Scotoma is a typical spot(s) caused by visual problems surrounded by the normal visual activity.The picture shows a case of Scotoma.It is said that this Scotoma is not a constant spot.It moves around in the vision.
Many Doctors say that Scotoma is the problem of collision of the vision of the two eyes. Even a normal person who is not seeing such light/dark spots in his vision has Scotoma unconsciously.For some people it is very much scintillating like no channel on old TV(see the pic below).
So,what's the reason for such conscious vision?.
The reason is not that one eye is not able to catch the picture .Lets take this way.we can’t see the objects that are extreme right with left eye but in normal vision we could see the whole objects??This is due to the optic nerve which sends the visual activity of both eyes to brain.And Brain overlaps the visionary part that is same in both images of respective Left and right eye and adds the right portion of right eye along with left portion of left eye.Each eye has its respective optic nerve.If there is a problem with this nerve then such problems arises in vision.So, visual signals absent in both eyes are mixed.If optic nerve got damaged enough which is unable to send signals then such spots will occur as these spots are not conveyed to brain By the Optic Nerve.
See this Scintillating Video of Scotoma.