What do flashing lights in your peripheral vision mean?

Migraine: The most likely cause for this symptoms is occipital migraine which causes a greying out of the vision the the corresponding peripheral 1/2 of vision of each eye with surrounding flashing lights lasting up to 2 hours and leaving with no trace. Persistent flashing in one eye only especially with lowered vision and floaters might be a retinal detachment and needs ophthalmological evaluation soon.