They may be more obvious with eye movement, or in a dark room. Some people with migraine headaches may experience a different type of flash. These are usually shimmering, jagged lights that are present constantly for a period of fifteen minutes or so. They typically appear in the center of your field of vision, and progress slowly towards the edges of the visual field. A headache that is commonly throbbing, and on one side of the head, may follow the disappearance of the flashes.
Many people have floaters.
Floaters that you have had for years, and that show little change, are usually not serious. It is the sudden onset of a new floater or floaters that may be serious. The onset of flashes may also be serious. The vitreous is a jelly-like material that fills most of the space inside the eye. As we age, the vitreous often liquefies and may collapse.