I have just read about a visual effect called a scintillating scotoma.
Many variations occur, but scintillating scotoma usually begins as a spot of flickering light near or in the center of the visual field, which prevents vision within the scotoma area. The affected area flickers but is not dark. It then gradually expands outward from the initial spot. Vision remains normal beyond the borders of the expanding scotoma(s), with objects melting into the scotoma area background similarly to the physiological blind spot, which means that objects may be seen better by not looking directly at them in the early stages when the spot is in or near the center.
[...]The visual anomaly results from abnormal functioning of portions of the occipital cortex at the back of the brain, not in the eyes nor any component thereof, such as the retinas.3 This is a different disease from retinal migraine, which is monocular (only one eye).
It is a phenomenon that is believed to occur in the brain.
Scintillating scotomas are most commonly caused by cortical spreading depression, a pattern of changes in the behavior of nerves in the brain during a migraine.
That article is short and includes an impressive looking GIF of an optical image of the brain showing a wave-like pattern of blood volume which seems to come from the YouTube video Spreading Depolarization viewed using IOS. Santos et al. Neuroimage 2014 which links to Santos, E. et al. Radial, spiral and reverberating waves of spreading depolarization occur in the gyrencephalic brain NeuroImage, 99, 1 Oct. 2014, pp244-255.
The first Wikipedia article also says:
Symptoms typically appear gradually over 5 to 20 minutes and generally last fewer than 60 minutes, leading to the headache in classic migraine with aura, or resolving without consequence in acephalgic migraine. Many migraine sufferers change from scintillating scotoma as a prodrome to migraine to scintillating scotoma without migraine. Typically the scotoma resolves spontaneously within the stated time frame, leaving no subsequent symptoms, though some report fatigue, nausea, and dizziness as sequelae.
I don't completely understand the language there; I am not sure if it means that scintillating scotoma is always associated with some kind of migraine, or if they also occur as isolated events.
QUESTION: Why is it believed, or how is it known (if it is) that scintillating scotomas are caused by cortical spreading depression? Are these always related to some kind of migraine?