From the Mayo Clinic website:
My ocular migraines were never associated with pain, and I could never figure out what triggered them. I got several in a relatively short period of time, and then after that I didn't have another one until last night. The one last night went away really quickly, though - much more so than the ones from a few years ago - though I have no idea why. I took two aspirin as soon as I realized what was going on, so perhaps that did it. In any case, it's all quite odd. Thank goodness it clears up pretty quickly - it's an uneasy feeling (to say the least) to watch your field of vision slowly shrink.
Ocular migraines are characterized by abnormal visual sensations. They occur most often in people with a history of classic migraines. Ocular migraines are sometimes followed by head pain.
When an ocular migraine starts, you may notice a small, shimmering spot near the center of your field of vision. Initially, you may only be aware that something isn't quite right with your vision.
However, within a few minutes, the shimmering spot expands. You may become aware of a distinct visual abnormality accompanied by patchy vision loss - usually affecting both eyes. The shimmering area may also be bordered by silvery or colored zigzag light patterns. This zigzag pattern eventually expands into the outer part of the visual field. Within 15 to 30 minutes, the visual abnormality travels far out into the side vision and then disappears.
The cause of ocular migraines isn't clearly understood. But they're thought to be due to abnormal stimulation of nerve cells (neurons) at the back of the brain. Like classic migraines, ocular migraines tend to occur irregularly but repeatedly. You may experience several ocular migraines within a week and then not have any for months or years. Some people have heightened sensitivity to light or sound before the start of an ocular migraine.
Ocular migraines typically need no treatment. However, if they're often followed by headaches, you doctor may recommend medication to relieve headache pain.