seeing a bright flash, like the afterimage of a flash bulb, when we blink, and
sometimes, on upgaze. This is transient symptom for most of us, lasting anywhere from a few days to a few weeks. When the "spot" in our visual field passes over a dark background, the spot appears light. When the spot passes over a light background, it appears dark.
So far, the doctors haven't been able to diagnose us, and perhaps that means it is simply some idiopathic phenomenon.