my mother has been experiencing very rapid/quick episodes of temporary loss of vision.

Mom describes the episodes like this, she knows when the "episode" is about to happen by a feeling coming upward from the lower part of her chest and then all of a sudden she has a black out of vision - loss of vision but is completely aware, that is, she has not blacked out because she is concious, it creates a temporary "loss of vision" and last usually about 5 seconds. She is very upset after the episode, and we are terrified. My mother has seen neuro- specialists and has also had MRI and CT scans but no one seems to understand what this is.